&EPA
United States
Environmental Protection
Agency
Office of
fcadiatton Programs
Washington DC 20460
EPA 520/4-7W07C
Rsdiatiarj.
Technical Support of
Standards for High-Level
Radioactive Waste
Management
Volume C
Migration Pathways
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TECHNICAL SUPPORT OP STANDARDS FOR
HIGH-LEVEL RADIOACTIVE WASTE MANAGEMENT
TASK C REPORT
ASSESSMENT OF MIGRATION PATHWAYS
EPA Contract No. 68-01-4470
Prepared by
Arthur D. Little, Inc.
Cambridge, Massachusetts 02140
March-July 1977
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DISCLAIMER
This report was prepared as an account of work sponsored by the
Environmental Protection Agency of the United States Government under
Contract No. 68-01-4470. Neither the United States nor the United
States Environmental Protection Agency makes any warranty, express or
implied, or assumes any legal liability or responsibility for the accu-
racy, completeness, or usefulness of any information, apparatus, product,
or process disclosed, or represents that its use'would not infringe
privately owned rights.
ii
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ACKNOWLEDGMENTS
Many Individuals contributed to the vork done under the direction
of Arthur D. Little, Inc., for the U.S. Environmental Protection Agency
under Contract No. 68-01-4470. John L. Russell and Daniel Egan of the
Office of Radiation Programs at EPA served as constant guides in the
process of our vork. Dr. Bruce S. Old, James I. Stevens, and David I.
Hellstrom of Arthur D. Little, Inc., were Program Director, Program
Manager, and Assistant Program Manager, respectively, of the overall
project. Key individuals involved in each of the reports prepared
under the four tasks were:
TASK A
TASK B
JDonald Korn
"Arthur D. Little,
Task Director
Inc.
Robert McWhorter,
Michael Raudenbush,
and, Lester Goldstein
S.M. Stoller Corp.
_Edwin L. field
Arthur D. Little, Inc.
Task Director
TASK C
Robert McWhorter and
Michael Raudenbush
S.M. Stoller Corp.
P.J. O'Brien
Arthur D. Little, Inc.
Task Director
TASK D
Dr. Ronald S. Lantz
Intera Environmental
Consultants, Inc.
Dr. John Gormley
D'Appolonla Consulting
Engineers, Inc.
JDonald S. Allan
Arthur D. Little. Inc.
Task Director
Ajit Bhattacharyya and
Charles R. Hadlock
Arthur D. Little, Inc.
ill
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FOREWORD
A major Federal effort Is underway to develop methods for disposal
of high-level radioactive waste in deep geologic repositories. An impor-
tant element of this program is the development and promulgation by the
U.S. Environmental Protection Agency (EPA) of environmental standards
for the management of these wastes.
In anticipation of its efforts to develop these standards, EPA
recognized that it would be necessary to estimate the expected and
potential environmental impacts from potential geologic repositories
using modeling techniques based upon as thorough an understanding as
possible of the uncertainties involved in the quantities and charac-
teristics of the wastes to be managed, the effectiveness of engineering
controls, and the potential migration and accidental pathways that might
result in radioactive materials entering the biosphere. Consequently,
in March 1977, the EPA contracted with Arthur D. Little, Inc.,for a study
to provide technical support for its development of environmental regula-
tions for high-level radioactive wastes. This study was divided into
the following four tasks:
Task A - Source Term Characterization/Definition
Task B - Effectiveness of Engineering Controls
Task C - Assessment of Migration Pathways
Task D - Assessment of Accidental Pathways
The information presented in the reports on these tasks was developed
principally during the period March 1977 to February 1978. In the case of
this report, Task C, the information contained in it was prepared during
the period March-July 1977. There are many national and international
programs underway to develop additional data, especially in the fields
of waste forms, knowledge of geology and geohydrology, and risk assess-
ment. The information presented in these reports has been developed
on conceptual bases and is not intended to be specific to particular
conditions at geologic repositories.
iv
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TABLE OF CONTENTS
Page
Acknowledgements ill
Foreword iv
List of Tables viii
List of Figures x
C-1.0 INTRODUCTION 1
C-2.0 SUMMARY AND CONCLUSIONS 2
2.1 ASSESSMENT OF GEOLOGIC SITE SELECTION FACTORS 2
2.2 ANALYSIS OF MIGRATION POTENTIAL 4
2.3 DOSE-TO-MAN CONSIDERATIONS 5
C-3.0 ASSESSMENT OF GEOLOGIC SITE SELECTION FACTORS 8
3.1 IMPORTANT INTRINSIC PROPERTIES OF GEOLOGIC MEDIA 10
3.1.1 Hydrogeologic Properties 11
3.1.2 Geochemical Properties 16
3.1.3 Thermal Propert ies 27
3.1.4 Engineering Properties 28
3.2 IDENTIFICATION AND DISCUSSION OF SITE SELECTION
FACTORS 28
3.2.1 Lithology 30
3.2.2 Stratigraphy 36
3.2.3 Structural Geology 39
3,2.4 Hyjd rog eo logy 43
3.2.5 Tectonics 48
3,2.6 Erosion and Denudation 54
3.2.7 Mineral Resources 55
3.3 CATEGORIZATION AND RANKING OF SITE SELECTION
FACTORS 56
3.3.1 Categorization of Factors 57
3.3.2 Ranking of Factors 59
3.4 CONCEPTUAL GEOLOGIC MODELS 60
3.4.1 Bedded Salt 61
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TABLE OF CONTENTS (cont.)
Page
3.4.2 Salt Domes 64
3.4.3 Shale 66
3.4,4 Igneous Rocks (Basalt Model) 68
C-4.0 ANALYSIS OF MIGRATION POTENTIAL 71
4.1 MODES OF WATER CONTACT 71
4.2 EVALUATION OF LEACH RATES 78
4.2.1 Introduction 78
4.2.2 Empirical Leach Models 79
4.2.3 Theoretical Leach Models 82
4.3 GEOSPHERE TRANSPORT THEORY ASSESSMENT 83
4.3.1 Hydrology - Radionuclide Transport Model Review 85
4.4 MIGRATION OF RADIONUCLIDES IN THE GEOSPHERE 90
4.4.1 Introduction 90
4.4.2 Results JJsing Analytical Model 95
4.4.3 Results Using Numerical Model 104
4.5 COMPARISON OF THE EFFECT OF LOW-GRADE URANIUM
ORE VS. AN HLW REPOSITORY 125
C-5.0 DOSE-TO-MAN CONSIDERATIONS 128
5.1 OBJECTIVES AND OVERVIEW 128
5.1.1 Introduction and Purpose 128
5.1.2 Options and Considerations for Making Radiation
Dose Assessments 129
>
5.1.3 Conditions of Exposure and Persons or Populations
at Risk 132
5.1.4 Implications of the Use of a Linear-Nonthreshold
Dose-Response 136
5.2 BASIS FOR DOSE-TO-MAN CALCULATIONS 137
5.2.1 Introduction 137
5.2.2 Repository Release Scenarios 138
5.2.3 Pathway Models 142
vi
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TABLE OF CONTENTS (cont.)
5.2.4 ExposureRoutes
5.3 RESULTS Of RADIATION DOSE CALCULATIONS
5.3.1 Introduction
5.3.2 "Maximum."Individual Radiation gose
5.3.3 "tfaximum" PopulationRadiation Dose
5.3.4 "Reasonable" Population Dose
5.4 EVALUATION Of DOSE-TO-MAN CONSIDERATIONS
5.4.1 WorldwideImpactofIodine-129
5.4.2 Comparison with Natural^ Background Radiation
5,4.3 Comparison with Radium-226 Currently Present
in the Colorado River
References Cited
APPENDICES
Appendix C-I
Appendix C-II
Glossary
Radioactive Walste Migration
Model
Appendix C-III Parameters of Study
Appendix C-IV "Maximum" Individual and
Appendix C-V
Population Dose Commitment
Sample Calculations
"Reasonable" Population Dose
Methodology and Sample Calculation
Page
150
157
157
158
162
164
166
166
168
170
173
C-I-1
C-II-l
e-iii-i
C-IV-JL
c-v-i
vii
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LIST OF TABLES
Table No. Page
C-l Example Intrinsic Properties for Typical
Repository and Associated Lithologies 12
C-2 Permeability of Various Rock and Soil Types 15
C-3 Estimated Distribution Coefficients (K
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LIST OF TABLES (cont.)
Table No. Page
C-17 Physical Data and Dose Commitment Factors
for Significant Radlonuclldes 139
C-18 Radlonucllde Concentrations Following Diffusion
Into & Potable Aquifer (Scenario 1) 140
C-19 Radlonucllde Concentrations in Aquifer at
Aquifer Entry (Scenario 2) 144
C-20 Environmental and Biological Transfer Coefficients
for Dose Commitment Calculations 146
C-21 Information for River Model "A" for Reasonable
Population Dose Calculations 148
C-22 Information for River Model "B" for Reasonable
Population Dose Calculations 149
C-23 Fifty-Year Dose Commitment Calculated for the
Case of Diffusion Into a Potable Aquifer
(Scenario 1} 159
C-24 Dose Commitment Calculated for Aqueous Transport
by Flow into a Potable Aquifer (Scenario 2) 160
C-25 Summary of Total "Maximum" Individual and
Population Dose Commitments for Direct Ingestion
of Contaminated Aquifer 161
C-26 Summary of Reasonable Population Dose Commitments
by Significant Radionuclides and Pathways 165
C-27 Comparison of "Reasonable" Population 50-Year
Dose Commitments for Ra-226 from Scenario 2
and Ra-226 In the Colorado River 172
ix
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LIST OP FIGURES
FigureJfo. Page
C-l Ranges of Distribution Coefficients for
Various Rock Types 21
C-2 Radioactive Waste Repository Conceptual
Model in Tectonized Shale 41
C-3 Radioactive Waste Repository Conceptual
Model in Salt Dome 44
C-4 Regions of the World in Which Major Earthquakes
and Volcanoes Occur 52
G-5 Radioactive Waste Repository Conceptual
Model in Bedded Salt 62
C-6 Radioactive Waste Repository Conceptual
Model in Non-Tectonized Shale 67
C-7 Radioactive Waste Repository Conceptual
Model in Basalt 70
C-8 Two Flow Regimes 73
C-9 Cross-Section Data for Evaluation of Flow
Regimes 74
C-10 Calculated Interbed Velocities for Two Flow
Regimes 76
C-ll Comparison of Laboratory and In-Situ Leach Data 81
C-12 Comparison of Leach Rates for Two Leach Models 84
C-13 Numerical Representation of Geologic Models 91
C-14 Effect of Leach Time and Migration on Nuclide
Concentration 101
C-15 Schematic Illustration of Maximum Individual
and Upper Limit Population Dose by Diffusion
to a Potable Aquifer (Scenario 1) 141
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LIST OF FIGURES (cont.)
FigureNo. Page
C-16 Schematic Illustration of Maximum Individual
and Upper Limit Population Dose by Aqueous
Flow Transport to a Potable Aquifer (Scenario 2) 143
C-17 Schematic Illustration of "Reasonable"
Population Dose (Scenarios 1 & 2) 155
xi
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Page Intentionally Blank
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C-1.0 INTRODUCTION
In providing technical support of standards for high-level radio-
active waste management, this Task C report has three principal objectives:
• To assess geologic site selection factors.
* To review available information and quantify the potential
for the migration of nuclides through the geosphere to
the biosphere, and finally
• To consider dose-to-man implications of a repository for
high-level waste (HLW) containing large quantities of
radionuclides in high concentrations that might become
dispersed into the biosphere over geologic times.
The Task C work implicitly assumes that no repository is likely
to meet all criteria that would ensure absolute integrity of the
repository over geologic time or even over one or two centuries.
Rather, repository integrity will involve:
.» Selection of a site whose characteristics minimize, but
do not entirely remove,the probability of nuclide
migration,
* Site/repository engineering solutions to problems that
cannot be avoided in the site selection process, and
* Site monitoring to allow remedial action should events
require it.
Thus, Task C attempts to summarize the influences on nuclide
migration potential and thereby identify critical inadequacies in the
data and analytical method.
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C-2.0 SUMMARY AKD CONCLUSIONS
C-2.1 ASSESSMENT OF GEOLOGIC SITE SELECTION FACTORS
The extent to which geologic media can serve as barriers to radio-
nuclide migration depends principally on the geologic properties of the
media and on the possibility of catastrophic geologic events. Such
factors as lithology, stratigraphy, structure, hydrogeology, tectonics,
erosion, and mineral resources—and the interrelationships among these
factors—must be identified and evaluated from the perspective of radio-
nuclide transport or containment.
The "ideal" land site for a high-level radioactive waste repository
would be geologically stable and isolated from aquifers and from the
biosphere. The rock would be capable of adsorbing radionuclides and
diffusing heat. Since an "ideal" site will not exist in nature, this
task report identifies geologic factors that bear importantly on siting
decisions in general.
Siting factors rely on a detailed knowledge of hydrogeologic t geo-
chemical, thermal, and engineering properties of the repository material
and of the surrounding rock. Of principal importance are the basic
physical rock properties, e.g., permeability, porosity, and fracture
systems that determine the basic hydrologic parameters of a rock suite,
flow rate through the rock most particularly. Other rock properties,
e.g., geoehemical and thermal, operate to modify (largely to retard) the
rate of hydrologic transport of various species, e.g., chemical consti-
tuents as typified by radionuclides. Engineering properties of the
repository host rock taken as a whole, are critical to the repository
itself and are, therefore, the most vital factor in the sense that they
are the initial determinant of repository integrity.
In examining a specific site, a number of basic studies are required,
each designed to yield information sufficient to identify and evaluate
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site seismicity characteristics, rock type, and distribution; the struc-
ture or physical arrangements of the rocks; tectonic processes, e.g.»
volcanlsm; and surficial processes such as erosion. Inventories of
mineral resources and other near-site activities such as oil and gas
drilling could also bear on site suitability.
The notion of site factor ranking should be avoided in other than
site-specific cases since, at any given site, one or another factor may
predominate. To suggest a generic ranking assumes a uniformity and sim-
plicity in nature that cannot be supported. For example, in a salt dome,
the plastic behavior of salt under thermal stress over long periods of
time is of greatest importance. In layered or bedded salt, the distri-
bution and thickness of interlaminations of clays and shales with the
salt, as well as Joint or fault systems, are of primary significance.
In basalt terrains, interlayers of rubble that may be aquifers are impor-
tant, as well as joint or fault systems that could serve as conduits
from a repository to the biosphere. These several examples do not sug-
gest that other factors can be neglected or in fact that in any given
instance another factor or several factors might not predominate. These
factors reflect the inherently complex character of crustal rock, which
makes Investigation difficult. This complexity accounts as well for
success, or lack thereof, in drilling oil wells, unpredictable rock fall
behavior in mines, and the profound difficulties encountered in earth-
quake prediction.
In order to facilitate understanding of these processes, general-
ized models were developed that formed the conceptual background, util-
ized subsequently in the Task C Report for analysis of migration from
the repository through geologic media to the biosphere and for evaluation
of the potential dose to man.
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C-2.2 ANALYSIS OF MIGRATION POTENTIAL
The objective of this sub-task was to select a suitable model for
quantitative"prediction of the migration of radionuclides from a breached
HLW containment through the geologic environment and into communication
with biosphere pathways to human consumption. The intention of the task
was not to develop entirely new physical and analytical models, but rather
to review the applicability of existing models, and identify and select an
appropriate one for examination of parameters. The physical parameters of
the model ware to be compatible with the several characteristics of acceptable
geologic media for siting of an HLW, and were to be definable as independent
variables.
A review of available models identified one as being generally
adequate. Sensitivity analyses were performed on the several param-
eters of this model, and appropriate ranges of values were found for
each parameter.
The simplified model for migration of radionuclides from the HLW
through the geosphere assumes ultimate failure of the primary contain-
ment system, and proceeds to estimate the concentrations of Individual
radionuclides that will escape from the repository by (a) leaching into
groundwater, (b) transport in and/or through the groundwater in the vicinity
of the repository upward to an overlying aquifer, and (c) lateral transport
in the aquifer to points at various distances from the repository site.
Mechanisms considered to affect transport between the repository and the
overlying aquifer include permeability of the rock, convective water flow
through fractured rock and, alternatively, diffusion of dissolved nuclides
through non-flowing water in that region. Mechanisms affecting transport
in the aquifer include convective flow and retardation of ions by selective
adsorption.
Factors considered to affect the concentrations of individual radio-
nuclides occurring at various locations in the aquifer and at various
points in time (up to 100,000 years after emplacement) include the mass
of each nuclide originally present in the HLW, radioactive decay and
growth, dilution by groundwater, and selective adsorption within the
aquifer. Calculations were performed, leading to predicted concentrations
4
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of specific radionuclides at various points in the aquifer up to 8.0 km
(5 miles) distant from jthe repository. Only a few of the actinide and
fission product nuclides were predicted to exist at significant concen-
trations at distances beyond 1.6 km (1 mile) from the repository at any
time up to 100,000 years after emplacement. These nuclides were used
as the basis for calculating dose-to-man effects. In order to compare
the concentrations of nuclides predicted to migrate to various points
at distances away from the repository with those that might be expected
to be leached away from a naturally-occurring uranium ore deposit of
comparable size and in the same location, calculations were made using
the same assumptions of factors affecting migration, and assuming a
0.2% uranium ore deposit at the site of the HLW. The circumstances
assumed for the HLW case (Scenario 2) resulted in approximately equal
concentrations of Ra-226 in the aquifer as for the uranium ore case.
C-2.3 DOSE-TO-MAN CONSIDERATIONS
Considerations of alternatives for geologic emplacement of high-level
waste (HLW) lead to a limited range of possible circumstances for transport
of radionuclides away from the repository site, once the primary contain-
ment system has been breached. Two sets of conditions were selected fo'r
modeling the circumstances for escape of radionuclides to the biosphere.
Those sets of conditions, labeled "Scenario 1" and "Scenario 2," were
used to predict the concentrations of specific radionuclides occurring
in potable aquifers at various distances from the repository site and
at various times in the future.
Given the predicted levels of various radionuclides in the potable
aquifer under the conditions assumed for "Scenario 1" (diffusive trans-
port to the aquifer) and "Scenario 2" (direct connective flow transport
to the aquifer), the possible rates for exposure of people to the radiation
dose resulting from ingestion of contaminated food or of the water itself
were examined. Calculations of anticipated human radiation dose commit-
ments were made for three different hypothetical cases:
* Human ingestion, for a period of one year, of water taken from
the contaminated aquifer at a point directly over the breached
repository.
5
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* Human Ingestlon of vegetables, milk from cows, and beef from
cattle that have served as biological transfer agents of radlo-
nuclides from contaminated water, as well as some direct inges-
tion of the water itself. Use of water from the river, in this
case, is predominantly for agricultural purposes, so radiation
dose commitments to the affected population are dominated by
crop-ingestion mechanisms. The contaminated water in this case
is assumed to be from a hypothetical river (River Model "A") that
intersects the contaminated aquifer at a point 1.6 km (1 mile)
from the breached repository,
* Conditions identical to the above, except that more of the water
from the hypothetical river (River Model "B") is used for direct
human consumption than is the water from River Model "A."
Radionuclide concentrations in all three cases are calculated -On the
basis of each of the two assumed mechanisms for aquifer contamination,
corresponding to Scenario 1 and Scenario 2, respectively.
The results of the analysis are given in terms of estimated 50-year
radiation dose commitments to individuals and to populations of 250
people for the "maximum" dose assumption and 10,000 people for the
"reasonable" dose assumption, respectively. Ingestion of the radionu-
clides is assumed to be for a period of one year, and dose commitments
are calculated for whole body, for bone, and for thyroid.
The principal radiation dose effects were found to be from a small
number of radionuclides, namely, strontium-90, iodine-129, radium-226»
neptuniuffi-237 and plutonium-239 and -240. Several other nuclides were
included in the exercise, based on their apparent Importance in terms of
concentrations predicted in the aquifer at various times and places.
When Ingestion and dose commitment factors were included, however, only
the named nuclides were found to contribute significantly to the dose-
to-man estimates. The relative importance of each of these nuclides was
found to change with time after emplacement. Comparisons were made of
the predicted radiation dose effects of nuclides escaping from the HLW
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with chose of the natural background radiation. Except for the extra-
ordinary circumstances postulated for calculations of the "maximum"
individual dose commitment for Scenario 2, none of the modeled cases
led to predicted dose commitments as large as that due to the natural
background.
Comparisons were also made between the dose commitments predicted
froifl the case of River Model "A" and those calculated from actual con-
centrations of radium in the Colorado River. (Characteristics of River
Model "A" were based on actual characteristics of the Colorado River
mainstream.) The radiation dose commitments estimated from present
actual radium concentrations were found to be 10,000 times greater than
those predicted for the maximum Ra-226 concentrations expected from the
HLW leakage.
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C-3.0 ASSESSMENT OF GEOLOGIC SITESELECTION FACTORS
Task C-l assesses deep geologic site selection factors and
provides conceptual geologic models for conditions where HLW reposi-
tories may be located. The scope includes evaluation of the existing
data on site selections, with no new development work. Much of the
Information is based on extensive studies conducted by contractors for
the Energy Research and Development Administration, especially Battelle
Northwest Laboratories and the U.S. Geological Survey,
The following are included in this subtask:
• Evaluation of available pertinent published
reports from the United States and abroad and
discussions with recognized authorities in
various applicable technical areas.
* Determination and discussion of intrinsic proper-
ties of geologic media which can serve as "barriers "
and/or "paths" to radlonuclide migration.
* Identification of geologic site selection factors,
Including litholoey, stratigraphy, structure,
hydrogeology, tectonics, erosion, and mineral
resources.
• Development of an Investigative sequence for evaluating
site selection factors, considering their
direct impact on radionuclide transport and inter-
relationships within the total geologic setting.
• Identification of general, but representative,
geologic settings for developing illustrative
conceptual geologic models of likely disposal
areas, including bedded salt, salt domes, two
types of shale, and a profile consisting primar-
ily of basalt.
The major objective of disposal of HLW in deep geologic media is to
provide containment until the radionuclides decay to an innocuous
level. For this study, the required containment period is considered to
be on the order of one million years. Containment is considered to mean
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that no wastes (radionuclides) escape to the biosphere in concentrations
deleterious to man or his environment.
The ideal site is one with an assured tectonic, hydrologie, and mechanical
stability, which is isolated from any type of pathway-to-groundwater
aquifers (rock or soil strata containing water that reaches the ground
surface in any natural or manmade manner) and from the biosohere. The geologic
materials should be capable of (1) adsorbing any radionuclides that
begin to travel from their emplaced position and (2) diffusing
heat without impairing the integrity of the natural formation.
Also, the ideal site would preserve for the longest period the integrity
of any other containment methods used; namely, the vitrification of
waste and the encapsulation of the solidified radionuclidee into metal
containers, as discussed in the Task B report. Individual
sites that satisfy all individual objectives may not exist in
nature, but a perfect site is not required so long as it has a proper
combination of the related factors. This subtask identifies those
geologic factors that will determine a site's suitability and shows how the
factors can be balanced to satisfy the basic disposal objective. Non-
geologic factors,including thermal and engineering properties, are dis-
cussed, but to a lesser extent.
Control of radionuclide release from a repository must account for
several ground escape mechanisms:
* Catastrophic but nongeologic events, such as a mete-
orite impact, that could disrupt the repository.
• Catastrophic, geologic-related events, such as
major fault displacement or volcanic eruption.
• Gradual processes which occur over long time
periods.
This geologic site selection study covers only the latter two types of
occurrences. Gradual release mechanisms include transportation of
radionuclides through the geosphere. The transport mechanism can con-
ceivably be through moving or stationary groundwater, moving or sta-
tionary gas within the pore spaces of, the geologic media, or by diffusion
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through the solid material itself. Available studies identify ground-
water as the most important mechanism. Consequently, dispersion with
moving groundwater and/or diffusion through stationary pore water are
considered as the only significant mechanisms, These phenomena are largely
controlled by the hydrogeologic and geochemical properties of the
media as introduced in Section C-3.1.
Catastrophic geologic events are not dependent upon the Intrinsic
properties of the host rock at the repository. Discussions of tb^se
factors are included in Sections C-3.2 and C-3.3, where the site selec-
tion factors are introduced and evaluated.
Section C-3.4 identifies conceptually several types of settings for
deep geologic repositories. It should be recognized that no unique
site factor ranking scheme is preferable to another since the site fac-
tors are not analytically related in a. mathematical or absolute sense.
The basic utility of arraying factors is to assure completeness of the
investigation.
C-3.1 IMPORTANT INTRINSIC PROPERTIES OP GEOLOGIC MEDIA
Unless a catastrophic event occurs, the escape of radionuclides will
be dependent on such forces as the pressure, temperature, and chemical
conditions surrounding the repository as well as on the basic intrinsic
properties of the geologic media. Depending upon site-specific
factors, one or several of the following intrinsic properties may
take on greater importance.
The intrinsic properties of geologic media considered to be most
important in relation to radioactive waste storage are;
* Hydrogeologic properties, which determine the forms in
which water, water vapor, or gases exist in pore spaces
and the rate(s) at which they will flow through a
geologic medium.
10
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• Geochemical properties.which play a major role in
determining (1) the rate at which radionuclide
movement is retarded or accelerated, (2) how the
geologic material will behave when internal en-
vironmental conditions are changed, and (3) the
nature of chemical reactions that may result from
contact of radionuclldes with the geologic media.
* Thermal properties, which determine the physical
and chemical behavior of the media when heated and
the rate at which heat can be dissipated,
* Engineering properties, which determine local
structural tsetiavior at the disposal opening before
and after it is backfilled and, more importantly,
how the media will respond to displacement changes
from larger-scale geologic events.
the following sections further describe these intrinsic properties and
how they influence radionuclide migration potential. Table C-l presents
typical ranges of certain intrinsic properties of several rock types.
C-3.1.1 Hydrogeologic Properties
Two basic hydrogeologic conditions govern the existence and/or movement
of groundwater:
• The bydrologie properties and behavior of
intact, unfractured or unbroken rocks (primary
characteristics); and
• The hydrologic properties and behavior of
disturbed zones (fractures, faults, joints,
stylolites, solution channels, etc.) where
the extent of disturbance, and not the rock
itself, predominates (secondary characteristics).
Groundwater moves from levels of higher pressure to lower pressure.
The velocity of the flow (for laminar flow) is proportional to the
pressure drop per unit length of distance traveled, i.e., the hydraulic
gradient. This relationship between the hydraulic gradient and the
(2)
groundwater velocity is expressed by a relationship called Darcy's Law.
11
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TABLE C-l
EXAMPLE INTRINSIC PROPERTIES FOR TYPICAL REPOSITORY AND ASSOCIATED LITHOLOGIES
PARAMETER SALT LIMESTONE SHALE SANDSTONE BASALT GRANITE
Porosity (percent) <] 15 10
Permeability lO^-lO'9 lO^-lO'9 lO^-lO'9
(cm/sec)
Thermal Conductivity n 17 = o o 7
(jiij i / ' On\ J-j— JL/ J— o j— /
millical/cm sec C)
<5-30
10~2-10"7
3-10
1-40
io-5-io-9
3-4
0.05-3
ID'10
6-9
Source: Ekren, E.B. et al., Geologic and Hydrologic Considerations for Various
Concepts of High-Level Radioactive Waste Disposal in Conterminous
United States. Open-File Report 74-158, U.S. Department of Interior
Geological Survey, Richland, Washington, 1974.
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Darcy's Law for a one-dimensional case Is described by the equation;
v - K Ab.
where
V - velocity of flow (cm/s),
K - permeability of the medium and indicates the
ability of the fluid to move through the medium
under a hydraulic gradient (cm/s),
Ah » difference in hydraulic head (cm),
L - the distance between the two points from which the
hydraulic head change is measured (cm).
The ratio of the difference between hydraulic head (Ah) and the distance
L is called the hydraulic gradient (I). To express the flow rate (Q), or
the quantity of water flowing through a unit cross-sectional area (A)
per unit tine, Darcy's Law can be written:
Q - AV » KIA (2)
The restriction that Darcy's Law is valid only under conditions of
laminar flow and that it requires modification when turbulent flow
prevails should not have impact on analyses for a suitable repository.
Even though turbulent flow conditions may exist on a microscopic basis
as groundwater moves through geologic media, the macroscopic outcome of
flow in overall low permeability regimes (such as should exist in the geologic
media surrounding any seriously-considered site for a repository) may be
adequately described by the laws governing laminar flow (Darcy's Law).
The permeability used in the above equation may be (1) the primary
permeability of the porous rock medium; or (2) the secondary permeability
of the rock if flow is predominantly through fractures, fissures, or other
types of small discontinuities.
13
-------
The magnitude of the primary permeability is most dependent upon the
grain size, the degree of crystallization, and/or cementation and compac-
tion. The lower end of the permeability ranges shown for various rock
types in Table e-2 is primary permeability.
Secondary permeability (resulting from cracks and fissures in the rock)
is likely to be of greater concern when considering potential ground-
water flow through strata surrounding a HLW repository. Secondary
permeability must usually be determined in the field from well-planned
boring drilled to intersect the predominant fractures in the
rock. Careful attention must be given to the fracture orientations to
facilitate recognition of preferred directions of permeability. Estab-
lished relationships exist for deducing overall permeability values from
field permeability tests and pump tests. Also, if appropriate measure-
ments can be obtained from rock corings, the following equation can be
used to estimate the secondary permeability value in fractured or fis-
sured rock:
K* - J2L 0>
12vb
where
K1 = equivalent permeability of a planar array of
parallel smooth cracks (cm/s),
g = gravitational acceleration (981 cm/s^),
e = opening of cracks or fissures (cm),
V = the coefficient of kinematic viscosity
(0.0101 cm2/s for pure
water at 20 °C.,)
b = spacing between cracks (cm).
For calculating flow through fractured or fissured rock, the permeability
in Equation (1) should be replaced by K* (secondary permeability).
14
-------
TABLE C-2
PERMEABILITY OF VARIOUS ROCK AND SOIL TYPES
Permeability, in
cubic centimeters
per second per
square centimeter*
Permeability, in
gallons per day
per square foot
Degree of
permeability
Soil type"'"
Rock type
Probabla yield, to •
mil la gallon* p«r
•Inuta
102 101 1.0 10"1 10"2 10"3 10~4 10~5 10"6 10"7 10~8 10"9
1 1 1 1 1 1 1 1 1 II
106 105 10* 103 102 101 1.0 10" l 10"2 10~3 10~4 10~5
1 1 II II II
*?* High Moderate Low *«**
nigh low
Very fine sands; silts; mixtures
Clean sands; clean of sand, silt, and clay; glacial Homogeneous clays
ean sand and gravel till; stratified clay deposits; below zone of
mixtures etc . weathering
>3,000 1,000 100 10 <1.0
11 t 1 1
* Multiply by 1.04 x 10 to obtain darcy units.
'From Terzaghi, K. and R. B. Peck, Soil Mechanics in Engineering Practice. John Wiley and Sons, Inc., New York, New York, p. 566, 1960.
Source: Schneider, K.J. and A.11. Platt (editors). High Level Waste Management Objectives.
Battelle Pacific Northwest Laboratories, Pvichland, Washington, BNWL-1900,, May 1974.
-------
The flow velocity defined by Equation (1) is actually only an apparent
velocity across a given cross section, considering that the water flows
in a straight path. Actually however, the water flows in a very tortuous
path around individual grains or fissures and into and out of- pore and
opening sizes of various sizes and configurations. Accordingly, in
order to describe properly phenomena that can retard the migration of
chemical elements through geologic media (discussed in Section G-3.1.2),
it is necessary to introduce another term, interstitial velocity, that
describes groundwater flow. As an example for porous media, inter-
stitial velocity is defined by the equation:
V = V/e (4)
w
where
V • interstitial velocity of groundwater flow
w
(cra/s),
e =« porosity (the ratio of the volume occupied
by pores to the total volume of a geologic
material).
Interstitial velocity is always greater than the Darcy velocity (Equa-
tion 1) when considering a specific hydrogeologic flow regime.
C-3.1.2 Geo chemica1 Properties
Geochemical properties of rock at a repository are Important for two
reasons:
• Properties such as ion-exchange capacity,
mineralogy, and natural water (pore water)
chemistry can remove nuclides (sorption) from
slowly moving water, reducing the threat to
man even if absolute impermeability cannot be
confirmed.
* Rock mineralogy, solubility, water content, and
composition are important considerations in
assessing potential waste-rock interactions-
considering both the impact on the waste and on
the rock.
16
-------
C-3,1.2.1 Sorption Capacities of Geologic Materials
Section C-3.1.1 indicates that if water and a hydraulic gradient exist,
some small flow can occur, even in very tight rock formations. This sec-
tion shows that geologic media can act to impede radionuclide migra-
tion to the biosphere, even if flow does occur. Dissolved radionuclides
may enter into complex physicochemical reactions with the geologic
materials by phenomena such as adsorption, Ion exchange, 'colloid filtra-
tion, reversible precipitation, and irreversible mineralization. These
mechanisms are usually combined into one general teijm called "sorption."
In effect, the sorption phenomena cause ions to move at much lower velo-
cities than the medium (groundwater in this case) transporting them.
Sorption is expressed in terms of distribution coefficients, K, for
(4) a
porous media and K for fractured media, as follows.
fm x Lv
and
where
where
K, * distribution coefficient for porous media In
ml/g,
fin = the fraction of total activity adsorbed on the mineral,
Lv «• volume of solution equilibrated with Mw (ml),
fs = fraction of total activity in solution «• 1-fm,
Mw = weight of mineral (g).
v - fm * Lv
Ka ~ fs x Fa <6>
K = distribution coefficient for fractured or faulted
a
media in ml/cm2
Fa « surface area of both sides of the fracture
(cm2).
1?
-------
The K, and K values are dependent in a complex way'on such parameters as
M. 3.
pH of the groundwater, the specific nuclides present, concentration and
type of dissolved ions, temperature, oxidation-reduction potential (redox
potential, !„), presence or absence of organic material, and others.
The effectiveness of any geologic medium to act as a retarder for a
particular condition is expressed as the retardation factor, R,. For
a
a particular element, R, is defined as the ratio of the water velocity
to the nucllde migration velocity (dimensionless term). The retardation
factor is related to the distribution coefficients by the following
relationships:
For porous media, R, *• 1 + K,p/e (dimensionless) (7)
For fractured media, R, = 1 + K Rf (dimenaionless) (8)
where
3
p * bulk density of the medium ( g/cm ),
e - porosity as defined in Section C-3.1.1 (dimensionless),
Rf ** the surf ace-to-volume ratio of the fracture (cm /ml).
The magnitude of radlonuclide migration retardation that can be realized
may be expressed by relating the velocity of ions moving through the geologic
medium to the interstitial velocity of water flow by the following equation:
V
V. - f- (9)
1 Rd
V. - the velocity of the ionic species,
and all other terms are as previously defined.
As discussed below in relation to Table C-3, retardation factors can
vary from as low as 1.0 to as high as 50,000 or more. For this range,
the velocity of the ions can be the same as the water transporting them
or as little as 10~ times as fast.
18
-------
TABLE C-3
ESTIMATED DISTRIBUTION COEFFICIENTS (ICj) AND
RETARDATION FACTORS (R.) IN A TYPICAL DESERT SOIL
ATOMIC NO.
1
4
6
11
17
18
19
20
26
27
28
34
36
37
38
39
40
41
42
43
46
48
50
51
53
55
61
62
63
67
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
ELEMENT
Tritium
Beryllium
Carbon
Sodium
Chlorine
Argon
Potassium
Calcium
Iron
Cobalt
Nickel
Selenliun
Krypton
Rubidium
Strontium
Yttrium
Zirconium
Niobium
Molybdenum
Technetium
Palladium
Cadmium
Tin
Antimony
Iodine
Cesium
Fromethium
Samarium
Europluu
Holmlum
Thallium
Lead
Bismuth
Polonium
Astatine
Radon
Francium
Radium
Actinium
Thorium
Protactinium
Uranium
Neptunium
Plutonium
Anericium
Curium
Berkelium
Kd
-------
In the mechanism that permits sorption to function, exchangeable ions
are released from the host rock into the water in exchange for radio-
nuclides. As an example, the adsorption of cesium or strontium ions
by clay can release sodium or potassium ions into the water. Thereafter,
those released Ions, together with ions originally present in the groundwater,
compete with radionuclides for additional available exchange sites.
This process allows radionuclide migration to be greatly retarded for
many conditions, but not necessarily eliminated.
Distribution coefficients are variable and uniquely determined for only
a single set of physical and chemical conditions. Figure C-l illustrates
ranges of distribution coefficients for tuff, granite, limestone and
dolomite, basalt, and soils in relation to cesium, mixed fission pro-
ducts, strontium, and iodine. For the cesium, mixed fission and
strontium cations (positively charged ions), granite possesses the
1 3
lowest distribution coefficient (10 to 10 ml/g)» while tuff and
1 45
soil exhibit the highest coefficients (10 to 10 * ml/g). It is
noteworthy that for the iodine anion (negatively charged ion), the
distribution coefficient is close to 1 for the rock types, but from
1 35
about 10 to 10 * ml/g for soils. Although very little research
has been conducted on anionic solutions, the available data indicate
that the mechanisms controlling retardation of cations may not readily
apply to retardation of anions.
As further related examples, Table C-3 lists estimated distribution coef-
ficients for a typical desert soil for a wide variety of nuclidea.
This table shows that the K, can be as low as zero for tritium, various
noble gases, iodine and a few other elements or as high as 4000 and
15,000 ml/g for protactinium and thorium, respectively.
Table C-4 indicates distribution coefficients for different soil materials
and for clay samples taken at various depths for plutonium, americiura,
yttrium, strontium,and cesium. For these materials, strontium ha* t
very low K , ranging from 2 ml/g in river sand to about
20
-------
'10s
EFFICIENTS
a 6
« *
o
o
2
O
p
3
CD
tr
fc
S
o
o
-
o
!
r
^«f
CESfUM
MIXED-FISSION
PRODUCTS
STRONTIUM
IODINE
NOTE:
DISTRIBUTION COEFFICIENT VALUES
FOR IONS SHOWN
LEGEND
TUFF
GRANITE
LIMESTONE AND DOLOMITE
iASALT
fill SOILS
SOURCE:
SCHNEIDER, K. J. AND A M. PLATT (EDS >
«
i An»*vfATTELLE PAC(F'C NORTHWEST
LABORATORIES, RICHLAND, WASHINGTON, MAY 1974,
FIGURE C-l RSMGES OF DISTRIBUTION COEFFICIENTS
FOR VARIOUS ROCK TYPES
21
-------
TABLE 0-4
DISTRIBUTION COEFFICIENTS *Kff) FOR DIFFERENT
SOIL MATERIALS AND CLAY
DISTRIBUTION COEFFICIENTS FOR DIFFERENT SOIL MATERIALS (rat/g)
MATERIAL
Clay
Sands tone
Caprock
River sand
Pu Am
4 4
10 5 x 10
2 x IO3 2 x IO4
5 x IO2 . 3 x IO3
2 x IO2 4 x IO2
Y
3
HT
io3
6 x IO2
6 x IO2
Sr
2.5-10
2.5
4
2
Cs
2
2 x 10
2 x IO2
6
10
DISTRIBUTION COEFFICIENTS FOR VARIOUS NUCLIDES MEASURED
FOR CLAY SAMPLES TAKEN AT VARIOUS DEPTHS (mt/g)
SAMPLE
DEPTH
(meters)
20 - 3U *
55 - 60 +
100 - 125 t
120 - 130 t
200 - 225 t
245 - 275
300 - 325
328 - 348
Pu
io4
5 x IO3
6 x IO3
3
5 x 10
3
7 x 10
6 x IO3
3
8 x 10
3
9 x 10
3
8 x 10
Am
8 x IO4
6 x IO4
5 x IO4
4
6 x 10
4
2 x 10
9 x IO4
4
4 x 10
4
4 x 10
4
5 x 10
Y
1.6 x IO3
8 x IO2
6 x IO2
2
7 x 10
«
6 x 10
8 x IO2
2
7 x 10
2
7 x 10
2
8 x 10
Sr
21
20
12
4
5
7
4
3
2.5
Cs
3 x IO2
2 x IO2
2 x IO2
2
2.7 x 10
2
3 x 10
1 x IO2
2
1.1 x 10
2
2 x 10
2
2.5 x 10
NOTES:
* Quaternary clay.
+ Young Tertiary clay.
+ Older Tertiary clay.
SOURCE: Modified from Hamstra, J. and B. Verkeck. Review of Dutch Geologic
Haste Disposal Programmes. International Conference on Nuclear
Power and its Fuel Cycle, Salzburf,, Austria, 2-3 Hay 1977. IAEA.
CN36/2G9, Netherlands Energy Research Foundation ECN Patten, the
Netherlands, 1977.
22
-------
20 ml/g in clay material. Plutonium and amerieium have the largest dls-
3
tribution .coefficients, reaching their maxlmums in clay at around 10 to
4
10 ml/g. The distribution coefficient values due to sorption are higher
for all of the radionuclides in clay material. No relationships between
K. and depth are apparent from the data in Table C-4.
None of the distribution coefficients presented above as examples can
be used directly in determining the retardation factor in fractured or
fissured media. Little work has been done to determine these values and
representative values are not reported in the literature. However, the
distribution coefficient for fractured media for a particular nuclide
should be expected to be significantly lower than the distribution
coefficient for porous media for the same ion based only on the geometry
of the two types of flow.
Retardation factors are also presented in Table C-3 for the corresponding
distribution coefficients. These are calculated for specific media
porosities, and bulk densities for illustrative purposes only. Consid-
ering the R, for thorium (50,000) as an extreme example and using Equation (9),
it is apparent that the velocity (V.) at which thorium would move through
-5
the tested material is a factor of 2 x 10 lower than the interstitial
velocity (V ) of the groundwater transporting it. For this case, radon
would move with the same velocity as the groundwater and cesium would
-3
move at only 10 times the rate of the groundwater.
The presence of organic material in soil has been found to reduce K.
values significantly because of complexlng for some radionuclides.
The influence of organic matter on radionuclide retention is an im-
portant consideration that needs considerable attention at this time.
Recent studies with cobalt have shown that while cobalt has 2. relatively
high K, value in soil free of organlcs, the K value drops to nearly
zero as a result of the cobalt complexing with organic material.
23
-------
The oxidation potential (!„) la Important for many of the radionuclides.
Depending upon oxidation state, some of the nuclides can exist in a
cation or anion form. In anion form, or a complexed form such as
mentioned above, the K, value may be rather low. In the cation form,
the same nuclide may be strongly retained by the rock or be limited
by its solubility. As a consequence, it is extremely important in
making K, measurements to recognize the oxidation potential of the
natural or modified groundwater environment. Examples of nuclides
that could be affected in terms of retention and/or solubility include
many of the actinides as well as technlcium.
The above discussion shows that lithologic types possessing high
"sorption" capabilities could have significant advantages for purposes
of radionuclide retention in the geosphere, all other factors being
equal. The sorption capabilities expected for fractured or fissured
media where groundwater flow could be most important to containment
may be significantly lower than the examples presented in Tables C-3 and
for porous media, but this phenomenon would still be very important in
the overall potential for geologic materials to act as barriers to radio
nuclide migration.
C-3.1.2,2 Potential High-Level Waste-Rock Interactions
The two general categories of interactions considered under this section
are:
• Reactions caused by the geologic materials comprising
the repository horizon or their associated pore fluids,
which have a corrosive effect on the waste containers
and/or the waste form.
* Reactions induced by heat and radioactivity generated
by the waste material,which have a negative impact on
the repository stratum.
As discussed in Task B of this study, HLW may be placed into a repository
as a vitrified solid and/or in metal canisters. Both of these factprs
add containment capability beyond that provided by the geologic media.
Accordingly, there is benefit in maintaining the integrity of the
primary containment systems, particularly the vitrified state, as long as
24
-------
possible, thereby effectively restricting the mobility of most of the
nuclides. However, the host materials or their associated pore fluids may
be naturally corrosive, and chemical reaction (corrosion) rates usually increas
with temperature, A special discussion of this issue with respect to salt foil
in Section C-3,1.2.3. Therefore, the longevity of both the waste containers "an
the waste form will be a function of the geoehemical environment present in the
repository. The significant geochemical parameters are those associated with
chemistry, heat, and radioactivity.
Typically, high ionic strength solutions (solutions with large amounts
of dissolved solids) have a higher corrosion potential (or dissolving
ability) than solutions with lesser concentrations of dissolved
solids, ' ' Consequently, groundwater high in dissolved solids
may be expected to attack and dissolve both the HLW and
its container more rapidly than would dilute groundwater. An example
of a high-ionic-strength solution is sodium chloride brine, which can be
associated with salt deposits and is not uncommon in many geologi-
cally old sandstone aquifers. An overall evaluation of site suitability
should consider how the site conditions will affect the time required
for the waste form to deteriorate and for the nuclides to become "free"
to move if a transport medium or means exists.
The other major category of potential waste-rock interactions is con-
cerned with the effect of waste-generated heat and radioactivity on the
repository stratum. The most important effects concern the dehydration
of hydrated mineral phases in surrounding rocks, the resultant water
expulsion, and the potential for rock melting should insufficient heat
dissipation occur.
Reactions that occur by either of the above mechanisms at elevated
temperatures are similar to igneous and metamorphic processes that
occur in nature at great depths in certain zones of the
earth's crust. As the temperature Increases, originally stable
minerals eventually recrystallize to a different group of minerals,
which are stable under the new pressure/temperature conditions. In the
presence of water, this recrystallization is generally rapid and exten-
sive, and the water is often driven away from the reaction site (as a
hydrothermal solution), carrying with it volatile components of the
25
-------
reacting rock mass. In a repository, the expulsion of water (as a
liquid or vapor) could provide a mechanism for transporting waste
components as well as a wide variety of trace elements. For example, ions
of elements with radioactive isotopes (as in HLW) are known to occur naturally
in hydrothermal solutions that have passed through several hundred meters of
nearly impermeable rock during geologic times and have eventually been
dispersed to the environment through such mechanisms as groundwater
mixing and hot springs. ^ These natural phenomena Indicate that the
amount and characteristics of pore water and the hydrated mineral phases
of the host rock must be considered in site evaluation.
lor the unlikely situation in which rock materials adjacent to fhe re-
pository actually melt from heat buildup, it may be expected that re-
crystallization will occur as the molten mass migrates a short distance
away from the heat source. If radionuclides are incorporated into the
melted rock material they would become part of the crystal structures of
newly formed mineral phases, providing a relatively insoluble form for.
the radionuclides.
C-3.1.2.3 Salt Solubility
The geologic occurrence and solubility of salt can lead to geochemical
consequences that could bear significantly on the integrity of an HLW
repository. The potential problems seem more acute in the case of
bedded salt as opposed to diapiric salt. The former is characterized
by heterogeneity, with interbeds of thin shales, other evaporites, and
limestone, the result of depositional processes. The geometry and precise
geologic detail of the interbeds are difficult to predict or even to
map over any significant distance.
The Ad Hoc Panel^ ' noted, with respect to bedded salt that, "On a
microscopic scale it can be seen that many halite crystals contain liquid
(brine) or solid (anhydrite) inclusions. A recent report by Roedder
(12)
and Belkin estimates the volume of these inclusions to be less than
1% but "an additional possibly even greater volume % fluid is present
in situ, filling intergranular pores'," The Panel noted the work
of Bredehoft ' et al., which showed that... "at 200°C, water in rock
salt including potassium and magnesium components can contain approximately
26
-------
70 wt% of dissolved salts. This implies that as little as 1 wt % H?0
in a ,salt bed may yield fluid that is 3 wt 1 because of the high con-
centration of the solution. The dissolving power of such brines should
not be underestimated." The Panel directs attention to... "the importance
of knowing the water content of salt beds proposed for repositories, par-
ticularly with the background and experience at Lyons, Kansas, where
considerable volumes of water migrated in an unpredlcted manner,"
Salt domes of the pierceraent variety are often referred to as ideal for
an HLW repository. The salt is generally thick, though laterally
limited, and the lithology tends to be uniform and typically free of
open fractures. Most, though not all, domes appear to be remarkably
dry and to have been so for millions of years. Technology is available
for discriminating between "dry" and "wet" domes.
C-3.1.3 Thermal Properties
Considerable heat generated by the HLW materials will increase
the ambient temperature at the depth of the disposal
horizon. Therefore, the geologic formation should possess thermal
properties that (1) promote heat dissipation, (2) do not have mineralogic
and pore water characteristics that will create unacceptable behavior
changes or water migration during heating, or (3) do not become unstable
because of heat buildup prior to the time that backfilling occurs.
The thermal conductivity of the rock is a primary consideration. A
relatively high thermal conductivity is desirable to dissipate the heat
produced by radioactive decay efficiently and steadily with disposal
canisters located at a reasonable spacing. Typical thermal conductiv-
(14)
ities of potential host geologic rocks are shown in Table C-l. These
values indicate that salt has the highest thermal conductivity of the
rock types listed, followed by quartzite, granite, shale,and tuff in
decreasing order.
Salt will also exhibit higher rates of creep at elevated temperatures
than other rock types. This creep can create an increased engineering
problem during disposal operations. On the other hand, it may be very
beneficial because it promotes sealing and healing of fractures around
the disposal cavity and of backfilled salt rock. Accelerated deforma-
27
-------
tion at elevated temperatures of more brittle rock could be detrimental
because it might lead to increased fracturing.
C-3.1.4 Engineering Properties
The basic engineering properties of rock, such as strength and stiffness,
have obvious importance to the basic "mining engineering" requirement to
establish repository openings and maintain their integrity until back-
filling is accomplished. This factor is very important from an economic
viewpoint, but does not have significant impact on the ability to assure
containment.
Elastic-behaving rocks, like salt and some shales, will provide some
additional containment attributes as opposed to brittle-type rocks, if
movement of the repository formation does occur due to any external
event. The more flexible rocks can deform to a greater degree without
cracking, whereas brittle rocks usually fracture under any significant
geologic movements. Fracturing to any extent can serve to reduce the
containment capability of the repository horizon.
C-3.2 IDENTIFICATION AND DISCUSSION OF SITE SELECTION FACTORS
The perspective on geologic sj.te selection criteria depends, in the absence
of specific guidelines, on the viewpoint of the investigator(s). For example
• The designer responsible for assuring safe
working conditions in the repository may be
most interested in rock strength.
» The ventilation engineer may be most interested
in pore water and its liberation into a high
temperature environment——but only until each
area is backfilled.
• The health physicist will be most interested
in the overall containment capabilities of the
total site for a very long period.
Clearly, the design team must carefully evaluate each site situation so
that all required actions to ensure site integrity are provided for, in-
cluding comparative evaluation(s) with alternative sites.
The natural complexity of geologic environments and the interrelated and
interdependent nature of rock features will make each potential site
unique and require that each site be evaluated individually. Con-
23
-------
sequently, exact values, or even ranges of values, for site selection
criteria cannot be specified in a generic study such as this. The
purposes of the following discussions and Section C-3,3 are to identify
site selection criteria applicable to most sites and establish a
rationale for their relative ranking on a site-by-slte basis.
The "design life" for containment for at least several hundreds of thou-
sands of years must be emphasized throughout the site selection process.
Obviously, when present-day site evaluations are conducted, the ability
to predict the timing and magnitude of particular geologic occurrences
diminishes for extreme time periods. On the other hand, however, even
a million-year period is considered relatively short in most studies
of the geologic past.
The newness of investigations for underground disposal of HLW and their
greater-than-normal dependency on the potential occurrence of geologic
events permit the listing of site selection factors to be arranged In
several logical ways. The following arrangement seems logical for this
study;
Site Selection Factors
Geologic Properties:
• Lithology (Rock Types)
» Stratigraphy
Thickness and Lateral Extent of Strata
Depth of Repository Horizon
Uniformity and Homogeneity of Geologic Media
Nature of Overlying, Underlying and Flanking Strata
* Structural Geology
Folding
Joints and Faults
Major Plastic Flows
• Hydrogeology
Regional Hydrogeology
Local Hydrogeology
Geologic Events:
• Tectonics
Tectonism
Seismicity
Diapirism
Volcanism
29
-------
* Erosion and Denudation
Water Flow
Glaclatlon
* Mineral Resources
Each of these items is related to time considerations and to intrinsic
rock properties. Two additional descriptive categories—geometry and geography-
were also considered as major elements in the selection factor list. For this
study, however, they are both incorporated into more traditional geologic
terms. A different list, incorporating these and other logical breakdowns
would not conflict with the following discussions. Mineral resource consider-
ations are obviously not a geologic event. They are listed in the event
category, however, because in the context of an HLW repository, mineral
resources represent a condition that could lead to an event with some
impact. Each property, event, and term is defined and/or discussed in the
following subsections.
Establishing the site values for each of the above factors would require very
extensive and probably site-descriptive studies including seismic surveys,
core drillings, and others. A question arises with respect to the potential
for rendering the site reusable by the very studies designed to illustrate
its integrity. Clearly, a balance must be struck reflecting this conflict,
which implies that a residual but acceptable level of uncertainty (e.g., risk)
will remain.
C-3.2.1 Lithology
Lithology is a very Important site selection factor,in that it encom-
passes the detailed description of the rock's basic characteristics,
including mineralogic composition, grain size and cementation, and other
properties that determine how the material will behave when exposed to
external forces or changes in environmental conditions. Rock types
that are representative of lithologies being considered for HLW repos-
itories include salt, shale (argillaceous materials), limestone, and
igneous rocks. The following paragraphs summarize the salient features
for each of these lithologic types or groups. , Individual characteristics
of any particular site will often vary from these capsule discussions,
but the basic features discussed represent a meaningful starting point.
30
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C-3.2.1.1 Salt
Salt (NaCl or halite) is an cvaporite type rock., originally formed in
basins that became isolated or partially isolated from the sea. The
Isolation allowed evaporative concentration and then salt precipitation
to occur. Salt is often found as relatively pure beds, but thin layers
are sometimes interbedded with other rock types or other evaporites.
Stratigraphic and structural features of salt formations are discussed
further in the next two sections. It is notable here that both bedded
salt and dome salt (formed from diapirism of deep bedded salt) are being
considered for HLW disposal.
Several basic properties of salt that are attractive for the location
of a repository are:
* Low porosity,
* Low permeability,
* Plastic behavior for healing fractures, and
* High thermal conductivity.
Some other characteristics of salt can cause special concern:
* High solubility,
* Mineral impurities in some deposits,
» High corrosion potential, and
* Low ion-exchange capacity.
Another unique feature of salt is the potential presence of brine
cavities filled with salt-saturated water. It has been shown that upon
heating, the brine cavities can migrate toward the source of heat.f1*)
The mechanism for this migration is that the material at the hot side of
the cavity (the side nearest the heat source) goes into solution, while
salt precipitates at the cold side. In some locations, salt brine
migration may be the only possible source for the natural introduction
of water into a repository.
Another potential natural source of water in many salt deposits may be
from hydrous mineral phases present as impurities in the formation.
31
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Water present in many of these minerals such as polyhalite and gypsum can be
released at only moderate temperatures. Gypsum, for example, will yield
approximately 75% of its water when heated to 190 or 200°C. Water repre-
sents about 21% of the gypsum; if this mineral is present in a large quantity,
it could release a significant volume of water into the repository. However,
phase behavior of minerals under repository pressure and temperature condi-
tions will not be the same as for ambient conditions. Therefore, water release
based on assumptions of ambient conditions may, or may not, model conditions in
or near the repository.
C-3.2.1.2 Shale
Shale is a general term for lithified clay or mud, although most shale
does contain varying amounts of silt and sand material. Some shales
have several properties that , make them attractive for an HLW repository.
including:
* Low permeability,
• Relatively good plastic behavior (though not
as good as salt),
* High ion-exchange capacity, and
• Wide geographic distribution of nearly homogeneous
material.
Potential negative characteristics of shale that must be investigated
include:
* Many argillaceous rocks, especially those with
relatively good plastic behavior, contain large
quantities of hydrous minerals, such as montmo-
rillonite that will release water when heated.
* Some shales contain petroleum or large amounts
of carbonaceous material that may yield combus-
tible carbon compounds when heated,
• Many shales are interbedded with more permeable
rock types such as sandstone, limestone,and coal.
• Most shales possess relatively low thermal
conductivity.
From the important viewpoint of water flow, marine shale sequences are
generally free of interbedded coarser-grained rocks and can be less
32
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permeable than continental (interbedded) sequences. Shales are not
self-healing like salts, however, and investigations to evaluate the
potential for groundwater flow must consider the potential for faulting
or fracturing in the future, as well as the impact of frac-
tures, joints or faults on secondary permeability (Section C-3.1.1).
C-3.2.1.3 Limestone
Limestone is a carbonate rock composed primarily of the mineral calcite
(CaCO«), usually with varying amounts ot the mineral dolomite, CaMg(GO»)2,
When the latter is dominant, the rock is generally referred to
as dolomite. Limestone can be attractive because:
* It sometimes exists over large areas as a thick
homogeneous stratum,
* Its basic matrix is fine grained, so the rock
can have a very low primary permeability.
Several characteristics of limestone that may not be favorable are:
* Carbonate rocks typically exhibit greater
solubility than other non-evaporite type
sedimentary rocks. Solutioning can greatly
increase secondary permeability and, in turn,
the overall capacity of a limestone formation
to transmit groundwater.
• Limestone is usually strong but brittle, so
fracturing may occur from regional tectonic
movements.
• Limestone does not possess significant sorptive
properties except In areas of solutioning where
clay minerals are commonly concentrated. Since
these areas of solutioning also represent avenues
for groundwater flow, the sorptive capacity pro-
vided by clay filling may partly counterbalance
the slow groundwater movement through the solution
channels.
A somewhat special consideration associated with disposal of
HLW in carbonate rock is the potential for release of carbon dioxide
gas (CO,.,) from the calcite and dolomite mineral phases. This could be
33
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caused by excessive heat buildup and could ultimately result In the
availability of gas as a transport mechanism.
C-3,2.1,4 Igneous Rocks
The tern igneous rocks refers to a group of rocks formed by
the solidification of molten rock. This group may be subdivided into
intrusive-type (plutonic) rocks, referring to those rocks which have
solidified below the earth's surface at various depths, and extrusive-
type (volcanic) rocks, which solidified after eruption onto the earth's
surface.
Intrusive rocks can range from fine-to coarse-grained, depending upon the
depth of intrusion, rate of cooling,and presence of gaseous constituents;
extrusive rocks are fine-grained or glassy and consist either of
rocks formed from lavas that flowed onto the earth's surface or from
volcanic ash that either flowed or was deposited on the earth's surface.
Table C-5 shows the general classification and principal mineral assemblages
of common plutonic and volcanic rocks. It also shows the approximate
percentages of silica contained in each general type of rock.
Igneous rocks vary in hardness and strength, depending Upon crystal
size, the mode of eruption,and the presence or absence of voids. Plu-
tonic rocks have generally low porosities and permeabilities, whereas
the volcanic rocks vary in these properties from very low to extremely
high. Plutonic rocks consist largely of granite, granodiorite, diorite,
and gabbro. These rock types can generally be considered similar from
the standpoint of mechanical strength and overall suitability for waste
disposal. Granites, however, have the lowest melting points and are
somewhat weaker than the other rocks in this category. Volcanic rocks
may be chemically similar to the intrusive varieties but differ in their
textures and physical properties.
Favorable characteristics of igneous rocks for the location of an HLW
repository are:
34
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TABLE C-5
GENERAL CLASSIFICATION AND PRINCIPAL
MINERAL ASSEMBLAGES OF IGNEOUS ROCKS
>66 PERCENT SILICA 52-66 PERCENT SILICA <52 PERCENT SILICA
1 U (ACID) (INTERMEDIATE) (BASIC)
Plutonic Granite Granodiorite Diorite Gabbro
(medium- and
coarse-grained).
Volcanic Rhyolite Quartz latite Dacite and Basalt and basaltic
^ (fine-grained or and rhyodacite andesite andesite
*•" glassy lava or
tuff).
Major minerals— Major minerals— Major minerals— Major minerals—
quartz, potassium quartz, sodium- calcium-sodium, calcium-sodium
feldspar, mica. calcium, feldspar, feldspar, mica, feldspar, pyroxene,
mica, amphibole. amphibole. olivine.
Source: BNWL-1900(3)
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High mechanical strength,
Low primary permeability,
Low porosity,
The occurrence of thick rock bodies (with th*
exception of certain volcanic flow deposits).
Unfavorable characteristics for the location of an HL¥ repository
* Potential for high secondary permeability due
to jointing and faulting,
• Low sorption potential (except for tuffaceous
deposits),
» Relatively low thermal conductivity for heat
dissipation.
For purposes of this study, basalt has been chosen to represent a typical
igneous rock type. Section C-3.4 will discuss a typical geologic setting
in which basalt occurs in suitable geometry for location of a repository.
C-3.2.2 Stratigraphy
When considering any particular lithologic unit for location of an HLW
repository, it is necessary to understand the host material itself and
Its relationship with surrounding geologic materials and condi-
tions. The study of the overall spatial and temporal relationships of
rock formations is called stratigraphy.
Determination of stratigraphy is accomplished by a wide variety of
studies, including review of existing site and regional geologic infor-
mation, drilling and field testing programs,and detailed data analysis.
Important stratigraphic relationships are typically presented graphically
as two-dimensional geologic cross sections or as block diagrams that
provide a three-dimensional perspective^ The pertinent characteristics
of the conceptual cross sections prepared for this study are discussed
in Section C-3.4.
36
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Important stratigraphic elements relative to HLW repository sites are
thickness and lateral extent of the host formation, depth of the dis-
posal zone, uniformity and homogeneity of the formation, and the nature
and extent of underlying, overlying,and flanking strata. These param-
eters are discussed In the following paragraphs.
C-3.2.2.1 Thickness and Lateral Extent of Strata
A host rock formation continuous for very long distances In
both the horizontal and vertical direction would be Ideal for almost all
design criteria. This type of situation will probably not occur natur-
ally, however, and the actual spatial extent of a host formation will be
a major Investigation and design factor. For example, current detailed
siting Investigations are considering both bedded salt and shale of
moderate thickness and great lateral extent, and salt domes with
pronounced lateral limits but thicknesses greater than 10,000 feet. The
minimum acceptable thickness and lateral extent of any particular geologic
formation will depend on such characteristics as the thermal properties
of the formation. A critical criterion for formation size is that It b«
sufficiently large (thick and/or extensive) to assure adequate heat
dissipation. For example, thinner deposits of salt may be utilized,
whereas other rock types, such as basalt, may need to be considerably
thicker because of their lower heat dissipation properties.
Lateral extent and thickness of the repository formation may also be
important because of enhanced containment capability provided by high
sorptive properties of certain rock types. The greater the pathway, the
greater the opportunity for radionuclide retardation by sorption processes
dlscusssed in Section C-3.1.2.
The minimum thickness or lateral extent criterion may also be dictated by:
* The distance required to protect the repository
from groundwater flow or to account for unknowns
about the material1 s purity that could affect
groundwater flow.
37
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« The distance from the mined cavity to the extremities
of the formation to minimize the potential that
localized cracking or straining could open pathways
beyond the desirable containment zone.
C-3.2.2.2 Depth of Repository Horizon
The necessary depth of a waste repository horizon will vary depending
upon many factors. Those factors associated with the host rock are:
Location of suitable host rock,
Erosion and denudation potential,
Local groundwater conditions,
Depth of weathering,
Depth of open fracturing,
Ambient temperatures at depth, and
Short- and long-term mine stability considerations.
The actual depth criteria for a specific location will be site-related.
Mined openings in shale could possibly be extended deeper than in salt
because of the differences in physical properties. Brittle
rocks, such as limestone, granite,and basalt, can undoubtedly be mined
more easily at greater depths than would be practicable for shale or
salt. These "engineering" limitations must be considered in the con-
text of all site selection criteria. The most common
depths for disposal may range from about 300 to 1500 meters, but
shallower or deeper rocks may be suitable under certain site conditions.
C-3.2.2.3 Uniformity and Homogeneity of Geologic Media
All other factors being equal, rocks having a high degree of uniformity
and homogeneity are superior for emplacement of radioactive wastes. The
major reason for this conclusion is that greater accuracy of all conclu-
sions from investigations and analyses may be achieved when site varia-
bility is reduced; i.e., the chance of missing an Important factor la
greatly reduced when site conditions are uniform. Nonetheless! sites
that are neither uniform nor homogeneous may be suitable* if other
conditions are particularly attractive.
-------
The acceptable degree of inhomogeneity will vary, depending on such fac-
tors as the nature and characteristics of rocks or minerals included in
the host rock and the location of potential pathways from the repository
area. For example, it is common to find thin partings of shale,
several meters apart within a thick limestone or salt stratum. The
nature and location of these partings are important in mining. When
widely spaced, they can be used during the mining to form a. smooth and
continuous roof. On the other hand, depending on the nature and spacing
of the partings, they may contribute to roof instability, act as pathways
for groundwater flow, or cause localized fracturing from differential
expansion with temperature changes. Also, changes unrelated to layer-
ing, such as the occurrence of hydrated minerals, which can react adverse-
ly with waste products or be a source of water under high temperature
conditions, may be significant. The importance of inhomogeneity must be
decided on a site-by-site basis.
C-3.2.2.4 Nature of Overlying, Underlying and Flanking Strata
It is implied throughout the above paragraphs that acceptable conditions
of the host rock will often depend directly on the surrounding environ-
ment. Accordingly, it is necessary to determine the detailed stratigra-
phy of the formations overlying, underlying, and flanking the repository.
For example, thick beds of shale and/or other plastic-behaving rocks
may be desirable adjacent to the repository formation if they are:
relatively impermeable to circulation of groundwater; have high radio-
nuclide retardation potential; and can be deformed without fracturing.
Probably the least desirable rocks surrounding the repository are
highly fractured and contain large quantities of groundwater.
C-3.2.3 Structural Geology
The subject of structural geology characterizes and describes features
of rock materials that have been created by earth movements. In addition
to descriptions of existing conditions, structural geologic studies pro-
vide valuable interpretative information that can greatly facilitate pre-
diction of future events. The three major structural geologic aspects of
interest for this study are:
39
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Folding of strata, which describes the spatial
orientation of bedding;
Fractures including-primarily joints and faults,
occurring ;Ln otherwise continuous geologic
materials.
Major plastic flow of materials, such as dia-
pirism in salt domes.
C-3.2.3.1 Folding
The degree to which strata have been folded is important because of the
resulting inclination of bedding and fracturing of the rock where It has
been forced into a curved configuration. Figure C-2 presents an example
of moderately folded shale and limestone. In this cross section, the
rocks have been deformed into an anticlinal structure (a fold which is
convex upward).
From repository design and operations viewpoints, the preferred dip of
rocks (or strata inclination) phould not exceed a few degrees, A nearly
horizontal alignment facilitates hauling the excavated rock and
subsurface transport of the HLW materials. This consideration
can be of lesser importance for very thick, relatively homogeneous rock
where the opening does not necessarily have to follow the stratigraphic
horizons or beds.
Sudden and frequent reversals of dip are not only undesirable for
design and operations, but the abrupt folding that caused the reversals
may have created undesirable secondary permeability (Section C-3.1.1) in
the rock. This is particularly true for hard and brittle rocks in which
stress conditions at the time when folding occurred have usually in-
creased the frequency of fractures in the formation, resulting in a
significant increase in the potential for groundwater movement. For
plastic-behaving rocks like salt, deformations accompanying moderately
40
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-SROUNO SURFACE
/•LMCSroNC-
1067
1220 u-
SOO
IOOO
SCO
2000
a
Z
I
bj
Q
500
1000
500
000
Horizontal Scale
JW4 3048 4572 6096 7620 Meters
LtaEMD
Source: D'Appolonia Consulting Engineers Inc., for Arthur D. Little, Inc.
•C-2
I Umestone
| Shah
• Fault Arrowi Indictw
Relative Mownwnt
E
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to steeply dipping beds may have little or no effect on water movement
as fractures are healed with time. The resistance to fracturing of
shales lies between salt and the more brittle rocks, such as limestone
and basalt.
C-3.2,3,2 Joints and Faults
Joints and/or faults are common geologic features in most rocks. Joints
are fractures in rock that have experienced no relative movement between
the two sides, whereas faults are fractures along which relative movement
(vertical or horizontal) has occurred. The discussion in this
section deals with existing fault conditions, while Section C-3.2.5
discusses the potential for future faulting.
Joints are very common in all types of lithologies, ranging from volcanic
rocks to unconsolidated sediments. Joints are most commonly oriented ap-
proximately perpendicular to bedding, but they can be oriented in any
direction, depending upon the stresses or deformations that caused the
rock to crack. They may be open, having a space large enough for water
to pass, or they may be closed or filled with impermeable secondary
minerals, such as calcite or quartz. The open type are of primary
concern because of their ability to transmit groundwater.
Some degree of jointing will usually be present In relatively shallow
shales, limestones, basalts, and granites. The frequency of jointing and
the probability of joints being open usually decrease with depth. Their
characteristics at any depth must always be determined by boreholes and
field or laboratory testing. Sometimes open joints are found at depths
equal to or greater than those typically considered for HLW
repositories.
Ideally, a potential repository area should be virtually free of faults,
whether active movement has occurred recently (within the past 20,000-
30,000 years) or not. Minor, recently inactive faults in an area
42
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might be acceptable if it is possible to show that (1) the faults were
due to some tectonic force that is not likely to reoccur, (2) all
disturbance was outside of the actual disposal host rock or, (3) the
rocks are sealed against potential groundwater flow.
Any existing faults that have occurred in the recent geologic past may
be sufficient cause to make the site unacceptable. At a minimum, re-
cently active faults require extensive investigations to ensure that
the host formation will not lose significant containment capability
through faulting during the facility's designed life.
Major lineaments (lengthy features on the earth's surface that may be
straight or slightly curved and can be observed on aerial photographs
and satellite imagery) also fall within the broad category of frac-
tured rock. These lineaments may represent areas of increased
jointing in subsurface rocks or they may represent major faults. Unless
such features can be positively shown to have no effect on containment,
they should be avoided in selecting areas for waste repositories,
C-3.2.3.3 Major Plastic Flows
Major structural features associated with upward plastic flow of geo-
logic materials considered in this study are salt domes formed by an
uplift mechanism called diapirism. Figure C-3 illustrates a geologic
cross section of a salt dome. Note how the intruded strata flanking the
salt are folded and faulted due to the uplift pressures, Diapirtsra is
discussed in Section C-3.2.5.
C-3.2.4 Hydrogeology
The concept of investigating and designing any facility with scientific
confidence in its integrity for time periods of perhaps hundreds of
thousands of years adds an unusual dimension to all aspects of selecting
a site based on geologic characteristics. The Impact of this extreme
time constraint is possibly greatest for the consideration of ground-
water movement. For typical water supply or dewatering evaluations,
43
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GROUND SURFACE
152 - :=^^r^==r===--==^=r
S. 457
Horizontal Scale
457 915 Meters
915 L
Sandstone —^=- Fault,
Arrows
Indteatt
MoveiTMnt
Source: D'Appolonia Consulting Engineers, Inc., for Arthur Di. Little, Inc.
FIGURE C-3 RADIOACTIVE WASTE REPOSITORY
CONCEPTUAL MODEL IN SALT DOME
-------
water flow is considered relative to its volume per minute, day, or year.
However, over time periods of thousands or hundreds of thousands of years,
water flow that is imperceptible on a per-day or even a pet-year basis
may be significant. In other words, rock permeability is a relative con-
cept and the aspect of time available for flow to occur is significant.
For a repository', a small volume of flow over each one thousand or ten-
thousand-year period could be important, if it could transport unaccept-
able levels of radioactive materials from their repository setting. To
/
evaluate the required detailed hydrogeology will require extensive inves-
tigations. Relationships must be established among such factors as the
following:
* Regional groundwater conditions,
* Local (at the site) groundwater conditions,
• Intrinsic hydrogeologic properties of the
disposal zone and associated strata.
Elements of intrinsic hydrogeologic properties were described in Section
C-3.1.1, and associated geoehemical characteristics for retarding radio-
*
nuclide travel were addressed in Section C-3.1.2. These basic data are
not repeated in the following broader description of hydrogeology.
C-3.2.4,1 Regional Hydrogeology
For this study, regional hydrogeology relates to the existence and ex-
tent of all aquifers in the vicinity of a possible repository that
contain water that could flow above, below, or beside the repository
given any natural or man-caused event during its designed containment
life. Depending on site location and the degree of detail of the invest-
igation, the study of regional hydrogeology can cover an area from several
hundred acres to hundreds of square miles. The area of interest probably
will be greatest where extensive sedimentary deposits are considered as
the host rock.
45
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Important regional questions follow:
* Where are any aquifers and aquicludes located
in relation to the host formation?
* Where is the recharge area and what is the re-
charge mechanism for each aquifer?
* Is there natural flow in each aquifer system? If so,
what is the gradient causing flow and where are
the discharge locations?
* Where and why is there flow between separate aquifers,
including consideration of boreholes for water supply,
gas and/or oil extraction, or mineral investigations?
• What is the quality of groundwater in each
aquifer; is it presently being used or does it
have potential for exploitation by man for
irrigation, drinking, or other purposes?
• Are there mechanisms that could provide a con-
nection between any aquifers and the disposal
formation, including geologic events that nay
occur at significant distance from the repository?
* How will extreme changes in the total hydrologic
(precipitation-runoff-infiltration-evaporation)
cycle in the region during the very long contain-
ment period affect the regional hydrologic system?
Both extremely wet and extremely dry periods must
be considered.
Investigation of these regional conditions must always be carefully
planned and executed. The program must then be continuously modified
as new data are evaluated and new methods are proved. It is not prac-
tical in a short period to determine regional hydrogeology by specific
field investigations. Conclusions must be developed based on:
• The large volume of stratigraphic and structural
geologic literature available,
• Logs of wells drilled in the area,
• Geologic data obtained from aerial photographs
and imagery interpretations along with field
verification of important observations and
evaluations, and
46
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• Site investigations at selected areas through-
out the region of interest.
Sometimes the data will be sufficient to rule out completely large areas
otherwise considered as potential repository sites. More often, however,
the regional study will identify important factors for more detailed
local and site-specific studies.
C-3.2.4.2 Local Hydrogeology
the study of local hydrogeologic conditions brings together the regional
character of aquifers and aquicludea and the intrinsic properties of the
host rock. Such a study is used to determine if:
» Groundwater can reach the extremities of the
host formation,
* Groundwater can pass through and escape from
the host rock with the potential for trans-
porting radionuclides, and
* Any radionuclides can leave the repository and go
to an aquifer that may eventually transport
them out of the containment zone.
The key evaluations will usually be to determine if: (1) there is
any type of gradient (usually pressure gradient) that could move ground-
water through the containment area; (2) material permeabilities are such
that flow will be significant; and (3) benefits or detriments may be
realized from the geochemistry of the system. In the final analysis, the
most Important hydrogeologic consideration is whether or not groundwater
can transport an unacceptable level of radionuclides to the biosphere.
Local hydrogeologic studies begin with the regional information and use
specific information derived from boreholes, such as pressure measure-
ments, water level measurements, and water quality and temperature analyses.
47
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Hydrologic and geochemical modeling analyses and other site-specific
Investigations appropriate to defining existing and potential flow
conditions are also employed. Detailed knowledge of stratigraphy
and structural features at the repository site (Sections C-3.2.2
and C-3.2,3) are necessary to the success of any local hydrogeologic
study,
Special note is made that the plugging of open boreholes or sealing
around installed piezometers and other instrumentation will "be very im-
portant for any repository investigation. New methods for sealing be-
tween aquifers encountered in a single borehole are being developed for
this purpose.
C-3.2.5 Tectonics
This general site selection category deals with significant earth move-
ments (geologic events) that could disrupt the repository in some manner
that would reduce containment. The discussion Is separated Into four
subtopics:
* Tectonism, which generally includes all types of
movements of the earth's crust. For repositories
in rock formations, the definition can be limited
to (1) faults, which cause abrupt disruptions at
the fault displacement and Increased brokenness
at considerable distances and (2) less abrupt
movement caused by folding, which can Increase
the brokenness of rock, change erosion patterns
and rates, and generally modify regional or
local hydro logic systems.
* which for repository site selection
purposes is related to (1) the frequency of
earthquake activity that occurs in an area and
(2) the magnitude of movement that occurs as
the result of an earthquake. The first relation
ship is closely related to the faulting
of tec ton ism and Is not discussed separately.
The second relationship will have significant
importance on the engineering design of above-
ground and underground facilities during the
relatively short operating life of a repository,
but little influence on containment for the very
long period after backfilling.
43
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* Piapirism. which for this study relates mainly
Co the creation of salt domes by the upward
flow of this plastic rock from a deep-bedded
deposit.
* Volcanlsm, which refers to processes whereby
magma (molten rock) and its associated gases
rise into the earth's crust and are extruded
into the atmosphere and onto the earth's surface
where rocks are formed as the magma cools.
C-3.2.5.1 Tectonism and Seismicity
Tectonism in the context of this study is related to fault disruptions
that actually cause abrupt movement between two portions of the
earth's crust and/or significant folding of the crust with a resulting
increase in the brokenness of rock formations.^ Basic questions requiring
consideration in site selection are:
* How often, if ever, are significant movements
to be expected?
• What will the magnitude of movement be?
» Where will the movement be located?
• What are the potential Impacts on the
containment capability of a repository?
The second and third questions overlap significantly with seismology or
selsmiclty investigations that make possible predictions based on eval-
uation of past conditions and the geologic setting. For purposes of this
study, discussions of tectonlsm and seismicity are combined.
An earthquake is a natural phenomenon in which significant rapid and/or
violent movement of the earth's surface occurs. Because earthquakes
have occurred in most areas of the United States during the relatively
short period of recorded history (approximately three centuries), they
are of immediate concern in selecting a repository j-gite, particularly in
light of the desired long containment time period. The magnitude of earthquake
disturbance and the location and depth of movement are quite variable for
different parts of the country. Even in a given area, the potential for
49
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rock movement or fracturing as a result of a nearby earthquake will
depend on such local factors as stratigraphy (Section C-3,2.2) and
stiffness or elasticity (Section C-3.1.4) of the affected rocks.
The principal tasks in selecting a suitable site will Include:
• Quantification to the extent possible of the
earthquake and faulting history of the site
region,
* Establishment of a geologic rationale for the
historical behavior and using this ac a basis
for predicting probable future activity.
Areas of high seismic or faulting risk, such as portions of the west
coast of the United States, will have very low acceptability and
areas of low risk will appear favorable if other desirable geologic
characteristics are present. It Is possible that almost any area
may be acceptable from the seisoicity viewpoint where It can be
reasonably shown that movements at or near the repository cannot
occur with sufficient magnitude to reduce containment capability.
The prevailing geologic explanation for regions of the world within un-
usually high earthquake activity is centered on two concepts—continental
drift and sea floor spreading which have been unified in
(18)(19)
the theory of plate tectonics . Continental drift was proposed in
1912 by German meteorologist Alfred Wegener who argued that the earth's
crustal rocks could flow laterally if they could flow vertically. Thus,
he postulated that all the continents had been joined 200 million years
ago, after which they had broken apart. Alternative views to Wegenerfs
were postulated by Wilson ' and earlier by DuToit as well as others
to dispose of various problems that Wegener's model raised. Within
the past 20 years the theory of continental drift was placed on a firmer
foundation by development of the sea floor spreading hypothesis originally
(22)
proposed by Dr. Harry Hess of Princeton. This concept of sea floor
spreading was substantially confirmed by the work of F.J. Vine and D.H.
(23)
Matthews of the University of Cambridge. Magnetic data from the
vicinity of the oceanic ridges confirmed that sea floor spreading has
been going on during the past 200 million years at rates of between 2
and 18 centimeters per year. That is, crust has been more or less
continuously created during the past 200 million years, accounting for
50
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most of the present areas of the oceans. Since evidence suggests that
the earth has not expanded more than 21 during the last 200 million
years ,-• there arises the implication that crust has also been destroyed.
Plate tectonic theory accounts for this implication.
The significance of plate tectonics to the waste disposal Issue lies
largely with the seismlcity/earthquake phenomena related to plate
{18)
movement/boundaries. Dewey points out that "Most earthquakes
occur in narrow zones that join to ,form a continuous network bounding
regions [the plates] that are seisnlcally less active." Figure C-4
illustrates the distribution of these zones and indicates the larger
plates by name.
The concept of plate tectonics also relates to tectonism,which deals
with the structural folding of geologic formations over geologic time
Active folding or curving of a geologic formation is not usually a re-
pository design consideration, because even if in progress, it occurs
very slowly. Folding, however, may be a consideration for a repository
requiring controlled.containment for geologic periods of time.
Major considerations are:
* Can slow geologic deformations occur that have
the potential to cause the rocks to fracture
(crack), opening avenues for radlonuclide
escape?
• Are the host rocks and surrounding formations
sufficiently flexible (e.g.,salt and shale)
and self-healing (e.g., salt) to withstand
slow movements without any change in their
containment capabilities?
C-3.2.5.2 Diapirism
Dlapirlsm refers to the process in which geologic structures are formed
as the result of the upward plastic flow of rock from deep to
shallower levels. For the selection of HLW disposal sites in the United
States, diapirlsm relates mainly to the salt domes in Texas and
Louisiana and in the Paradox Basin of Utah. These salt don.es
formed as a result of tectonic downwarping of a bedded salt deposit,
after which upward plastic flow occurred through overlying strata. The
conceptual geologic model shown in Figure C-3 and discussed in Section
C-3.4.2 Illustrates this type of geologic setting.
51
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«6k NORTH
AMERICAN
PLATE
LEGEND
f ^ MAJOR EARTHQUAKES
^^^ OTHER SEVERE
VZafffl. EARTHQUAKES
••=" VOLCANOES
B««l on: H. Williarm, Scientific American, November 1955 (24) and J.F. Dewey, Scientific American, May 1972.
FIGURE r_4 RFninMS OP THE WORLD IN WHICH MAJOR FABTUOIIAVBC Aiun \ir\\ OAKI/-.C
-------
Geologic considerations for a salt dome are significantly different than
vfor bedded salt or other rocjk,types. The domes often have a lateral
extent of only about one or two kilometers, but are continuous vertically
downward for several kilometers. The selection of a dome site will
largely be dependent upon the desirable lithologlc properties of the salt
(Section C-3.2.1), Stratigraphy (Section C-3.2.2) and regional and local
hydrogeology (Section C-3.2.4) will be less important for evaluating
basic containment potential,
A site selection criterion for any salt dome will,be the current status
of any local or nearby diapiric activity, or the potential for renewed
activity during the design life of the repository.
Another criterion of lesser importance will be the assessment of possi-
ble diapiric formation in regions that have no historic geologic
evidence of such structures. This consideration Is related to the pos-
sibility of disruption of the repository by upward flow of material.
The prerequisites for such an event are a plastic-behaving rock buried
(25)
at great depth (up to 8000 meters or more), ' under the required
(2£)
temperature conditions (at least 300°C) and beneath the
groper geologic strata. An early check on the potential for the neces-
sary combination of factors will likely rule out the possibility of
diapirism occurring at most sites which do not now have any history of
such events.
C-3.2.5.3 Volcanism
Areas in which volcanism might reasonably be expected to occur within
the next million years should be avoided in selecting a HLW reposi-
tory site. Obviously, the high degree of violent activity
associated with a volcanic eruption would affect the containment capabil-
ity of a repository stratum in close proximity to the volcano.
53
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.
Two basic types of landforms can result from volcanic activity. In
some areas, molten rock has been extruded through extensive fissures in
the earth's surface and has built large plateaus (fissure eruptions),
In other regions, molten rock escaped from within the crust through
vents; around these vents ejected materials built up into landforms
called volcanoes.
A volcano is considered active if It is presently erupting or is expected
to do so. If it Is not active but may well resume in the future, the
volcano may be termed dormant. If no signs of activity are present and
no activity is anticipated in the future, the volcano is considered
extinct.(28>
Ninety-eight percent of all active volcanoes are located along plate
boundaries where significant earthquake activity Is also concentrated.
The Western quarter of the United States Is such a zone and Includes
Lassen Peak in California, which was last active in 1917.
C-3,2.6 Erosion and Denudation
HLW repositories must be located in areas and at depths where waste will
not be exposed as a result of erosional processes. Major erosional
agents include rivers and streams, landslides and other mass movements;
rain, which erodes by Impact and surface flowj frost action, which loosens
surface materials; chemical weathering of surface materials; wind,.which
erodes by deflation and sand blasting; and glaciers,which erode by
grinding and plucking. For this study, the important factors are erosion
river flow and denudation by glacial action.
Erosion is a continuously occurring process, the immediate results
of which are nearly imperceptible, except In the case of mass movements.
However, over geologic time periods erosional effects can be quite
perceptible—those highly evident results that can have marked effects
on landforms are connoted by the term denudation. In evaluating erosion
54
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and ""denudation In the overall assessment of the suitability of a site
for a HLW repository, careful consideration must be given to present
erosion rates and the potential for substantial increases in erosion
rates over long time periods.
Rates of erosion vary according to specific topographic location, rock
hardness, rainfall, evaporation rates, type,of vegetation*and other fac-
tors. As mentioned in Section C-3.2.5, factors such as tectonism can
cause broad uplift of land surfaces, which can lead to radical changes in
erosion rates. For example, the Grand Canyon portion of the Colorado
River has eroded to a depth of 2000 m during an estimated one million
O fi\
year period. The overall rate of erosion in major river basins of
the United States averages about 60 m/million years. Erosion
rates range from about 40 in/million years in the Columbia River
(14)
basin to about 165 m/million years in the Colorado River basin.
Since repositories will not be placed in areas where major
ground movements or river erosion can be hypothesized, erosion will
usually not be a major site selection factor.
For many sites, potential glacial activity may be the most important
factor for evaluating erosion or denudation potential. Glaciers often
deepen and widen valleys previously formed by streams; where tremen-
dous thicknesses of ice occur, valleys may be deepened to great depths.
Examples are the fiords of Norway,which are estimated to have been
(27)
incised to depths of 1300 m by glacial activity.
Four glacial periods have occurred in the northern United States during tne
past million years. The occurrence of one or more additional glacial
periods during the next million years must be considered as a potential
factor in the development of an HLW repository.
C-3.2,7 Mineral Resources
The attractiveness of many sites will be affected by past or potential
future interest in a mineral resource. Mineral resource considerations
55
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include surface and underground mining of solids, solution mining, and
the withdrawal of subsurface fluids, such as oil, gas, and water. Each
identified resource requires consideration of any past exploration or
recovery activities and an assessment of the potential for exploitation
during the life of the repository.
The exact nature and locations of all existing mines and wells near
any site must be identified and their effect on the confinement ability
of the site must be analyzed. This analysis should include effects of
subsidence from mining or pumping, the extent and location of induced
permeability, caused by hydrofracturing (fracturing rocks by Injecting
water under high pressure), and the extent of exploration testing (e.g.*
borings, test wells, underground blasting for geophysics) that could have an
impact on containment potential. At some locations, the measurement
of existing conditions may present the greatest difficulty in certifying
a site's suitability.
Any potentially valuable mineral in the formations above,
adjacent to,or below the host rock presents two selection considerations:
* The existence of the mineral raises the possi-
bility that man may disturb the containment in
the distant future by exploring for or extracting
the mineral, and
* A necessary and valuable resource may be lost to
man because of its location close to a repository.
C-3.3 CATEGORIZATION AND RANKING OF SITE SELECTION FACTORS
The discussion sequence in Sections C-3,1 and C-3.2 ranged from the
detailed examination of the intrinsic properties of the host rock to the larger-
scale evaluations of geologic settings and finally, geologic events.
A logical procedure for locating and investigating potential repository
sites moves in the opposite sequence, from considerations of the large
scale settings or events to intrinsic properties. The following sections
present an appropriate starting procedure for most HLW repository site
selection programs.
56
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C-3.3.1 Categorization of Factors
Table C-6 summarizes geologic Information categories that are pertinent
to selecting a site for a HLW repository. The categories are arranged
to suggest a logical investigative sequence (top to bottom in the table).
The first stage of the investigation involves assessing the potential
for the occurrence of significant geologic events (Including those as-
sociated with mineral resource exploitation or loss) in candidate re-
gions. Areas with a high probability for such an event should be
eliminated from consideration; no further work would be performed rela-
tive to those sites.
The second phase of the investigative sequence would concentrate on the
evaluation of the geologic setting for the remaining sites. This
^
phase of the study will (1) eliminate other sites on the basis
of negative regional or local jteolojjic conditions, and (2) better
identify the most promising specific locations within acceptable
areas. It is important to begin a relative ranking of sites at
this stage in order that further detailed studies can be
tailored to the most promising areas, in an appropriate
manner. Each element within the geologic setting category (Table C-6)
«s
should be fully addressed during the ranking process.
The final step in site selection will be to gather specific local data
for the best site(s), These investigations must provide detailed stra-
tigraphic, structural, and hydrogeologic descriptions, as well as
quantitative information about the host rock's lithology and intrinsic
properties. These final data, with appropriate analyses and evaluations,
permit the identification of site limitations, special design, operation
and/or abandonment requirements, and allow final site ranking bap«-
-------
TABLE C-6
GEOLOGIC INFORMATION CATEGORIES FOR
SELECTION OF A SITE FOR HIGH
LEVEL RADIOACTIVE WASTE DISPOSAL
INFORMATION
CATEGORIES
ELEMENTS IN EACH CATEGORY
Geologic Events
Teetonism
Seismlcity
Diaplrism
Voleanlsm
Erosion and Denudation
Glaciation
Mineral Resources
Geologic Setting
Lithology
Thickness and Lateral Extent of Host Rock
Depth of Host Rock
Uniformity and Homogeneity of Host Rock
Nature of Strata Surrounding Host Rock
Regional and Local Folding
Joints and Faults
Major Plastic Flows
Regional Hydrogeology
Local Hydrogeology
Intrinsic Properties
Primary Permeability
Secondary Permeability
Porosity
Ion Exchange Capacity
Mineralogy and Solubility
Pore Water Chemistry
Water Content
Thermal Properties
Engineering Properties
58
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C-3.3.2 Ranking of Factors
It is not practical to provide a quantitative ranking of geologic site
selection factors presented in this study, because the emphasis required
for each factor will vary, depending upon the host-rock type and the
regional and local geologic setting. However, it is reasonable to
discuss a logic for ranking the factors to provide a starting point for
any specific study.
The importance of any of the geologic event factors will depend on the
area being investigated. Not all geologic events will have the same
likelihood of occurring in specific geographic/geologic regions. Example
questions regarding geologic event possibilities follow;
• Does the region include ariy sites near geologic
plate boundaries (Section C-3.2.5) where the
occurrence of volcanic action or major faulting
is highly likely?
• Does the area contain conditions of known, active
diapirism?
» Is it probable that future glaclation stages
will cover the area?
Part of the overall ranking of events should consider the probability
that certain events will occur in some areas. For example;
* Mineral resources will be a significant considera-
tion for any area.
* Major faulting or folding will always be a con-
sideration, but catastrophic earthquakes will not
be a significant consideration everywhere.
• Glaciatlon, diapirism.and volcanism will be more
geographic and can be quickly ruled out for cer-
tain areas.
Again, it is emphasized that the very long containment requirement
dictates that each investigation must Initially consider all possible
geologic events.
59
-------
The following listing suggests a sequential procedure for considering
geologic setting factors:
* Does the proper lithologic formation exist to
function as a host rock for a HLW repository?
* Are there any structural features that may
eliminate the site's potential?
* Do the stratigraphic and structural features
present any unusual groundwater characteristics
that must be considered?
• Are the structural and hydrogeologic conditions
suitable for containment?
Ranking of the relative importance of intrinsic properties during the
detailed site investigation phase will also be highly dependent on
individual site conditions. A reasonable sequence of considerations
for evaluating intrinsic properties follows:
• Does any mechanism exist such as groundwater
flow that could transmit radionuclides?
* Are there chemical or theraochemieal properties of the waste
that could accelerate exposure of radionuclides
to the host formation and cause an undesirable
reaction with the host formation?
* Are there geochetnical properties of the host
formation and surrounding geologic media that
could retard the migration of radionuclides if
a transport mechanism does exist?
• Can the host formation and geologic surrounding*,
dissipate heat from the waste and are there any
potentially serious limitations of the host for-
mation at high temperature?
• Can the host material be engineered to obtain the
required operating and abandonment characteristics?
C-3.4 CONCEPTUAL GEOLOGIC MODELS
The conceptual models discussed in this section are Illustrated in
60
-------
Figures C-2 through C-7. The models are general and broadly represent
various repository settings; nature is vastly more complex. Thus,
the models are provided only as a basis for discussing site-selection
factors for deep geologic disposal of HLW.
All of the conceptual models are presented at similar scales and each
hypothetical repository is located at a depth of about 500 meters.
This depth is used solely for descriptive purposes, but is generally
considered reasonable for many sites. The depth at any given site will,
as prevously discussed, depend on the lithologic, mechanlcaL and chemical
properties of the host rock and the general geologic setting. The con-
ceptual models chosen for discussion follow:
Tectonized Shale (Figure C-2)
Salt Domes (Figure C-3)
Bedded Salt (Figure C-5)
Non-Tectonized Shale (Figure C-6)
Basalt (Fieure f--7)
C-3.4,1 Bedded Salt
Salt deposits are considered as a leading candidate for HLW repositories.
The conceptual geologic model for a repository (Figure C-5 is an essen-
tially horizontal, relatively uniformly bedded salt deposit. At many
locations, the salt body would be characterized by an interbedded sequence
of salt (halite), gypsum, anhydrite, limestone, and in most cases,
shale. Only two shale layers are shown in the conceptual model to
Illustrate some interbedding.
As discussed in previous sections, salt has several very attractive
physical and chemical characteristics for a disposal site. Salt pos-
sesses a very high thermal conductivity on the order of 15 millicalories
per centimeters-second degrees C. This property, coupled with a
large lateral extent of a bedded salt formation, would be a viable mech-
anism for dissipating the heat generated by the waste. The low permeabil-
ity (often much less than 10 centimeters per second) and the low
Interstitial porosity (less than 1 percent) provide a significant bar-
"V
rier to fluid movement other than localized internal water.
61
-------
152
306
S
.§
I
457
610
762
/fEPOS/TORY \ •
1524 3048 4572 6086 7620 Mttan
Shalt
| LJmaton*
Fault, Arrowt Indioti
nalatim Momraant
Source: D'Appolonia Consulting Engineers, Inc., for Arthur D. Little, Inc.
FIGURE C-5 RADIOACTIVE WASTE REPOSITORY CONCEPTUAL
MODEL IN BEDDED SALT
-------
The presence of shale interbeds could retard radionucllde transport.
Although shale has a moderate Interstitial porosity on the order of 5%s
—4
it has a low permeability (usually less than 10 and often as low
-6 -ii
as 10 to 10 cm/s) and a high sorption capacity, which would inhibit
the migration of radionuclides from the repository horizon. If the
shale contained hydrous mineral, phases, a potential negative impact
results from the release of water by heat generated from the waste decay.
The conceptual model shows that the salt-shale sequence is overlaid by a
limestone unit. If relatively unbroken and without unfilled solution
channels or joint systems, the limestone should have a low permeability
to retard radionuclide migration to the overlying sandstone aquifer.
The sandstone aquifer shown overlying the repository is postulated to
have a relatively large lateral extent. Often such a layer can transmit
sufficient water 'for industrial and/or municipal uses. The aquifer is
shown at a depth, however, that would probably be uneconomical for farm
and domestic use, especially with the presence of a thick, water-bearing
sand near the surface. Since the region in which this model is postulated
is characterized by flat lying rocks, the sandstone aquifer will not
crop out for at least tens of kilometers.
The sandstone aquifer in the conceptual bedded salt deposit Is overlaid
by a thick shale unit with very low permeability. This unit represents
a significant barrier to radionuclide migration. As a result, waste
contamination to the biosphere could occur only where the sandstone
crops out, where the sandstone unit has been tapped for a water supply
in an area where the shale is excessively broken, or where a borehole
could allow vertical migration of fluids.
63
-------
A very old, inactive fault Is shown on the right side of the conceptual
model. The fault cuts several of the stratlgraphlc units, including the
salt and the sandstone aquifer. The effects of the fault on the aquifer
could be that it would prevent horizontal radionuclide migration, but it
might enhance vertical migration to at least the overlying shale unit.
For the case shown, the clays and silts forming the shale were deposited
after the tectonic activity that caused the fault had ceased.
A deep aquifer has been shown below the salt formation. This represents
an additional potential escape route similar to the upper sandstone, but
with much less potential for reaching the surface. The pressure gradient
between the upper and lower sandstone aquifers also represents a basis
for establishing whether water migration between the two, through the host
rock, should be a design consideration.
C-3.4.2 Salt Domes
A conceptual model for a salt dome waste repository is shown in Figure C-3
The unique features of this model are:
• The intrusive nature of the salt into the
surrounding geologic formations; and
* The large amount of faulting and fracturing
present in the formations adjacent to and
overlying the salt dome.
As discussed in Section C-3.2.5, salt domes in the United States are
concentrated primarily in the coastal regions of Texas and Louisiana,
and to a lesser extent in the Paradox Basin, located largely in Utah,
The salt was originally deposited as a bedded sequence of halite, gypsum,
anhydrite and shale. As a result of deep burial and differential pressures,
the salt intruded into the overlying sediments, eventually forming
, j
salt domes.
64
-------
The fracturing and faulting shown overlying the salt dome are the result
of the forceful Intrusion of the salt into the overlying sediments.
These faults would not be expected to undergo movement once active dome
emplacement has ceased. However, movements could occur from excessive
withdrawal of subsurface fluids, such as water, oil, gas, or brine.
The dome in the conceptual model (see Figure C-3) is essentially
circular in plan view, with the diameter on the order of 3000
meters. The salt dome is assumed to extend to a depth of at least
4000 meters, which is not uncommon for Gulf Coast and Gulf Interior
Basin salt domes. The large vertical extent of the salt dome will
assist in the dissipation of heat generated by radioactive decay.
The salt dome is overlaid by caprock consisting of anhydrite and gypsum,
with minor amounts of calclte and sulfur. This type of caprock is
common and generally has very low, if any, measurable permeability.
Caprock originates from accumulation of the less soluble minerals of the
salt body as its original top was dissolved during emplacement.
The salt dome penetrates, and is in part overlaid by, a thick shale
formation. The shale will have the same type of mitigating effects
on radionuclide migration as the shale discussed for the bedded salt
model, except that its permeability may be greater as a result of
fracturing,which occurred during the emplacement process.
The lower aquifers are both faulted and folded upward as a result of the
doming process. Because sections of the aquifer have become effectively
cut off from circulation and abut against the salt dome, stratlgraphic
traps can exist that may contain water, gas, and/or oil under pressure.
Conditions above a salt dome can vary greatly, depending on its depth
and the last time that the dome was active in geologic time. These
near-surface conditions should not be important at a properly selected
dome because the principal geologic barrier against radionuclide migra-
tion in-%this model is the salt.
65
-------
If it Is hypothesized that some radlonuclides escape from the salt,
their pathway to the surface would be dependent on the site-specific
conditions and where the breakthrough point occurs. As with the above
discussion for bedded salt:
» Pathways to the surface could be greatly re-
tarded by shale and clay layers that have
low permeability and porosity and a high
radionuclide retardation capacity.
* The number and location of sandstone zones,
the amount and extent of brokenness of the
surrounding rock and the potential for bore-
holes can contribute to increased pathways;
again depending on site-specific conditions.
C-3.4.3 Shale
Some shales have properties thau make them attractive for a waste
repository. The geologic potential of a shale body for a waste storage
site depends on dimensions of the formation, llthology, mineralogy,
mechanical and hydrologic properties, structural and seismic history and
the extent of drilling and/or mining.
The conceptual model for non-tectonized shale (Figure C-6) shows a relatively
simple stratlgraphic sequence composed of a limestone unit overlaid
by a sequence of shale and sandstone strata. Each sandstone unit is
considered to be an aquifer. The two lower aquifers have been offset by
an old, inactive fault, which would limit the horizontal flow through
these strata. The uppermost sandstone aquifer was deposited after
movement of the fault had stopped, and it is considered continuous
throughout the region. This upper aquifer is typical of conditions
that often provide a primary water supply for both domestic and muni-
cipal use.
The principal factors for mitigating radlonuclide migration from this
first conceptual repository in shale are associated directly and almost
exclusively with the thick shale units; i.e. low permeability, favorable
sorption characteristics, and resistance to fracturing.
66
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GROUND SURFACE
152
305
^457
610
762 L
1900 H
Horizontal Scale
1524 3048 4572 6096 7620 Meters
LECCNO
|m Sandstone
j I ; i ;| Limestone
-^*- Fault, Arrows IndicaM
Relative Movement
Source: D'Appolonia Consulting Engineers, Inc., for Arthur D. Little, Inc.
FIGURE C-6 RADIOACTIVE WASTE REPOSITORY CONCEPTUAL MODEL
IN NON-TECTONIZED SHALE
-------
The second conceptual model In shale (Figure C-2)has been constructed in
a tectonieally-defortned shale unit, on the flank of a small anticline.
The anticline is flanked by two high-angle faults. An obvious detrimental
feature in this case is the presence of an extensive fault system that
could present direct pathways for migration if groundwater were introduced
into the repository. This site would be acceptable for a repository in a
geographic location with an arid climate, if absence of water in the
subsurface could be reliably predicted for the life of the repository.
Even if water were to occur deep in the subsurface, the thickness and
geochemical properties of the shale unit might be sufficient to retard
nucllde migration. Substantial data would be required to demonstrate
this conclusion.
Even without a widespread groundwater system, water could be generated in
a shale repository through the dehydration of hydrous mineral phases of
the rock. If this volume of water were small, no detrimental effects
would be produced. Again, however, detailed studies would be required
to demonstrate the actual volume of water that could be developed
considering the temperature and size of the repository, and to evaluate
any possible paths for radionuclides to be released from the repository
along faults or fractured zones in the tectonically-affected rock.
C-3.4.4 Igneous Rocks (Basalt Model)
One of the most promising of the igneous rock types being considered for
an HLW repository is basalt. The most attractive property of basalt
appears to be its high strength for creating the mined cavities.
Potential undesirable properties requiring careful investigation and
evaluation are: basalt's relatively low thermal conductivity and its
relatively high linear thermal expansion coefficient, which could either
produce new fractures or expand existing fractures. Also, some basalts
contain minerals that will decompose If heated in the presence of water.
This is especially the case with pyroxenes and amphlboles.
68
-------
A conceptual model of a repository in basalt la Illustrated In Figure C^-7,
The thick deposits were extruded as sheet type (plateau) flows. These
are the fissure eruption type volcanic deposits mentioned in Section
C-3.2.5. The basalts shown are interbedded with fluvial sand deposits,
which in some cases could also be lacustrine clay and silt deposits.
In general, the clay and silt deposits would not be laterally extensive,
but would form isolated lenses or pods between basalt flows. The sand
units as well as rubble layers overlying some of the basalt flows
(29)
often contain large amounts of water. In one afea, however, it
is suspected that this water may not be connected to the surface} this
is suggested by the age of the water, which has been dated as at least
40,000 years. If this condition can be proved, it may be possible to
conclude that these "trapped aquifers" are likely not connected to a
recharge area and would have little likelihood of transmitting nuclldes
to the biosphere. However, all considerations discussed for shales in
Section C-3.4.3 would still be important on a regional and local basis for
basalt.
69
-------
6ROUMO SURFACE-
^
i-'-/~-'- /i^-v /X't.*'>--~.-*' ' ' ~ '•*_-/ ~'y_'-—_
152
306
487
610
762 U-
SAND
:
"•^^y^jf^^v ^'*Mti{2&^&^^
-
,^\^Xv:'.v>-^"-X,vix>^>
-------
C-4.0 ANALYSIS OF MIGRATION POTENTIAL
The primary objective of this subtask is to review the available
information concerning, and quantify the potential for, migration of
radionuclides through the geosphere. The greatest concern for such
migration appears to be related to groundwater movement. The initial
groundwater flow conditions (perhaps better described as deep subsurface
water movement) at potential disposal sites are extremely important.
The leaching characteristics of the high-level radioactive waste (HLW)
are also important to the migration.
The approach used in the nuclide migration analysis Involves
several steps, including:
(1) Review of the available data on waste leaching and
of empirical and theoretical models describing leach rate,
(2) Review of the available models of radionuclide transport
in the geosphere, and
(3) Selection and use of transport models for sensitivity
analysis in order to identify the range of parameters
important in nuclide migration.
The calculations for this report are shown to be applicable to the
conceptual models for each of the five potential geologic repository types
(summarized in Section C-3.4). A mathematical approximation of these
geologic strata was then used for nuclide migration calculations. Details
of this approach and the results are discussed in Sections C-4.3 and C-4,4,
C-4.1 MODES OF WATER CONTACT
The pathway used throughout this analysis Involves nuclide migration
to the earth's surface through geologic strata. In analyzing this pathway,
two processes have been examined: flowing groundwater and diffusion through
static groundwater. This section discusses the modes of possible water contact,
the probable natural water flow rates? and the mechanisms whereby water
might come into contact with HLW after the closing of a waste repository.
It will be important to have minimal or no natural water movement in
the strata and interbeds of any potential repository. Since it is highly
probable that in deep repositories there will be water-bearing strata
71
-------
either above or below the potential repository area, water movement in
the potential repository strata and interbedded strata can be expected
to occur unless:
(1) the repository formation has no permeability to water, or
(2) the permeable strata above and below the repository are in
hydrostatic equilibrium (i.e., water pressures differ
only by the difference in elevation multiplied by water
density).
There are probably no strata that satisfy the first criterion, but
the combination of extremely low permeability and little, if any, hydro-
static imbalance could create a situation of virtually no flow in a
repository. At least the water velocity could be below the usual
threshold of hydrologlc flow measurements.
two kinds of hydrologic flow regimes were postulated to calculate
the natural water velocity that might exist in a potential repository
stratum. These calculations were made with a conventional two-dimensional
hydrologic flow model. Although no specification of site type - bedded
salt, salt dome, granite, basalt, shale, etc. - is intended, the example
examined corresponds more obviously to a bedded formation than to other
types. Nevertheless, the intrusive types of geology could still have
water-bearing rocks above and below the potential repository area, thus,
In both flow regimes, two aquifers were assumed to exist: one above and
one below the potential repository strata. Two cases were investigated for
each flow regime, illustrated in Figure C-8. The cross-section data
applicable to each case are presented in Figure G-9.
In the first regime (Cases 1 and 2), flow in the two aquifers was
assumed to be isolated until it reached the upstream point of entry to
the region of interest In the model. At this point, a hydrostatic im-
balance equivalent to 305 m of water (achieved by assuming an increase in
upstream pressure in stratum 3 to 1403 m of water) was assumed to exist
between the upper and lower aquifers (a condition that could occur at
specific sites, although such sites would not be likely candidates for
repositories). Depending upon the permeability of the repository of the
72
-------
Recharge
Recharg
Recharge
\
\
Case* 1 and 2
Flow Regime 1
Upper Aquifer
X
Repository and Interbeds
Lower Aquifer
Caaes 3 and 4
Flow Regime 2
FIGURE C-8 TWO FLOW REGIMES
t
t
Upper Aquifer
' ' x
' X X
Repository and Interbeds
Lower Aquifer
)ischarc
Oischarc
Discharge
73
-------
Upstream
Potential
(m of Water)
782
Base Case (No Vertical Flow)
Stratum No. Thickness, m Permeability, cm/s
— •- 1 Aquifer 30 1(T5
Porosity, %
15 —
Downstream
Potential
(m of Water)
m n
Repository and
930 — *~ 2 Interbeds
1ogg „ ,,*. 3 Aquifer
Flow Regime Case No,
1 1
1 2
2 3
2 4
30S 10"7 1
30 10'5 15
——168
»-336
Change From Base Case
Upstream Potential in Stratum 3 Increased to 1403 m HjO
Stratum 2 Permeability Decreased to 10 cm/s
Same as Case 1
Same as Case 3 but with Stratum 2 Permeability Decreased to
10"9cm/s
FIGURE C-9 CROSS-SECTION DATA FOB EVALUATION OF FLOW REGIMES
74
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repository strata and iriterbeds, this hydrostatic pressure imbalance
dissipates as vertical flow occurs. The object of this set of calculations
was to ascertain the magnitude of the vertical velocity and the distance
at which the hydrostatic imbalance is dissipated as a function of assumed
repository strata permeability.
The second set of cross-sectional runs (Cases 3 and 4) used the
same basic geometry, but flow was assumed to enter and leave only the
upper aquifer, with no recharge or discharge into the lower layers.
Depending upon the permeability of the repository strata and interbeds,
flow in the upper aquifer moved downward and spreads to the lower
aquifer as well. The primary objective of these calculations was to
ascertain the magnitude of the vertical velocity that might exist in
the repository strata and interbeds.
The permeabilities used for the three strata were characteristic
of the range for all of the strata types considered for HLW repositories.
The values used for each case are illustrated in Figure C-9,
The calculated vertical velocities (Darcy or superficial, rather
than interstitial) in the repository strata are shown in Figure C-10.
The vertical velocities at the upstream end for the high-permeability
cases (1 and 3) are larger by at least an order of magnitude than those
of the low-permeability cases (2 and 4). But the hydrostatic pressure
imbalance decreases more rapidly in the high-permeability cases, and the
magnitudes of the vertical velocities decrease accordingly. Because of
the more rapid dissipation of the hydrostatic Imbalance in the high-
permeability case, there is a region where the vertical velocities in
the interbeds are actually higher for the low-permeability interbed
case than for the high-permeability case, as shown in Figure C-10. In
Cases 3 and 4, the vertical velocities reverse sign at the midpoint
between the recharge and the discharge ends. The velocities are symmetrical
around the zero velocity point and are higher near the discharge for
Case 3 (higher permeability) than for Case 4. In both these cases, the
calculated vertical velocity at the midpoint of the interval is less
than 10~ ft/day (Darcy velocity).
75
-------
10
Upstream
0.4 0.6
Fractional Distance
0.8
Downstream
10
,-5
> 10'1
jo
10
8
o o
o
Upstream 0.2
0.4 0.6
Fractional Distance
o o
O
0.8
Downstream
FIGURE C-10 CALCULATED INTERBED VELOCITIES FOR TWO FLOW REGIMES
76
-------
The calculated vertical velocities can be compared with what might
be termed "significant" flows. One definition of a significant vertical
flow is one that would still allow a salt bed to exist over geologic time.
A salt leach front (the point at which most of the soluble salt has
dissolved) will move at only a fraction of the natural water flow. This
leach-front velocity can be expressed approximately as:
*
where:
v = Darcy velocity of water,
v = leach front velocity,
p = water density,
w
p = solid salt density,
s
X • weight fraction of solid salt.
s
It should be noted that the salt porosity, $ , could probably be assumed
to be zero without significant loss in generality.
For typical values in the above expression, the leach front could
move about one-tenth as fast as the water velocity. At a natural
Darcy water velocity of 10~ ft/day, the salt leach front would
move only about 30 cm (1 ft)/30,000 years. Leach rates higher by two
orders of magnitude could probably be significant, but could probably not
exist in or near existing salt beds. Nuclide migration at natural flow
i_'7
rates as low as 10 ft/day should not be significant!since it would
require almost 3 million years to flow 30.5 m (100 ft).
Another type of water movement in the repository strata, although
caused by the presence of the repository, might be termed "natural"
flow. In considering potential repositories in certain existing salt
beds, the presence of small pockets or volumes of saturated or super-
saturated brine have been noted. These brine-saturated regions could
migrate slowly toward the repository because of the temperature gra-
77
-------
dient imposed by the radioactive wastes. Additional dissolution of
salt on the high-temperature side of the gradient, along with recrystal-
lization on the low-temperature side, could cause migration of the
liquid. Although this migration could create substantial water contact
with the waste, there would be no additional driving force for water
migration away from the repository.
Similarly, water movement due to thermal stress has been observed
for potential repositories in salt and shale. Again, these low-velocity
flows would move toward the potential repository (heat source) rather
than away from it.
Even if highly selective siting criteria are applied, there can be
extremely slow natural migration of water through the repository strata.
If this occurs, then the velocity of this flow would have to be so low,
that, even if the nuclides were in soluble form and no sorption took
place, the migration would not be significant.
A repository containing HLW creates a substantial heat source.
The heat stress thus imposed may cause some fracturing of the repository
and interbeds and open up a more permeable channel to an adjacent
aquifer. Similarly, penetration by exploration or other types of holes,
or failure of the repository entry shaft seal could create a permeable
area above the repository. The importance of these areas of increased
permeability in terms of nuclide migration is discussed in Section 4.4
of this report.
C-4.2 EVALUATION OF LEACH MIES
C-4.2.1 Introduction
Most waste forms for the radioactive nuclides are heterogeneous,
slightly porous solids. Leaching of the HLW form may include several
simultaneous, interdependent physical mechanisms. The movement of
nuclides may occur inside crystal lattices, along crystal grain boundaries,
through porous regions, along pore surfaces, and through interparticle
voids and may Involve other complex processes at the molecular level.
This movement is generally accompanied by Ion-exchange type chemical
78
-------
reactions, vaporization and condensation, and dissolution and precipitation
of leached isotopes. Typically, leaching rates are higher during initial
contact with water, dropping significantly shortly thereafter.
Factors that affect leachability are porosity and fractures in the
solid, temperature of the solid, chemical composition of the storage
matrix material, solubility of the isotope being leached, and charac-
teristics of the leaching solution such as pH, salinity, etc. Porosity
and fractures determine the solid surface area directly in contact with
the fluid. Solubility of the waste form depends upon temperature; a
temperature increase of 25-100 C can increase the leach rate and the
total amount of material dissolved by factors of 10-100. Most laboratory
measurements have been made at temperatures at or near 25 C, although
the waste material burled in a geologic formation will probably be at
higher temperatures. In the leach model used for transport calcu-
lations, a correction has been made to account for this temperature
effect. Because of the differences in solubility, leachabilities of
various isotopes in the same waste form material are not the same.
This is especially true of the actinides, as will be discussed in
Section C-4.4.
Principally because of differences in experimental techniques,
available leaching data are not always in agreement. The International
Atomic Energy Agency (IAEA) adopted a standard procedure for measuring
leachability of solids and expressing leach rates. Most of the published
2
leach rate data are expressed in grams of solid leached/cm of surface area/day
3 2
IAEA has recommended plotting the volume of solid leached as cm /(cm )(day),
or cm/day units, or alternatively as the fraction of initial radioactivity
leached vs. the square root of time. A good summary of test methods and
leach data has been published by Mendel,
C-4.2.2 Empirical Leach Models
The most commonly used empirical model for quantity leached is
expressed as followsJ
79
-------
1/2
Q - at ' + bt (10)
where
Q ™ cumulative quantity leached,
t * time, and
a, b » empirical constants.
The fit of this model to much of the experimental data shows that the
1/2
variation of Q is proportional to t for short time periods and linear
for longer times. The above expression for cumulative quantity leached
would indicate a constant leach rate is approached at longer times.
Much of the data show this behavior after leach' periods of only
2500 days or less.
Leach data obtained at Oak Ridge National Laboratories have been
fitted to the equation:
Q - at" (11)
A similar expression was used by Zagar and Schillmoeller for their data
(31)
for leaching from glass. As pointed out in this work, when a is 0.5,
the constant "a" can be related to a dlffusivity in the porous solid.
Investigators at Atlantic Richfield Hanford Company have fitted
experimental data with a polynomial equation, attributing each term
(32)
in the equation to the leaching effect from a separate mechanism-
These empirical leach models fit much of the laboratory leach data.
An important aspect of the problem, however, is how consistent these
laboratory data fit in situ waste leach rates. An in situ test has been
^33^
conducted at the Chalk River site in Canada over the last 15 years.
Data from this work appear to be consistent with the laboratory
leaching data on the same glass form, as shown in Figure C-ll. A constant
leach rate was achieved in the in situ test after 6-8 years in place.
These data show an extremely low leach rate, stabilizing at
-11 2
about 5 x 10 g/cm /day.
Leach rates at actual waste repositories would be even more complex,
because of the substantial temperature effect from the radioactive decay.
Based .upon the expected temperature decline and changes in surface area
as a function of time, an empirical model for the in situ waste form was
derived in Task B, where:
80
-------
10"
10
,-7
& 10*
•o
CM
E
o
en
u
*^
a
10
,-10
10"
11
o
O Laboratory
• Field
»O
• *
_L
1,000
I
4,000
2,000 3,000
Time, Days
Adapted from: Merritt, W.F. High Level Waste Glass: Field Leach Test.
Nuclear Tech., 32, January 1977.
FIGURE C-11 COMPARISON OF LABORATORY AND IN-SITU
LEACH DATA
81
-------
(1) For glass forms (standard and devitrifled) the fraction leached
is proportional to the logarithm of time after water contact
has occurred.
(2) For calcine forms, the fraction leached increases linearly and
relatively rapidly after water contact has occurred. No benefit
in terms of a decrease in leach rate due to decline in decay
heat was assumed.
In addition, many of the radioactive nuclides in the actinide chains have
extremely limited solubility. Nuclides in other than the actinide chain can
also exhibit solubility limitations. Under oxidation-reduction conditions
existing in the natural groundwater, several nuclides might exist in a
relatively insoluble form. Such nuclides include Tc-99 and Np-237, which
under some circumstances would not be strongly adsorbed. In the actinide
chains, U, Pu, Th, Am, and others might also be in a very insoluble form.
Consequently, even though the waste form leaches, the nuclide may be present
in an immobile, insoluble form. The importance of this solubility limitatior
in the migration of the actinide series has been examined and the results
are discussed in Section C-4.4.
C-4.2.3 Theoretical Leach jtodels
Virtually all of the theoretical modeling efforts are solutions to
one-dimensional differential equations in linear, cylindrical, or
(34)
spherical coordinates. These equations describe diffusion in a
slightly porous solid with decay and formation of radioactive isotopes;
in these equations, dissolution of the solid is important. The resulting
solutions are complex because of the form of the equation and the Initial
and boundary conditions.
Solutions to the one-dimensional differential equations for theoret-
ical leach rate are consistent with the empirical models . ' These
solutions show an initial leach rate that decreases rapidly with time and
later becomes constant. The advantage of the theoretical solutions is
that the constants can be interpreted as physical quantities, such as
the diffusion coefficient.
82
-------
The approach in modeling geosphere transport has been to use the
leach rates defined in Task B, Two computer runs investigating different
assumptions about waste leaching were made. One run was consistent with
Figure B-42 of the Task B report for the amount of glass leached as a
function of time. That curve represented a leach rate decreasing with
time only because of the changing surfaee-to-raass ratio. No separate
correction for the temperature effect was made. The second run assumed
that the initial leach rate had been increased by a factor of four by the
repository temperature increase. In this run, leach rates beyond 4000
years were less than those shown in Figure B-42 to remove some of the
conservatism. The fraction of the waste leached as a function of time
for both runs is plotted in Figure C-12.
C-4.3 GEOSPHERE TRANSPORT THEORY ASSESSMENT
As previously mentioned, the objective of this study has been to
investigate non-site-specific parameters important in mitigating
radionuclide transport in the geosphere. Forrauljtion of the cases to be
examined with the model is strongly dependent upon this non-site specificity,
A distinction should be made regarding the range of values for In-
dividual parameters (permeability, porosity, adsorption, etc.) needed to
describe the classes of generic sites and the uncertainty in the identity
of these same parameters once a specific site is selected. In the former
case, the parameters might need to span the range of salt, basalt, granite,
and other types of formations. For the specific site case, a different
range of parameters may be needed to describe the uncertainty remaining
subsequent to a measurement program. Modeling by the petroleum Industry
supports this conclusion.
These petroleum models solve a broad range of equations for simulta-
neous three-phase flow, energy considerations in enhanced recovery
mechanisms, multicomponent oil phases, and other complexities. In addition,
a number of laboratory measurements, as well as in situ tests, are made in
83
-------
.014
Leach Rate
0.0865 %/yr.
'Variable Leach Rate" Model
Constant Leach Rate" Model
Total Leach Tim«
65,000 Years-x
4,000
8,000 12,000 I6f000 20,000
Time From Start of Leach, Years
24,000
FIGURE C-12 COMPARISON OF LEACH RATES FOR TWO LEACH MODELS
84
-------
order to provide background data for these models. Significantly few, If
any, purely predictive models have been developed. On the contrary, the
models are first required to fit existing history concerning a specific
reservoir. Use of such models generally Involves four steps:
(1) Compile data concerning both the geologic description of the-
reservoir and the fluid properties.
(2) Utilize these data in the model, and attempt to match model
output with the reservoir performance history. (This would
include production of oil, gas, and water at each well, along
with measured pressures with the wells either producing or
shut-in).
(3) Adjust these data as necessary and to the extent consistent
with the geologic description in order to obtain the best
match to the existing reservoir history.
(4) Predict future reservoir performance.
Petroleum industry practice is cited to emphasize that uncertainty
in both field and laboratory measurements is well recognized. This does
not imply that the measurement Itself is inaccurate, but rather that it
is impossible to use these point-value measurements (whether laboratory
or In situ) as necessarily representative of a geologic transport
response over an area of the size of a repository. Mathematical predic-
tions for non-site-specific areas or for geologic strata for which the
available data are limited must have considerable uncertainty. Sensitivity
calculations are, therefore, required for a range of the parameters Impor-
tant to the transport process.
The complexity of a model suitable for such non-site—specific calcula-
tions is an important consideration. There is a potential for error in
using too sophisticated a model, thereby perhaps hiding some of the
uncertainty that actually exists, or in using too simple a model that may
not include potentially important parameters. The results of a rather
complete review of models that could have application in radioactive
nuclide transport will soon be published!"
C-4.3.1 Hydrology -Radionuclide Transport Model Review
An extensive review of hydrologlc models has been completed by the
(16\
Holcomb Research Institute with the support of EPA . Results from this
85
-------
survey are not yet available in published form, but summaries and conclusions
from that review were discussed with members of the review team.
This review was quite comprehensive and conducted on a worldwide basis.
Questionnaires were mailed to groups known to be working in the field of
hydrology soliciting information concerning the types of models developed
and available. Later, workshop sessions were held at three locations in
the United States in order to obtain additional information regarding
actual applications and limitations of existing models. Those models
useful for geosphere radioactive nuclide transport are discussed here.
Any model to assess nuclide transport has two essential parts,
(1) the aquifer flow; and (2) the transport, dispersion, sorption (including
precipitation), and radioactive-decay of nuclides in this flow. These
two parts can generally be uncoupled and solved independently. Two critical
factors must be considered, however, before such uncoupling can be permitted.
The first critical factor to be considered is the concentration of the
radionuclides. • If the geologic strata represent a viable repository,
nuclide concentrations will have to be so very small that they will not
affect the density or viscosity of the water.
The second factor is the amount of heat that will be generated as a
result of the energy released by decay of the radioactive nuclides. The
density and/or water viscosity could be affected by increases in temperature
and could initiate natural convection. Since the leached nuclides should
be at trace concentrations in the water, however, the heat generation within
the flowing water should be negligible, thus removing this possible barrier
to uncoupling. This latter effect would not normally be negligible, but
instead of coupling the water flow and nuclide transport, it would instead
couple flow with energy balance.
The following comments are a summary of the review of available
models for geosphere nuclide migrations:
(1) A number of hydrologic flow models are available that consider
a rather complete range of spatial dimensions, inhomogenelties,
anlsotropy, and other factors.
86
-------
(2) A number of constituent transport models are also available
that are less general than the flow models but are quite
usable for consideration of trace-level constituents that
undergo simple radioactive decay.
(3) Only two models appear to include both the decay and growth
exhibited during parent-daughter radioactive chain interactions.*
These two models are quite different in approach.
One, developed at Battelle Northwest Laboratories, represents
an analytical solution to the set of one-dimensional partial
differential equations describing dispersion, sorption,
convection and radioactive growth, as well as decay, of each
(37)
nuclide. Since it is based upon an analytical solution, this
model is limited to the following;
(a) Dissolution of the waste form at a constant rate
(radioactive decay of the waste form is included).
(b) Water flow at a constant velocity.
(c) Trace levels of the nuclldes are present, so non-
interactive equilibrium sorption can be assumed.
(d) Hydrodynamlc dispersion is treated as constant and
one-dimensional in the axial flow direction.
* Other constituent transport models have been used by various authors
to analyze radionuclide migration; however, these do not appear to
have included the Interaction of complete parent-daughter chain reactions
(e.g., see (35) and (38)). It should be noted that most models that
include nuclide source and decay terms could be used for full parent-
daughter chains by a successive application of the nuclide balance
through the entire chain. Examples of models that have used this
concept Include (39).
87
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When restricted to constant flow velocity, the flow model can be
uncoupled from the transport code, and the constant velocity
would be reduced to a simple steady-state flow approximation.
For situations where the water velocity is not constant, however,
the model could be used by assuming a representative, composite
average flow.
The other model, which includes complete geosphere transport
and decay chains, is one recently developed by INTERA Environmental
Consultants for Sandia Corporation on behalf of the U.S. Nuclear
Regulatory Commission (NRG). ' A brief summary of this model
appears as Appendix C-II of this report.
This model represents a numerical finite-difference solution
to the set of partial differential equations describing geosphere
transport. It uses second-order, correct spatial finite-difference
forms to eliminate numerical dispersion, although in doing so,
it places a limitation on the size of the grid block and the
time step that can be used. The model, similar to the one
previously described, allows for hydrodynamic dispersion,
sorption, convection, and the nuclide chain radioactive
decay and growth. This model offers considerably more flexibility,
in that it is multidimensional and handles inhomogeneous geologic
strata or groups of strata. Moreover, the model couples the nuclide
transport code with a complete multidimensional water flow model.
This model still assumes trace levels of radioactive constituents,
so the sorption properties can be treated as if in equilibrium
and non-interacting. Furthermore, the model can be used to examine
thermal convection effects or a constituent that affects water
density, since it includes a total-energy balance and a major-
constituent balance.
For the present application involving geosphere transport, most of
the assessment was made for a one—dimensional geosphere pathway. As a
consequence, either the analytical transport model or the numerical
transport model described above could have been used for the calculations.
Actually, both models have been used for this study: the analytical model
88
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for simple sensitivity calculations, and the more flexible numerical model
for the actual inhomogeneous migration path from the repository to the
aquifer. Two scenarios have been calculated with the numerical model to
provide concentrations to the biosphere pathway models for dose-to-man
calculations In Section C-5.
Most, if not all, of the models Included in this review and discussed
above use a "continuum" approach (transport through a non-fractured porous
medium). Much of the transport in the Immediate vicinity of a potential
repository may be in multiple-channel fractured media, however. Further-
more, many potential repository rock types can exhibit natural permeability
and/or flow characteristics based upon a fracture system. Nevertheless,
many of the natural fracture systems containing petroleum reserves have
been modeled using a continuum approach. Gas production in a number of
places has also been modeled with a continuum approach through a dual
porosity system, with the fracture system characterized by significant perme-
ability despite a low porosity. Such fractures communicate by a dlffusional
mechanism (capillarity, heat conduction, or component diffusion) with a
rock matrix that has porosity, but essentially no flow permeability.
The Office of Waste Isolation recently sponsored a workshop on
"Movement of Fluids in Largely Impermeable Rocks". The discussion
focused on water movement, largely through fracture systems, in low-
permeability rocks. One of the tasks in this workshop was to investigate
the role of numerical modeling In evaluating transport of chemical species
and heat in such an environment. The participants concluded that longer-term
effects (1000-1 million years) could perhaps be simulated by continuum
models. Shorter-term effects, however, might require models that include
the actual fracture networks.
The mathematical description of laminar water flow in fractures
is not fundamentally different from that of continuum flow through a
porous rock. That is, in both cases the flow velocity is proportional
to the hydraulic gradient (Darcy's Law). However, the permeability term
in Darcy's Law then becomes proportional to a power (usually the square)
of the fracture spacing. Thus, the question of fracture model versus
the continuum model for water flow is not a difference in model type
but rather a difference in how to characterize, from either deterministic
or stochastic considerations, the fracture spacing, width, and geometry.
89
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Mathematical description of the nuclide transport would be
described essentially in the same way, whether the flow were in fractures
or in a porous medium. If the matrix blocks are not porous, only transport
in the fracture system would be calculated. Nuclide adsorption on the
fracture face would need to account for a different ratio of surface area
to volume than for porous systems. In the case where the matrix blocks
include effective porosity, a dual porosity approach would be necessary.
In this case, diffusion of nuelides into the matrix blocks slows the net
transport rate of the nuelides in the fracture. In the present study, a
continuum model has been used to calculate the water and nuclide transport
in both porous and fractured systems. The effective porosity of the
fracture systems has been assumed to be small, and no diffusion into matrix
blocks has been included. This is a conservative approach, in that it
allows the nuelides to move slightly faster than they actually would.
In the modeling used for this analysis of geosphere migration for
fracture systems (whether naturally present or created subsequent to
repository installation) the fractures are assumed to offer the permeabilitj
for transport'but to have quite low porosity. Consequently, the rate of
nuclide migration in these fractured regions can be significantly higher
because of a higher interstitial or transport velocity. Another factor
that increases the transport rate of the radionuclides is the reduced
adsorption that could take place on the fracture surface.
C-4.4 MIGRATION OF RADIONUCLIDES IN THE GEOSPHERE
C-4.4,1 Introduction
C-4.4.1.1 Modeling Concepts
In Section C-3,4, conceptual models were developed that are repre-
sentative, in a generic sense, of geologic media that could serve for HLW
storage. These potential repository strata include bedded salt, two
types of shale, basalt, and an intrusive salt dome. For nuclide migration
modeling, the essential aspects are the presence of adjacent aquifers
above and/or below the repository strata and the communication that exists
or could develop between the repository formation and these aquifers.
The interbedded layers between the repository strata and the aquifer(s)
also are an Important part of this flow regime.
Each of the conceptual geologic models shown in Section C-3.4 is
amenable to a similar type of calculational model for geosphere transport.
This is illustrated in Figure C-13 as one-dimensional migration both in
90
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Upper Aquifer
Repository Strata
and Interbeds
\ I
\ I
\ I
I /
/ /
Higher Permeability
Communicating Area
Lower Aquifer
(3} Schematic Vertical Section of Model Area
Aquifer Flow
Flow or Molecular
Diffusion Only
Flow or No Flow
From Lower Aquifer
(b) Model Dimensions
Potential Discharge
/ To Biosphere /
150 Grid Blocks 1100m x 15m x 52m
(3600* x 50'x 176')
5 Grid Blocks 1100 m x 1100 mx30m
(3600 x 3600x100 Ft.|
FIGURE C-13
NUMERICAL REPRESENTATION OF GEOLOGIC MODELS
91
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the repository and interbedded strata, and in an aquifer. Although the
geometry modeled appears more characteristic of the bedded geology, even.
the intrusive geologic models are not significantly different. The path-
way still involves movement through a region of higher permeability, which
movement has been induced in the repository strata, with the nuclldes
finally reaching an aquifer for horizontal migration. For modeling, there
is little difference between the five geologic types as long as a continuum
approach is used to represent a fractured area. In the numerical trans-
port model the following factors can be included;
(1) Extremely low-velocity natural water flow in the
repository strata,
(2) Possibility of communication, because of thermal
stress or disruptive events, from the repository
to the upper, lower, or both aquifer(s).
(a) This communication could consist of liquid
contact, but no flow, with liquid diffusion
of nuclides to the aquifer.
(b) The communication could allow a small flow
from the aquifer down Into the repository
and returning to the aquifer because of the
natural gradient in the aquifer,
(c) The communication could allow flow from one
aquifer to the other through the repository
if there is a hydrostatic imbalance between
the aquifers.
(3) Sorption of nuclides can be included or neglected in the
repository and interbed layers independent of sorption
in the aquifer.
(4) Discharge from the geosphere to the biosphere can be
calculated at many points along the pathway within the
aquifer.
In the analyses, properties of the communication area, as well as those
of the aquifer, were varied in order to show the importance of these
effects in geosphere transport.
92
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C-4.4.1.2 Sensitivity Analysis Modeling
Both the analytical and the numerical models were used to perform
sensitivity analyses. Two types of parameters were Investigated! those-
that affect the transport mode, and those that affect the leaching
characteristics. The parameters tested and the model used for each test
were as follows:
Transport Mode Parameter Model
Repository Bed Thickness Analytical
Permeability Analytical
Flow Gradient Analytical
Adsorption Numerical
Dispersivity Analytical
Porosity Analytical
Leach Characteristics
Leach Time Analytical
, _ Analytical
Leach Rate
Numerical
Nuclide Stability Numerical
Containment Numerical
Distance to Biosphere Analytical
»,
The ranges of values of the parameters that affect the transport
mode are shown in Table C-7. These values have been taken from Tables
C-l and C-2 for low- to moderate-permeability sandstone formations.
Assuming that site selection criteria would eliminate the upper end of
the permeability range, the range included in Table C-7 seems reasonable.
A similar tabular listing of ranges for the geosphere transport parameters
was subsequently determined by a panel of personnel from the U.S. Geologi-
cal Survey, the EPA, and INTERA. This list is summarized in Appendix C-
III. The range of parameters is similar In most respects to those in
Table C-7» although some of the parameters cover a wider range.
C-4.4.1.3 Nuclide Concentration Modeling
The purpose of this modeling was to calculate nuclide concentrations
for use as input for assessment of the radiation dose to man in Section
C-5. Two sets of calculations were made. Scenario 1 corresponds to a
liquid dlffusional pathway in the repository and interbed strata. It
also assumes that the permeability of the repository and interbedded
93
-------
TABLE C-7
GEOSPHERE MIGRATION PARAMETERS AFFECTING TRANSPORT
Parameter Range
Variable
Thickness, m (ft)
Permeability*, cm/s
(ft/day)
Flow gradient,
m/m (ft/ft)
Adsorption, 1C, (ml/g)
Dispersion, m (ft)
Porosity %
Aquifer
15-45 (50-150)
Communication Area
150-450 (500-1500)
(2.8 x 10~3-2.8 x 10"1) (2.8 x 10~6-2.8 x 10~4)
0.01-0.1
(0.1-1.0)x(normal)**
6-60 (20-200)
15
0-1.0
(0-2.0)x(normal)**
60 (200)
1-10
*Vertical permeabilities were one-tenth that listed.
tFracture permeabilities in communication area varied from 10 to
10~-* cm/s, and porosity values (Including fracturing) were always
taken as 0.01.
**See Table c-3.
94
-------
strata has been Increased by fracturing to 100 times higher than normal.
The scenario also uses the most permeable aquifer and highest flow gradient
to develop the greatest case flow condition in the aquifer. The 100-fold
Increase in permeability for an interbed region permeability (conductivity)
of 10 ctn/s assumes that a fracture of 0.1 nan every 3 m has been created
throughout the region above the repository. The leach rate has been chosen
as that for a standard glass waste form, with no solubility limitation.
The throwaway cycle waste started leaching at a waste age of 200 years.
"Normal" adsorption (defined by the retardation factors) was used.
Scenario 2 utilizes the same aquifer flow conditions, but is based
upon what might be termed a "worst probable" case. It consists of
connecting a lower aquifer to an upper aquifer by flow through the
repository.
Analysis of these two scenarios identified the nuclides most
important in the migration pathway. These nuclides tend to have
similar characteristics, such as:
• low adsorption,
* long half-life,
• they, or their parent components, are present in
large initial amounts.
Migration of most important nuclides, such as: Np-237, 1-129,
Sr~90, Ra-226, Cs-137, Tc-99 (when no adsorption is assumed) and some
of the plutonium and uranium nuclides, is discussed in Sections 4.4.3.2
and 4.4.3.3.
G-4.4.2 Results Using Analytical Model
With the analytical model, the sensitivity analysis examined
primarily the following input variables:
* leach time,
* hydrodynamic disperslvlty (a multiplier that is applied
to velocity to produce a total dispersion coefficient),
• water migration rates (in turn a function of permeability,
porosity, and hydraulic gradient),
95
-------
* repository and interbed thickness, and
« distance to discharge to the biosphere.
Two output values are important: (1) the initial arrival time at a
given distance from the repository, and (2) the dilution in nucllde
concentration that occurs during migration. The arrival time is most
affected by water migration rate and nuclide adsorptivity. Dilution is
affected by both these parameters and is also sensitive to leach time
and hydrodynamic dispersivity.
The partial differential equation describing nuclide transport and
the analytical solution for a single decaying adsorbing nuclide are
shown in Appendix C-n. When decay is not significant over the travel
time of the nuclide, the analytical solution4 can be reduced to an
extremely simple form for evaluation of arrival and dilution time:
Initial Arrival time, T
LR,
T . — a. (12)
V
w
where
L - distance to biosphere discharge,
R, = adsorption retardation factor, and
the interstitial water velocity,
Dilution
C - C erf [=£ -~l 4*£/L ] (13)
o R L
where
C = the maximum concentration at L,
C * the boundary concentration at the repository,
T, = the leach time,
a = the hydrodynamic dispersivity, and
erf (x) = the standard tabulated error function.
The approximate dilution expression above can be derived from the
analytical solution given in Appendix C-II by noting that peak concentra-
tion will occur at the midpoint of the traveling body of material. These
expressions can be used to obtain a sensitivity analysis of several
variables .
96
-------
In all calculations, the repository geometry was assumed to be
constant and consistent with the repository handling wastes from reactors
charged with 50,000 MTHM. For heat dissipation, the site was assumed to
have a loading of 150 kW/acre. In the calculations for this study, the
waste was assumed to occupy a 300-acre site.
Table C-8 summarizes the 1-129 arrival times and relative concentra-
tions at specified points. Iodine adsorption is zero. The two water
velocities correspond to the highest levels listed In Table C-7 for
conditions in the aquifer, I.e., a hydraulic permeability of 10~ cm/sec,
a gradient of 0.1 m/m, and a porosity of 15%; and to the lowest values,
which are 10 cm/sec, 0.01 m/m, and 15Z respectively. The travel times
to a point 1.6 km (1 mile) distant from the repository range from 77 years
up to almost 77,000 years.
The masses of the significant actinlde and fission product nuclides
initially present in the repository are listed in Table C-9. The mass of
1-129 present for the throwaway cycle is 1.16 x 10 g. The flow for a
15 m (50 ft)-thlck aquifer over an 1100 m (3600 ft)-wide repository would
be approximately 144,000 liters/day and 144 liters/day for the high- and
low-flow cases, respectively. Consequently, if all of the 1-129 is assumed
to be leached at the high flow rate in one year, the average concentration
would be 0.22 g/liter and if leached in 100 years, the average concentration
would be 0.0022 g/liter. This results from the initial concentration being
Inversely proportional to the product of water velocity multiplied by leach
time. The effect of leach time on initial nuclide concentration is Illustra-
ted schematically In Figure C-14. Thus, for a leach time of one year and a
dlsperslvity of 3 m, the maximum concentration of 1-129 at 8 km (5 miles) from
the repository would be 0.0084 g/liter. On the other hand, for the longer
leach time of 100 years, even with no dispersion in the geosphere transport
path, the maximum concentration would be 0.0022 g/liter. This is simply an
indication that, for a soluble, non-sorbed, long-lived nuclide such as 1-129,
the important reduction in nuclide concentration is due to the leaching rate
limitation. In case of the larger dispersivity of 30 m, the concentrations of
1-129 at 8 km (5 miles) distant from the repository are 0.0026 g/liter and 0.0019
97
-------
TABLE C-8
1-129 ARRIVAL TIMES AND RELATIVE CONCENTRATIONS
Interstitial Water
velocity (cm/day)
5.75
Leach Time
(yr)
VO
00
.00575
Dispersivity
(m)
\100
' 1
100
1000
r
(30
h
(30
)30
3
O/\
Distance-1.6 km (1 mile)^
Arrival Time Relative
(yr) Concentration*
0.085
77
76,674
Distance-8 km .(5 miles)
Arrival Time Relative
(yr) Concentration*
0.038
0.027
1.000
0.996
8.5 x 10
2.7 x 10
0.0085
0.0027
0.085
0.027
~5
~5
383
383,368
0.012
1.000
0.860
3.8 x 10
1.2 x 10
0.0038
0.0012
0.038
0.012
~5
~5
*Concentration at specific distance relative to concentration at aquifer entry.
-------
TABLE C-9
RADIONUCLIDE AMOUNTS FOR ORIGEN OUTPUT
{ I !" «'
(a) Actlnides and daughters
Initial Mass in Place*
Retardation
Nuclide
Cm-246
Am-242ra
Pu-242
U-238
Pu-238
U-234
Th-230
Ra-226
Cm-245
Pu-241
Am- 2 41
Mp-237
U-233
Th-229
Cm-247
Am-243
Cm-243
Pu-239
U-235
Cra-248
Pu-244
Pu-240
U-236
Th-232
Half-Life
(Yr)
4700
152
379,000
4,51xl09
86
247,000
80,000
1600
9300
13.2
458
2.14xl06
162,000
7340
1.6xl07
7370
32
24,400
7.1xl08
470,000
S.OxlO7
6580
2,39xl07
1.4xl010
Factor ,
Rd
3300,
10,000
10,000,
14,300
10,000
14,300
50,000
500
3300
10,000
10,000
100
14,300
50,000
3300
10,000
10,000
10,000
14,300
3300
10,000
10,000
14,300
50,000
(Case 1)
Throwaway
1.48xl03
2. 26x10*
2.24xl07
4.72xl010
6.50xl06
9.15xl06
2.91xl02
1.48xlO~2
1.25xl04
3.91xl07
2.53xl07
2.35xl07
3.60xl02
2.10xlO~2
1.99X101
4,46xl06
3.60xl03
2.70xl08
4.02xl08
1.41x10°
2.56xlO~5
l.llxlO8
2.05xl08
6.70X101
(8)
(Case 2)
UQ_ Recycle
1.48xl03
2.26X101
1.12xl05
2.36xl08
2.14xl05
5.90xl04
5.85X101
5.50xlO~3
1.25xl04
1.96xl05
3.28xl06
2.33xl07
7.20X101
9.60xlO~3
1.99X101
4.46xl06
3.60xl03
1.36xl06
2.01xl06
1.41x10°
2.38xlO~5
9.10xl05
l.OSxlO6
1.17X101
(case 3)
Mixed Oxide
3.18xl05
4 . 95xl02
2.57xl06
2.26xl08
3.87xl06
t
2.19xlO~7
1.97xlO~U
4.46xl06
1.67xl06
5.25xl07
6.90xl06
2.59xl02
1.35xlO""3
4.20xl03
1.27xl08
3.62xl04
3.76xl06
9.50xl07
2.44xl02
4.12xlO~3
1.68xl07
1.54xl05
1.47x10°
*Based on 50,000 MTHM charged to the reactors, and a 10-yr cooloff period.
tValue not listed-in ORIGEN.
99
-------
TABLE
C-9
(Continued)
(b) Fission Products
Initial Mass in Place*
Nuclide
H-3
C-14
Kr-85
Sr-90
Ic-99
1-129
Cs-134
Cs-137
Sm-151
Eu-154
Half-Life
(Yrs)
12.3
5730
10.8
27.7
2.1xl05
1.7xl07
2.05
30.0
87
16
Retardation
Factor,
Rd
1
10
1
100
\3300
1
1000
1000
2500
2500
(Case 1)
Thrgwaway
2.15xl03
1.71xl02
7.65xl05
2.l2x\Q7
4. 20x10 7
7
1.16x10'
3.53xl05
4,96xl07
2.14xl06
1.79xl06
(g)
(Case 2)
U02 Recycle
1.72xl02
1.71xl02
0
2.12xl07
4
1.46x10^
3.53xl05
4.96xl07
2.14xl06
1.79xl06
(Case 3)
Iftxed Oxide
2,27xl02
2.17X101
0
1.23xl07
A
1.86x10
1.55xl05
S.lOxlO7
3.62xl06
1.24xl06
*Based on 50,000 MTHM charged to the reactors, and a 10-yr, cooloff
period.
100
-------
Concentration
Low Concentration
Long Leach,Time
High Concentration
Short Leach Time
Large Dilution
During Migration
Small Dilution
During vMigration
r
Migration Distance
BEFORE GEOSPHERE MIGRATION AFTER GEOSPHERE MIGRATION
FIGURE C-14 EFFECT OF LEACH TIME AND MIGRATION ON NUCLIDE CONCENTRATION
-------
g/liter, respectively, for the 1-year and 100-year leaching periods.
Even In this case, the reduction in concentration due to reduced leach
rate has been larger than that due to dispersion during migration.
At lower rates of nuclide migration, the reduction in concentration
in the geosphere pathway due te dispersion, becomes relatively more important,
For the low rate illustrated in Table C-8, the end result is somewhat
surprising. It indicates that the increased dilution due to longer leach
times is exactly compensated for by decreased dilution during the geosphere
pathway migration. This can be deduced from the dilution equation, since
the boundary concentration leaving the repository (for this idealized
"constant leach rate" case) would be inversely proportional to the water
velocity. The reduction in concentration during migration is proportional
to the water velocity, which for small values is a linear proportion.
Thus, for small water velocities (or high retardation factors), there
will be an interdependence between time and flow rate. The above effect
can be noted In Table C-8, where the result for a high flow rate and one-
year leach period is identical (in final concentration) to a flow rate
lower by a factor of one thousand and a leach period of 1000 years.
Table C-8 shows that for long-lived nuclides such as 1-129, half-
life has little effect. (1-129 has a half-life of 1.7 x 1Q7 years.) If
Np-237 were not strongly adsorbed, the result for this nuclide would be
virtually Identical to that for 1-129. A high degree of adsorption
(R, = 100) reduces the concentration of this isotope after migration
d
through a distance of 8 km (5 miles) in the aquifer to on,ly about 1.2%
of that leaving the repository, assuming high water flow, 100-year
leaching time, and a dispersivlty of 30 m. Neglecting adsorption, (i.e.,
assuming R^ «• 1), the concentration of Sr-90 at 8 km (5 miles) is reduced
_c
to 3.5 x 10 of the initial concentration leaving the repository. Appli-
cation of the probable adsorption value for Sr-90, which is about equal to
that for Np-237, would produce a further reduction in concentration at
8 km.
These results lead to the following conclusions:
(1) If the leach time is more than 251 of the nuclide migration
time, hydrodynamlc dispersion will be relatively unimportant.
102
-------
Reduction of concentration during geosphere transport will
result primarily from radioactive decay.
(2) Retardation of migration by adsorption in the geosphere is
important in reducing concentrations of nuclides, as well as
in delaying nuclide arrival time at specific points.
This effect on concentration is due to the increased time
permitted for dispersion to occur.
The effect of total distance—the thickness of the repository
itself and interbeds—between the repository and an aquifer is also
important. The type of analysis examined above can be applied to this
variable. Although the vertical path length in the reposltory-
interbed section would be considerably shorter than the horizontal
distances investigated above, the vertical permeability would also
be much lower, and thus there is a compensating effect. Therefore,
a 150 m (500 ft) thickness for repository and interbed strata will
be similar in its effect on nuclide travel time to the distance between
0 and 1.6 km (1 mile) in the aquifer, as illustrated in Table C-8,
Another variable concerns the effectiveness of the containment,
i.e., when leaching starts. For most radionuclides, later leach
initiation is advantageous, because concentrations have then decreased
because of radioactive decay. A few daughter nuclides, e.g., radium-226»
however, exhibit increases in concentration for up to 100,000 years
and subsequently start decreasing. In such circumstances, the early
release could reduce concentrations at a later time. Unless Ra-226
became the most critical nuclide from the standpoint of dose, early
release would be disadvantageous. The concentrations of most, if not
all, of the other critical nuclides decrease with time and their
earlier release would be highly disadvantageous.
No detailed set of calculations has been made for the fuel
cycles other than throwaway (Case 1, Task A Report). In general,
wastes from the other cycles have less of the critical nuclides, e.g.
1-129, Ra-226, Np-237 and plutonium Isotopes. A few nuclides are
Increased, e.g. Am-243. If such nuclides as Am-243 contribute a
103
-------
significant amount to the potential dose rate, these cycles should be
* v
investigated.
The calculations discussed above used the analytical model previously
described. The rezualning calculations were made with the numerical
nuclide transport model.
C-4.4.3 legultsUsing Numerical Model
G-4.4.3.1 Parameters for Scenarios 1 and 2
The waste form placed into the repository was assumed to be from
the throwaway cycle. The amount of each nuclide used in the migration
calculations is summarized in Table C-9, along with half-life and
retardation coefficient. Two additional types bf fuel processing options
are also listed: uranium oxide recycle and mixed-oxide recycle (Cases 2 and
3 of Task A report). The nuclides tested include four actinide chains
and the fission products that are important from the standpoint of amounts
of each initially present, as well as of potential health effects.
The parameters used for Scenarios 1 and 2 are summarized in Table C-10.
A comment should be made regarding "normal" adsorption. The sorption levels
used were taken from the estimated distribution coefficients (K,j) in typical
desert soil shown in Table C-3. These values have been compared with
measurements for clay, sandstone, caprock, and river sand soil types
(Table C-4) and, despite some differences, there are no consistent trends
for either actinldes or fission products. In some cases, the distribution
coefficients in sandstone are higher than the equivalent retardation
factors listed in Table C-9, and in other cases they are lower. The
sorption distribution coefficient (Kj), rather than the retardation factor (R_ several
hundred)^2 as well as the zero value given in Table C-3. Corresponding
R,j values of 3300 and 1,0 have been used for illustrative purposes in
calculating concentrations of this nuclide.
104
-------
TABLE C-10
PARAMETER VALUES USED IN SCENARIOS 1 AND 2
Variable
Thickness, m (ft)
Permeability, cm/sec (ft/day)
Flow gradient, m/m (ft/ft)
Adsorption (Kd in ml/g)
Dispersivity, m (ft)
Porosity %
Aquifer
15 (50)
10~4 (2.8 x 10"1)
0.1
Normal
30 (100)
15
Communication Area
150 (500)
10~6 (2.8 x 10"3)
0.1
0
30 (100)
1
105
-------
The grid used in the mathematical model consisted of five 30-m
(100-ft) blocks representing the repository and interbeds and 150 52-m
(176-ft) blocks representing the aquifer. Potential discharge points
were examined in the aquifer at a point above the repository and at
each mile distant from the repository. Both the concentration of each
nuclide and the aquifer flow rate were summarized at each of these
potential discharge points,
C-4.4.3,2 Scenario 2 Results
Migration of actinides and fission products as a function of time
and space for the worst probable case (Scenario 2) are summarized in
Tables G-ll and C-12, The four actinide chains are shown in Table C-ll
for 300, 500, 6000, 30,000, and 100,000 years. Selected fission products
are shown in Table C-12 for 300, 500, and 1000 years.
Two leach rate models have been used. One assumes a "constant
leach rate", with a correction for the changing surface-to-mass
ratio as a function of time. The resulting leach rate decreases approxi-
mately exponentially with time. In deriving this leach model, a very
conservative leach rate was assumed, in order to account for early
temperature effects and possible glass devitrification later.
The second, "variable leach rate", model was a modification of the
first, used to define a more severe case during early time. The possible
temperature effects were accounted for by increasing the leach rate for
the first 4000 years, thus spanning the most important period with respect
to fission products. After 4000 years, a lower leach rate was used to
account for the probability that devitrification might not occur over
the leach time. The comparison of the fraction leached as a function of
time was given earlier in Figure C-12. Although the waste form might
break down more rapidly than the leach rate model indicates, solubility
would then undoubtedly limit a number of the actinide concentrations in
the water leaving the repository. This effect was Investigated in
another model analysis.
106
-------
TABLE C-ll
SCENARIO 2—WORST-CASE FLOW
ACTINIDES AND DAUGHTERS AFTER 300 YEARS
Concentration in g/1 at
Nuclide _ 0 km (mile)*
Cm-246 l.lxlO~?"°
Am-242m 1.4x10 ,
Pu-242 5.9xlO~f
U-238 8, 8x10" J
Pu-238 1.8x10";
U-234 2.8x10" .
Th-230 6.4x10 IT
Ra-226 8.6x10
Cm-245
Pu-241 2.3x10";:
Aa-241 1.1x10 ?"
Np-237 1.0x10"?.
U-233 7.5x10":^
Th-229 1.6x10
Cm-247 1.6xlO~i2
Am-243 1.2x10 ,
Cm-243 7.9x10"^
Pu-239 7.1x10;?
U-235 7.5x10
Cm-248
Pu-244 1.8xlO
Pu-240
U-236 3.9x10^
Th-232 1.1x10
Concentrations at other discharge points are
negligible (less than one atom per liter).
107
-------
TABLE C-ll
''Continued)
SCENARIO 2— WORST-CASE FLOW
ACTINIDES AND DAUGHTERS AFTER 500 YEARS
Nuclide
Cm-246
Am-242m
Pu-242
U-238
Pu-238
U-234
Th-230
Ra-226
Cm-245
Pu-241
Am-241
Np-237
U-233
Th-225
Cm-247
Am-243
Cm-243
Pu-239
U-235
Cm-248
Pu-244
Pu-240
U-236
Th-232
0 km (mile)
,-10
3.9x10
1.8x10
2.2x10
3.2x10
2.4x10
1.0x10
4.1x10
8.4x10
3.4x10
6.3x10
3.0x10
3.0x10
4.2x10
1.6x10
-13
-6
___*|
-8
-6
-10
-11
-9
-12
-6
_4
-10
-13
7x10
1x10
1x10
2.6x10
2.8x10
4.0x10
1.1x10
1.0x10
1.4x10
6.3x10
-12
-7
-15
-5
-5
-13
-16
-5
—5
-11
Concentration, (g/liter) at Following Distances
from Aquifer Entry*
1.6 km
(1 mile)
3.2 km
(2 miles)
4.8 km
(3 miles)
6.4 km
(4 miles)
8.0 km
(5 miles)
2.2x10
1.0x10
2.4x10
-14
-20
-24
7.0x10
-25
*Concentrations at other discharge points are negligible
(less than one atom per liter).
108
-------
TABLE G-ll
(Continued)
SCENARIO 2—WORST-CASE FLOW
ACTINIDES AND DAUGHTERS AF1ER 6000 YEARS
Concentration, (g/liter) at Following Distances
from Aquifer Entry*
Nuclide
Cm-246
Am-242m
Pu-242
U-238
Pu-238
U-234
Th-230
Ra-226
Cm-245
Pu-241
Am-241
Np-237
U-233
Th-229
Cm-247
Am-243
Cra-243
Pu-239
U-235
Cm-248
Pu-244
Pu-240
U-236
fh-232
0 km (mile)
-10
5.0x10
-6
8.1x10 *
1.3xlO~f,
3.9x10":™
•>«fo
4.1x10 °
2.1x10 1:
1.3x10
_9
2.8x10 *
— 1 /
1.4x10 I
—9
6.3x10 I
2.4x10 £
1.9xlO:°
1.6x10
l.lxlO"11
5.3xlO~7
— 1
2.0x10 r
6 . 4x10
-13
7.3x10 "
«.1 S
6.6x10 r
1.2x10 e
— S
7.8x10 I
8.9x10
1.6 km 3.2 km 4.8 km 6.4 km 8.0 km
(1 mile) (2 miles) (3 miles) (4 miles) (5 miles)
4.4xlO"18
-24
2.8x10
-5 -5 -5 -6 -1
7.4xlO_g 5.0xlO_^Q 1.5xlO_^Q 3.1x10 ,- 4.9x10 '.
1.8xlO~ . 6.7xlO~1? 1.6x10" . 2.8xlO~ , 4.1xlO~1A
9.7xlO~ 2.8xlO~ 5.8x10 9.6x10 1.4x10
*Blank Indicates extremely small concentration, less than one atom per liter.
109
-------
TABLE C-ll
(Continued)
SCENARIO 2—WORST-CASE FLOW
ACTINIDES AND DAUGHTERS AFTER 30,000 YEARS
Concentration,(g/lltei) at Following Distances
from Aquifer Entry*
Nuclide
Cm-246
Am-242m
Pu-242
U-238
Pu-238
U-234
lh-230
Ra-226
Cm-245
Pu-241
Am-241
Np-237
U-233
Th-229
Cm-247
Am-243
Cm-243
Pu-239
U-235
Cm-248
Pu-244
Pu-240
U-236
Th-232
0 km (mile)
4. 9x10" 12
-6
5.5x10 «
1.1x10
3.4xlO~|
9.3x10":
6.6x10
5.2x10"^
2. 7x10" "
1.2x10" J
3.6xlO~
4-2xlO"XU
4.9xlO~*2
1.4x10"
7.1xlO"2
3.4xlO~"
8.7xlO~fi
2.8xlO~
7.3x10"^
1.8x10
1.6 km 3.2 km 4.8 km 6.4 km 8.0 km
(1 mile) (2 miles) (Smiles) (4 miles) (Smiles)
8.0xlO~14
1.2xlO~21
1.9x10""
1.8x10"! 4.2xlO~I 5.5x10 q 3.7xlO~:*n 1.6x10 :*n
2.7xlO~:[ 2.3x10"^ 1.5x10"^ 6.6x10 ," 2.3x10 \
2.5X10"1 1.7x10 •• 8.7x10" 3.4xlO~ l.OxlO""
S.lxlO"2-1
2.9xlO~23
*Blank indicates extremely small concentration,
less than one atom per liter.
110
-------
TABLE C-ll
(Continued)
SCENARIO 2—WORST-CASE FLOW
ACTINIBES AND DAUGHTERS AFTER 100.000 YEARS
Nuclide
Cm-246
Am-242m
Pu-242
U-238
Pu-238
U-234
Th-230
Ra-226
Cm-245
Pu-241
Am-241
Np-237
U-233
Th-229
Cm-247
Am-243
Cm-243
Pu-239
U-235
Cm-248
Pu-244
Pu-240
U-236
Th-232
-7
0 km (mile)
4.3xlO"18
6.7x10
2.7x10
7.1x10
9.5x10
6.9x10
6.4x10
3x10
3x10
7x10
2x10
—7
-8
-8
-14
-17
•15
-14
-8
3.6x10
•10
5.9x10
9.4x10
1.1x10
1.9x10
-14
-11
-3
-2
3.7x10
7x10
1x10
8x10
3.3x10
-15
-15
-9
-5
-8
Concentration, (g/liter) at Following Distances
from Aquifer Entry*
1.6 km
(1 mile)
2.8x10
-21
1.2x10
3.3x10
-19
-20
1.6x10
-12
8x10
0x10
7.
4,
1.4x10
1.
2.
3.5x10
1x10
7x10
-17
•20
-18
-9
-9
-11
2.8x10
2.4x10
1.4x10
1.4x10
1.6x10
1.1x10
5.6x10
9.1x10
-15
-19
-15
-15
-16
-18
-23
-21
3.2 km
(2 miles)
4.8 km
(3 miles)
6.4 km
(4 miles)
8.0 km
(5 miles)
2.1x10
-18
1.1x10
2.7x10
3.6x10"
-9
•11
8.7x10
2.8x10
3.6x10
—7
-9
-11
2.5x10
2.9x10
3.7x10'
—o
-9
-11
4.2x10
2.9x10
3.6x10
-9
•11
* Blank indicates extremely small concentration, less than one atom per liter.
Note: This table includes four separate actinide series, Nuclides omitted
from the above results should be assumed to be in secular equilibrium
with their parent component, i.e., the activity of each nuclide would
be equal. For example, an equilibrium concentration of Rn-222 would
be present along with the values listed for Ra-226.
Ill
-------
TABLE C-12
SCENARIO 2—FISSION PRODUCTS AND C-14
Time, yr
300
500
1000
Nuclide
C-14
Sr-90
Tc-99Rt
Tc-99N
1-129
Cs-137
Sm-151
C-14
Sr-90
Tc-99R
Tc-99N
1-129
Cs-137
Sm-151
C-14
Sr-90
Tc-99R
Tc-99N
1-129
Cs-137
Sm-151
0 km (mil<
-L • / XX U —
2.6xlO~°
— i 1
6.7x10 ,
5.1xlO~7
1.4x10";
1.2x10 I
— X
2.0x10
_Q
1.3x10 *
5.3x10 "
2.4xlO-f
3.2x10 ^
8.7X.O-:*
4.8x10 Q
—8
1.6x10
5.6xlO~ s
3.2x10 ^
4.6xlO~^
1.5x10
4.2x10 ,.
—1 u
1.4x10 ,!:
— 1 0
6.6x10
Concentration, (g/liter) at Following Distances
from Aquifer Entry* ^^^
1.6 km
(1 mile)
3.7x10
1.2x10
-14
3.6x10
1.0x10
-4
-4
3.2 km
(2 miles)
1.6x10
4.7x10
1.3x10
-4
-4
7.1xlO~10 1.9x10
1.6xlO~* 1.8x10 *
4.5x10 5.0x10
4.8 km
(3 miles)
8.7x10
2.4x10
-11
-11
3.6x10
1.0x10
-4
-4
5.7x10
-23
2.0x10
5.6x10
-4
-5
6.4 km
(4 miles)
1.8x10
4.9x10
-15
-16
6.9x10
1.9x10
-5
-5
2.3x10
6.3x10
-4
-5
8.0 km
(5 miles)
9.8x10
2.7x10
-21
-21
2.0x10
5.5x10
-7
2.6x10
7.2x10
-4
-5
*Blank indicates extremely small concentrations, less than one atom per liter.
Tc-99R represents an assumed chemical form of Tc-99 that is highly sorbed (Rd=3300).
Tc-99N represents a non-sorbed chemical form (R
-------
This second leach model is represented by the following equation:
Fraction leached - 0.173 In (1 + t/200) (14)
where
t = time in years after first contact with water (200 years).
This equation shows that all the glass is leached at 65,000 years, with
the leaching period continuing over most of the time interval shown.
The rate, however, decreased substantially from the start of leaching
at 200 years until the end of leaching at 65,000 years.
The results summarized in Table C-ll show significant amounts of
several nuclides entering the aquifer at 6000 years. For example, U-238
and Pu-242 would have 1900 g/day and 1.2 g/day, respectively. These rates
** 0
were obtained by multiplying the concentrations, 1.3 x 10 g/liter and 8.1
x 10 g/liter, respectively, by the water flow rate In the aquifer, 1.44 x 10
™~2
liter/day. The concentration of U-238 of 1.3 x 10 g/liter translates
to 4300 picocuries/liter (pCi/liter) and of Pu-242 to 47,000 pCi/liter.
However, significant levels of these nuclides never reach a point 1.6 km
(1 mile) from the repository even after 100,000 years or more. The worst
actinide migration occurs with Np-237. About 3.5 g/day are still
reaching the aquifer at 6000 years. The peak potential discharge rate
of Np-237 at a distance 1.6 km (1 mile) from the repository would be
about 10 g/day; at 3.2 km (2 miles), about 9 g/day; and at 4.8 km (3 miles),
about 8 g/day. Because U-233 and Th-229 are daughters of Np-237,
concentrations of these nuclides are found to have moved substantially
farther than would have been expected.
The migration of fission products is summarized in Table C-12 for
times of 300, 500, and 1000 years. Significant amounts of 1-129 are
present and are not adsorbed; hence this nuclide has substantial migration.
Table C-13 lists the maximum value of 1-129 in both g/liter and pCi/liter,
calculated at various distances from the repository.
In Scenario 2 permeability of the repository strata was increased
by two orders of magnitude. The highest interstitial horizontal permeability
listed for this strata was 10 cm/sec. The corresponding vertical
113
-------
TABLE C-13
PEAK VALUES OF 1-129
Distance from
Repository -km
(miles)
Concentration,
.g/llter
pCi/liter
Time (yr)
1,6 (1)' 3.2 (2) 4.8 (3) 6.4 (4) 8.0 (5)
1.34xlQ~4 1.30xlO~4 l,27xlO~4 1.25xlO~4 1.2QxlO~4
21,840 21,190 20,700
389 470 551
20,380 19,560
632 700
114
-------
permeability was 10 cm/sec. This case could be thought of as representing
a substantial fracture zone above the repository, with a uniform set of
fractures spaced 3m (10 ft) apart and with a fracture width of 1,5 cm
(0.05 ft); the relative cross-sectional area is then approximately
(0.025)(10)(4)/100 - 0.01. If these fractures had a permeability of
10 cm/sec, the communication area above the repository would yield a net
permeability of about 10~ cm/sec and a net porosity of about 1%, which were
the values used in Scenario 2. This scenario simply sets out a conceptual
illustration of what has been simulated—namely, a "worst case" geosphere
migration, since a fracture area has created the communication pathway.
C-4.4.3.3 Scenario 1 Results
Permeability and flow gradient are important variables controlling
the base flow in the aquifer. Probably more important to the geosphere
migration pathway, however, is the permeability of the communication
area between the repository and the aquifer. Section C-4.1 discussed
the magnitude of the natural flow that might occur in potential repository
strata. These velocities would be quite low and probably not important
from the standpoint of nucllde migration. More important would be those
events that could lead to a more permeable area occurring between the
aquifer and the repository:
(1) Breakdown of the shaft entry seal over long time periods;
(2) Thermally-induced fractures around the repository caused by
heat from radioactive decay;
(3) Seismic activity.
In Scenario 2 a rather permeable area above the repository was assumed.
The porosity for the repository strata was assumed to be 1% and thus
vertical migration rates included in Scenario 2 calculations are
indicative of fractured media. This flow would undoubtedly exceed the
actual flow, even considering the thermal convection effects.
Scenario 1 is nearly identical to Scenario 2, except that in order to
examine a low end of the range for vertical migration rates of nuclides
from the repository, the strata fluids were assumed to have zero vertical
velocity. Liquid contact was postulated, but nuclides were assumed to be
115
-------
transported from the repository only by molecular diffusion In the liquid
—fi 9
phase* A net diffusion coefficient equal to 10 cm /sec was used in
^
this calculation. This net diffusivity is made up of an open-stream
—6 2
molecular diffusion coefficient of 5 x 10 cm /sec and a tortuosity
factor equal to five.
The diffusion pathway in the repository strata should represent a
lower bound on the transport of nuclides from the repository to the aquifer
(unless these strata are also strongly adsorbing). However, in the absence
of a solubility limitation for the nuclides, even a diffusional concentra-
tion gradient will increase until the amount of nuclides diffusing from
the repository equals the leach rate. Although there is a much longer
time transient involved before the nuclide fluxes into the aquifer are
equal to the leach rate, the amounts entering thi aquifer by either diffu-
sion or flow would eventually be the same—in the absence of a solubility
limitation. Calculations have been made to ascertain how long this time
transient might be.
The results are tabulated in fable C-14 for the actinide series at
6000 and 30,000 years. Np-237 has a concentration at the 0 km (mile)
location of about two orders of magnitude less than the Scenario 2 flow
at 6000 years and about a factor of five less at 30,000 years. Np-237
was selected for the comparison because of its low adsorptivity and its
relatively large contribution to dose.
The fission product results for the diffusion pathway (Scenario 1)
are tabulated in Table C-15. In this case, 1-129 has a concentration at
the 0 location of about eight orders of magnitude less than in Scenario
2 at 500 years, and six orders less at 1000 years. In Scenario 2, the
peak 1-129 concentration had already gone beyond the 6.4 km (4-mile)
migration point in the aquifer at 1000 years. At 1000 years in Scenario
1, the concentrations are still increasing, but are nearly at their peak
values. The condition results from an interaction between the increasing
diffusion gradient and the decreasing leach rate.
These results indicate that over the time periods of interest for
either fission products or actinides, the concentrations migrating in an
aquifer would be at least a factor of five lower for the diffusion pathway
116
-------
TABLE C-14
SCENARIO 1—DIFFUSION PATHWAY - ACTINIDES AND DAUGHTERS AFTER 6000 YEARS
Nuclide 0 km (mile)
Concentrations (g/liter) at Following Distances
from Aquifer Entry*
1.6 km
(1 mile)
Cm-246 4.0 x 10~13
Am-242m
Pu-242 6.8 x 10-9
U-238 1.0 x 10~5
Pu-238 1.3 x 10~20
U-234 3.4 x 10-9
Th-230 2.0 x ID-11
Ra-226 1.7 x 10~11 6.2 x 10'
-24
3.2 km
(2 miles)
4.8 km
(3 miles)
6.4 km
(4 miles)
8.0 km
(5 miles)
Cra-245
Pu-241
Am-241
Np-237
U-233
Th-229
5.9 x 10~1Z
2.8 x 10~15
3.4 x 10~12
3.2 x 10" 7
3.5 x 10-11
1.5 x 10-13
4.4 x 10~9
1.1 x 10-14
1.3 x 10~17
5.0 x 10~12
9.9 x 10~18
9.5 x 10~21
Ctn-247
Am-243
Cm-24 3
Pu-239
U-235
Cm-248
Pu-244
Pu-240
U-236
Th-232
1.6 x IQ-
6.8 x 10-1°
6.7 x 10-8
1.0 x 10-7
1.2 x 10
4.7 x 10
1.6 x 10
5.8 x 10
-15
-18
-8
-8
3.4 x 1
-------
TABLE C-14
(Continued)
SCENARIO I—DIFFUSION PATHWAY - ACTINIDES AND DAUGHTERS AFTER 30.000 YEARS
Nuclide
Cm-246
Am-242m
Pu-242
U-238
Pu-238
B-234
Th-230
la-226
Cm- 24 7
Am-243
Cm-243
Pu-239
U-235
Cm-248
Pu-244
Py-240
U-236
Th-232
0 km (mile)
3.5 x 1Q-13
2.7 x 10"7
4.7 x l
-------
TABLE C-15
SCENARIO 1—DIFFUSION PATHWAY - FISSION PRODUCTS AND C-14
Nuclide
0 km (mile)
Concentration (g/liter) at Following Distances
from Aquifer Entry*
1.6 km
(1 mile)
3.2 km
(2 miles)
4.8 km
(3 miles)
6.4 km
(4 miles)
8.0 km
(5 mile;
300 Years
C-14
Sr-90
Tc-99Rf
Tc-99N
1-129
Cs-137
Sm-151
4.3 x 10~19
1.2 x 10- J-J
2.4 x 10~20
1.6 x 10~1J
4.4 x 10~14
7.2 x 10~20
1.4 x 10~18
1.3 x 10'
,-25
3.1 x 10
8.6 x 10
r!3
,-14
4.7 x 10
1.3 x 10
-14
r!4
500 Years
C-14
Sr-90
Tc-99R
Tc-99N
1-129
Cs-137
Sm-151
1000 Years
C-14
Sr-90
Tc-99R
Tc-99N
1-129
Cs-137
Sm-151
1.4 x 10
4.4 x 10
2.
4.
x 10
x 10
,-17
,-20
,-18
,-12
4.0 x 10
1.5 x 10
-20
r!7
7.6
3.3
x 10
,-24
6.8 x 10~17
2.4 x 10
,-10
4.4 x 10'
,-22
1.9 x 10
1.3 x 10~12 5.3 x 10
-12
r!3
6.9 x 10
1,9 x 10
-13
-13
x 10~16 1.4 x 10~17 6.8 x 10~20
1.7 x 10
-10
1.2 x 10
-10
6.5 x 10"""- 4.7 x 10-n 3.3 x 10'11
8.7 x 10
1.4 x 10'
-24
-17
* Blank indicates extremely small concentration, less than
one atom per liter.
t Tc-99R represents an assumed chemical form of Tc-99 that is
highly sorbed (% - 3300).
Tc-99N represents a non-sorbed form of Tc-99 (Rjj * 1).
119
-------
(Scenario 1) than for water flow through the repository (Scenario 2).
Because of the high concentration levels necessary for the diffusional
transport to be effective, solubility limitations will also be important
and will further reduce the concentration reaching an aquifer.
C.4.4.3.4 Sensitivity Analyses Using the Numerical Model
C-4.4.3.4.1 Effect of Leach Rate, Containment, and Nuclide Solubility
For the aetinides, solubility limits the actual leach rate below
that utilized in Scenario 2. In order to examine this effect, the
leach model used in Scenario 2 includes the limitation that If a nucllde
concentration exceeds its solubility, only the soluble portion leaves the
repository. This was accomplished by including a material balance of
nuclides in the repository. These nuclides could exist In the solid
waste form (assumed here to be glass), In soluble form, or in an insoluble
form (precipitate) after the glass matrix has dissolved. The case chosen
for illustration is the actlnide series Gm-246 and Am-242m. The solu-
(43)
bility values used to limit these components are given In Table G-16.
The figures taken were for the most soluble form of each nucllde at a
neutral pH in fresh water, since this condition gave the highest
"^
solubility values.
v The results are quite dramatic. The glass was completely leached
at 65,000 years, but even at 100,000 years substantial amounts of uranium
and plutonium remained in an insoluble form in the repository. These
nuclides would continue to leach very slowly over long periods of
time. In Scenario 2, 1900 g/day of U-238 reached the aquifer at a time
of 6000 days; when the solubility limit was imposed, however, less than
_o
3 x 10 g/day reached the aquifer. There Is a similar, but not as
drastic, reduction in the concentration of plutonium reaching the aquifer.
The solubility limit will undoubtedly be an important influence in
controlling the leaching rate of the actinides.
The migration of fission products should be most sensitive to the
leach rate and to the effectiveness of containment. In order to investigate
120
-------
TABLE C-16
ACTINIDE SOLUBILITY
Sat'd NaCl Water
Actinlde (g-moles/llter) (g-moles/liter)
U log [U02 CL+] - -28,7 log [UO^] - - 9
Np log CNp02+] - - 3 log [NpOj4*] - - 3
Pu log [Pu02Cl] - -11.8 log [Pu 02+] - -12,3
Am log [Am OH**] - -18.4 log [Am OH"""] - -18.4
Cm values were taken to be similar to those for Pu.
121
-------
this effect, a case was assumed where the important fission products were
completely leached in a 25-year period. Leaching was started 50 years
after the repository was closed. The results can be contrasted to
-4
Scenario 2 in which the maximum 1-129 concentration was about 1.3 x 10
g/liter at a distance 1 mile from the repository (see Table C-13). With
the more rapid leaching rate and shorter containment period, the 1-129 concen-
tration at this same distance peaked at a concentration more than 20 times higher,
••» ^
2.5 x 10 g/liter. The concentration of Sr-90 entering the aquifer never
^Q
exceeded 3 x 10 g/llter in Scenario 2, but with the shorter leach time and
containment, Sr-90 reached a level nearly 10 times higher at 2 x 1Q~ g/liter.
Sr-90 still would not present a problem, however, once migration into the
aquifer takes place because of its high adsorption coefficient.
In Scenario 2, a standard glass form with leaching starting 200
years after emplacement was assumed; water contact was present when the
repository was sealed and the containment then broke down in 200 years.
The "variable leach rate" model used gave higher leach rates than the
"constant leach rate" model for periods up to 4000 years but then leaching
declined. The total leach time was 65,000 years. (See Figure C-12).
A set of sensitivity calculations was performed using the same
conditions as in Scenario 2, except that the "constant leach rate" model
was used. In this instance, the waste form leached completely in 20,000
years. Nevertheless, the results showed only small variances in nuclide
concentrations from those resulting from the assumptions of Scenario
2. At a similar discharge point no nuclide concentrations differed by more
than a factor of 2. The fission product concentrations were lower (as
was expected) since the crossover point in leach rates is 4000 years; the
concentrations of radlum-226 and neptunium-237 were higher for the
"constant leach rate" model than for the "variable leach rate" model,
but by less than a factor of 2.
As mentioned previously, solubility would undoubtedly limit the actual
leach rate of actinides in either case. Thus, the ultimate difference
between the two leach models would be even smaller.
122
-------
Another factor associated with the source terms Is the method of
reprocessing, if any, of the waste form before it is placed in the
repository. Three alternatives were discussed in Task A; (1) no
reprocessing, i.e., the "throwaway" cycle; (2) uranium recycle only
(plutoniura is also separated but not recycled), and (3) mixed uranium
and plutonium oxide recycle (see Table A-3). In the latter two cases,
the important radionuclides removed are plutonium, uranium, and iodine.
Slight reductions in other nuclides also occur in these cases where recy-
cling is used. A few of the heavy nuclide concentrations are increased,
Am-243 perhaps being most important. The relative hazard from a recycling
case should always be considerably less than in a "throwaway" case, and
thus no geosphere calculations were made for the recycle cases. An impor-
tant reduction in the amount of 1-129 from that initially present is obvious
from Table C-8. This reduction of nearly three orders of magnitude in
the initial amount present in the waste form should decrease the 1-129
concentrations along the geosphere pathway proportionately. The
nuclides whose quantities have increased from the amounts initially
present in the waste form are all actinides; from the previous calcu-
lations it appears that solubility would limit concentrations of these
nuclides in the geosphere transport pathway.
C-4.4.3.4.2 Effect of Adsorption
One highly uncertain factor is the adsorptive character of the
formation, including both the repository strata and interbeds, and
the aquifer. In Scenario 2, normal adsorption in the aquifer and none
in the communication area was assumed. In order to examine the
importance of adsorption, computer runs were made using different
sorption values.
In a low-sorption case, the aquifer was assumed to have much less
adsorption than In either Scenarios 1 or 2. The actlnide chain beginning
with curium-246 was used in this illustration. Adsorption values based
on one-tenth the original distribution coefficients (Table C-9) were
used for the aquifer. Again, no adsorption was assumed in the repository
or in the communicating area.
123
-------
The results for this case Indicate that even with much lower
adsorption in the aquifer, the actinide series migration will be effectively
retarded. The two components of most concern from this actlnide chain would
be radlum-226 and plutonium-242. At a distance of 1.6 km (1 mile) from
the repository at 6000 years, the concentration of Ia-226 had increased
to 1.9 x 10~9 g/liter (2000 pCi/£), which is a large Increase (almost
nine orders of magnitude) over that of Scenario 2. Pur-242 concentration
had also undergone a large relative Increase, but was still extremely
small. It should be emphasized that under idealized conditions of a
constant leach rate and virtually no decay, the adsorption level in the
aquifer primarily affects time of arrival rather -than the concentration
level of & nucllde. In the above cases, the effect is larger, since the
leach rate is decreasing with time and the nuclldes are either decaying
or being formed. As a result, the concentrations can be affected
substantially.
In the high-sorption case, the communicating area was assumed to
have substantial adsorption (as it would if the repository were
placed In shale). In this case, twice the original distribution
values were used in the communication area. Normal adsorption was
used in the aquifer.
In this case, substantial decreases in nucllde concentrations
resulted from strong adsorption in the repository strata. The residence
time of components, such as Ra-226, increased to 50,000 years compared
with roughly 50 years in the no-adsorption condition of Scenario 2.
Decay, dispersion, and leach-rate changes are Important over this
increased residence time.
124
-------
C-4.5 COMPARISON OF THE EFFECT OF LOW-GRADE URANIUM ORE VS. AN HLW
REPOSITORY
For this comparison of the HLW with a natural uranium ore body, the
assumption is made that the site of the HLW is occupied by a 0,2% uranium
ore body, which la in the same location In the ground and is subject to
the same influences and conditions as assumed for Scenario 2.
For the calculation, the natural ore is assumed to consist of U-238
and its important daughter components contained in the repository volume
(300 acres by 15 m [50 ft] thick). After several Initial calculations,
four nuclides (U-238, U-234, fh-230, and Ra-226) were found to be
sufficient to represent the entire decay chain. The amount of each
nuclide leaving the ore was limited by its solubility. The limiting
solubilities are as follows:
Nuclide Limiting Solubility, moles/1
0-238 1 x 10~9 (oxide)
U-234 1 x 10~9 (oxide)
_o (441
Th-230 1 x 10 y (hydroxide)* '
Ra-226 7 x 10~5 (carbonate)*
For comparative purposes, the aquifer and flow through the ore deposit
were assumed to be the same as for the Scenario 2 repository calculation.
There is a distinct difference between nuclide transport from the
ore and that from an HLW repository. In the HLW, there is an actinlde
decay chain, beginning with Cm-246, which contains U-238 as a daughter.
This decay chain also produces Ra-226 by a separate route that does not
involve U-238. As a consequence, in an Intact repository from which no
nuclides have escaped, Ra-226 attains its maximum concentration between
100,000 and 200,000 years after the HLW has been removed from a reactor.
The amount of Ra-226 would then decrease slowly toward the equilibrium
value produced by the decay of U-238 alone.
(44)
*Estimated by analogy to the solubility of barium carbonate.
125
-------
In the ore deposit, the amount of Ra-226 Initially present would be
only that quantity produced by the decay of the U-238. However, since
the ore would not be confined by the kinds of barriers used in an HLW
repository, Ra-226 would be removed rapidly because of solubility in
water and the resulting equilibrium concentration of Ra-226 would be
controlled only by its rate of formation from decay of U-238.
For a moderately low-grade ore deposit (0.2% U-238), the area
equivalent to a repository would contain about the same mass of U-238
as does the throwaway cycle HLW. In this analysis, these amounts are
4.72 x 10 g for HLW and 7 x 10 g for the ore deposit. The following
table summarizes the Ra-226 concentrations arriving at various distances
in the aquifer,
Ra-226 Concentrations (g/liter)
Distance; Time, {years) From Ore From HLW
6,000 7.1 x 10~8 1.3 x 10~
0 km
(0 mile)
1.6 km
(1 mile)
30,000 7.3 x 10"8 6.6 x 10~8
100,000 7.3 x 10~8 6.6 x 10~8
6,000 2.5 x 10 1? 4.4 x 10"1
30,000 2.1 x 10~13 8.0 x 10~14
100,000 2.6 x 10"13 1.8 x 10~12
In the above comparison, the concentrations of Ra-226 from the ore
deposit are quite similar to those from the HLW; directly above the
repository, the concentrations are virtually the same. At 1.6 km
(1 mile) distance, however, the concentrations of Ra-226 from the HLW
are smaller than those from the ore for times up to 30,000 years but
become larger after that. The reason for this is that the HLW contains
substantial initial inventories of Pu-238 and U-234 (both parent nuclides
of Ra-226). As these parent nuclides decay, significant quantities of
Ra-226 are created. In the ore deposit, only U-238 decay produces
Ra-226| thus, the concentrations at later times approach a constant
value. The magnitude of this equllibrum concentration depends primarily
on the travel time and upon the decay rate of Ra-226.
126
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One additional factor should be mentioned. In the above analysis,
water was assumed to have come in contact with the ore (or with the HLW)
at the moment calculation was started. It is equally reasonable to
assume that water would have been flowing through the ore continuously
over geologic time. In that event, the aquifer might be saturated with
respect to adsorption of U-238 and its daughter products. This
"saturation" would then cause water from this aquifer to have essentially
a constant ratio of U-238/Ra-226. This is in reasonable agreement with
(45)
an extensive data set collected by the U.S. Geological Survey. ' The
largest number of samples typically had concentrations of 1 x 10 g/£
12
U-238 and 1 x 10 g/i of Ra-226. For distances beyond roughly 1.6 tan
(1 mile) from the ore, the U-238 that is first dissolved in the water
and then adsorbed downstream can produce more Ra-226 in situ than is
formed in the ore and then transported to the same point.
127
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C~5*0 DOSE-TO-MAN CONSIDERATIONS
C-5.1 OBJECTIVES AND OVERVIEW
C-5.1.1 Introduction and Purpose
A repository for high-level radioactive wastes will contain large
quantities of radionuclides in high concentrations. Placement of the wastes
in deep geologic formations and in physical forms and containment structures
designed to minimize the chances of escape and transport in the environment
Is being considered as a way of protecting the public from the potential
hazards associated with such wastes.
The objective of this section of the Task C report is to provide a
basis on which to estimate the magnitudes of radiation doses that might result
from releases at any time in the future. More specifically, this part of the
project provides estimates of doses to individuals and populations as a
result of radionuclides entering various environmental media and being trans-
ported through the biosphere via likely pathways. A further objective of
this task is to compare the calculated radiation doses resulting from the
potential repository leakage with radiation dose guides applicable to the
public and with radiation doses resulting from naturally-occurring radionu-
clides, and to assess the importance of these predicted radiation doses.
V
Because of the long half-lives of some of the radionuclides that would
be placed in a geologic repository, the potential toxicity of those materials
persists for long periods. In general, the few nuclides that persist in the
repository will be the same as those that occur naturally in the earth, e.g.,
the uranium, thorium, neptunium, and actinium decay series. The long life
of the high-level waste (HLW) necessitates the consideration of impacts on
the biosphere, including radiation doses, over equally long time spans.
Indeed, the National Environmental Policy Act of 1969 requires that such
projections be made.
128
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C-5.1.2 Options and Considerations for Making Radiation Doge^Assessments
In assessing the radiation impact of HLW on the public, the procedure is
to postulate a pathway by which radioactivity from an HLW repository can get into an
individual and to use a metabolic model to calculate the absorbed dose (rad),
or preferably the dose equivalent (rem, i.e., the absorbed dose multiplied
by a quality factor) to a specific organ or to the whole body of the exposed
person(s). The dose equivalent is the important radiological quantity
for estimating the health effects resulting from a particular radiation
exposure.*
For an individual, therefore, the magnitude, and hence the acceptability
or risk, of the radiation impact can be compared to the dose limits recommended
by national and international organizations. For the case of a group of persons,
i.e., a population, a similar direct comparison is also desirable. This can
be done by invoking the concept of a dose equivalent tothe population (the
unit used is person-rem), consisting of the sum of the dose equivalents to
each individual of the population. The collective effect of the dose equivalent
to this population is assumed to be independent of the dose equivalent distribu-
tion in the population. The condition that permits such a dose equivalent
comparison is a linear-nonthreshold dose-response model of the biological ef-
fects of radiation. Such a model is mathematically convenient, is unlikely to
underestimate the health effects, and permits direct comparisons to dose limits
and dose equivalents from other sources, especially when the dose equivalents
are at or below the dose limits or are equivalent to those from natural causes.
(See Section C-5.1.4 for further discussion of the model and its implications.)
Assessment of the radiation Impact can be made by comparing the concen-
trations in drinking water of radionuclldes derived from HLW with the maximum
permissible concentrations recommended by national and international organiza-
tions. Such a comparison can be useful, especially if drinking water is the
main route of entry into persons. However, the recommended concentrations are
based on dose limits and again the comparison is really one of dose equivalents.
*While prediction of health effects is a subject that is beyond the scope
of this report, it is assumed that dose estimates will eventually be used
for this purpose.
129
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Further assessments can be made by comparing the dose equivalents due to
the HLW with dose equivalents due to natural background, especially that
due to internal radiation from radionuclides such as carbon-14» potassium-40,
and the uranium and thorium series.
Comparisons based on relative hazard indices are sometimes made as an
alternate approach and to avoid the necessity of making absolute health esti-
mates. Such Indices include:
(1) The total volume of water required to dilute the radioactivity
in the HLW to the maximum permissible concentration for
drinking water, or
(2) The total number of "annual intake limits" contained in the
HLW for either the radiation worker or for a member of the
general public, or
(3) The ultimate number of cancer deaths that might be produced by
a given quantity of HLW, if distributed in a large population
in such a way as to produce the maximum biological effect.
While these indices may serve as extreme boundary conditions or may
be .intellectually interesting, they are not very useful and can even be
misleading, since they omit all the transport processes and metabolic path-
ways that affect different radionuclides in different ways in the final
calculations of dose equivalent.
It is possible to make a relative comparison between important HLW
radionuclides and natural radionuclides, provided that the natural radionu-
clide is assumed to be placed in the repository along with the other radio-
nuclides. The comparison will not be a proper one if the natural radio-
nucllde is not exposed to the same transport processes as the HLW.
C-5.1.2,1 Prediction of Pathways
The transport processes described in Section C-4 are based on geological
considerations that are associated with long time periods in the past and
that will be generally applicable for the long time periods into the future
associated with HLW disposal. Thus, the transport processes described and
discussed at present will presumably be unchanged for equivalent periods in
the future.
130
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The analogous situation for predicting the pathways by which the
radionuclides will disperse in the biosphere and enter people, and even
for predicting human metabolism,does not have the same sound basis of
constancy with time. Society and the activities of people have been changing
continually and quite rapidly in relation to time periods as long as those
involved in HLW management. However, to make a dose equivalent calculation,
a pathway and metabolic model must be postulated describing the conditions
under which the radionuclides are introduced into people and how they are
metabolized. If society and the activities of people are known, realistic
models can be postulated and the calculations made for the short term. For
the long term assessment, on the other hand, it is nearly impossible to predict
what society and the activities of people will be and, hence, no applicable
models have been postulated and no appropriate calculations can be made.
This inability to predict pathways and metabolism for the long term
poses great limitations for dose calculations. This concept is important
enough that it is described in greater detail in Section C-5.1.3 in order to
give a broader perspective to the dose equivalent calculations that are made
in a more conventional manner and are presented in Section C-5.2 and C-5.3.
C-5,1.2.2 Assumptions and Conditions for the Dose Calculations
In spite of the difficulties of long-term predictions, estimates of
dose equivalents are needed to make an evaluation of the potential consequence
to populations in the future.
Dose equivalent calculations are expressed in terms of dose commitment.
The latter is defined for the present purpose as the dose equivalent accumu-
lated over a period of 50 years following the intake of radionuclides for a
period of not more than one year. The dose equivalent, as defined, will vary
with the age of the individual and is, therefore, age specific. The repre-
sentative age chosen for this report is 21 years.
The models for the calculations involve two types of transport from
the repository, three types of pathways to people, and three types of dose
commitment to these people. The transport scenarios both Involve water
getting into the repository; in Scenario 1 there is water contact but no
flow, and the radionuclides diffuse to an overlying aquifer; in Scenario 2,
there is water flow from an aquifer below the repository, which carries the
radionuclides to an overlying aquifer. The model pathways to man include
131
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direct access to the water in the aquifer and via two types of river fed by the
aquifer, one simulating a river like the Colorado River and the other simulating
a "midwestern river." In both the river models, radionuclides reach man through
direct ingestion of river water and through crops, cattle, and milk. The path-
ways involving direct access to the aquifer results in a "maximum" Individual
dose commitment that is believed unrealistic and serves principally
as an upper limit value. For the river pathways, "maximum" and "reasonable"
population dose commitments are calculated. The former is analogous to the
"maximum" individual dose and again is an upper limit; the latter is an attempt
to represent realistic conditions whereby radionuclides will be introduced
into a population served by a particular body of water.
The radionuclides used in the dose calculations are those that persist
or grow over long time periods and that contribute significantly to the total
dose commitment. The times for the dose calculations are those times in the
future at which the concentrations of the radionuclides reach maximum values.
Thus, the dose calculations are made for present-day people, with present
metabolism and present modes of access to the radionuclides, using those radio-
nuclides that would be present in the future, at their maximum concentrations,
if they were indeed to move from the repository to a point accessible to such
people. The calculations must, therefore, be considered only as an illus-
trative exercise, based on a specific set of assumptions, which cannot in any
way be considered as predictive of actual occurrences in the future. Finally,
all the dose commitment values that are presented can be considered to represent
conservative values, partly because water has been assumed to enter the repository
partly because of the use of the linear-nonthreshold dose-response model, and
partly because of the transport and pathway assumptions. The expected dose
commitments in a real situation would almost certainly be lower than those
predicted by these illustrative models.
C-5.1.3 Conditions of Exposure and Persons or Populations atRisk
C-5.1.3.1 Usefulness of Modeling Short-Term Transport and Health Effects
Computerized simulation of the dynamics of environmental transport and
redistribution of radioactivity has been most useful and valid in the studies
of potential impacts of specific facilities at specific locations. In these
situations, the existing environment can be investigated and substantially,
though not completely, quantified for use as input to a computer model.
132
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The model can be used to evaluate critical pathways and thereby identify
the most appropriate locations for subsequent sampling and analysis. The
model can also be verified after its use by comparing measured concentrations
with those predicted by the model.
The extensive computer modeling of health effects resulting from the
release of radionuclides to the environment, as well as of the potential
mechanisms by which such releases night occur, has been valuable in pointing
out areas that had been overlooked in previous evaluations of the long-term
impact of nuclear energy. For example, the potential health impacts of
nuclides such as earbon-14, teehnetium-99, iodine-129, and noptuniutn-237
were identified primarily as a result of computer modeling.
In nonspecific studies of facility types or typical operations, however,
computerized simulations are of limited value simply because they are so
critically dependent upon the assumptions of environmental conditions that
are used as input data. Acceptance or rejection of the results of such
modeling will depend upon whether one accepts or rejects the assumptions
used as input to the model.
C-5.1,3.2 Problems in Modeling Long-Term Transport and Health Effects
The validity of detailed models for projecting transport and health
effects many millenia into the future is certainly questionable. Relevant
demographic and socio-economic factors can be hypothesized for such distant
times, but cannot be forecast. The discussions in the following sub-sections
serve to illustrate this fact.
An extensive review of 83 documented computer codes for assessing
environmental transport and radiation doses from radioactivity released to
(49)
the environment was published recently. The codes evaluated exhibited a
rather wide range of sophistication and capabilities. The environmental
transport models and supporting data represented by these computer codes are
generally similar in two respects: most of them are designed to evaluate
releases from point sources (i.e., specific surface facilities) directly into
the atmosphere or into a body of surface water, and most consider only the
first cycle of radioactive materials through environmental pathways. No
documented computer codes were found that were designed to handle releases
from dispersed underground sources or to consider recycling of extremely
long-lived radionuclides in the biosphere.
133
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C-5.1.3";2.1 Population Distributions
Geographic distributions of populations tend to change as environmental
conditions and resource values change. For example, anyone making predictions
of present population distributions based on conditions existing more than 500
years ago would not have extrapolated beyond his own country or at least his
own continent. As recently as 100 years ago, few people could have foreseen
the city of Phoenix, Arizona. Even Las Vegas, Nevada, as it exists today,
would have been considered unlikely as recently as 50 years ago, before the
construction of Hoover Dam. In general, the fastest-growing cities in the
United States during recent decades either did not exist or were inconsequen-
tial a century ago.
At the present time, the most rapid growth in the United States is
occurring in the southwestern ("sunbelt") regions. The principal limitation
to growth in those areas is the supply of water. However, some of those
same areas may be considered for location of large Federal repositories for
radioactive waste. If a practical system were devised to divert water from
the northwestern region of the county, or from the Hudson Bay area of Canada,
to the southwestern portion of the United States, it is entirely conceivable
that extremely large metropolitan areas would develop in the Southwest. At
the present time, there is no way of predicting with a high degree of assurance
the population growth or distribution for long periods into the future for those
areas in which HLW repositories may be considered.
C-5.1.3.2.2 Agricultural Distribution
The geographic distribution of agriculture in the United States has
changed significantly during the nation's history. Two hundred years ago,
few would have predicted that the Imperial Valley of California would be a
major agricultural center. Also, few at that time would have predicted that
the plains of Kansas, Nebraska, the Dakotas, etc., would be agriculturally
important to the United States and to the world. Just as the Imperial Valley
in California was changed by large-scale irrigation practices, so also could
large portions of the southwest be changed by the importation of large quantities
of water from more northern regions of the United States, or possibly Canada.
As with population distribution, there is no assured way of predicting what
the agricultural distribution might be around an HLW repository at times far
into the future.
134
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C-5.1.3,2.3 Agricultural Technology
Basic agricultural technology may change as a result of efforts to
feed larger numbers of people on smaller parcels of land. This would be
particularly true if continued population growth caused more agricultural
land to be taken out of use for other purposes, such as residential, com-
mercial, or industrial uses. During the past century, especially during
the past few decades, a number of significant changes have been taking place
in agricultural technology, particularly In the areas of increased auto-
mation and artificial enhancement of food production.
In recent years, many revolutionary procedures have been developed for
producing the world's food supplies with smaller commitments of land and
in more stringently controlled environments. Thus, it is entirely plausible
that within a few decades, or at most within a few centuries, present-day
concepts of transport of contaminants through food chains will be obsolete
and irrelevant. It Is possible that food production will be very closely
controlled and managed in order to maximize its quantity and quality. At
that time, the uncontrolled outdoor environment may have little or no inter-
action with the controlled agricultural environment.
Dietary and nutritional practices will undoubtedly change as the result
of changes in agricultural technology and distribution networks for food
products. In the past few decades, food chains have become more truly world-
wide, as opposed to being localized within agricultural communities. At the
same time, however, there has been a massive increase in the control of food
ingredients and distribution.
C-5.1.3.2.4 Resource Conservation
As natural resources become more expensive to obtain, increased emphasis
is placed on recycling. Not only mineral resources, but also organic materials
are being recycled; a common example is that of wood and fiber products, such
as paper.
Another resource that is being conserved and "recycled" in some parts
of the country, out of sheer necessity, is water. As a nation we are rapidly
approaching "limited pollution11 releases with the long-range objective of
"zero release." Admittedly, it will be a long time before we arrive at "zero
release," but as we approach this condition, the importance of contamination
from outside sources will diminish.
135
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C-5.1.3.2.5 Medical Technology
One of the most important factors affecting the significance of radi-
ation dose to the human race in the centuries and millenia to come is the
effect of progress in medical technology. It is a matter of record that the
progress achieved during thp past several decades has already lengthened human
life expectancy and increased the average age of the population. The success
of medical science has also been felt in the increased numbers of people in
the procreative age group who have survived what wou?d once have been a fatal
disease and who are now capable of passing that hereditary defect on to their
offspring. It is entirely conceivable that, within the next century, the
somatic diseases caused by radiation will be treated'•success fully, so the per-
centage of people who have sustained some physiological, and perhaps even gen-
etic, damage and who are still able to produce children will increase. If,
in fact, the adverse health effects of radiation prove to be either prevent-
able or curable by advances in medical science, the impact of the radiation
doses resulting from long-term storage of HLW may be substantially altered.
Although none of these possible eventualities would affect the radiation
doses received by future populations, the consequences of those doses could
obviously be very different.
C-5.1.4 Implications of the Use of a Linear-Nonthreshold Dose-Response
The linear-nonthreshold hypothesis is defined as: "the assumption that
a dose-effect curve derived from data in the high dose and high dose-rate
ranges may be extrapolated through the low dose and low dose-rate range to
zero, implying that, theoretically, any amount of radiation will cause some
damage." To permit simplified calculations of the health impacts of
small individual radiation doses delivered to large populations, it is
customary to assume that every increment of dose represents the same impact,
regardless of when, where, how, and to whom it was delivered. At doses or
dose-rates to individuals that are small fractions of those due to natural
sources, the question of the validity of this assumption is unresolved.
Inasmuch as a straight-line extrapolation from the high-dose region down-
ward through the origin would overestimate response effects in the low-dose
136
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region, compared with a curvilinear form, it is generally agreed that the
"llnear-nonthreshold" model provides a "conservative" basis for public
policy making. In addition, since this model, like all linear models, Is
intrinsically easy to understand and apply, there is a practical reason for
preferring it to a more complicated one. The BEIR Committee, (50) therefore,
recommended its adoption, in spite of the fact that experimental evidence
indicated that some non-linear function would more accurately repre-
sent the true dose-response relationship for human health effects. The
Committee justified its recommendation on the basis that a "non-linear
hypothesis for estimating risks in support of public policy would be
impracticalin the present state of knowledge*, and partly because "the
linear hypothesis, which allows the mean tissue dose to be used as the
appropriate measure of radiation exposure, provides the only workable*
approach to numerical estimations of the risk in a population."
A further advantage of using the llnear-nonthreshold model arises In
calculating limits of population radiation doses for circumstances where
population distributions and radionuclide concentrations may not be well
defined.
C-5.2 BASIS FOR DOSE-TO-MAN CALCULATIONS
C-5.2.1 Introduction
The preceding section has identified and emphasized the uncertainties
and Inadequacies Inherent in modeling future events and consequences that
require detailed knowledge of the biosphere and of human living conditions
In times far Into the future. In spite of such limitations, there is a
distinct need for making reasonable projections related to the effects of
current actions on future generations. In order to make such reasoned,
though limited, projections, one must rely on making explicit assumptions
and proceeding thereafter to evaluate consequences of an action or actions
based on those assumptions.
For this report the dose-to-man consequences of HLW emplacement in
deep geologic formations have been exemplified by stipulating a "most
likely" release mechanism and by stipulating conditions whereby "maximum
individual dose" and "reasonable population dose" would occur. The general
*Emphasls added
137
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mechanisms and assumptions have been described briefly in Section 5.1.2.2.
The most likely release mechanism is contact with, and subsequent leaching
by, groundwater. Dose-to-man calculations have been made for two sub-
categories of this release mechanism; transport by passive aqueous diffu-
sion and by active flow of radionuclides in potable groundwater. In the
following sections, the assumptions made relative to the two release mech-
anisms (scenarios), the three pathways, and the three categories of dose
calculations are discussed in greater detail.
C-5.2.2 Repository Release Scenarios
The two release scenarios used for this task are those termed ''normal"
as specified in Section C-4.4. Both scenarios consider groundwater in
contact with the repository waste materials, leaching of the waste, and
subsequent transport through the geosphere to the biosphere. Leaching
by groundwater appears to be the most likely failure mode for a waste
repository. In fact, some workers have concluded that eventual contact
(37)
between groundwater and the repository will occur. The following
sections describe the two groundwater release scenarios upon which dose-to-
man calculations are based. Table C-17 presents physical data and calculated
50-year dose-commitment factors for a number of long-lived radionuclides
that will be present in the waste.
C-5.2.2.1 Diffusion to a Potable Aquifer (Scenario 1)
In this release scenario, radionuclides are transported from the
waste repository by way of diffusion in the water through the repository
horizon, from which some of them gain access to an over-lying potable aquifer
(See Figure C-15). Table C-18 shows the concentrations of important radio-
nuclides 'in the aquifer directly above the repository at time periods of
300, 1000, and 30,000 years (times refer to age of waste following emplace-
ment). The assumptions on which these concentrations are predicted include
the effects of sorption and dilution in the aquifer but not sorption in
the repository strata. After entering the aquifer, radionuclides move down
the hydraulic gradient with the groundwater. Concentrations of important
radionuclides (i.e., those that are predicted to appear in significant quan-
tities) one mile down gradient from the repository area are also shown In
138
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TABLE C-17
PHYSICAL DATA AND DOSE COMMITMENT FACTORS
FOR SIGNIFICANT RADIONUCLIDES
Nuclide
0-14
Sr-90
Tc-99
1-129
Cs-13?
Sm-151
Ra-226
Th-230
U-233
U-234
U-235
U-236
U-238
Np-237
Pu-238
Pu-239
Pu-240
Pu-242
Am-241
Am-243
Half-Life
(years)
5.7 x 103
2.8 x 101
2.1 x 105
1.7 x 107
3.0 x 101
8.7 x 101
1.6 x 103
8.0 x 104
1.6 x 105
2.5 x 105
7.1 x 108
2.4 x 107
4.5 x 109
2.1 x 106
8.6 x 101
2.4 x 104
6.6 x 103
3.8 x 105
4.6 x 102
8,0 x 103
Specific
Activity
(Ci/£)
4.5 x 10°
1.4 x 102
1.7 x 10~2
1.6 x 10~4
8.8 x 101
2.8 x 101
1.0 x 10°
2.0 x 10~2
9.7 x 10~3
6.2 x 10~3
2.2 x 10~6
6.4 x 10~5
3.4 x 10~7
7.3 x 10~4
1.8 x 101
6.3 x 10~2
2.3 x 10""1
3.9 x 10~3
3.3 x 10°
1.9 x 10'1
fcr\
Ingest ion 50-Year Dose
Commitment to Adult
(rem/M Cl intake)*
Whole Body
5.65 x 10~4
1.67 x 10-1
4.92 x 10~5
9.32 x 10~3
4.32 x 10~2
2.84 x 10~6
3.11 x 101
5.70 x 10~2
5.24 x 10~2
5.13 x 10~2
4.82 x 10"2
4.92 x 10"2
4.50 x 10~2
5.54 x 10~2
1.72 x 10~2
1.91 x 10~2
1.91 x 10~2
1.84 x 10"2
5.42 x 10"2
5.30 x 10~2
Bone
2.8 x 10~3
8.3 x 10°
1.2 x 10~4
[7.5 x 100"1"
Thyroid]
8.1 x 10~2
6.9 x 10~5
3.0 x lO1^
2.0 x 10°
8.6 x Hf1
8.3 x 10"1
7.9 x 10"1
7.9 x 10"1
7.6 x 10"1
1.4 x 10°
6.8 x 10"1
7.9 x 10"1
7.9 x 10"1
7.3 x 10"1
8.2 x 10"1
8.2 x 10"1
intake at age 21.
+2.8 x 101 for child.
tfrom ICRP Publ. 10.
139
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TABLE C-18
RADIONUCLIDE CONCENTRATIONS FOLLOWING DIFFUSION
INTO A POTABLE AQUIFER (SCENARIO 1)*
Aquifer Entry
Nuclide
C-14
Sr-90
Tc-99Rf
Tc-99Nf
1-129
Cs-137
Sm-151
U-235
U-236
Np-237
Pu-239
Pu-240
Am-243
Ra-226
300 Years
4.3xlO-19
1.2X10"19
2.4xlO~20
1.6xlO"13
4.4xlO~14
7.2xlO"20
1.4xlO~18
2.7xlO~15
1.4xlO~15
4.6xlO~14
2.5xlO~15
l.OxlO"15
-17
4.1x10
8.7xlO~21
1000 Years
7.6xlO-16
3.3xlO~24
1.4xlO-16
2.4xlO-10
6.5X10"11
8.7xlO~24
1.4xlO~17
8.1xlO~12
4.4xlO~12
l.lxlO"10
7.1xlO~12
2.6xlO~12
-13
1.0x10
6.4xlO"16
30,000 Years
tt
tt
1.7X10"11
**
**
tt
tt
5.5xlO~6
3.1xlO~6
2.3xlO~6
1.5xlO~6
6.3xlO~8
-9
3.4x10
2.8xlO~9
One Mile Below Repository
300 Years 1000 Years 30,000 Years
1.3xlO-25 1.4xlO-17 tt
tt
_ _ _
3 . lxlO~13 1 . 7xlO~10 **
8.6xlO~14 4.7xlO~1:L **
tt
- tt
_ - _
_
l.OxlO"23 2.0xlO~6
_ - _
_ _ _
_ _ -
5.5xlO~16
100,000 Years
tt
tt
tt
**
**
tt
tt
tt
tt
tt
tt
tt
tt
1.2xlO~13
*A11 values are representative of the throwaway cycle and are in units of
**Concentrations at these time intervals are less than concentrations at 1000 years.
tTc-99R represents an assumed adsorptive (Rjj=3300) form of Tc-99.
ttTc-99N represents a non-adsorptive (R
-------
INDIVIDUAL
DIRECT.1NGESTION
NVK^vw^W-^yKv/A^rfjW •
SOIL
POTABLE AQUIFER
POPULATION
DIRECT INGESTION
WATER SUPPLY WELL
SOIL
OF
GROUNDWATER
FLOW
T
VERY LOW
PERMEABILITY
REPOSITORY STRATUM
WITH INTERBEDS
[
Dl FFUSION OF RADIONUCLIDES
TO POTABLE AQUIFER
HLW REPOSITORY
"**.'. I * T>/?ai/r/^£IK SLOW LEACHING IN REPOSITORY
_____ * . * •
LOWER AQUIFER
Source; D'Appolonia Consulting Engineers, Inc., for Arthur D. Little, Inc.
FIGURE C-15 SCHEMATIC ILLUSTRATION OF MAXIMUM INDIVIDUAL AND UPPER LIMIT
POPULATION DOSE BY DIFFUSION TO A POTABLE AQUIFER (SCENARIO 1)
141
-------
Table C-18 for time periods of 300, 1000, 30,000 and 100,000 years
following waste emplacement.
C-5.2,2.2 Flow to a Potable Aquifer (Scenario 2)
In this scenario, radlonuclldes are transported from the waste reposi-
tory by groundwater from an underlying aquifer flowing through the repository
to an overlying potable aquifer (See Figure O16). When the nuclide-bearing
groundwater reaches the aquifer, movement is downgradient within that aquifer.
Table C-19 presents concentrations of important radionuclides in the aquifer
directly above the repository and in the aquifer one mile down gradient from
the repository. Concentrations at time periods of 300, 500, 6000, and
30,000 years are shown for the area overlying the repository site and
for 300, 500, 6000, 30,000, and 100,000 years one mile down gradient from
the repository. These concentration estimates are based on assumptions
that include sorption and dilution in the aquifer.
C-5.2.3 Pathway Models
The intake of radionuclides by people is postulated to occur not by
direct access to the repository but by access through some pathway to the
aforementioned aquifer. Three routes have been chosen for consideration,
.one of which (direct access to the contaminated" aquifer) would require con-
currence of several specific conditions or events and may therefore be rather
unlikely, while the other two (river models) are intuitively more plausible
and realistic. In each of the three cases, contamination of the aquifer
was assumed to occur by either the Scenario 1 or Scenario 2 mechanisms.
Information important to the use of all models for calculating dose com-
mitments Is listed in Table C-20.
C-5.2.3,1 Direct Access to Aquifer
In this case the assumption is made that water is obtained from a
well that has been drilled Into the aquifer, i.e., the resulting pathway
is that of direct ingestion of the contaminated water continuously for a
period of one year. This is a worst-case situation, most likely to be
prevented either by appropriate security or by the remoteness and lack of
142
-------
INDIVIDUAL
DIRECT INGESTION
POPULATION
DIRECT INGESTION
-WATER SUPPLY WELL
SOIL
*" POTABLE AQUIFER ^
^
f
SOIL
— ^ -^--DIRECTION OF
Z~^ GROUNDWATER
"* FLOW
VERY LOW
PERMEABILITY
REPOSITORY STRATUM
WITH INTERBEDS
~L ^*
^—* ^
3t jr iiii-i *^-^
AQUEOUS FLOW TRANSPORT
OF RADIONUCLIDES
HLW REPOSITORY
RELATIVELY RAPID LEACHING IN REPOSITORY
LOWER AQUIFER
Source; D'Appolonia Consulting Engineers, Inc., for Arthur D. Little, Inc.
FIGURE C-16 SCHEMATIC ILLUSTRATION OF MAXIMUM INDIVIDUAL AND UPPER LIMIT
POPULATION DOSE BY AQUEOUS FLOW TRANSPORT TO A POTABLE AQUIFER
(SCENARIO 2)
143
-------
TABLE C-19
Radionuclide Concentrations* in Aquifer at Aquifer Entry (Scenario 2)
Nuclide
C-14
Sr-90
Tc-99Rf
Tc-99Nf
1-129
Cs-137
Sm-151
U-233
U-234
U-236
Np-237
Pu-240
Pu-242
Am-243
Ra-226
300 years
1.
2.
6.
5.
1.
1.
2.
7.
2.
3.
1.
2.
5.
1.
8.
7 x
6 x
7 x
1 x
4 x
2 x
0 x
5 x
8 x
9 x
0 X
9 x
9 x
2 x
6 x
10-9
10-8
10-11
10-4
10-4
10-8
10"8
10-11
10-7
10-6
io-4
10-6
10-7
10-7
10-12
500 years
1.3
5.3
2,4
3.2
8.7
4.8
1.6
4.2
1.0
1.4
3.0
1.0
2.2
4.1
8.4
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
10-9
10-10
10-10
10_4
10-5
10-10
io-8
10-10
10-6
10-5
10-*
10-5
10-6
10-7
10-11
6000 years
4.5
2.3
6.4
1.9
4.1
7.8
2.4
1.2
8.1
5.3
1.3
tt
tt
x
x
X
tt
tt
X
X
X
X
X
X
X
X
10-10
10-5
10-6
10-8
10-6
10-5
10-5
10-5
10-6
10-7
10-8
30,000
tt
tt
2.1 x
4.2 x
1.3 x
tt
tt
3.6 x
3.4 x
7.3 x
1.1 x
2.8 x
5.5 x
1.4 x
6.6 x
years
10-10
10-6
10~6
10-8
10-6
10-5
10-5
10-6
10-6
10-7
10-8
t,
tt
All values are representative of the throwaway cycle and are
In units of g/£,
Tc-99R represents an assumed adsorptive (Rd-3300) form of Tc-99.
Tc-99N represents a non-adsorptive (SdHl) form of Tc-99.
Concentrations at these times are not available.
144
-------
TABLE C-19 (Continued)
Nuclide Concentration in Aquifer One Mile Beyond Repository ^Scenario 2j
Nuclide 300 years 500 years 6000 jrears
C-14 - 1.2 x 10-14 ft
Sr-90 tt
Tc-99Rf -
TC-99N1" 1.3 x 10~4 3.6 K 10~* 2.3 x 10~5
1-129 3.7 x 10~5 1.0 x 10~4 6.5 x 10~6
Cs-137 tt
Sm-151 - - ft
U-233 - 1.0 x 10~20 1.8 x 10-9
U-234 -
U-236 -
Np-237 - 2.2 x ID'14 7.4 x 10~5
Pu-240 -
Pu-242 -
Am>-243 -
Ra-226 - - 4.4 x 10"18
30,000 years
tt
tt
3.1 x 10~19
4.2 x 10-6
1.3 x 10"6
tt
tt
2.7 x 10~9
-
-
1.8 x 10~5
-
—
—
8.0 x 10'14
100,000 years
tt
tt
8.5 x 10~14
tt
tt
tt
tt
2.7 x 10~9
-
9.1 x 10~21
1.1 x 10~9
5.6 x 10~23
1.2 x 10~19
2.4 x 10~19
1.6 x 10"12
*
All values are representative of the throwaway cycle and are in
units of g/&.
lc-991 represents an assumed adsorptive (R
+EPA-520/4-73-002.
See Appendix C-V for explanation of transfer coefficients.
146
-------
usefulness of the repository area, it provides, for reference purposes,
an extreme case of human exposure to a contaminated aquifer, and produces
an "unrealistically" high radiation dose commitment to affected individuals.
C-5.2.3.2 River Model "A"
This river model is constructed using data typical of the Colorado
River mainstream. Water from the aquifer is assumed to enter the river
one mile beyond the repository. Main water diversions (for population
use) along the river are taken at five major locations.
The river model has an upper and lower basin; the upper basin has
more rapid dilution via tributaries, and the lower basin has a relatively
constant flow and,dilution factor. For this reason, two diversions were
used to represent use in the upper basin and one "total diversion" to
represent use in the lower basin, A dingle point diversion in the lower basin
(at the river mouth) was used because the dilution factor throughout the
lower basin is approximately constant. Other upper-basin characteristics
are high agricultural use and relatively small populations ingesting water.
Dilution factors applicable at each diversion were calculated using a
general materials balance. Use along the river has been taken either from
existing Colorado River data, as stated in Table C-21, or calculated from
standard use and yield values.
C-5.2.3.3 River Model "B"
This model is for a hypothetical river having a population and use
distribution proportional to the river flow (i.e., proportional to the
dilution factor). Water from the aquifer is assumed to enter the river
one mile beyond the repository. The model was previously used for a
(52)
purpose similar to its application in this assessment. This hypo-
thetical river can be considered more typical of a mldwestern river than
River Model "A" in that it is more heavily used for drinking water and
less used for agricultural supply. Where available data are insufficient,
the same assumptions and water use calculations made for River Model "A"
are applied in River Model "B". Information important in using River
Model "B" for calculating a reasonable population dose is listed in
Table C-22.
147
-------
TABLE C-21
00
INFORMATION FOR RIVER MODEL "A"*
Distance
to
Diversion
(miles)
Upper Basin
0
50
100
Lower Basin
550
700
800
River
Flow
(cfs)
2,400
4,000
6,700
13,700
12,300
11,500
Dilution
Factor
1.0
1.7
4.5
10.4
10.5
11.3
11.3
Diversion
(cfs)
_
1,600
700
1,500
1,600
4,000
7,100
FOR REASONABLE
Population Total
Drinking Irrigation
Water Acreage
(persons) (acres)
_ _
1 x 10* 1.5 x 105
1 x 105 6.6 x 10*
4 x 105
5 x 106
1 x 104
5.4 x 106** 1.5 x 106
POPULATION DOSE CALCULATIONS
Sprinkler
Irrigation
Acreage
(acres)
_
1.5 x 10A
6.6 x 103
_
_
_
1.5 x 105
Total+ Dairy?
Crop Beef I Cow
Yield Feed Feed
(kg/yr) (kg/yr) (kg/yr)
_ _ _
4.5 x 108 3.9 x 108 4.0 x 107
2.0 x 108 1.8 x 108 2.0 x 107
_ - _
_
_ - -
1.2 x 1010
, . Garden
Beef Milk* Vegetable
Yield Yield Yield
(kg/yr) (H/yr) (kg/yr)
_ _ _
3.9 x 106 3.2 x 107 2.0 x 107
1.8 x 106 1.6 x 107
_ -
_
_
1.2 x 101(H+
Assembled using Colorado River mainstream data.
Assumes 10 percent to be contaminated by sprinkler irrigation.
^Assumes 9C percent of animal feed is for beef cattle, 10 percent is for dairy cows.
Assumes 1 kg beef per 100 kg feed.
Assumes 80 i milk per 100 kg feed.
++
Assumes all crop yield is for human Ingestion.
Used one "total" diversion for the lower basin.
Note: Dashes indicate instances where the information was not used for calculational purposes
and consequently is not presented.
-------
TABLE C-22
VO
INFORMATION FOR RIVER MODEL "B"*
FOR REASONABLE
Distance
to
Diversion
(miles)
0
15
45
80
150
250
350
450
River
Flow
(cfs)
200
220
360
700
2,000
5,200
10,000
16,000
Dilution
Factor
1.0
1.1
1.8
3.5
9.9
26
50
81
Population
Drinking
Water
(persons)
^
3
4.3 x 10
7.0 x 103
1.8 x 104
1.3 x 105
s
3.3 x 10
5
6.3 x 10
6
1.1 x 10
POPULATION
Total
Crop
Yield
(kg/yr)
•K
5
2.0 x 10
3.2 x 105
8.4 x 105
6.0 x 106
7
1.5 x 10
7
2.9 x 10
7
4.8 x 10
DOSE CALCULATIONS
Beef
Feed
(kg/yr)
_
4
3.0 x 10
4.7 x 10*
1.2 x 105
8.6 x 105
6
2.2 x 10
6
4.2 x 10
6
6.9 x 10
Dairy
Cow
Feed
(kg/yr)
^
4
6.2 x 10*
9.8 x 10
2.5 x 105
1.8 x 10
f
4.6 x 10
g
8.7 x 10
7
1.4 x 10
Beef+
Yield
(kg/yr)
2
3.0 x 10
4.7 x 102
1.2 x 103
8.6 x 103
4
2.2 x 10
4
4.2 x 10
4
6.9 x 10
Milkt
Yield
U/yr)
_
A
5.0 x 10
7.8 x 104
2.0 x 10
1.4 x 106
ft
3.7 x 10
6
7.0 x 10
7
1.1 x 10
Garden
Vegetable
Yield
(kg/yr)
.
s
1.1 x 10
1.8 x 105
4.7 x 105
3.3 x 106
ft
8.6 x 10
7
1.6 x 10
7
2.7 x 10
*Assembled using river in "GESMO" (NUREG-0002, Vol. 3).(52)
Assumes 1 kg beef per 100 kg feed.
^Assumes 80 £ milk per 100 kg feed.
Note: Dashes indicate instances where the information was not used for calculational
purposes and consequently is not presented.
-------
C-5.2,3.4 Accumulation of Radionuclldes in Surface Soil
Continual irrigation with contaminated water produces a long-term
buildup of contaminants in surface soil. At the same time, contaminants
are continually removed by erosion, by uptake into plants, or by leaching below
the root zones of plants (effective removal from food chains). There are
few data available to indicate the natural residence time of various elements
in tillable soils. For this study, a soil exposure time of 100 years was
used to allow for reasonable buildup of radioactive contaminants through time.
For calculational purposes the midpoint of this time, 50 years, is used
as an average value for transfer coefficients from soil to plants. This
is believed to be a conservative assumption, sincesit is unlikely that most
nuclides will accumulate without dispersion for that period of time.
C-5.2.4. Exposure Routes^
For the postulated "most likely" mechanisms for release of radio-
nuclides fromanHLW repository (i.e., leaching and transport by groundwater)
the "maximum" individual and population doses can be approximated without
detailed pathway modeling. This results because, for most of the radionu-
elides, direct exposure routes lead to much higher radiation doses than
V
do the indirect exposure routes. The indirect routes, however, are the
more likely ones whereby man could be exposed to HLW radionuclides.
One possible exception to the direct route being more important is
iodine-129 for which the radiation dose via the milk-to-human pathway can
be approximately 100 times greater (per unit of liquid ingested) than the
dose from direct ingestion of the radioiodine in drinking water. However,
such extreme differences between doses delivered by direct and indirect
pathways occur only with the simultaneous existence of two or more rather
unlikely maximizing circumstances, for example, in order to have an annual
intake approximately 100 times greater than would be achieved by direct
ingestion of the contaminated water, a child would have to drink nothing
but milk from cows whose only source of feed was vegetation grown on land
totally Irrigated with contaminated water at an irrigation rate of one meter/
150
-------
year and with a retention of radlolodine on the edible portions of the plant
of 25% of the total amount sprinkled. In similar fashion, a child obtaining
all Its vegetable food products from land Irrigated with contaminated water
under the same circumstances as described above would also receive a radiation
dose approximately 100 times greater than that delivered by direct Ingestion
of the contaminated water. Such circumstances are not considered realistic
for purposes of dose estimations.
Models, assumptions, and data have been prepared or collected by the
U.S. Nuclear Regulatory Commission (NRC) for the purpose of calculating
radiation doses to individuals and populations. This information was
evaluated and was determined to be adequate, reasonable, and realistic for
the purposes of this study. Consequently the NRC models, assumptions and
data have been utilized for dose calculations; these will be addressed
more explicitly In the subsequent sections.
The conversion factors for the radiation dose to an Individual, per
unit of radioactivity ingested or inhaled, are based on the Oak Ridge
(53)
computer dose program INREM as described In ORNL-5003 and as tabulated
in ORNL-4992/ ^ The dose conversion factors tabulated in ORNL-4992 are
50-year dose commitments to an individual at age 21. These conditions are
felt to be a suitable approximation for an average population when all
other uncertainties In the assumptions are considered.
Generalized environmental transfer and bloaccumulation coefficients for
a rather complete range of radionuclides have been used for environmental
modeling as shown in Table C-22 for River Model "B".'51' These coeffi-
cients are appropriate for use where site-specific data are not available;
this certainly is the case for considerations relative to HLW repositories
In geologic formations.
C-5.2.4.1 "Maximum" Individual Eadiation Dose
The "maximum" Individual dose is calculated on the assumption that
an Individual is present at the point ot maximum environmental concentra-
tion for a specified radlonuclide and at the point in time for which that
nuclide reaches the maximum concentration. It Is assumed, however, that
the individual Ingests the contaminated material at an estimated maximum
151
-------
rate (730 liters per year) and is an individual of average age in the popu-
lation. In this respect, the calculation should not be considered the "maxi-
mum conceivable individual dose" but is more appropriately thought of as a
"maximum dose to the typica^ individual."
The "maximum" individual dose calculations are based on the assumption
that an individual would obtain all drinking water from a well penetrating the
contaminated aquifer at the point of highest concentration In the vicinity
of the waste repository (see Figures C-15 and C-16). Thus, the maximum in-
dividual dose in this case is calculated by assuming that it results from
continuous consumption of water by a 21-year-old individual for a period of
one year, that the water is contaminated to the maximum level in the aquifer,
and that it produces a 50-year dose commitment during the lifetime
of the individual. For purposes of Calculating a 50-year dose com-
mitment, an ingestion period of one year is considered to be the same as a
single instantaneous ingestion. The location of the well in the aquifer
is assumed to be directly above the repository where the radionuclides in the
aquifer are at the maximum concentration following initial dilution. The
times used for estimating the radionuclide concentrations in each scenario
are those that produce the maximum radionucllde dose commitments for fission
products and actinides, respectively.
C-5.2.4.2 "Maximum" Population Radiation Dose
A "maximum" population dose Is closely related to the "maximum"
individual dose. In most cases of release of radionuclides into the envi-
ronment, the assumed conditions of release generate certain limitations
to the total population radiation dose that can be delivered, at least by
direct exposure pathways. Analogous to the "maximum" individual dose,
the "maximum" population dose is designed to represent an upper limit of
radiation dose to a population present at the time and location for which
the maximum impact might occur from a release of radionuclides from a
waste repository. However, the population density and the environmental
transfer coefficients are assumed to be approximately average for commun-
ities that might be exposed, rather than the extremes that might produce
the "maximum conceivable" population dose. In the case of a contaminated
water body such as an aquifer, a surface reservoir, or a surface stream,
152
-------
the "maximum" population radiation dose is equal to the "maximum" individ-
ual radiation dose (i.e., the radiation dose to an individual drinking a
maximum quantity [730 £/yr] of the contaminated water exclusively) multi-
plied by the population (250 persons) that could be supplied by the water
body (see Figures C-15 and C-16), and reduced by a factor of two to allow
for a lower annual water intake of 370 £/person. The upper limit of popu-
lation that can be supplied by a particular water body was estimated on
the basis of current par-capita domestic water consumption and the total
outflow volume of the hypothetical aquifer.
C-5.2.4.3 "Reasonable" Population Radiation Dose
"Maximum" radiation doses to individuals and populations represent
upper bounds for the analysis of health consequences. However, they do
not represent what might cautiously be called "most reasonable estimates"
of radiation doses that might be expected from a geologic HLW repository.
It is highly unlikely that any Individual or population group would be
present at precisely the point of maximum environmental concentration in
the event of release of radionuclides from a repository. It seems more
realistic to postulate that populations would be exposed at some distance
from the repository and through indirect pathways rather than by the most
direct pathways. To develop such realistic population radiation dose
estimates for unspecified repository locations and indeterminate times
into the future, it has been necessary to make many simplifying assumptions.
"Reasonable" population radiation doses are calculated on the basis
of the few radionuclides that actually reach the biosphere under the
assumed exposure mode, i.e., by discharge of a contaminated underground
aquifer into a surface stream. These few critical radionuclides are
assumed to follow typical pathways and reach typical populations (based
on current societal and environmental conditions) during the course of
their migration downstream toward the ocean. The typical environmental
pathways are simplified, primarily to emphasize the fact that the cal-
culations should not be thought of as highly accurate but as subject to
some uncertainty. Population distributions and densities, agricultural
practices, etc., are assumed to be those common in much of the United
153
-------
States at present. No reasons could be found for anticipating particular
population densities or agricultural practices at times in the distant
future,
The "reasonable" population dose commitments for each scenario have
been calculated on the basis of the contaminated aquifer Intersecting a
surface stream one mile down-gradient from the repository. Ingestion
pathways used in these calculations include!
(1) Direct ingestlon of contaminated river waterj
(2) Ingestlon of contaminated crops;
(3) Ingestlon of contaminated beef; and
(4) Ingestion of contaminated milk.
These four pathways are Illustrated schematically in Figure C-17
and are applied to the two river models.
The "reasonable" population dose commitment represents the 50-year
dose commitment to a population resulting from one year of intake via all
four pathways. Although the population is different for each pathway,
the population dose commitment (person-rem) can be added for different
pathways because of the assumed linear-nonthreshold dose relationship.
In almost all cases for which "reasonable" population radiation
doses are calculated, as well as for some of the cases for which "maximum"
population radiation doses are estimated, it was noted that size of
population may not necessarily be of great importance when using the
assumed linear-nonthreshold model. In general, when local meteorological
or hydrological parameters are unknown, it could be assumed that radio-
nuclides released to the environment would be distributed and diluted in
approximate proportion to the area Involved; i.e., the environmental
concentration would be inversely proportional to the area over which the
material was spread. On the other hand, lacking definite population dis-
tribution data, making an assumption of uniform population density would
imply that the affected population would increase in proportion to the
affected area. Thus, if the transfer coefficients from environmental
media to human beings, remain constant, the total, population
radiation dose would tend to remain constant, regardless of the area
or the population over which a radioactive contaminant is spread.
While there are exceptions to this generalization, it is nevertheless
154
-------
Surfoce Stream
POTABLE
AQUIFER
REPOSITORY
STRATUM AND
INTERBEDS
SCENARIO 1 - DIFFUSION
SCENARIO 2 - AQUEOUS FLOW
TRANSPORT
LOCAL
POPULATION
LIVESTOCK
Ingestion
\
VEGETATION
Ingestion
SURFACE WATER
LIVESTOCK
Ingestion
Seepage
SUBSURFACE WATER
Migration
RADIONUCLIDES
Source: D'Appolonia Consulting Engineers, Inc., for Arthur D, Little, Inc.
FIGURE C-17 SCHEMATIC ILLUSTRATION OF "REASONABLE"
POPULATION DOSE (SCENARIOS 1 & 2)
155
-------
useful in arriving at upper bound population radiation doses without
specific data on population distributions.
The population radiation dose from food crops grown on land
contaminated with irrigation water is inherently limited by a number
of factors. The total amount of land that can be irrigated and, hence,
the total amount of crops that can be grown are limited by the volume
of water available. If the volume of water is increased, the concen-
tration of radionuclides in the water, and subsequently in crops, decreases
proportionately, and the transfer of radionuclides to the total food chain
remains essentially constant (the transfer coefficient from soil to
vegetation being assumed to remain constant). Thus, the total
distribution of radionuclides to the population via first-cycle food
chains is inherently limited. The upper bound population dose commit-
ment is obtained by assuming total ingestion of the leafy vegetables
that can be grown regardless of the number of people consuming the foods.
G-5.2.4.4 Time Intervals for Dose Commitment Calculations
For the calculations of dose commitment, it is necessary to make
some assumptions regarding duration of the exposure. It could be
assumed, for instance, that future generations will be unable to
detect and evaluate radioactive contaminants in water. In that case,
exposures could go on indefinitely. On the other hand, it could be
assumed that, with continued expansion of human knowledge and technology,
the identification and elimination of environmental contaminants would
be essentially assured. Thus, for any group of exposed peaple, the
duration of the exposure could range from days to a lifetime.
The age at which an individual ingests a radioactive material
determines, in part, the dose commitment from that intake. Detailed
calculations of population radiation doses for assumed age distributions
and life expectancies are not warranted, as previously discussed. Age
21 is the youngest age of intake that could typically be considered
average for a large population and still permit a lifetime dose
commitment of 50 years. By using the 50-year dose commitment
for an intake at age 21 for the entire population, instead of an
156
-------
age-specific calculation for an assumed population, a realistic estimate
of the "reasonable" population radiation dose is attained.
All dose commitments are normalized on the basis-of a one-year intake
of water or food products. This duration of exposure is not unreasonable
and is also convenient for changing to other time periods if so desired.
Changes in the duration of exposure will produce proportionate changes in
the dose commitments. However, the conclusions in Section 5.4 would not
be affected even if the duration of exposure were changed by one or two
orders of magnitude,
C-5.3 RESULTS OF BADIATION DOSE CALCULATIONS
C-5.3.1 Introduction
All dose calculations presented in the following sections are de-
veloped from the geologic transport conditions described in Section C-4.
Since the distribution of radionuclides in the biosphere is assumed to
be independent of the time or rate of release into a potable aquifer
(except as time affects the repository Inventory), any change in geologic
assumptions would result in calculated doses that would be changed only
by a linear scaling factor.
For all of the postulated scenarios and pathways, only a few
radionuclides contribute essentially all of the potential radiation dose.
The results of the radiation dose calculations are present for those
times at which each of these "important" nuclides reaches a maximum con-
centration in the aquifer. However, the total radiation dose given for
the whole body and for each organ at each of these times includes not
only the radiation dose from the one "important" radionuclide that has
the maximum concentration at a particular time, but also radiation doses
from each of the other "important" radionuclides, whatever their concen-
trations at this time.
The tabulations shown in the following pages reveal that the "impor-
tant" radionuclides from the standpoint of radiation dose commitment are
limited to strontium-90» iodine-129, radlum-226, neptunium-237, and plu-
tonium-239 and -240, each of which has a maximum contribution at a
different point in time. Among the nuclides that were considered as
potentially Important, but were subsequently shown,to be of relatively
minor significance (under the circumstances assumed for the purposes of
157
-------
this study), are teehnetium-99, cesium-137, uranlura-236, and americlum-
243,
C-5.3.2 "Maximum" Individual Radiation- Dose
The "maximum" radiation doses from various radionuclides that
would be delivered to an individual who obtained all drinking water
(73Q£/year) from the aquifer directly above a repository are shown in
Tables C-23, C-24, and C-25. A sample calculation is Included in
Appendix C-IV,
C-5.3.2.1 Diffusion to a Potable Aquifer (Scenario 1)
For the Scenario 1 (diffusion) case, the "maximum" dose commit-
ments are very small (Table C-23). The dose commitments from fission
products were calculated at 300 years, a time when the concentrations
of these nuclides in the aquifer are assumed to be at a maximum.
s
Some of the actinide elements also contribute substantially to
the total dose commitment at 300 years. The "maximum" Individual whole
-9
body radiation dose at 300 years is 9 x 10 rem, consisting principally
of almost equal contributions from stronium-90 and plutonium-239 and
-240. The same three radionuclides contribute most of the bone dose,
_7
calculated to be 4 x 10 rem. The thyroid dose resulting from iodine-
0
129 at 300 years would be 4 x 10 rem.
After 30,000 years, the fission products would no longer contri-
bute significantly to the dose commitment of the whole body or
bone. The thyroid dose commitment is maximum at 1,000 years, with a
—5
value of 6 x 10 rem, and decreases with longer periods of time, be-
coming negligible with respect to the other dose commitments at 30,000
years. Scenario 1 indicates that the concentration of actinldes in the
aquifer would increase continuously up to a maximum at approximately
30,000 years. At that time, the "maximum" Individual whole body dose
commitment Is calculated to be 60 rem, contributed primarily by
Ra-226. The maximum bone dose commitment at that time is calculated to
be 100 rem, primarily from Ra-226 and Pu-239.
C-5.3.2.2 Flow to a Potable Aquifer (Scenario 2)
For Scenario 2, understandably large values for the "maximum" indiv-
idual dose commitments are obtained (Table C-24). At 300 years, the
calculated "maximum" individual dose commitment is 500 rem to the whole
158
-------
TABLE C-23
F1FTY-YEAK POSE COmiTHEMT CALCULATED TOB THI CASE
OF DIFFUSTOH IMfO * POTA8LB AQUIFER (5CEMAKIO 1)*
"Haxl.«im" Individual Dose'*'
Tine • 300 years
Whole Body (rero)
Bone (reft)
Thyroid (ren)
Fiasion Product*
BadiuB
Aetigld**
Sr-90 Te-99 1-129 Ca-137 ia-226 U-236 llp-237 fu-239 Fu-240 A»-24}
2x10
1x10
mo'10 5xio-u2xio-10
2x10
-10
4x10
,-10
0-8
2x10'
2x10'
,-10
,-10
1x10
4x10"
-9
2x10
9x10
3x10
1x10
3x10
5x10
,-10
,9
9x10
4x10"
4x10
Time - 1,000 years
Whole Body (rem) -
Bone (rem) -
Thyroid (ram)
Tine - 30,000 years
Whole Body (reia)
Bone (ren)
"Maxiaum" topulatiort Dole** t
Tlae - 300 years
Whole Body (person-r«a) 3x10'
Bone (organ-rem)
Thyroid (organ-ren)
TiJM " 1,000 years
Whole Body (person-rein)
Bone (organ-rets)
Thyroid (organ-rea)
1x10
1*10
4x10'
ZxlO
5x10'
7x10
6x10
-5
1x10 7x10
JatlO"8
1x10
3x10
SxlO
,-8
-8
-5
9x10
.-6
7x10
-3
1x10
1x10
6x10
6X101
3x10
2x10
,-8
-8
2x10
2x10'
3xlO"6 ixlO"6 8xlO"6
SxlO
"5
IxlO"1 2x10°
"7
3x10
1x10"
SxlO1
2x10
4xlO"6 IxlO"5
4x10
8x10
-4
2xlO"1 2xlO"2
8x10
4xlO
r7
n-5
1x10
,-3
IxlO"2 3xlO"2 4xlO"2
4xlO
-1
2x10 6x10
9x10
1x10
3x10
7ilO'
6.10
-5
6x10*
1x10
5x10*
1x10
4x10
8x10'
7x10
-3
" 30,000 years
Whole Body (person-reta)
Bone (organ-rem)
"Reasonable" Population Doae***
River Model "A"
line » 1,000 years
Thyroid (organ-tea)
Tlae " 30,000 years
Whole Body (per»on-rem)
Time • 100,000 years
Bone (organ-rea)
8i»er Model "B"
Time - 1,000 yeara
Thyroid (organ-re*)
fine » 30,000 years
Klrele Body (person-ren)
Time - 100,000 year*
Bone (organ-res)
8x10
-3
SxlO
8xlOJ
SxlO3
9x10
,-2
3x10
,-2
9xlO~ 9x10
IxlO1 2xl02
3x10"
6x10
-1
2x10
7xl03
3x10 3x10
1x10
5x10
8x10
2*10*
8x10
,-3
9x10
SxlO
6x10
-2
3x10
,-2
All results are 50-year individual or population dose comltmenti rt»ulting fro« one-year ingcation intaka.
**
Population coneist* of 250 people aervcd from a tingle water iource.
•*«
Population consists of 10,000 people icrved froB • single Hater source,
+Due to rounding of miclide values to (me significant figure after totaling, totals shown may not agree
with tmclide valuea shows. •
individual and upper Itait populatioc doses represent extreae conditions,
Note: Dashes are shown were nuclide dose valuea have • aaall or negligible contribution to the total
dose cosaitaent.
159
-------
TABLE C-24
DOSE COHHITKEHT CM.CTOATED FOR AqUEOUS TRANSPORT BY
FLOW IHTO A fOTABLE AQUIFER. {SCEHMtlO 2)'
fission Products
Radium and Actlnidea
Sr-90 Tc-99 1-129 Cs-137 Ra-226 U-234 Hp-237 Pu-240 Pn-242 Aa-243
"Maximum" Individual Dose*
Time • 300 years
Whole Body (res) 4jsl02
*
Bone (re») 2x10
Thyroid (rem) -
Tine - 6,000 years
Whole Body (rem) -
Bone (rem)
Thyroid (re») -
"Maximum" Population Pose** t
fine « 300 years
4
Whole Body (parson-rea) 6x10
Bone (organ-ren) 3x10
Thyroid (organ-rem)
Time - 6,000 years
• Whole Body (person-rem)
Bone (orgaa-rem) -
Thyroid (organ-rem)* —
"Reasonable" Population Dose***
River Model "A"
fine " 500 years
Thyroid {organ-rem)
line " 6,000 years
Whole Body (person-Tea) -
fine " 100,000 years
Bone (organ-ren)
River Model "B"
Time *• 500 years
Thyroid {organ-res)
Time - 6,000 years
Whole Body (persot>-r*»3 -
fine » 100,000 years
Bone (organ-ren)
3x10
8x10'
3x10
7X101
1x10
6x10
1x10
4x10
8xl03
2x10
7x10
2x10'
1x10
2x10
2x10'
3x10
3xl02
2x10
4x10
3x10
SxlO1
1x10° 7xlO"X
2X10 2X10
4x10
1x10*
IxlQ
1x10"
4x10 2x10 2x10
1x10
2x10
9x10
4x102
4x10
3xl03
1x10
5x10*
5x10
2xl05
4x10
2X101
-1
9x10
IxlO1
4x10"
-1
5x10
2xl03
5x10
7xl03
TOTAL
5x10
2x10*
1x10
3x10
2xl03
6x10°
6*10
3x10*
2x10
5x10
2xl05
7x10
2x10
1x10
1x10
1x10
2x10
4x10
-1
4x10
-1
All results are 50-year Individual or population dose cosnrftiBents resulting Iron one-year Ingestloa intake.
**
Population consists of 250 people »erved from single water source.
Population consists of 10,000 people served fron single water source,
*Due to rounding of nucll.de values to one significant figure after totaling, totals shovn aay not
agree with nuclide values shown.
^Maximum individual and upper limit population doses represent extreme conditions.
Mote: Dashes are shown where miclide dose values have a imall or negligible contribution to the
total dose coaraitctent*
160
-------
TABLE C-25
I. "Maximum" Individual Dose Commitment
A. Whole Body Dose (rem)
B. Bone Dose (rem)
C. Thyroid Dose (rem)
II. "Maximum" Population** Dose Commitment
A. Whole Body Dose (person-rem)
B. Bone Dose (organ-rem)
C. Thyroid Dose (organ-rem)
SUMMARY OF TOTAL
MAXIMUM" INDIVIDUAL AND POPULATION
ITS FOR DIRECT INGESTION OF
CONTAMINATED
Scenario 1
Diffusion to Aquifer
300 Years
:ment
9 x 10
4 x 10~
4 x 10
nitment
» 1 x 10"
5 x 10
1 x 10
30,000 Years
6 x IO1
1 x IO2
(6 x 10~5)*
8 x IO3
2 x IO4
(7 x 10"3)*
AQUIFER
Aqueous
Scenario 2
Transport to
300 Years
5 x
2 x
1 x
6 x
3 x
2 x
io2
io4
io2
IO4
io6
io4
6,000
3 x
2 x
6 x
5 x
2 x
7 x
Aquifer
Years
io2
io3
10°
io4
io5
io2
* These are values for 1,000 years, which decrease with increasing time.
**Population consists of 250 people served from single source.
+ "Maximum" individual and population doses represent extreme conditions.
-------
body. Strontium-90 would be the nucllde responsible for this high
whole-body radiation dose and also for the calculated 20,000 rem to the
bone. The contributions from the actinlde elements at 300 years are
appreciable but still small compared with the dose commitments that
would be produced by the fission products.
After 6,000 years of direct flow through the repository, the
actlnide elements would reach maximum concentrations in the aquifer above
the repository. At this time, the "maximum" individual dose commitment
4
would be 300 rem to the whole body and 2 x 10 rem to bone. Under those
conditions, the major contributors to the calculated doses are Ra-226
for the whole body and Ra-226 and Pu-240 for bone. The dose commitment
to the thyroid at 300 years is 100 rem, which then decreases continuously
with increasing time.
C-5.3.3 "Maximum" Population Radiation Dose
C-5.3.3.1 Direct Access to a Potable Aquifer
The underground water flow rates developed in Section C-4 are
such that the maximum population that could be supplied by the contamin-
ated water in the aquifer above the repository would be about 250 people.*
The "maximum" population dose commitments were calculated, therefore, as
the product of the "maximum" individual dose commitments multiplied by
250, except that the value used for the individual dose is reduced by a
factor of two to allow for an assumed average individual water intake of
only 370 £/year. These are the values shown in Tables C-23, C-24 and C-25.
A sample calculation is shown in Appendix C-IV. If the aquifer volume
were larger, a larger population might be supplied, but the radionuclide
concentrations would be decreased proportionately. Consequently, the
"maximum" population dose commitment is bounded by the assumptions of
initial transport of nuclides from the repository into the potable aquifer.
C-5.3.3.2 Diffusion to a Potable Aquifer (Scenario 1)
For the diffusion case, (Table C-23) calculated population dose
commitments are extremely small at 300 years, i.e., 1 x 10 person-rem
*The assumption of 250 people supported by this aquifer is based on a
water flow rate of 1.44 x 10 £/day (Section C-4.4.3.2) and an individ-
ual water intake rate of 370 £/year.
162
-------
to the whole body; 5 x 10 organ-rera to bone; and 1 x 10 organ-rem
to the thyroid. After 30,000 years, when the actinldes have reached
maximum concentrations, the whole-body population dose commitment would
be 8 x 10 person-rem, mainly due to Ra-226, and the population dose com-
4
mitment to bone would be 2 x 10 organ-rem, mainly due to Ra-226, Pu-239,
and -240. The dose commitment to the thyroid at 300 years would be
_5 _3 . "
1 x 10 organ-rem, rising to 7 x 10 organ-rem at 1000 years and de-
creasing thereafter to become negligible with respect to the other dose
commitments.
C-5.3.3.3 Flow to a Potable Aquifer (Scenario 2)
For the flow case, the "maximum" population dose commitments are on
the order of tens of thousands of person-rem or organ-rem per year of
intake. The largest calculated value is 3 x 10 organ-rem to bone at
300 years, entirely due to strontium-90. The bone dose commitment at
6000 years was calculated to be 2 x 10 organ-rem, primarily from plu-
tonium-240, amerlcium-243, and some radlum-226.
It is not the purpose of this task to go beyond dose calculations
and project health effects. It should be emphasized, however, that any
translation of "maximum" individual or population dose commitments to
actual health effects based on the BEIR report's prediction of effects
of low levels of radiation on large populations'-^"' are inappropriate.
The maximum doses calculated for Scenario 2 are such that, if those con-
ditions occurred, the health effects would be acute rather than long
term. Furthermore, the size of the population would be too small to
justify the use of large-population statistics. Instead of multiplying
the total number of person-rem or organ-rem by the risk factors indicated
in the BEIR report, it would be more realistic to conclude that, at
these concentrations, Intake by 250 people over a period of a few years
would result in 250 fatalities.
Under the "worst case" conditions assumed in Scenario 2, "normal"
adsorption was assumed in the aquifer, but none in the repository and
the communicating channels to the aquifer. If "normal" adsorption were
to be assumed in either or both of the latter two regions, then the
163
-------
dose commitments to these 250 people would be reduced substantially,
and any Impact on them would probably be a long term, rather than acute,
one.
C-5.3.4 "Reasonable"PopulationBpse
On the basis of the assumptions stated earlier, i.e., discharge of
the contaminated aquifer into one of two river models at a distance of
one mile down gradient from the repository, and for the postulated con-
dition in which the only movement of radionuclides is by aqueous diffu-
sion (Scenario 1), the calculated population dose commitments are in the
-3
range of 8 x 10 organ-rem to the thyroid after 1000 years to 3 per-
son-rem to the total body after 30,000 years. These values are shown
in the lower half of Table C-23 and In Table C-26. Sample calculations
for the four pathways considered are included in Appendix C-V.
For the postulated flow case (Scenario 2), the calculated popula-
tion dose commitments are approximately three orders of magnitude higher
than for Scenario 1. (Note, however, that the differences between popu-
lation dose commitments calculated for Scenarios 1 and 2 are not as
great for the "reasonable" population dose commitments as they are for
the "maximum" population dose commitments.) The population dose com-
4
mitments to the thyroid are estimated to be 2 x 10 organ-rem after 500
years for River Model "A," and 1000 organ-rem after 500 years for River
Model "B," as shown in the lower half of Table C-24 and in Table C-26.
Although not shown in the tables, the population dose commitments from
iodine-129 would remain constant over the times shown for other nuclides:
6000, 30,000, or 100,000 years. The whole-body population dose commit-
ments for either of the river models reach a maximum at approximately
6000 years and are attributable to neptunium-237. The maximum popula-
tion bone dose commitments are calculated to result from radium-226,
which does not reach Its maximum by ingrowth from uranium and thorium
until at least 100,000 years.
The total population dose commitments from all pathways are con-
sistently lower for River Model "B" than for River Model "A" (Tables
C-23 and C-24), The contributions to population dose by individual
164
-------
TABLE C-26
River Model "A"
Direct Ingestion
Ingestion of Crops
Ingestion of Beef
Ingestion of Milk
Total: All Pathways
River Model "B"
Direct Ingestion
Ingestion of Crops
Ingestion of Beef
Ingestion of Milk
Tqtal: All Pathways
SUMMARY OF
REASONABLE POPULATION DOSE COMMITMENTS
BY SIGNIFICANT RADIONUCLIDES AND PATHWAYS
Scenario 1
129
x"l*
(1,000 y)
3 x 10~
8 x 10~3
8 x 10~
1 x 10"
8 x 10
5 x 10~
1 x 10"
2 x 10"8
7 x 10~
237M .
Np+
(30,000 y)
4 x
3 x
3 x
6 x
3 x
6 x
4 x
1 x
6 x
io-1
10°
io-4
10"5
10°
io-1
io"3
10"6
io-6
Ra+
(100,000 y)
2 x
6 x
2 x
4 x
9 x
3 x
1 x
9 x
4 x
io-2
io-2
io-3
io-3
io-2
io-2
io-4
io-6
io-4
129
-""^I*
(500 y)
6 x
2 x
2 x
3 x
2 x
1 x
3 x
4 x
2 x
IO2
io4
io1
io2
io4
IO3
io1
io"2
io1
Scenario 2
237M .
Np+
(6,000 y)
1 x IO1
9 x IO1
1 x 10"
2 x 10~3
1 x IO2
2 x IO1
2 x 10
5 x 10"5
2 x 10~
226i> ±
Ra+
(100,000 y)
3 x
9 x
3 x
5 x
1 x
4 x
2 x
1 x
6 x
io-1
io-1
io"2
io-2
10°
lO'1
io-3
io-4
io-3
5 x 10
-4
6 x 10
-1
3 x 10
-2
1 x 10-
2 x 10
4 x 10
-1
*Thyroid dose commitment (organ-rem).
-Whole body dose commitment (person-rem).
TBone dose commitment (organ-rem).
-------
pathways, however, are in some cases larger for River Model "B" than
for River Model "A" (Table C-26). For River Model "A," the population
dose commitments are dominated by the ingestion of crops; for River
Model "B," direct ingestion of water contributes essentially all of the
population dose commitment. All of these "reasonable" calculated dose
commitments are actually unrealistically high because they assume that
drinking water is not pretreated and that vegetables are neither washed
nor cooked. The differences between the relative dose contributions
from direct ingestion of water and from ingestion of crops are also
greatly influenced by the assumptions that are made concerning the use
of the river water. In both cases, however, the more indirect pathways
(meat and milk consumption) do not contribute significantly to the cal-
culated dose commitment.
C-5.4 EVALUATION OF DOSE-TO-MAN CONSIDERATIONS
Besides the specific calculations for the various radiation dose
categories presented in the preceding sections, other evaluations are
made in the following sections to bring the overall scope of the radio-
active waste problem into focus. For example, 1-129, because of its very
long half-life, should be considered beyond its Initial entry into the
biosphere. An ultimate limit to the population dose from 1-129 would be
achieved only if it were all diluted by the stable iodine present in
some fraction of the oceans' volume.
Naturally-occurring radium provides some clues as to the antici-
pated behavior of actinides in the biosphere. Little radium reaches the
biosphere and average radiation doses to humans from radium are extremely
small in spite of the large amounts in soils and rocks.
Comparison of the impact of the "most likely" leakage from an HLW
repository with that calculated for sources of natural radioactivity
indicates a very small impact from the waste in comparison with that
from natural sources.
C-5.4.1 Worldwide Impact of Iodine-129
Of the fission products contained in HLW, the one that seems most
likely to reach the biosphere as a result of leaching by groundwater is
1-129. This conclusion is based on the long half-life of 1-129 (17 million
years) and the relatively high mobility of iodine (compared with the few
other long-lived fission products) in groundwater. The 50-year dose
166
-------
commitment to the first generation exposed has been estimated by assuming
typical transfer coefficients to a population located in the vicinity
of the release point.
The question of ultimate population dose commitment is also of
interesti This concept has been called the "environmental dose commit-
ment," although the dose referred to Is to human populations and not
the "environment." The ultimate population dose commitment can be de-
fined as the sum of the doses to all individuals at all future times that
will result from a specified release of radioactivity to the environment.
Although extremely easy to define conceptually, this dose commitment Is
impossible to calculate meaningfully. The only quantitative data
available for the calculation are the physical decay rates for each
radlonuclide and the abundances and relative distributions of stable
elements In the biosphere.
The uncertainties involved in making estimates of future population
distributions, agricultural practices, etc., have been discussed in
Section C-5.1.3, Even if one assumes that human populations and the
general biosphere will be the same at all future times as they are now,
calculations of ultimate dose commitments are limited by a lack of data
on environmental dispersion rates. The population dose rate at any time
depends only on the total amount of radioactivity In human beings at
that time and not on the number of people actually exposed or in the
total population. (This assumes that age-dependent dose factors are
averaged over sufficiently large populations and are always in the same
proportions.) Calculations of ultimate dose commitments, therefore,
involve the estimation of the fraction of released radioactivity that
will be available to, or actually in, human beings at any future time.
The ultimate limit on population doses from 1-129 released to
the biosphere would most likely be due to the ultimate dilution by stable
iodine in the earth's crust and in the oceans. The mobility of iodine
in water is Indicated by the fact that the iodine is more easily leached
and transported underground than other nuclides. The same relative
mobility could be assumed in surface waters, indicating that most of the
iodine would eventually flow into the oceans. The average concentration
of stable iodine in the oceans Is 0.06 ppm (g/m ). The total area
of the oceans is 361 x 10 km and, if one assumes a mixing depth
of 0.1 km, the total volume available for mixing would be 36 x 10 km .
At a mean concentration of 0,06 g/m , this top 0.1-km layer of the
167
-------
oceans' surfaces would contain approximately 2 x 10 g of stable iodine.
8
This is approximately 10 times more than the total amount of 1-129 pro-
jected to be contained in geologic repositories (Table C-9) as a result
of nuclear power generation. Instead of the intrinsic specific
activity of 1.8 x 10 pCi/g, the effective specific activity of 1-129
would be reduced by roughly eight orders of magnitude to 2 pCi/g. With
a normal annual iodine intake of about 0.07 g (200 yg/day), the total
annual intake would be approximately 0.14 pCi.
The average individual thyroid dose from 1-129 (if it returned to
human beings after uniform dilution by the stable iodine in the ocean)
would be 1 x 10 rem/year or about 5 x 10 rem in 50 years. For a
world population of 4 billion, the population dose commitment would be
3 5
about 4 x 10 organ-rem/year or 2 x 10 organ-rein in 50 years. The 1-129
could be assumed to be diluted into a smaller volume of seawater (e.g., the
top 0.1 km over only 1% of the earth's ocean surfaces). Under such an
assumption, the average thyroid dose commitment to an individual could
be larger, but the population exposed would be proportionately smaller,
so the total population dose commitment calculated under such assumptions
would not be greatly different from the previous calculation.
As pointed out in an Environmental Protection Agency (EPA) report on
the concept of environmental dose commitment, no data are available
from which to base projections of redistribution of elements in the
biosphere over extended periods of time. ' All calculations contained
in this report are based on assumptions considered to be conservative
with respect to the biosphere as it exists today, and the estimated
dose commitments are believed to be upper limits for any reasonable
assumptions of redistribution. All calculations are made for the most
direct pathways to achieve maximum exposures and hence dose commitments
for any time after release of material from the repository.
C-5.4.2 Comparison with Natural Background Radiation
If radioactive wastes from a geologic repository are
contacted and leached by underground water, the subsequent transfer to
the biosphere and to human beings may be assumed to be similar to that of
naturally-occurring radionuclides. Radium is ubiquitous in the earth's
crust and occurs with a wide range of concentrations in many surface
streams- Uranium ore of commercial grade typically contains
radium at concentrations of hundreds of pCi/g, often in contact with
groundwater. Sedimentary rock and many surface soils contain radium
concentrations in the range of 0.1 to at least 1 pCi/g.
-------
Radium Is less mobile in the biosphere than many lighter elements,
including most fission products, but is considerably more mobile than
most of the actinide elements that comprise the major portion of the
very long-lived materials in radioactive wastes. Environmental transfer
data for the radionuclides of greatest significance are listed in
fable C-20. The fact that the values for the actitiides (represented by
neptunium-237) are generally smaller than those for radium indicates
the lesser mobility of these nuclldes in environmental media. Further-
more, the fractional uptake of the actinides from the human gastro-
intestinal tract to blood is estimated by the International Commission
on Radiological Protection to be 3 x 10 , compared with 0.3 for
radium. In this respect, the behavior of radium in the biosphere
constitutes a very conservative basis for comparison of the projected radiation
doses that would result from environmental contamination by actinides.
The extent to which radium in the environment is transferred to
people is indicated by data contained in a recent report of the National
Council on Radiation Protection and Measurements. The mean dietary
intake of radium from foods and drinking water is estimated to be 1.4
pCi/day. The mean skeletal content, based on analysis of bone ash, is
—3
approximately 5 x 10 pCi/g of bone. The same report provides an
absorbed dose conversion factor of 200 mrad yr. /pCl g in bone.
If a conservative value for the quality factor is assumed to be 10 for
radium and its decay products, the dose-equivalent conversion factor
becomes 2 rem yr. /pCi g for bone. Thus the mean skeletal dose
equivalent to individuals in the United States is estimated to be:
— 1 —1 —1 —2
5 x 10 pCi/g x 2 rem yr. /pCl g , or approximately 10 rem/ year.
For approximately 200 million persons, the population bone dose equiva-
f Q
lent is approximately 2 x 10 organ-rem/yr . or 10 organ-rem for 50
years.
For the postulated transport of waste materials from a repository
by flow (Scenario 2) , the maximum population 50-year dose commitment to
bone was calculated to be 3 x 10 organ-rem for an intake of one year
(Table C-24). For a 50-year Intake, the bone dose commitment amounts to
Q
1.5 x 10 organ-rem, a value comparable to that from radium in the
natural environment. This calculation, of course, represents a "worst
case" in that it involves the direct consumption of water that has
flowed directly through the waste repository into the aquifer and has
subsequently undergone a minimum loss of radionuclides by dilution or
adsorption.
169
-------
The total radiation dose to bone from natural sources of radiation
Includes external sources (cosmic and terrestrial) in addition to the
internal deposition of radium and other radionuclides. The total radia-
tion bone dose from natural sources is comparable to the whole-body
radiation dose and is on the order of 0.1 rem/yr. to average individuals
in the United States population. The total population dose commitment
to bone for 200 million people is, then, approximately 2 x 10 organ-
o
rem/yr. and 10 organ-rem/50 years.
As Indicated in previous sections of this report, the quantities
of actinldes that could be expected to reach the biosphere under any
reasonable scenario for leaching by groundwater", would be orders of
magnitude smaller than the activity of radium in the biosphere, even in
the immediate vicinity of a geologic waste ^repository. Although it is
possible to hypothesize extreme scenarios for which actinides (as well as
fission products) could reach the biosphere in relatively high concentra-
tions, the number of people who might be exposed to such concentrations would
be small. The largest population dose commitment to the whole body, cal-
4
culated for Scenario 2 at 300 years, is 6 x 10 person-rem for a 1-year
intake. This number is equivalent to 0.3% of the annual population
dose commitment from natural radiation sources. For all other expected
times or modes of release of radionuclides from a repository, (i.e.-,
transport by flow at other times or diffusion at any time) the upper-limit
population dose commitments are smaller. Hence, the 50-year dose commit-
ments calculated for "reasonable scenarios," i.e., by contaminated surface
water and irrigated food pathways, are negligible in comparison with radia-
tion doses from natural sources.
C-5.4.3 Comparison With Radlum-226 Currently Present in the Colorado River
A useful perspective is obtained by comparing the Ra-226 population
dose commitment calculated for Ra-226 concentrations presently occurring
in the Colorado River with that obtained for Ra-226 using Scenario 2
(aqueous flow) and River Model "A". To make this comparison, actual
Ra-226 concentrations in the Colorado River were used as input parameters
to River Model "A". Dose commitments for Ra-226 for Scenario 2 using
River Model "A" were calculated as presented in Section 5.2.3.2.
170
-------
Table C-27 Indicates the results of calculations that compare the
Colorado River with Scenario 2, River Model "A". The total dose commit-
4
tnent from all pathways from the Colorado River is 5 x 10 organ-rein,
while the dose commitment from Scenario 2, River Model "A" is 1 organ-reni.
According to this calculation, the Colorado River now contributes 10,000
times more radiation dose to the present population than would an HL¥
repository at the time of maximum concentration of Ra-226 in the biosphere
to a population existing in the same location 100,000 years following
waste emplacement.
The dose commitments calculated for the Colorado River are conser-
vative because they do not account for radium removal during water treat-
ment to remove hardness, which typically occurs before municipal utiliza-
tion of this water. Nevertheless, the projected radiation doses from the
proposed HLW release are still quite small compared with those arising
from the Colorado River now.
171
-------
TABLE C-27
COMPARISON OF "REASONABLE" POPULATION
50-YEAR DOSE COMMITMENTS FORRa^-226 FROM
SCENARIO 2 ANDRa-226 IN THE COLORADO RIVER
Exposure Pathway
Direct Ingestion
Ingestion of Crops
Ingestion of Beef
Ingestion of Milk
Ra-^226 Dose Commitment*
Colorado River
(organ rem)
2 x 104
3 x 104
2 x 102
5 x 102
Scenario 2
River Model "A"+
(organ rem)
3 x 10
-1
9 x 10
-1
3 x 10
-2
5 x 10
-2
Total: All Pathways
5 x 10
1 x 10
*Radium dose commitments for the Colorado River are based on actual radium concentrations in the
Colorado River at the present time used as input to River Model "A".
+Results for River Model "A" are the same as presented in Table C-26 for Scenario 2; the time period
is 100,000 years following waste emplacement.
-------
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177
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Page Intentionally Blank
-------
APPENDIX C-I
GLOSSARY
Actinides; A series of elements in the periodic table, beginning with
actinium (element No. 89) and continuing through lawrencium (element
No. 103). The series includes uranium (element No. 92) and all of
the man-made transuranium elements. All are radioactive.
Aquifer; Rock or soil strata containing water that reaches the ground
surface in any natural or man-made manner.
Argillaceous: Referring specifically to argillite, a rock formed from
clays, mud, and silt, but more compacted and solidified than shale
or mudstone. Although the distinction between shale and argillite
is imprecise, it is used here to imply that the rock has sufficient
strength and rigidity to be mined, and that it has not been meta-
morphosed into slate, which has a platy cleavage.
Background radiation; The radiation in man's natural environment,
including cosmic rays and radiation from the naturally radioactive
elements, both outside and inside the bodies of men and animals.
It is also called natural radiation.
Basalt: A hard, dense, dark fine-grained igneous rock.
Bedded salt; Salt occurring in strata, commonly Interspersed with
layers of shale or limestone.
Biosphere; Zone at and adjacent to the earth's surface where all
life exists.
Brecciapipe; A geologic formation composed of broken rock, breccia,
filling a vertical cylindrical hole, or pipe. Such structures have
also been identified as Collapse Structures, Pseudo-Karst Formations
and Dry Sinkholes. Their development is assumed to require the dis-
solution of underlying rock by circulating groundwater, and collapse
of the rock above into the cavity thus formed.
Curie: The basic unit to describe the intensity of radioactivity in a
material. The curie is equal to 37 billion disintegrations per second,
which is approximately the rate of decay of 1 gram of radium. A curie
is also a quantity of any nuclide having 1 curie of radioactivity.
The prefixes milli-, micro-, nano-, and pico- are frequently used and
indicate quantities of 10~3 curie, 10~^ curie, 10~9 curie, and 10"12
curie, respectively.
Darcy's Law; The relationship between the hydraulic gradient and the
groundwater velocity.
C-I-1
-------
DiapiriBin; The forceful intrusion of one geologic material into another,
overlying material.
Distribution coefficients (KH) and (Ka); Sorptions or mechanisms causing
ions to move at lower velocities than the groundwater transporting them.
Kd = distribution coefficient for porous media; Ka * distribution coef-
ficient for faulted or fractured media.
Fatfltr A break in the continuity of a rock formation, caused by a shifting
or dislodging of the earth's crust, in which adjacent surfaces are dif-
ferentially displaced parallel to the plane of fracture.
Geosphere; The solid earth, including rocks, soil, and water in the
ground, but excluding oceans (hydrosphere) and air (atmosphere),
Half-life(radioactive); The time required for one-half of an Initially
radioactive material to undergo nuclear transformation; the half-life
is a measure of the persistence of radioactivity and is unique to each
radionuclide.
High-level waste;- The highly radioactive waste resulting from the
reprocessing of spent fuel to separate uranium and plutonium from
the fission products. The term includes the high-level liquid
wastes (HLLW) produced directly in reprocessing, and the solid
high-level wastes (HLW) which can be made therefrom,
Hydraulic gradient: The pressure drop per unit length of distance
traveled.
Igneous rocks; A group of rocks formed by the solidification of molten
rock. The group consists o'f intrusive (plutonic) rocks which have
solidified below the earth's surface and extrusive (volcanic) rocks
which solidified after eruption.
Interstitial velocity: Groundwater flow through the pores and openings
and around individual grains of geologic media.
Ion; An atom or molecule that has lost or gained one or more electrons.
By this ionizatlon, it becomes electrically charged. Examples: an
alpha particle, which is a helium atom minus two electrons; a proton,
which is a hydrogen atom minus its electron.
Isotope: One of two or more atoms with the same atomic number (the sane
chemical element) but with different atomic weights. Isotopes have
very nearly the same chemical properties, but different nuclear
(radioactive-decay) properties. Thus, for the element carbon, for
example, the isotope of atomic weight 12 (C-12) and the Isotope of
atomic weight 14 (C-14) behave identically in chemical reactions;
but whereas C-12 is not radioactive, C-14 is radioactive, decaying
with a 5730-year half-life to stable nitrogen (N-14) with release
of a beta particle.
C-I-2
-------
Laminar flow; Non-turbulent flow of fluid in layers near a boundary.
Limestone; A carbonate rock composed primarily of calcite, usually
with varying amounts of dolomite.
Lithology; The study, description and classification of rock.
Nuclide: A general term applicable to all atomic forms of the elements,
The terra is often used erroneously as a synonym for "isotope," which
properly has a more limited definition. Whereas isotopes are the
various forms of a single element (hence are a family of nuclides)
and all have the same atomic number and number of protons, nuclides
comprise all the isotopic forms of all the elements.
Oxidation-reduction potential; Energy gained in the transfer of one
mole of electrons from an oxidant to hydrogen, expressed in volts.
Permeability; The ability of a fluid to move through a medium under a
hydraulic gradient,
Rad ipactivitv_; The spontaneous decay or disintegration of an unstable
atomic nucleus, usually accompanied by the emission of ionizing
radiation.
Radioisotope; A radioactive isotope. An unstable isotope of an element
that decays or disintegrates spontaneously, emitting radiation. More
than 1300 natural and artificial radioisotopes have been identified.
Radionuclide: A radioactive nuclide. Thus, carbon-14 (C-14) is a radio-
nucllde because it decays radioactively to nitrogen-14 (N-14).
Radwaste; A contraction of the term "radioactive waste."
Redox potential; Oxidation-reduction potential (q.v.).
Rein; A dose unit which takes into account the relative biological
effectiveness (RBE) of the radiation. The rem ("jrpentgen equiva-
lent man") is defined as the dose of a particular type of radiation
required to produce the same biological effect as one roentgen of
(0.25 Mev) gamma radiation. A 1-rad dose of alpha particles is
approximately equivalent in its biological effects to 10 rads of
gamma radiation, and hence may be expressed as 10 rems. A milli-
rem (mrem) is one thousandth of a rem.
Retardation _f_actor_(R<^)_; Ratio of the water velocity to the nuclide
migration velocity.
Salt dome; A geologic salt formation in which a plug of salt has been
thrust up through rock at some depth, leading to a subterranean
"cylinder" of salt which may be a mile or more in diameter and
several miles deep.
C-I-3
-------
Shale: Llthifled clay or mud, usually containing some sand and silt.
Seismicity; Caused by an earthquake or earth vibration.
-Stratigraphy: The study of rock strata, especially of their distribution,
deposition and age.
Styolite; A small columnar rock development in limestone and other
calcareous rocks.
Tectonic; Pertaining to the formation of the earth's crust; the forces
involved in or producing such deformation and the resulting forms.
Tuff; A rock composed of compacted volcanic ash varying in size from
fine sand to coarse gravel.
C-I-4
-------
APPENDIX C-I.I
RADIOACTIVE WASTE MIGRATION MODEL
The radioactive waste migration model used in this study is a
single-phase, numerical model for transport of fluid, en&rgy, salinity,
(29)
and radioactive nuclides in porous media . Basic laws of physics
are used to describe mathematically the tiuclide transport in geologic
formations. The model includes radioactive decay and generation (by
decay of other components) ^and adsorption of nuclides.
The complete "flow system" is described by three coupled partial
differential equations for conservation of total fluid mass, energy,
and an inert component (e.g. brine). The equations are coupled through
fluid properties—density and viscosity. Calculation of radioactive
waste transport, in addition to the above three equations, requires
separate solutions for each of the radioactive constituents. Conservation
of mass of the species dissolved in the fluid phase and adsorbed on the
rock as well as radioactive decay and generation from other nuclides
are included.
Basic assumptions and model features can be summarized as
follows:
(1) Three-dimensional, transient, laminar flow.
(2) Three-dimensional Cartesian (x, y, z) or two-dimensional
(r, z) coordinate description,
(3) Fluid density can be a function of pressure, temperature
and concentration of the inert component. Fluid viscosity
can be a function of temperature and concentration.
(4) Waste dissolved in fluid is completely miseible with
the in—place fluids.
(5) Aquifer properties are varied with position.
(6) Hydrodynamic dispersion is described as a function of
fluid velocity.
(7) The energy equation can be described as "enthalpy in -
enthalpy out = change in internal energy of the
system". This is rigorous, except for kinetic
and potential energy, which have been neglected.
(8) Boundary conditions allow natural water movement in
the aquifer, heat losses to the adjacent formations,
C-II-1
-------
and the. location of discharge, withdrawal, and observation
points within the system. Pressures, temperatures, and
concentrations at these points are related to corres-
ponding grid block centers through a wellbore model.
(9) Fluid properties are independent of radioactive nuclide
concentrations (trace quantities).
(10) Radioactive nuclide source rates can be described through
a leach model integrated in the waste migration model.
The leach rate can be described by a logarithmic or a linear
equation. The radionuclide concentration in fluid can
be limited by solubilities.
(11) Equilibrium adsorption and desorption exist. The
equilibrium adsorption constant is related to adsorption
distribution constant, rock density, and porosity. The
adsorption distribution constant is a function of rock
or formation type only.
Suppose x, y, z to be a Cartesian coordinate system and let Z(x,y,z'
be the height of a point above a horizontal reference plane. Then the
basic equation describing single-phase flow in a porous medium results
from a combination of the .continuity equation
y.pu + q' . _ JL (ep>* (I_l)
O t'
Net
Convection Source Accumulation.
and Darcy's law in three dimensions.
u - - k (Vp - pgVZ) (1-2)
The result is the basic flow equation,
V.£JE. (vp - pg?Z) - q' = |^ (eo) (1-3)
The energy balance (defined as enthalpy in <* enthalpy out - change in
internal energy) is described by the energy equation,
Detailed definitions'of all terms are given in the Nomenclature.
C-II-2
-------
- pg?Z»
Net Energy
Convection
Conduction
Heat Loss
to Sur-
rounding Strata
q'H
Enthalpy In
with Fluid
Source q1
(1-4)
Energy In Accumulation
Without Fluid
Input
A material balance for the solute results in the solute or concentration
equation
- pg?Z)
Net Solute
Convection
.p| -VC
Diffusion
q'C
Source
ft <>ec>
Accumulation
(1-5)
A similar material for N radioactive components results in N component
equations
- pg?Z)]
qC,
I K..R.C.pe
Net Component 1
Convection
N
Diffusion of Source of Generation of
Component i Component 1 Component 1 by
decay of other
isotopes
(1-6)
Net Decay of
Component i to
Other Isotopes
Accumulation
where
KkiPECi
and assumes the approximation
The equilibrium adsorption constant E. * 1
C-II-3
-------
The system of equations, along with the fluid property dependence
on pressure, temperature, and concentration, describe the reservoir flow
due to discharge of wastes into an aquifer. This is a nonlinear system
of partial differential equations, which must be solved numerically using
high-speed digital computers. The equations are coupled with each other
through fluid property dependence. Since the radioactive components are
assumed to be present in trace quantities only, and the fluid properties
are independent of these concentrations, the equations predicting the con-
centrations of radioactive components are uncoupled from the other equations.
The solution procedure is simple. First, the three differential
equations for the flow system are solved over a numerical time step.
Since the radionuclide transport equations are uncoupled from the first
three equations, the calculated flow solution (pressures and velocities)
is then used to solve the radioactive constituent equations. The
radioactive nuclide transport equations are actually coupled through
decay and generation terms. These are uncoupled by solving the
equations in a systematic order, starting from parent constitutent with
no generation terms.
The model has been tested by comparing results with an analytical
solution for a one-dimensional, homogeneous, constant -velocity, and
three-component system . The solution for an individual decaying
nuclide would have the form:
ux
^/% - e"E erfc^% ] (1-7)
For a step change, input superposition can be used with the above equa-
tion. A comparison of the numerical finite difference model with the
analytical ORIGEN computer model for uranium-238 is shown in Figure C-II-1.
The solid lines are ORIGEN results and the data points indicate the
numerical model results. Table C-II-1 lists the nuclides, their half-
lives and initial amounts present. Considering the range in component
amounts and the time frame, the match between the two model results is
excellent. This comparison indicates that the decay chains, including
components with large contrast in half-lives, can be accurately described
by the numerical model. Although this comparison deals only with decay,
the same problem was solved for each nuclide migrative with the same degree
of absorptivity. The comparison showed that the numerical model was capable
of the same accuracy when dealing with migrations as with decay.
-------
-ORIGEN
• Intern Model
TIME,YEARS
FIGURE C- II—1 COMPARISON OF ORIGEN AND INTERA MODEL RESULTS FOR
STATIC FLOW CONDITIONS
C-II-5
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TABLE C-II-1
Nuclljte
Cf-25G
Cor-246
Am-242m
Am-242
Cm-242
Pu-242
Pu-238
U-238
Th-234
Pa- 234m
Pa-234
U-234
Th-230
Ra-226
Rn-222
Po-218
Pb-214
Bi-214
Po-214
Pb-210
Bi-210
Po-210
Pb-206
DECAY CHAIN USED
Half-Life
(Years)
13.08
4711.0
152.0
1.825 x 10~3
0.4463
3.79 x 105
89.0
4.51 x 109
6.598 x 10~2
2.225 x 10~6
7.7 x 10~4
2.47 x 105
8.0 x 104
1600.0
1.04 x 10~2
5.8 x 10~6
5.095 x 10~5
3.746 x 10~5
6.338 x 10~12
21.0
1.372 x 10~2
0.3778
Stable
FOR COMPARISONS
Initial Mass
Present, (g)
1.5 x 10~4
101.0
319.0
3.83 x 10~3
0.784
6.15 x 104
1.26 x 104
5.73 x 107
8.32 x 10~4
1.93 x 10~8
6.69 x 10~6
1.02 x 104
0.405
2.94 x 10 5
0
0
0
0
0
6.49 x 10~8
2.94 x 10""11
7.39 x 10"10
0
OF MODEL RESULTS
Daughter
Nucllde
Cm-246
Pu-242
Am-242
| Cm-242
\Pu-242
Pu-238
U-238
U-234
Th-234
Pa-234m
(Pa-234
\U-234
U-234
lh-230
Ra-226
Rn-222
Po-218
Pb-214
Bi-214
Po-214
Pb-210
Bi-210
Po-210
Pb-206
— _
Fraction
0.9921
0.9997
1.0
|0.82
to. 18
1.0
1.0
1.0
1.0
1.0
to. ooi
\0.999
1.0
1.0
1.0
i.r
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
C-II-6
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APPENDIX II
NOMENCLATURE
C - concentration of the radioactive/trace component
A
C - concentration of the inert contaminant
Cf, - concentration of the radioactive/trace component in
adsorbed state
E ~ dispersivity tensor (hydrodynamie + molecular)
g - acceleration due to gravity
H - fluid enthalpy
k - permeability
K - constant decay
R, - retardation factor
d
K, - adsorption distribution constant
£ - distance between adjacent grid block centers
p - pressure
q' - fluid source rate (withdrawal)
q. - rate of heat loss
q, - rate of energy withdrawal
t - time
T - temperature
1 - transmissibility
_u - velocity vector
U - internal energy
Z - height above a reference plane
E - porosity
y - viscosity
p - fluid density
p_ - formation density
p_ - bulk density of rock and fluid
C-II-7
-------
Subscripts
c - component (mass)
H - heat (energy)
R - rock (formation)
w - water (fluid)
C-II-8
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APPENDIX C-III
PARAMETERS OF STUDY
A meeting was held on January 9, 1979, with personnel from EPA, the
Geological Survey and INTERA. The objective of this meeting was to
establish reasonable bounds on the parameters (for such variables as per-
meability, porosity, flow gradients, and distribution coefficients) that
could significantly affect geosphere transport of radionuclides, By this
date, EPA had developed a pi oabilistic risk sodel for estimating the
consequences associated with a high-level radioactive waste.repository.
In utilizing this risk model, sensitivity studies were to be performed
covering a range of the important parameters. This appendix summarizes
the results of these studies for comparison with the values listed in
Tables C-7 and C-10.
The panel concluded.that use of site criteria would establish at
least three factors. These are:
(1) Yields of ground water in aquifers or flow units
adjacent to the repository site would be extremely
low.
(2) The original ground water environment in the vicinity
of the repository would be a reducing one rather than
an oxidizing one.
(3) No organic material would be considered for storage in
the high-level repository.
The simplified diagram below characterizes and defines the variables
of interest in the geosphere pathways.
C-III-1
-------
In the above diagram, $Q, -,
are the potentials at the
surface, aquifer entry from repository, aquifer discharge, and repository,
respectively, k- and e, are the permeability and porosity of the aquifer
d the depth from surface to repository, and L the pathway length in the
aquifer.
The following summarizes the values for the above variables:
>8 km (5 mi)
0.001 - 0.3
1(T5 - 10~3 cm/sec
(2.8 x 10"2 - 2.8 ft/day)
(Max)
10"4 - 10'1 m/m
—2
(Note: 10 had almost
equal support for the
upper limit.)
0 - *3)/d
0 - 10"2 m/m
vertical permeability
from repository to surface
0.01 - 0.3
(porosity for
vertical pathway)
C-III-2
-------
In addition, the panel established the following limitations:
(1) Travel time In the aquifer should be greater than
1000 years (siting criteria would eliminate lower
travel tiaes).
(2) Adsorption coefficients should be assumed as follows:
Nuclides Rd
(Retardation Factor)
C-14 1
Kr 1
Ic 1
1-129 1
Sr 10, 100
Cs 10, 100
All actinldes over a range of 1, 10, 1000 (fully correlated)
(3) Limit the nuclldes leaving the repository by solu-
bility in a reducing environment.
The above criteria (3) would undoubtedly limit by solubility Tc» and the
actlnide nuclldes. It might also limit others depending upon anlons
present.
The above parameter ranges were provided by the panel to give guid-
ance in the range of variables for EPA'a sensitivity analysis. In some
cases, e.g., the adaorptivities, other values have been measured and
reported. However, Investigation over the above range should provide
guidance in establishing the range of nucllde concentrations that might
reach the biosphere.
C-III-3
-------
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APPENDIX C-IV
"MAXIMUM" INDIVIDUAL AND POPULATION
DOSE COMMITMENT SAMPLE CALCULATIONS
"MAXIMUM" INDIVIDUAL DOSE COMMITMENT
The "maximum" 50-year individual ingestion dose commitment is cal-
culated as the product of the drinking water activity (pCl/£) times the-
maximum annual Individual intake (liters) times the 50-year adult inges-
tion dose factor (rem/pCi), For Ingestion of strontlum-90 at 300 years
in Scenario 2 (aqueous transport), the whole-body dose commitment is:
3.6 x 106 pCl/£ x 730£ x 1.66 x 10~7 rem/pCi - 4.4 x 102 rem
"MAXIMUM" POPULATION DOSE COMMITMENT
The "maximum" population 50-year Ingestion dose commitment is cal-
culated as the product of the drinking water activity (pCi/A) times the
average annual individual intake (liters) times the population (persons)
times the 50-year adult ingestion dose factor. For ingestion of
strontium-90 at 300 years in Scenario 2 (aqueous transport), the whole-
body dose commitment is:
3,6 x 1C6 pCi/t x 370£ x 250 persons x 1.66 x 10~7 rem/pCi
4
* 5.6 x 10 person rem
C-IV-1
-------
Page Intentionally Blank
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APPENDIX
"REASONABLE" POPULATION DOSE METHODOLOGY AND SAMPLE CALCULATION
Case: Aqueous Flow Transport (Scenario 2), River Model "A" for
neptunium-237 at 6,000 years.
From Table C-19, Np-23/ has a concentration In the aquifer at one
mile downgradient of the repository equal to 7.4 x 10 g/£. Using the
specific activity of Np-237 (7.3 x 10 Ci/g), the activity in the
aquifer as it intersects the river is;
7.4 x 10"5 g/A x 7.3 x 10"4 Ci/g - 5,4 x 10"8 Ci/A or
54,000 pCi/H (1)
From Section C-4,4.2, the total discharge of the aquifer Is 144,000
liters per day (0.06 cfs). The river discharge at the aquifer intersect
(from the river model) is 2,400 ft /sec (cfs). The Initial activity
of Np-237 in the river is then:
(1 A/day - 4,1 x 10"7 cfs)
54,000 pCi/A x 0.06 cfs -1.3 pCi/A (2)
2,400 cfs + 0.06 cfs
A^ Direct Ingestion Dose Commitment
The 50-year population dose commitment resulting from one-year in-
gestion of the river water is calculated as the product of the activity
at the point of diversion for drinking water use (pCi/Jt) times the
average individual's annual intake (£) times the population drinking the
water (persons) times the 50-year adult ingestion dose factor for Np-237
(rem/pCi).
For the first population (i.e., that at the first diversion only),
drinking water below the aquifer intersect, the values used are:
* Dilution factor =1.7 (from river model)
* Activity of diversion = 1.3 pCl/£ * 1.7 = 7.6 x 10
* Average individual's annual intake = 370 liters (54)
4
* The population drinking river water = 1 x 10 persons
(from the river model)
* The 50-year adult ingestion dose factor for Np-237 *
5.54 x 10"8 rem/pCl (54)
C-V-1
-------
The 50-year population dose commitment is:
.54 x 10" rem/pCi
(3)
7.6 x Itf1 pCi/£ x 370JI x 1 x 104 persons x 5.54 x 10"8 rem/pCl -
0.2 person-rent
The total population do»« commitment resulting from direct inges-
tion of contaminated water is shown in Table C-26 for important'radionuclides
at the time of their maximum concentration in the model river. These dose
commitments result from a summation of population dose commitments at each
river diversion for each nuclide.
B. Ingestion of Contaminated Crops
The 50-year population dose commitment resulting from ingestion of
one-year's crop irrigated with river water was calculated as the product
of the crop activity (pCi/kg) times the annual garden vegetable yield
(kg) times the ingeetion dose factor. The crop activity for irrigated
crops is the sum of the activity resulting from direct deposition on the
vegetation and the activity resulting from uptake through the soil. The
equation for estimating plant activity from irrigation follows; (54)
r
r . r T r
, '^e B
1 - e _L . v
-At.]
1 - e b -
Cv V Y A, ' PA
L v E _i
-^
n
(4)
r A [
_»
where C * crop activity (pCi/kg)
GW = irrigation water activity (p€i/£)
I = irrigation water application rate (fc/m -hr)
r » fraction of activity retained on crops (0.25 for sprinkler
irrigation)
2 2
Y - agricultural yield (2.0 kg/m for vegetation, 0.75 kg/m for
crop)
B = the transfer fraction from soil to vegetation for each element
(pCi/kg inplant \
pCi/kg in soil I
2
the effective soli "surface density" (240 kg/m for 15.24 cm (6-inch
depth)
X - radioactive decay constant (hr ) for each nuclide
X_ » effective removal rate constant (X_ « ). + A where X
E E w w
is the weathering loss (0.0021 hr*"*)
C-V-2
-------
t " time that crops are exposed to contamination during the
growing season (hrs)
t. - midpoint of soil exposure time (hrs)
D
t, «• time between harvest and consumption (hrs).
h
The first fraction in the brackets accounts for the plant activity
resulting from direct deposition and the second fraction represents the
soil uptake activity.
For all cases, the nuclides leaving the aquifer and contaminating
the river have sufficiently long half-lives that their removal by radio-
active decay in crops or soils is negligible. Consequently, the above
equation may be simplified based on the following relationship: As a
nuclide's half-life approaches infinity, \ approaches 0.
limit A - X - 0.0021 hr"1 (5)
t n £. W
A -* 0
(6)
(7)
The equation then reduces to:
(8)
The above equation must be expanded as follows to consider the 10%
sprinkler and the 90% ditch irrigation contribution to the vegetation
activity:
-i t-
B t.
C - 0.1
V
-------
The following terms are constants In the above equation:
r - 0.25 (sprinkler irrigation)
X - 0.0021 hr"1
w
t - 1000 hrs
e
P - 240 kg/m2
t. • 50 years or 4,4 x 10 hrs
For neptunlum-237 at the diversion in River Model "A" corresponding to a.
1.7 dilution factor, C -0.76 pCi/H, I (from river model) - 0.23H/m -hr,
Bv - 2.5 x 10~3 (1), Yy - 2.0 kg/m2 (1)
Evaluation of terms in equation (9) results in the following:
,. 'VX » ..... -0.0021 x 1 x 103. , ,
*<*-• - 1 Q.25(l-e - 1 , 52 taAg'1 (10)
g UU|
X Y 0.0021 x 2.0
w v
C - 0.1 (0.76(0.23) [52 4- 4.6]} 4- 0.9 (0.76(0.23) (4.6)}
-1.71 pCi/kg <12>
And the resulting dose commitment for a population of 10,000 persons Is:
7 R
1.71 pCi/kg x 2.0 x 10 kg* x 5.54 x 10 rem/pCi - 1.9 person-rein.
As in the dose commitments resulting from direct ingestion, the
population dose commitments resulting from ingestion of contaminated
crops are summed for all diversions along the model river.
C, Ingestion of Contaminated Beef
The 50-year population dose commitment resulting from ingestion of
contaminated beef is the product of the dose factor (rem/pCl) times the
beef activity (pCi/kg) times the total population Intake (kg) for one
year.
The beef activity, C, , is calculated from the activities of the
cattle drinking contaminated water and eating contaminated feed grain
using the following equation (1):
C, - F, (C I 4- C 1 ) (13)
o b w w v v
*Value from Table C-21( kg of vegetables per 10,000 people,
C-V-4
-------
where C - activity in drinking water (pCi/i)
w
intake of drinking water (£/day)
C » activity in feed grain (pCi/kg)
I * intake of feed grain (kg/day)
F. « beef transfer factor (day/kg).
-4
I = 50 8,/day, Iy - 50 kg/day and Ffe for Npr237 - 2.0 x 10
(day/kg). <54)
The only difference between C for the human ingested crops and C
for livestock ingested crops is that the vegetative yield (Y ) used for
2 v
the livestock crops is 0.75 kg/m (1). This value gives a C of:
" -0.0021 x 1 x 10 ,
X Y 0.0021 x 0.75
w v
Cy - 0.1 (0.76(0.23) (139 + 4.6)} + 0.9 (0.76(0.23) (4.6)}
- 3.23 pCi/kg (15)
Thus, for the beef grown at the first diversion in Elver Model "A,"
Cb - 2.0 x 10"4 day/kg [(0.76 pCi/£ x 50 Jl/day) + (3.23 pCi/kg
x 50 kg/day)], - 0.04 pCi/kg (16)
For all beef pathways, a beef yield of 1 kg/100 kg feed crops was
used (Cattle Marketing Information Service). This gives a total beef
yield (population intake) » 3.9 x 10 kg* for the sample calculation.
The resulting dose commitmeiit for a population of 10,000 persons is thens
0.04 pCl/kg x 3.9 x 10 kg x 5.54 x 10 rem/pCl - 8.6 x 10 person-rem.
D. Ingestion of Contaminated Milk
The 50-year population dose coiaaitoent resulting from ingestion of
contaminated milk involves a methodology Identical to that for contaminated
beef ingestlon (see equation (13)). The parametric differences for the
sample calculation are:
1-60 A/day (54)
F - 5.0 x 10~6 day/A (54)
\ *
*Value from Table C-21.
C-V-5
-------
thus C - 5.0 x 10~6 t(0.76 pCi/fc x 60 it/day) -I- (3.23 pC,l/kg x 50
* • _3 - -.•".••,-•• ' '• •• •'.
» 1 x 10 pCi/Jl
Bie resulting dose commitment for a population of 10,000 people Is then:
7 —3 —8 ^
3.2 x 10 £/yr x 1 x 10 pCi/£ x 5.54 x 10 rem/pCi - 1.8 x 10 person-reo.
'. U S, COVERflwmPBINTINC OFFICt 19?9-281-UJ/128
C-V-6
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TECHNICAL. F.EPO1T DATA
1. flEPOHT NO. 2.
EPA 520/4-79-007C
4. TITLE AND SUBTITLE
Technical Support of Standards for High-Level
Radioactive Waste Management, Volume C -
Migration Pathways
7. AUTHOFHSJ
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Arthur D. Little, Inc.
Cambridge, Massachusetts 02140
12. SPONSORING AGENCY NAME AND ADDRESS
Office of Radiation Programs
U.S. Environmental Protection Agency
Washington, D.C. 20460 •
Sr^SlPIENT^ ACCESSION NO.
>";«]? i 0 0 2 -j /
5. REPORT DATE
1979
6, PERFORMING QHGAMIZATION CODE
8. PERFORMING ORGANIZATION REPQF
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-01-4470
13. TYPE OF REPORT AND PERIOD COVI
March - July 1977
14. SPONSORING AGENCY CODE
/W/?- KG
15, SUPPLEMENTARY NOTES
16, ABSTRACT
This report is the result of work performed under the third part (Task C) of
four-part contract to gather technical information to evaluate environmental
acceptability of various options for disposal of high-level wastes. The other tas
are: A-Source Term Characterization; B-Effectiveness of Engineering Controls; and
D-Assessment of Accidental Pathways.
Task C report has three principal objectives; to assess geologic site selecti
factors; to review available information and quantify the potential far the migrat
of nuclides through the geosphere to the biosphere, and to consider dose-to-man
implications of a repository for high-level waste containing large quantities of
radionuclides in high concentrations that might become dispersed into the biospher
over geologic times. Task C attempts to summarize the Influences on nuclide migra
potential and thereby identify critical inadequacies in the data and analytical
method.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
radioactive waste disposal
high-level waste disposal
high-level waste depository
radionuclide pathways
18. DISTRIBUTION STATEMENT
t
*
b. IDENTIFIERS/OPEN ENDED TERMS
19, SECURITY CLASS (This Report)
20. SECURITY CLASS f This page )
c. COSATI Field/era
21. NO. OF PAGES
22. PRICE
EPA Form 2220-1 (He». 4-7?) PREVIOUS EDITION is OBSOLETE
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