RISK ASSESSMENT OF DISPOSAL OF HIGH-LEVEL
RADIOACTIVE WASTES IN GEOLOGIC
REPOSITORIES
August 1985
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Radiation Program*
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Chapter 1 - INTRODUCTION
1 • 1 Backeround
The national program for the disposal of high-level radioactive.
waste Is governed by the Nuclear Waste Policy Act of 1982. Three
federal agencies have central roles In this program. The
Environmental Protection Agency is responsible for setting general
environmental standards that govern the level of performance to he
expected of a waste disposal system. The Nuclear Regulatory
Commission is responsible for implementing the standards promulgated
by EPA and issuing licenses for the construction and operation of a
waste, disposal system. The Department of Energy is charged with the.
responsibility for developing and implementing a waste disposal
system. These federal agencies, as well as other organizations and
individuals, have been involved in developing or evaluating waste
disposal technology for many years. The studies carried out by each
agency have been appropriate to the particular responsibility of that
«gency in the entire waste disposal program. The purpose of thic
report Is to summarize the risk analysis studies that have been
carried out by the Environmental Protection Agency to provide the
information necessary to set generally applicable environmental
standards.
The analysis of the long-term risks f-om the disposal of
high-level radioactive waste is an important component in determining
the suitability of any proposed waste disposal system. Other
important components include the cost and logistics of a waste
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disposal system, the environmental impact of such systems, •»..>! t .!,«-.
safety of euch facilities and system* during construction and
operation. Th« estimation of the risks associated with the disposal
of high-level , waste it a particularly difficult problem, and one for
which there exists no good precedent. The waste disposal system
itself involves the complex interaction of engineered systems (e.g.
metallic containers, vitrified waste forms, borehole seals) and
natural systems (e.g. geology and hydrology). The time frame over
which such systems must be evaluated extends far into the future. For
purpor «= of the principal EPA analyses, for example, estimates have
been made of the performance of waste disposal systems for at least
ten thousand years into the future. Predictions of such complex
systems for such long periods of time necessarily include large
/
uncertainties. Therefore, an additional component of the risk analysis
involves estimating or bounding such uncertainties; this makes the
problem even more difficult. The approach described in this report is
one that uses relatively simple mathematical models for the various
processes and systems involved, combined with bounding estimates
(i.e., intended to , overestimate risks) on parameters and processes
whenever there is considerable uncertainty associated with their
< *f*»l9'»» *•* ^/^^ 4
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give an overview o. c:,.» previous literature at the outset. Uh.n the
"A published it. draft F.nvironm.ntal Standard In 1982, it al.o
available a number of report, on studi.. carried out up to that
These reports included:
Rafnoaer.
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models developed by the EnVlrorjr.ental Protection Agency.
These models include the enviroiuiitmtwl transport of
radionuclides, their health effect* on the population, and a.
comparative risk analysis between the disposal high level
radioactive waste and existing uranium ore bodies. This
report presents a reasonably complete summary of the risk
analyses carried out through 1982 in support of the Agency's
efforts to §et environmental standards for high level
radioactive waste disposal.
Environmental Pathways Models for Estimating Population Health
Effects from Disposal of Hieh-Level Radioactive Wasts in Geologiq
520/5-80-002
This report documents the models developed and applied by
EPA to estimate the transport of radioactive materials
through the biosphere (e.g. through food chain or other
pathways readily accessible r the human population) . In
addition it summarizes other work done by EPA on the health
effects associated with the exposure of populations to
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radioactive materials. It was one of the principal source
documents, in conjunction with the ADL reports, for the
Integrated analysis presented in the Population Risk Report.
Population Riaks From Uranium Ore Bod^a, EPA 520/3-80-009.
This report was prepared by EPA in order to estimate the
risks to a human population from various kinds of uranium
or* bodies, such as are found throughout tha United States.
has provided comparative rick information that
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nas b«.en used by the Agency in r^ ,
J _ S ncy ln crying to decide how low the
ri.k goal from a repertory should be sec.
to
out .ub..qu.nt
«tudle3 „.
Th« original risk analysis described above was carried out
on che baiis of so-called "conceptual models" of geological
repositories for nuclear waste in varic-s geologic media.
These conceptual models were developed early in the
national program, when little site-specific information was
available on actual sites under evaluation. In an effort to
refine the conceptual models so that they would more
realistically describe the capabilities of geologic
disposal, a review was conducted of the site data collected
by DOE at various potential sites. On the basis of this
site specific data, modifications were made iq the
conceptual models. = re
Individual DO a.
A -tudy va. carried out by Sandi. Laboratories under
contract to EPA to use Sandia's computer code NWTF/DVK to
Mtimate potential individual do... at various points in
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time, as a result^ of a nuclear waste repository. (The
original analyses exclusively „,*
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treat, among other topiCB, the validity of the technical basis for the
EPA standard. The Vacte Isolation System Panel of the National
Academy of Sciences conducted an overall review of the entire nuclear
waste program and devoted particular attention to the approach that
had bean adopted by the EPA for regulating the disposal of high level
waste. Sandia National Laboratories, under contract to the Nuclear
to
Regulatory Commission, conducted a number of technical analysis
intended to test whether the EPA standard could actually be
implemented by the regulating organization, i.e., the NRC. The EPA
itself convened two subcommittee, of its Science Adviaory Board. The
first was. directed specifically at the high level waste standard and
its associated technical support material. The second was addressed
towards EPA1s regulation of airborne .radioactive emissions under the
Clean Air Act, but it too addressed fundamental questions relevant to
the high level waste standard. A summary of the conclusions of each
of these studies with respect to the risk analysis approach of EPA
is sunmaarized below:
Waate Isolation System Panel (WISP) Report
This report argued that the measure of risk used by EPA in
its risk analysis, namely, cumulative population dose over
ten thousand years, was inappropriate, for two reasons.
First, the time frame was considered to be arbitrary and
•hould have been extended indefinitely into the future, for
as long as a residual risk front the repository would exist.
Second, the measure used by EPA did not take adequate
account of the risk to individuals, some of whom might
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relatively large doses ev«u ti^ugh Liie cumulative
population do so might fall within acceptable limits. «••••*
Sandia/NRC
An issue that had been heavily debated during the
development of the draft Standard wa« the question of
whether it could actually be implemented by the regulating
agency, i.e., the NRC. The probabilistic nature of the
standard and the uncertainties associated with the complex
system being analyzed raised a question as to whether
adequate levels of certainty could be associated with a
licensee's application in order to verify compliance with
EPA standard. Sandia National Laboratory developed and
tested computer codes for generating the measures of risk
required by the draft Standard, as well as methods for
quantifying the associated uncertainty. On the basis of
this work, Sandia concluded that technology existed or could
be further developed such that there would be adequate
technical basis for testing compliance with the standard.
After this work was Completed, EPA contracted with Sandia
for additional model development and testing, as well as
application to the generic lithologies treated in the EPA
risk analysis.
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bciencc Advisory Board Subcommttte on ^e Htph Level Radioactive
.g Sfandarj
A special subcommittee of the Science Advisory Board
reviewed in detail the technical basis for the EPA Standard.
*
including both the risk analysis and consequence models.
There were four basic conclusions derived from this review.
First, it was noted that there were large uncertainties in
Che parameters and models used in the EPA work. Second, the
relatively restrictive level of risk imposed by the standard
night require excessive conservatism on the part of the
repository designers in order to demonstrate, with an
adequate degree of assurance, that the standard would be
met. This is essentially a corollary of the previous point.
Third, certain parameters used in the geocheraical analysis
should be modified in order to reflect the most recent
information on the behavior of appropriate, species in
groundwater systems. Fourth, certain environmental pathways
for radlonuclide migration and human exposure require some
modification in the models used by EPA in order to more
accurately reflect current knowledge.
Among these comments, the first and the third are relevant
to the risk analysis and hence are addressed further in this
report. In particular, additional discussion and measures
of uncertainty are developed here so as to lend further
perspective to the degree of uncertainty that might be
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associated with any risk estimates generated. The
*
geochemical values have also bean modified taking lnf<>
account the SAB comments, as well as information from the
WISP report and other sources. The appropriate degree of
conservatism in the standard is more a policy than a
technical issue, and it is discussed elsewhere in connection
with the final version of che standard itself. The
modifications Co the models for environmental pathways and
human exposure are addressed in the companion report to this
report.
Science Advisory Board Subcommittee on Risk Assessment for
Radionuclides
•N
The emphasis of this review was primarily on the consequence
models related to human exposure. This subject is not treated in this
c*H/
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analytical methods used to carry it out
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CHAPTER 2 - MODELING APPROACH
2.1 Generic Analysis
Th« risk analyses carried out in support of the development of
the High Level Waste Standard are intended to be "generic" in nature.
1C is important to understand what this means. In setting its
standards, the Agency needs to ensure that the performance levels It
requires «re realistic, that is, that there is a high likelihood that
they can be achieved by available or developing technology. Beyond
this, the Agency also wishes to use its regulatory authority to
encourage the selection of waste disposal systems that are close to
the beet that might reasonably be expected Co be available.
Therefore, an important aspect of the work carried out during the
development of the standard has been risk analyses intended to
estimate how well waste disposal systems might perform and how
sensitive this performance is to various parameters.
In developing this information, the Agency considered a wide
range of geologic environments and other parameters. In the early
stages of this work, the Department of Energy, which is responsible
for developing a geologic repository, had not yet collected much data
for its principal candidate sites. Therefore, Che Agency's
consideration of potential conditions at a repository was based on the
general literature on potential waste disposal environments, as well
mm the limited data that had been obtained by DOE in the early stages
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of its investigations. Morfe recently DOE hae developed much more
complete d*t» on specific sites that it is ovfllviaf.tng for suitability
as a repository. Vhlle the Agency utill intends that its risk
analysis b« considered as conceptual or generic, in defining the most
representative^ parameter* to use in such analyses, it has reviewed
carefully the developments by DOE at particular sites and it has based
its conceptual models largely on these data. Because of the generic
nature of the analysis, the results of the risk calculations do not
purport to represent the actual risk expected at these particular DOE
sites. They are still intended as rough estimates of potential
repository performance in a general sense, in the full realization
that further investigations at particular sites may show that such
sites have a performance somewhat different from the generic
calculations. At the same tine, by using the most recent information
generated by DOE and others at particular sites, the Agency has
attempted to ensure that its generic calculations are based on the
most reasonable parameters and hypothetical scenarios that would
control performance at a real site.
2.2 Waste Disposal System Model
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The waste disposal system considered in the Agency's analyses is
based on the current reference plan of the Department of Energy for
the disposal of high-level radioactive waste in mined geologic
repositories. Such repositories consist of underground mines or
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excavations whose working levels sr« between 1,000 and 3,500 ft. below
the surface. Rock formations in which «uch repositories are being
considered include: bedded aalt, *alt domes, basalt, granitic rocks,
and tuff. Each of these environments has its own special
characteristic*, which are discussed in later chapters of this report.
The radioactive waste* themselves would consist either of spent fuel
from nuclear power reactors or solidified reprocessing waste in a
relatively durable form, such as borosilicate glass. Such wastes
contain a wide rang* of radioactive element* ranging from highly
active fiaaion products with relatively short lives to long-lived
elements, such a* the transuranic*. The wastes themselves would be
packaged in one or more containers or canisters and these would be
placed in holes in the walls or the floor* of mined vaults in the
repository. After emplacement of the wastes, the repository would be
backfilled to enhance its mechanical stability and Co retard the
movement of fluid*. Its various connection* (e.g. shaft* and
boreholes) to the surface would also be severed and sealed. The
intent in selecting a location for a rep *itory would be to obtain a
highly stable geologic environment and one in which it would be.
difficult for groundwater to come in contact with the waste. In
addition, in order to avoid the preemption of important resources
nearby or the attractiveness of the site for future excavation, DOE
would attempt to locate the site where other resources are minimal,
and it would mark the site so that future generations would be aware
of the hazardous materials that had been disposed of there.
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Despite the care chat DOE may exercise during the development of
<*
a repository, it olwaye remains possible that there could be A futvir«
disruption that could lead to the release of wastes. The purpose of
the risk analyses carried out by the Agency la to identify the most
important mechanism* that could lead to such releases and to estimate,
on a quantitative basis, both their likelihood and their consequences.
While this analysis varies somewhat from one geologic environment to
another, the overall common structure can be represented as shown in
Figure 2.1. The components of the system to the right of the vertical
dotted line are referred to as the "accessible environment". The
Agency has focused particular attention on tha actual physical
quantity of radionuclidea that might enter the accessible environment
over various relatively long time periods in the future.
In order for wastes to reach the accessible environment, they
must be transported through the various components shown on the left
side of the diagram. In particular, radioactive material must be
released from the waste form itself, which, as noted previously, might
be -either a borosilicate glass, unraprocesced spent fuel, or some
s
similar type material. Having left the waste form itself, such
radionuclidea must pact through the canister, which would of course be
designed to try to retard such movement, and then enter the backfilled
openings of the repository. From the repository there are two general
ways in which they nay find their way to accessible environment.
There may be direct pathways to the land surface, such as might occur
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if future generations penetrated the repository during an exploratory
drilling program and accidentally contacted the wastes, or they might
migrate in slowly moving ground water to an aquafer within which they
could than b« transported some distance where they might thon have
access to the surface. (For example, many underground aquifers
ultimately discharge into surface water bodies.) The physical
description of the movement of radionuclides from the waste form,
through the canister, through the repository, and ultimately to the
accessible environment depends on the physical description of a number
of possible future scenarios that might alter the conditions of the
underground environment. Such processes are called "scenarios" or
"release mechanisms". They may affect any of the four components
indicated in Figure 2.1. For example, the nature of the waste form
might be altered by crushing or by intersection by a drilling tool.
Similarly, a canister might also be affected by a disruptive event, or
by degradation as the result of corrosion. A scenario might affect
the repository component by providing a. flow of groundwater through
certain parts of the repository. It might affect the availability of
pathways to the surface or to the aquifer in a number of ways, such as
by the rupture of a f*ult or by the creation of a new borehole. The
risk analysis reported here considers a number of «uch release^
nechanisms, their probabilities, and their consequences. The results
of the calculations for individual release mechanism are then combined
into an integrated representation of the riak from a hypothetical
repository. The actual measures used to characterize auch ri«ks are
discussed in subsequent sections.
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2 . 3 Time
, There is wide variation in the published literature on nuclear
waste disposal on the subject of the time frame over which disposal
alternatives should be evaluated or compared. If it were possible to
predict the future with reasonable assurance, then clearly the longer
the time frame, the more complete could ba the comparisons between
alternative systems. Some authors have attempted to compare nuclear
waste disposal sites or systems for periods of .up to one or even ten
million years. Such results, however, contain very large
uncertainties because of the difficulty of making such long time frame
predictions about either the natural or the engineered components of
the system. At the opposite extreme, other authors have only
considered site risk comparisons for periods on the order of 300
years, arguing that relatively reliable predictions can only be made
for time frames of this length and that the longer term performance is
both less important (due to the decay of the radionuclides) and
expected to be generally proportional to the performance calculated
for this period. ,
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In the course of performing numerous -risk analyses over a
relatively large range of time spans, the Agency has concluded that
the risks identified over relatively short time spans, such as a few
hundred to one thousand years, do not adequately portray important
differences among alternative sites or waste disposal systems. This
is because the groundwater travel times may be sufficiently long for
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of
slt,. that
. if th. .„!,...
be significant differences b.t-een che §lt..
or g.och.nlc.l ch.ract.ri.clc, of th... groun<.».t,r
«y«.«,. Th. priory risk ......»'.«.- ..rxi.d Me ln .upport of th.
tlvt,l... tUr.fore. h.v, be.n ba,.d on . el*. £,„ of «n
y»tt. It t. bellevecl th>t tMs
diff.r.nc.8 b«v«n on. ile. .na
of «ne .u^,.., systen annocher
r.strictlon to . t.n tho.s.nd y.« clm.
•tlU do« noc captur« th.
This U especially the „,. when the dominant r.dlonuclldes « ,oM'
Point in tin,. .„ nembera of th. rain. ^ whi^
different element, In th. ch.in h.v. different r.t.rd«lon r.te, In
the grounder .y.t». Depending on th. ri.k »...ur. used, It ,.y b.
t~.port.nt to consider .uch process,, m cor^.rlng .tt.._ „,.„,„._
in . nu.b.r Of the Agency's c.lcul.tlons. tl. fr.ne. Lnger th,n t.n
thous.nd years have «lso ,e.n considered.
in consldertng the long t.r. .f£.ct. ot . nuel..r
"po.ttory, the Agency h.. con.4d.red both total cumul.tlv. popuUtlon
effects, a, wll „ eff.ot. „ tnl3 ^M ^ ^ ^^
provide 8,Mwhat different Information. It 1. p(1..tbi., for .XMlpl.,
">«t a relatively unproductlv. groundw.t.r .upply COuld b.
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contaminated at 9on,e point in'jrhe future by nuclear waste, but due to
the limited availability of water from this supply, only a few
individuals would actually receive exposure to radioactivity as a
result. In this case the individual doses might be relatively high,
although the overall effect on the population could still be quite low
because of the small number of people involved. On tha other hand, it
is also possible that a nuclear waste repository could at some point
in the future lead to very low level contamination of a water supply
that serves a relatively large population. In this case the dose to
any individual in that population night be small, although the overall
effect on the population could be substantial because of the large
numbers of people involved. Because of these differences, in
assessing the risk from a repository, the Agency has developed and
applied techniques that can estimate both kinds of risks. Since the
ultimate effect of exposure of populations or individuals within the
population to ionizing radiation is negative health effects and since
th. hypothesis used by the Agency and others to estimate such health
effects is the linear mm-threshold hypothesis, the doses received by
individuals or by populations can ultimately be reduced to common
terms, namely, the total expected number of somatic or genetic health
effects. This is therefore the most fundamental risk measure of all.
While of course it is desirable to keep individual doses as low as
possible, the total cumulative release of radionuclides over ten
thousand years and the associated population done and health effects
1« the most complete measure of risk for evaluating alternative sites
or repository systems. Therefore, the greater emphasis in the
Agency's risk analyses has been on total integrated cumulative
releases over ten thousand years.
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/
Even within this framework, there are additional alternative risk ]/
raensures. The final risk me.Asur* of immediate importance is health
effects, Such health effects are calculated by fint estimating the
radiological dose to the population. The dose in turn is estimated by
calculating the released quantities .of specific radionuclides to the
accessible environment. According to the conceptual model for •
. the risk analyses as illustrated in Figure 2.1, one would begin with
releases from the waste form and go through a sequence of models that
would first calculate the physical quantities of radionuclides
released to the accessible environment and then, building upon these
numbers, would calculate doses to the population and ultimately health
effacts. These last two stages, namely, the calculation of population
doses and health effects, bring the calculations to their final point
of interest, but at the same time they introduce additional
uncertainties into the calculations because of the need for the
additional models. Therefore, for some calculations the Agency has
found it desirable to express risk in terms of quantities of
radionuclides released to the accessible environment. For other
calculations these numbers have been converted into dose or health
effects estimates^/Each such risk measure serves a useful purpose as
will be illustrated when the result of the risk analyses aro discussed«
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Che .election or a particular risk mea6ure for fl fiec Qf
calculations, the'-ode of presentation of eh. ri.k results .iao allow*
— flexibility, .inc. the various relea.. MCh.ni.M have associated
with them .on,, probabilities of occurrence (genially qmcc small) and
since the point in time at which, they can occur ha. an important
influence on their effects on th. population (b.cause of the gradual
decay of the radioactivity). The method of "risk profiles" or
"complementary cumulative* distribution functions" (CCDF) has been used
in the risk calculations. A hypothetical CCDF is illustrated in
Figure 2.2. In general a point on such a curve consists of
conscience and a probability value. Th. relationship between them is
given by the statement: "The probability shown t. che probability of
obtaining a consequence at least as large a. the consequence shown.-'
To simplify some of the calculations, two specific probability values
have been chosen and their corresponding consequences are calculated.
This is actually the form in which the numerical part of the high
level waste standard is presented
2.5 Computer Codes Utilized
A number of computer codec have been used a* tools in the
Agency's risk analyses. The central tool has been the program REPRISK
which ha* been under development at the Agency since 1978 and for
vhich separate documentation exits. This code will be described late?
in this section. REPRISK makes use of certain conversion factors that
relate the amount of radioactive material released to the accessible
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to population ReaUh effects. These ccnver3ion factors
-r« obtained by u.ing another EPA computer code called WESFDOSE. This
cod, considers a number of pathways for th. environmental transport of
radionuclides and is described in a companion document. For
calculations involving individual. doses and longer time frames than
ten thousand years, the computer code NWFT/DVM, developed by Sandia
under contract Co NRC, has been used. Thi. code models the transport
of decay chains whose element, have different retardation factors in
the groundwater system. A more complex groundwater code, SWIFT, has
also been us.d to support the EPA risk ..alyses, primarily to
validate so*. of the hydrologlc c.lculatlon, carrie
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After the repository inclosed and Sealed, there may be various
natural proc.s.«. or human activity chat could lead to rel.«,ses troir.
the repository. Th.se perturbations happen ~according to some
frequency or probability, eith.r constant or variable. REPRISK
incorporate, the characteristics of these perturbations and their
associated probability, to model radioactive releases. Four kinds of
"release mechanisms'1 are addressed:
1. Direct impact on a waste package with associated releases to
th. air and/or the land surface (e.g., volcano, meteorite,
drilling/dir«ct hit).
2. Direct impact on a wast* package.with associated releases Co
an aquifar (e.g., faulting, breccia pipes).
3. Disruption of the repository with associated releases to the
land surface (e.g., drilling/no hit).
4. Disruption of the repository with associated releases Co an
aquifer (e.g., normal groundwater flow, faulting, breccia
pipes, drilling/no hit).
Afl a result of each release mechanism acting at the site, some
portioi. of the radionuclide inventory will begin to travel to various
components of the accessible environment, e.g., rivers. Each such
release mode U.ds to several pathways to human exposure. The
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consequences of a radioactivity release to the accessible environment
4
are expressed In terms of 1) number of somatic health effects (fatal
cancers), 2) number of genetic health effect*, 3) ratio of released
amount to the release limit in 40 CFR 191, and/or 4) curias released
of each radionuclide.
Two time frames, are used by the model. One, called a dose
commitment period, is for modeling the occurrence of release
mechanisms «t the site. The other, a dose integration period, is used
for measuring the consequences of the releases. This way consequences
may be measured beyond the time when a particular perturbation may be
active at a site.
2.5.2 NWFT/EVM (
NWFT/DVM is computer code that w«s developed for somewhat noro
general purposes than those for which it was applied in connection
with the Agency's risk analyses. In the paragraphs that follow a
general description of the capabilities of NWFT/DVM is first
presented, followed by a description of the way It was applied by the
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Agency.
NWFT/DVM is essentially a pair of two computer codes, NWFT and
DVM, that are compatible in the sense that the output from the first
can be fed directly into the second. NWFT, the Network Flow and
Transport Code, is a simple network flow groundvater code in which the
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flow through each segmtnc of the network is modeled as one-dimenstonal
Darcian flow. It t* thus essentially a steady state flow code,
although the boundary conditions can be varied from one point in time
co another so as to iimulate time varying flow in a quasi steady-state
fashion. The elements of the ,network might b« parts of certain
aquifers or pathways connecting one aquifer with another. On a
smaller seal* there could be individual drifts in * rained repository,
filled boreholes, or any other physical pathways for groundwater
within which Darcian flow would be a reasonable model. Given the
appropriate hydrologic boundary conditions and parameters describing
the geometry, permeability, and porosity in individual segments of the
network, NWFT solves for the potentials atnrodal points and th«n
calculates the Darcian flow and the average interstitial fluid
velocity in each individual segment. DVM, the Distributed Velocity
Hethod Code, is used to model the transport of radionuclides In the
flow regime described by th« output of NWFT. In the absence of
transformations from one radionuclide to another du» to radioactive
decay, DVM essentially associates with each radionuclide a transport
velocity which is the fluid velocity divided by a retardation factor
appropriate for the nuclide in question. Thus in this case it would
be relatively simple, given the output of the flow calculations from
NWFT, to track within DVM the inventory of each radionuclide in
Individual s.egments of the network. The complication arises in the
case of radioactive decay chains, in which on« radionuclide may
gradually evolved into several others and these individual
radionuclides may have different retardation factors. In order to
account for this factor (as well as radioactive decay itself), DVM
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mak.. use of an effici.nr. mathematical technique for tracking the
transport of the varlou. nuclid.fi And maint.tning a current inventory
for each of the segments of the network. The details of this model
are de.cribed in th. code's original documentation and *re noe
important here. In addition to the., mathematical capabilities.
NWFT/DVM ha. an additional useful characteristic. The codes arc
relatively simple codes, allowing for many repetitive calculations at
minimal cost. They are in fact set up in order to allow a large
number of Monte Carlo simulations to be performed on the system, where
the individual parameters describing flow and transport can be modeled
by probability distributions from which representative valuas are
selected during each simulation. In addition, to sharpen the
usefulness of the Monte Carlo simulations, the codes are arranged so
that the sampling from the set of probability distributions for the
various sample parameters can be carried out by a sophisticated
technique called Latin Hypercube Sampling (LHS). In « certain
statistical sense, this technique maximizes the amount of information
that can be obtained from a limited numbei of Monte Carlo simulations.
The total capability of NWFT/DVM then might be described as follows.
The repository and surrounding geologic system can be described by a
relatively large number of parameters representing geometry, flow and
transport conditions, and release rates from the waste packages.
Uncertainties in each one of these parameters can be described by
aligning a probability distribution function to the paraneter, rather
than limply apecifying a particular point value. The computation can
b« run a large number of times in aMonte Carlo fashion, each time
selecting a value for each input variable from its probability
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distribution function. The output th.n. which m.y be Inventories in
individual compArtn,ent., r.l....a to th. .cee..ibl. environment, or
even associated health effects (obtained by u..r..uppll.d conversion
factors) can also be represented by « probability distribution
function. The nature of this . distribution function can aid in
understanding the uncertainties in the system's performance.
While the Agency believes that the techniques contained in
NWFT/DVM may prove to be extremely useful as the NRG evaluates a
propose repository system for licensing, only rather restricted use
was made of NWFT/DVM for the Agency's risk analyses. In' particular,
the Mont. Carlo capabilities were not utilized. Individual point
values for the analyses were chosen to be identical to those that were
applied with REPRISK, which does not have the Monte Carlo capability.
The resulting flow regimes were therefore essentially identical since
they are both based on sirapU networks described by Darcian flow. The
unique contribution of NWFT/DVM was in the DVM portion of the analysis
where, «or tiraeframe. longer than ten thousand years, certain
r.dionuclide decay chains were modeled so as to obtain groundwatcr
concentrations and health effects over the., longer periods of time.
x
As described in the documentation for REPRISK, there are some
simplified approaches to decay chains also contained in the EPA's own
codes. However, the availability of DVM enables none accurate
calculations of release, for time periods longer than ten thousand
y.ars, «;ter which the transport of decay chains begin, to become a
•ore important factor.
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2.6 Site Parameters
A number of parameters are used to described the geometry and
hydrologic conditions present at the site. The most important ones
*
are listed in Table 2.1 and can best be understood in conjunction with
the generic cross section shown in Figure 2.3, with certain exceptions
to be described shortly. The conceptual framework of the lithology at
the site is that the repository is located between two aquifers called
respectively the "upper aquifer" and "lower aquifer". For actual
applications of the model to simulate conditions present at one or
more real sites, the upper^ and lower aquifer do not generally
I
represent single hydrostrat/rfgraphic units, but rather represent
"synthetic aquifers" whose properties are defined to approximate the
combined properties of a number of transmissive units above and below
the repository horizon. For example, if a number of such transmissive
units are present above the repository at a particular site and if the
application of the generic models described here is intended to
represent conditions similar to those at that site, then one can
calculate the combined volumetric flows in the upper units and define
Appropriate hydrologic parameters so that the synthetic aquifer
represents the sane total flow. Similarly, by varying one or more
additional parameters it is possible to simulate the effective fluid
velocity in any one of the actual units. This will be illustrated in
subsequent sections when specific llthologles are discussed.
The general way in which the REPRISK model constrains the risk
analyses is that the upper aquifer is considered to be the pathway of
groundwater release of any radionuclides that leave the repository.
-------
(An upward gradient is considered to exist along any pathway that
might exint or develop between the lower .aquifer and upper aquifer.)
This is why there 10 greater emphasis in Tabl* 2.1 on the properties
of the upper aquifer. It is recognized, however, chat at potential
•
repository sites the hydrogaologic environment may be somewhat
different from that assumed in the generic model. For example, there
nay be no significant water bearing units balow the repository (as in
a number of crystalline rock sites), or above the repository (as in
the case of a repository in the unsaturated zone), or there may be a
prevailing gradient that is downward from the upper aquifer to the
lower aquifer, in which case the lower aquifer would appear to be the
more likely release pathway. These cases can all be accommodated
within th« modeling framework used by the Agency.
With respect to the specific hydrologic parameters listed in
Table 2.1, hydraulic conductivity is used in conjunction with Darcy's
Law to estimate volumetric flow rates through various 'components, such
as pathways from the repository to the upper aquifer or along the
upper aquifer itself. For further elaboration on the mathematical
equations referred to here and below, one may consult SPA-520/3-80-006
and the references cited there. Only Darcian flow has been treated in
the analyses, and work by the Department of Energy at specific sites
tands to confirm that the flow regimes are such that this appro*;h is
adequate. The porosity is used to convert volumetric flow rates into
average effective fluid velocities in the direction of movement. In
particular, the volumeric flow rate ia divided by porosity to otain
an effective fluid velocity,. It is important to make this distinction
when considering the time of arrival of contaminated groundwater at
-------
polnt „
., che
b. .f
other phenomena aasum.d to be present
present, ft8 described later in the
••ctlpn on release mechanisms.
pr.viou.ly
b.
tn
clted
8qul£.r,
.1.0
. st.ndlrd
V.«OUB t.mp.r.tur.1 .t. built ifs th. mo
-------
analysis modele indicates ' chat the result* of the Agency's
calculations are not highly lensitive to these engineering
assumption*, and therefore, Che Agency has adopted a tingle Bet of
assumptions to be applied to repositories at all potential types of
sites. These parameters are shown in Table 2.2. The repository
considered in the Agency's analyses is assumed to contain 100,000
metric tons of heavy metal equivalent (MTHM) in the waste, which
assumption can be used to normalize the results of the Agency's
analyses to repositories with different total amounts of buried
radioactivity
Because of the rectangular areal dimensions of the repository.
for certain types of releases it is important to make a further
assumption about the orientation of the repository with respect to the
flow direction in the upper aquifer. It is assumed that the longest
dimension of the repository, 4 kilometers, is in the same direction as
this flow pattern. The mined volume of the repository, as well as the
porosity of "he backfill, enter into calculations of the amount of
radionuclides that might desolve in the water that would gradually
seep into the repository after its closure. Such dissolution might be
United by solubility factors, and therefore, this water volume is
significant. It is also important because It can be used to estimate
the amount of dissolved radionuclides that might be withdrawn by an
exploratory well that penetrates the repository at some point in the
future. The case of a salt repository is somewhat different because
it is anticipated that such repository would gradually seal-up after
closure as a result of salt creep. In this case, therefore, the time
to such closure and the amount of moisture present at that time, are
-------
important for the risk analysis. The other parameters in Table 2.2
enter in connection with epecific release mech*ni«mu «nd will be
discussed in subsequent sections.
•
2.8 Waste Package Parameters
The vast* package consists of essentially two main components:
the waste form and the canister. Each of these may actually be
somewhat complex. For example, the waste form consists of an initial
Inventory of radionuclidea contained in some physical matrix, which
may be unreprocessed spent fuel, borosllicate glass, or some other
alternative. Thus the waste form might have a somewhat homogeneous
physical structure, as in the case of glass, or a more complicated
one. The canister may actually consist of a number of components,
such as a more or less concentric set of containers with spaces,
coatings or other materials between them. For the conceptual analyses
reported here, the Agency has decided to adopt a relatively simple
model for the waste package, consisting of a relatively homogeneous
waste form and a single canister. The waste package parameters
entering the analysis are shown in Table 2.3.
The assumed total initial inventory of radionuclides is shown in
Table 2,4. This is based on a number of detailed calculations and
s
considerations elaborated on in the Population Risk Report and
summarized below, The radionuclide inventory in the waste will vary
according to whether the, waste consists of spent fuel itself or
whether the spent fuel has been reprocessed to recover some of the
tranuranic components. Since reprocessing is not part of the present
-------
national civilian nuclear program, we have assumed a radionuclide
inventory that IB roughly equivalent to that associated, with direct
disposal of spent fuel. For a reference repository based on 100,000
MTHM, the total radioactivity may be characterized roughly as shown in
Figure 2.4. Thia figure is based on calculations using the ORIGEN
Code. The fuel assembly structural components are essentially
insignificant contributors Co the total radioactivity for all periods
of time and have been ignored in the analyses. However, it is clear
from che graph that Che fission products dominate the radioactivity
early in the life of che repository, and che actinides and their
daughter products dominate che inventory somewhat later. The choice
of radionuclides listed in Table 2.4 is certainly not all inclusive,
but ic does include the dominant radionuclides, as well as a range of
radionuclides with somewhat different properties. Thus, a risk
Analysis based on Che radionuclides shown provides a valid
»
representation of the risk from a repository.
The canister is essentially a protective container that should
inhibit the leaching or the disolution of che waste form and Che
consequent transport of wastes cowards Che accessible environment. In
the risk analyses fcpuTLed here, che performance of che canister has
been represented in an approximate fashion by a user-specified
caniscer lifetime. Up until this time is reached, no radionuclides
5
are assumed to be released from undisturbed waste packages.
-------
The release rate of radionuclides from the waste form, after the
beginning of th« period when the canister can no longer be relied
upon, may be governed by one of two limiting physico-chemical
processes. The first of these processes la leaching, which is
essentially controlled by the accessibility of the radionuclldes
within the waste form to any groundwator with which the waste form may
come into contact. A leach rate, specified in terms of fraction
loached per year, is the input parameter used to characterize this
aspect of radionuclide release. The second limiting process is that
of solubility. Even though the groundwater may have access to
radionuclides within the waste form matrix, the water may already
contain sufficient amounts of the dissolved material so that no more
can enter into solution without corresponding precipitation, resulting
in a constant concentration. Such solubility limits for Individual
radionuclides are taken into account in the risk calculations reported
here. The values are based on literature values from a number of
sources and will be discussed in later sections in connection with
Individual geologic environments.
2.9 R»l«*flfe Mechanisms
The release mechanisms by which radioactive waste may leave the
repository and be transported to the accessible environment have been
introduced in Section 2.2. Both there and In Section 2.5 It was noted
that release mechanisms may lead to the direct transport of
-------
racilonuclides to the land surface or to che atmosphere, or they may
l*«d to the groundvat.r transport of vast, away from the repository.
The purpose of this section is to review the release mechanisms one by
one and de.cribe the conceptual models that have been incorporatsd in
the ri.k analyses. A more detailed ' mathematical treatment of the
•quations used to implement the conceptual models may be found in the
Population Risk Report and further background on the release
mechanisa. themselves can be found In the Task D Report. Six classes
•4HHV have been modeled by REPRISK. The meteorite release
mechanism, however, has not been included in J^j^lculations <£^H
-^•Because lt ls conslder.d to be of negligible significance in
estimating the performance of a repository or the selection of a
repository «ite.
Except for the case of a repository in salt, normal groundwater
flow refers to the movement of water through the repository horizon,
•ccprding to the natural hydrologie conditions, perturbed, perhaps, to
«ome degree by the presence of the repository. During the
construction and operation of the repository, it is expected that
water in the aurrounding rock would gradually be drained so that the
rock will enter an unsaturated conditon near the openings. (In the
ca«« of tuff, the repository would be located in a rock mass that
would be unaaeurated at the start, and hence this discussion requires
•light modification for the tuff case.) After the end of the
operational parlod and sealing of the r pository, it is expected that
water would gradually seep back into pores and fractures In the rock
and establish a flow regime connected to the regional groundwater
-------
system. The resulting flow patterns may be icmewhat different from
•*
Choc* prior to the excav«tion of the repository. For example,
the heat generated by Che waste may modify the hydraulic conductivity
of the surrounding rock and may also change the properties of water,
naking it less dense and less viscous.* The lower density can lead to
a buoyancy effect that may cause an increased vertical hydraulic
gradient. The decreased viscosity enables the water to flow more
easily through the rock and hence allows for potential increases in
flow rates. In the case of a repository in tuff, normal groundwater
flow refers to the downward percolation of water through the
unsaturated rock towards the water table. This downward movement is
not expected to be influenced greatly by the presence of the
repository, because the limiting factor is essentially the amount of
water itself. Release mechanisms within the category of normal
groundwater flow lead to potential radionuclide releases to an aquifer
with consequent transport of the radionuclides within the aquifer.
The faulting release mechanism is intended to cover both the
cases of a new fault occurring at a repository site (and intersecting
the repository itself), as well as the case of the reactivation of an
old apparently stable fault. This reactivation is also assumed to
lead to an intersection with the repository itself. This is treated
In Che model as a vertical planer structure with increased hydraulic
conductivity over and above that of the original rock. Thus, the
fault can lead to an essentially new and preferential pathway for
groundwter aovement. Its hydrological properties are different from
-------
chose of th. original rock and, in fact, there could be rather high
volumetric flows through «uch featur... In addition to creating a
flow pathway, the model for faulting assumes that since thac this can
be a relatively viol.nt and di.ruptiv. event, the integrity of waste
packages within a certain distance of the fault la destroyed. The
result is th. earlier un.et of leaching (if this had not already began
by the tin,, of fault movement). A. in the case of normal groundwater
flow, the only releases by faulting are assured to be via a pathway
connecting th. repository with an aquifer, thereby, enabling
groundwater transport of the waste. Faulting has been treated aa a
random sto.cha.tic process for purposes of the Agency', risk analyses.
This is not to say that faulting is a random process, but only that
faulting at a real r.spository site shold b. .bl. to be bounded by a
similar physical process that occurs randomly. The likelihood of new
or reactivated fault, is estimated on the basis cf geometric arguments
and simple probability concepts, as described in the next chapter for
salt.
The breccia pipe release mechanism refer* to the development of a
localized dissolution feature and only applies to the case of a
x
repository in salt. Breccia pipes, or collapse chimneys, have been
known to develop from the base of the salt beds and proceed vertically
in a relatively rapid and sometimes abrupt fashion. Such &> breccia
pipe is attuned to provide a relatively high permeability zone that
can facilitate the vertical movement of groundwater. In addition, it
ia assumed to violate the integrity of waste packages that are located
within the collapse zone. A probability of occurrence of such
-------
features ii based on observed statiatical average! and ic assumed to
occur randomly. Since good site ••lection procedure* should •n«bl«
the selection of a site where the likelihood of occurrence of a
breccia pipe is much less than the average occurrence rate, it is
•
believed that the resulting calculation will be conservative, that is,
tend to overestimate the risk. This i» the intention with respect to
all the modeli used by the Agency.
The occurrence of future exploratory drilling at a repository
site cannot be ruled out, even though steps will be taken in the.
decommissioning of the repository to signal to future generations that
dangerous materials are buried there. The Agency has considered a
wide range of potential purposes for drilling in different geologic
media, and ha* estimated drilling rates which are intended to be upper
bounds on the future likelihood of drilling at a repository site. In
estimating these values, no credit hat been taken for the
communication to future generations of the presence of the repository,
except that for a limited time in the future it is assumed chat such
communication would be completely effective. This value is specified
in individual risk analyses and will be discussed later. The dominant
purposes for future drilling may vary from one kind of geology to
another. For example, for salt deposits in sedimentary basins, the
dominant drilling is expected to be in search of oil and gas, whereas
in a granitic terrain the dominant purpose might be exploration for
water or minerals. The basis for the selection of drilling rates is
discussed in connection with each lithology in subsequent chapters.
The drilling release mechanism contains a number of components.
-------
4t is possible X
release of radionuclides to ar aquifer. In the calculations JjBB^E
^^••IHHm^this third release mechanism has not been explicitly
included because earlier calculations showed that its contributions
were negligible by comparison to other forms of release.
Volcanism also has the potential to release waste from a
repository, either by transporting it directly to the surface or by
-------
translocating it In an underground volcanic structure, such as a sill
or a dike, which may also encounter an aquifer. Calculations reported
in the Population Risk Report and the Task D report suggest that this
latter mode of transport is overwhelmingly dominated by the fault
release mechanism, in tarns of likelihood of occurrence, and that it
is roughly similar in terms of consequences. Therefore, it has not
been included in the comprehensive risk analysis because its
contribution la essentially negligible. The release of radionuclides
to the surface, however, with magma, ash, or gases passing vertically
upward through the repository has the potential for the significant
distribution of radioactivity to the accessible environment, even
though its likelihood is relatively small in most cases. It has
therefore been included in the calculations for certain lithologles
that have been investigated in relatively close proximity to active
volcanic regions. This does not mean that the Agency believes that
volcanism is likely at any of the sites that are being investigated,
but only that it may be the dominant low probability high consequence
•vent and hence should be included in the calculations to give an
adequate perspective on the risk. In the case of the release of
radionuclidee by volcanism, there are components in the model for
waste release both to the air and to the land surface.
4
The release mechanisms are further discussed in subsequent
chapters, largely with a view to summarizing the specific parameters
that are used to characterize them for the risk calculations. As
noted earlier, further details on the rationale and the struct re of
the models can be found in the associated documentation.
-------
2'10
A companion report to th. present one discusses in much greater
.detail the analysis of the transport of radionuclide. through the
accessible environment, leading to exposure to th. human population.
b\t.
T*4J 2.5 li, tg the pathways that have been considered. A large
fraction of this modeling takes place not within the risk analysis
code REPRISK itself, but rather in the form of radionuclide
release/health effects conversion factors that are incorporated in
REPRISK on the basis of the moling described in the companion
report. Health effects conversion factors are provided for both
•omatic and genetic health effects, in the form of cancer fatalities.
Some of the environmental transport parameter, are included
directly in the REPRISK code. These ar« described in the code
Documentation package, a. well as in the Population Risk Report. They
are constant for all lithologies and will not be discussed in any
greater detail in the present report.
The results of risk analyses as discussed in this report contain
relatively large uncertainties in th. numerical results. Such
uncertainties are due to a number cf factors, among them:
o the long-time frame over which predictions are needed;
o the simplified nature of the models In comparison with the
real physical situation;
-------
o the generic nature of the modeling, i.e. the lack of
incorporation of detailed alte «p*cific dat* for *
particular real site;
o the use of simplified system models that do not capture all
the detail* of individual physical, chemical, And geological
processes.
The purpose of the risk analyst* has been to make rough approximations
of the capabilities of geologic disposal of radioactive waste.
Therefore, despite these uncertainties, the Agency believes that the
JK A> W-^» 4 ** fuAatft JUCnW W .
estimates ger rated ^HB*" provide an adequate technical basis for the
associated regulations.
In order to lend perspective to the uncertainties in the reported
calculations, the Agency has proceeded as follows. First, in
estimating parameters or In choosing models to represent various
processes, an attempt has been made to consistently overestimate
factors that contribute to risks from the repository. That is, we
have attempt* ' to calculate via our risk analyses upper bounds on the
expected performance of the repository. This is the same philosophy
that was adopted in "the Population Risk Report and in the ADL Task D
, s
Report, where some of the underlying models and parameters were
developed. Second, extensive use has been made of sensitivity
analyses in order to understand how much the results of the risk
analyses vary with the variation in certain model components or
parameters. For parameters that are particularly crucial in
determining the final risk results, we have devoted special attention
to choosing appropriate values. Third, in cases where it has been
-------
difficult to model on a generic basis the characteristics of a sice or
a process, we hav« or.r.ASfiionally chosen to use more than one value for
a parameter, often referred to in the supporting documents as a "first
estimate" and a "second estimate", The first estimate parameter is
Intended to represent a process or condition at a site that is chosen
to be particularly favorible with respect to this phenomenon, whereas
the second estimate value corresponds to a site which while it may be
favorable in an overall sense, may be somewhat less advantageous with
respect to the particular phenomenon under discussion. The Agency
realizes that the choice of a real site for a geologic repository
ft
will represent a compromise between a number of desirable goals and
thus, it is quite conceivable that a site selected using a full range
of siting criteria responsive to the intent of this and other
I
regulations could still include conditions that would be better-
represented by the second estimate values.
It is important to distinguish between the type of uncertainty
. . ., . ^Jr»>v/.
included in the generic analysis ^0BeV her« *nd c^» uncertainties
that would remain with real sites when they are characterized and
modeled in connection with the decision on where to put a repository.
Many of the uncertainties included here might better be characterized
•A<*»L A
as variabilities, •^preal site^ there might be a wide variation in
the property in question. The attempt in our risk analyses to include
such variation corresponds then to an uncertainty in the final results
aa to how well they characterize the performance of the repository. A
real site will include the additional uncertainties associated with
data collection, site complexity and difference of opinion about a
-------
specific sice's character is tic. Such uncertainties are not properly
within the scope of the work reported here.
It should be noted that in the accompanying 40 CFR 191 standard,
attention ha* been devoted to the residual uncertainties expected to
be present at the end of the analyses of the real site. Therefore,
the. results
have not been translated directly into the numerical values
of the standard; rather, an additional margin for error and/or
uncertainty has been included in the numerical values in the standard.
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CHAPTtR 3 • REPOSITORIES IN BEDDED SALT
3.1 Introduction
For almost 30 years 0alt deposits have been considered to be a
prime candidate for a nuclear waste repository. There are a number of
reasons for this. Salt deposits are common in several regions of the
United States and they are found at depths considered to be suitable
for a repository. By their very presence, they indicate relative
geologic stability and hydrologic isolation, since if g -undwater had
ready access to them, the salt would have been dissolved and carried
away. While it is the case that almost all known salt beds are
undergoing gradual dissolution by groundwater, the rates of such
dissolution processes are generally so slow that these deposits are
expected to remain substantially intact for millions of years. There
is, in addition, extensive experience in constructing underground
mines in salt, and there is the additional advantage that gradual
creep of the salt will aid in the reseallng and the ree»-ablishment of
total Isolation of a repository placed in such an environment. On the
other aide, there is the disadvantage that if some unforseen
circumstances arise that introduce groundwater into contact with the
aalt near or at the repository, then the effects might be more severe
because they could be aggravated by relatively rapid dissolution. In
addition, salt deposits are located in sedimentary basins that often
contain other valuable resources such as oil, gas, and potash. As a
result, the adoption of a site for a nuclear waste repository may
either preempt the resources present at the site, or may lead to
future risks from efforts to obtain those resources.
-------
The Department of Energy i« actively investigating sites for ft
repository In bedded salt deposits in the Paradox Basin in Utah «nd in
the Palo Duro Basin in west Texas. The evaluation of specific sites
is a long and complex process expected to take a decade or longer;
thus it ie not possible to predict* ac part of the present study how
wall a specific site in either of these basins might perform in
isolating radioactive wastes. However, based on data collected Co
date by the Department of Energy and others, it is possible to define
certain idealized conceptual models of repository sites in each of the
two basins so as to make rough first approximations of the potential
performance of such repositories and to identify some of the
parameters that are most critical in determining that performance.
Al/*-U* %V /w\*6yS uw-
This chapter contains a summary of ^fjjj^ff^j^ff^^y the Agency
so-called "generic" sites that are based on simplified models of the
general geologic and hydrologic conditions reported at promising
locations in each of these two basins.
In connection witl the publication of the Draft High Level
Standard in 1983, the Agency also reported on risk analyses associated
with the repositories in salt. Such analyses were based on two types
of salt environments. The Agency used a bedded salt model based on
data available at that time for a number of basins being investigated
for salt repositories. (At that time the Department of Energy had not
yet settled upon the Paradox Basin and the Palo Duro Basin as the most
likely locations for a salt repository.) The most extensive data
available at that time was from the Delaware basin in southeastern New
Mexico, and this provided important information to the Agency in the
-------
^uiib Liuction ot its models. I"n order Co compare the previous analvses
for bedded malt with those carried out in connection with the Final
High Level Waste Standard, this chapter also contains a brief review
of th« parameters and results considered there. Also in connection
with th« draft Standard, the Agency considered the possibility of a
repository in a salt dome. This is quite different from the bedded
salt deposits previously considered. Such salt domes are corampn in
the Gulf Coastal Region of the United States and are being actively
investigated by the Department of Energy. The Agency concluded in
connection with its previous analyses that there were sufficient
problems associated with the disposal of radioactive wastes in a sale
dome that it was unlikely that such a deposit would ultimately be
proposed by Che Department of Energy-for -the 'actual"construction of a
repository. While it is possible that the site investigations being
carried out by the Department of Energy may lead to different
conclusions (so that a sale dome repository may ultimately be found to
be acceptable), the Agency decided not to expend further efforts in
th« evaluation of the performance of a repository in a salt dome.
Therefore, this geologic formation is not discussed here in connection
with the risk analyses supporting the High Level Waste Standard.
X
Section 3.2
cuonarizea the important input parameters that have been used in the
Agency's risk analyses for beddad salt. These parameters are reviewed
for three cases: Salt A (based on data from the Palo Duro Basin), Salt
B (based on data for the Paradox Basin), and the original bedded salt
stratigraphy considered in connection with the draft Standard. In
each case there is a figure that presents a cross-section of the
-------
assumed stratigraphy at the site. There is also on accompanying table
which summarizes the input parameters used to ehftracr.nrize the site
for the risk analysis models. Following this, there is a sequence of
tables summarizing the release mechanisms that have been included in
the risk analyses for the site. Sirfce most of these data can best be
presented in the form of such tables and figures, there is minimal
Cextual discussion of additional details. However, as noted
previously, there is a more extensive discussion of Che actual
mathematical models used for the analyses in the Population Risk
Report, as well as in the model documentation packages themselves.
-------
3 2 Inrut Parameters
The Input parameters for the risk analyses carried out by che
Agency fall into several categories. These are listed below along
with a reference to the section of this or the companion documentation
in which more elaborate discussion may be found.
i
o Site Parameters. These are discussed in this section for
Salt A, Salt B and the original generic bedded salt sice.
o Repository Parameters. These are the same for al'
lithologies and have been presented in Section 2.7 with a
summary table given as Table 2.2.
o Waste Package Parameters. " These have been discussed in
Section 2.8 and summarized in Table 2.3. , Radionuclide
inventory is the same for all lithologies, and leach rate is
a generic parameter that is given the same values and the
sane range of values (for sensitivity analyses) for all
lithologies. Canister lifetime has the same baseline values
for all lithologies, but for certain calculations reported
later a shorter canister lifetime for a salt repository has
been used. .The solubility limits for different
radionuclides do depend on the site, but are listed for all
lithologies In Table 2.3.
o Release Mechanisms. The input parameters to characterize
specific release mechanisms for the salt repositories are
discussed in this section.
-------
Therefore. If water were to
-------
effect could drive groundwater cowards the upper aquifer, which is
thus modeled as the pathway along which total cumulative releases have
been calculated. There are a number of differences in the
calculations carried out for Salt A and Salt B as compared to those
used in connection with the Draft. High Level Wait* Standard and che
generic salt' site in Figure 3.3. In this last case, there i« a
significant natural upward gradient that would move groundwater from
the repository to che upper aquifer and which would be additive with
the additional effect* of thermal buoyancy. In addition, the
temperature profile used for the early calculations with the generic
model (Figure 3.3) are based on higher heat loadings than are now
being considered by the Department of Energy. Because of this change
in the Department's apparent plans, the thermal buoyancy effects in
the earlier risk analyses, reported, for example, in the Population
Risk Report, were much greater Chan those presently believed to be the
case.
Let us also note here certain similarities and differences in
connection with the release mechanisms Included in the present
analyses. For salt repositories, the analyses discussed hare include
five release mechanisms: Faulting, breccia pipes, volcanes, drilling
without hitting a canister, and drilling and hitting a canister.
Other release mechanisms have also been considered and will be
discussed at the end of this section. Tables 3.4 through 3.8
summarize the important Input parameters associated with these release
mechanisms. In the case of faulting, the probability of occurrence
in somewhat higher than the original values used in connection with
the, draft standard. The new values are based on correlations with
-------
selsmiclty in a published 'report of the U.S. Geological Survey,
released since the earlier analyses were carried out. These value*
suggest that faulting is more likely to occur and to affect the
repository in Salt B (Paradox Basin) than in Salt A (Palo Duro Basin).
The likelihood of occurrence of breccia pipes has not been changed
from the original generic analyses. In both the Palo Duro and Paradox
Basins, chere is evidence of localized dissolution features and in the
latter basin at least these are often associated with collapse
chimneys or breccia pipes. One should not* from the model input
parameters that no such features are allowed to affect the repository
for the first one thousand years after closure because of the
assumption that detailed site selection and site confirmation studies
would be almost assured of Identifying such features if they were
already under development. However, it is possible that either as a
result of natural forces or as a result of the perturbation of the
natural environment by the presence of the repository conditions might
develop that would foster such localized dissolution with the
propagation of a collapse chimney vertically towards the surface. The
probability of this is still quite low, however. Volcanos have been
included in the analyses in order to represent what may be the
limiting very low probability but relatively high consequence event.
The parameters used to characterize than and to estimate their
probability are identical with those in the original ganeric analyses
and ara discussed in the references.
The possibility of future human intrusion leading to some
disruption of the repository has been an Important concern for the
Agency in developing High Level Waste Standard. While institutional
controls nay be successful In preventing such intrusion for a limited
-------
period 'of time and long-term'markers and other means of communication
«%
nay preserve knowledge of the sire still further, th« Agency hus,
adopted for Its risk analyses the underlying assumption that such
measures are of limited effectiveness and hence cannot be relied upon
•
completely to eliminate the possibility of future intrusion.
Therefore, in the original generic analyses described in the
Population Risk Report and in the references cited therein, certain
reference future drilling rates for each lithology were adopted.
These same values have been used in the present analyses. Some of the
future drillholes might even intersect actual waste canisters,
bringing a portion of their contents to the surface. Other drillholes
will not Intersect the canisters themselves, but may bring
contaminated groundwater to surface. It is also ^ossible that in
abandoning such future drillholes, a more permeable pathway tight be
established between the repository and overlying or underlying
aquifers, but calculations' the Agency has carried out suggest that
this is of negligible importance compared with the direct release to
the surface.
In addition to the release mechanisms discussed here, a number of
X
other possible release mechanisms for a bedded salt repository have
been considered, Of particular concern has been the question of
whether there should be a so-called "routine release" nu =haniem
Included in the analyses. Such a mechanism is included in the
analyses for hard rock repositories where a small but non-zero amount
of groundwater movement is anticipated through the r«;aaitory
throughout its lifetime. The Agency has considered flow through
gradually degrading seals in shafts and boreholes, as well as
groundwater movement through localized areas of the repository that
-------
may have higher hydraulic conductivity than the numbers generally
assigned to intact ealt. For example, groundwater «cep« frequently
occur in underground salt and potash mines, not throughout the entire
aine, but only through localized feature* such as "dirty" areas of
c«lc, dissolution/collapse features,-or other geologic anomalies. On
the basis of the calculations that have been carried put for such
features, the Agency has found no evidence that they could contribute
co non-negligible releases from a salt repqsitory. QMBBHMBM0
sensitivity analyses
it was assumed that 5% o the area of the repository had a
hydraulic conductivity roughly 5 orders of magnitude higher than the
-16
hydraulic conductivity generally assigned to intact salt (10 ).
-------
These are the
.11
"Port on Envlronmantal Pathways
th.
char.ct.rll. th. .tt. „
•°" '"Portant r.Uaaa ..chaniama.
3.1. ,.2. and 3.3 ,hou th.
u.ed to dsfine ^ llBpUfl>d
«.!».«. A..Mt.t.d vtth thas, fi,uv . „. IabUs 3.1, 3.2, and 3 3
th. mo.t
th. r..ulta
on ApD.ndIx A to
be.n uaad co con.trucc th. generic, cro....ectton, tor SaU A and
.. Th. cro»..«cclon ahovn In Flgu« 3.3 u b.s.d „ th. ..rtu,
c.rtUd «, ln e0nn.ctlon Blth th« Dr.£t
. It too ,. discu.Md In Appe7 iix ^ bu£
on. .hould conault th. Population ,i.k R.port .nd th. ,„„
about th. almll.rltt.s and «„.„„... ,mong
. d.pth o£ .pprol!lMt.ly
« th. alt. thar. 1, . !.rg. productlv. aoutf.r, th. og.11.1. Aoulf.r,
»btch .h.uld of cour.. b. .„ lBport>nt cMc>rn ^ jtcin
in thi. raglon. Ho«.v.r. ch.r. l. . S.n.t4l downv.r<,
hydrauuc h..d v.iu.. « v.rlou. ^peh< M ^ r>iui
•PP.«. that any .roundw.t.r pathway, that .tght p... through th. .alt
to hav. . downward
-------
CHAPTER U - REPOSITORIES IN GRANITIC ROCK
4.1 Introduction
Granitic rocks are attracting, increased attention as potential
host rocks for a nuclear waste repository. Such rocks are widely
distributed throughout the United Settee, and thus offer the
possibility of b«ing found in connection with oeher desirable
characteristics for a repository site. At depth they can be extremely
"tight", fhe naturally occurring fractures being kept almost complete
closed by the high lithostatic pressure. Mined openings in granitic
rock are expected to be highly stable for well chosen sites, and there
is considerable experience in such underground excavations from
various kinds of hard rock mines end tunnels. Unlike salt, the
likelihood of associated valuable resources Is much lower; when they
are present they are often in veins at the boundaries of the granitic
bodies or plutons. Water wells are occasionally drilled into granitic
rock, but because of the general trend of decreasing permeability with
depth, such wells rarely exceed several hundred feet. An important
distinction between granitic rocks and most of the other host rocks
being considered for a repository is that they are not generally found
in a layered structure as are sedimentary rocks, or even basalt and
tuff, and thus the possibility of extensive aquifers at a depth below
the repository Is much less likely. This decreases the possibility of
« productive and high pressure source of water that could cause upward
flow and carry radionuclides towards the surface. On the other hand,
the certain presence of fractures and the water saturated condition
expected at depth virtually guarantee that there would be some
-------
|ro«mtwatsr movement through "R repository in granite. It may occur av
extremely low volumetric flow rates and velocities; but unlike sale,
which is virtually impermeable, it would b« present and must be taken
into account in estimating the performance of a repository.
*
The Department of Energy is actively investigating sites for a
repository in granitic rocks in the North Central and Northeastern
regions of the United States. The Department had previously carried
out a screening of the entire United States and had Identified these
regions as most likely to contain suitable repository environnu "s in
these kinds of rocks. The evaluation of specific sites is a long and
complex process expected to take a dacade or longer; thus it is not
possible to predict as part of the present study how well a specific
site in either of these regions might perform in isolating radioactive
wastes. However, based on data collected to date by the Department of
Energy and others, it is possible to define certain idealized
conceptual models of repository sites in each of the two regions so as
to make rough first approximations of the potential performa-".e of
such repositories and to identify some of the parameters that are most
critical in determining that performance. This chapter contains a
summary of Wf^fjf^^^^ by the Agency %^so-called "generic"
sites that are based on simplified models of the general geologic and
hydrologic conditions reported at promising locations in each of these
two regions.
In connection with the publication of the Draft High Level
Standard in 1982, the Agency also reported on risk analyses associated
with repositories and granites. At that time the Department of Energy
-------
had noc yet settled upon the North Central region or Northeastern
region as the most likely granitic terraines for eh« location »[ a
repository, and therefore, the parameters used by the Agency were
chosen from more general surveys Of granitic rocks throughout the
*
United States. In some regions the granites extend to or outcrop at
the surface, so that the only significant water bearing zones above a
granice repository might be very near surface deposits, generally
glacial in nature, and in an upper weathered layer of granite. In
other regions the granites might be rather deeply burled at hundreds
or even thousands of feet, so that a granitic repository might have
above it a sedimentary sequence that contained aquifers that would
themselves be at depth. In the earlier analyses carried out by the
Agency, this latter situation was in fact assumed, but after a review
of recent Department of Energy data of potential granitic sites, the
generic granite model has undergone some revision. The result takes
the form of two new models of a granitic host rock, one based on the
North Central region and one based on the Northeastern region. It
should be emphasized that these are still generic in nature and are
not intended to represent performance at a particular site.
Furthermore, the Department of Energy's investigations have not
proceeded to the investigation of specific sites, as they have in the
case of talt, and therefore the selection of data Is even nor* general
•nd hypothetical. It is the fancy's opinion that the parameters
represented here present a valid current estimate of the performance
of a repository in granite and that they fall in the range values to
b« expected at specific real
-------
^
tnput
Agency, rlak analyie, for granite
b. Pr.a.nt.d ln th.
««,.! dlscus.lon of
. ther. 1( .
Report, .. V.U „ in
th. ^j doculMntati<]n
-------
4.2 Input Parameters
The input param.ters for the risk analyse, carried out by the
Agency fall into several categories. These are ilsCed below along
with a reference to th. action of this or the companion documentation
in which more elaborate discussion may be found.
o
PKf PflrMnrrm. The., are discussed ln this section for
Granite A, Granite B and the original generic granite site.
&eP°iUory Palmers. These are the same for all
lithologiea and have been presented in Section 2.7 with a
•unwary table given as Table 2.2.
These have been discussed in
Section 2.8 and summariz.d in Table 2.3. Radionuclide
inventory is the same for all lithologies, and leach rat. is
a generic param.ter that is given the sane values and the
•am. range of values (for sensitivity analyse.) for all
lithologies. Canister lifetime has the same baseline values
for all lithologies, but for certain sensitivity analyses
different variations on canister lifetime have been
-------
considered, based' on the Agency's estimation of the
harshness of Che environment in which the cantstor would be
placed. The solubility limits for different radionuclides
do depend on the site, but are listed for all lithologies in
Table 2.3.
•
Q Release M»$himiams. The input parameters to characterize
specific release mechanisms for the granite repositories are
discussed in this section.
o Environmental and Health Effects Pqrfljflft^ars,. These are the
sane for all lithologies and are discussed in the companion
report on Environmental Pathways Analysis.
Thus the focus of this section will be on the parameters used to
characterize the site as well as those used to represent the
most important release mechanisms.
Figures 4.1, 4.2, and 4.3 show the geologic cross*sections that
have been used to define the simplified models used in the Agency's
analyses. Associated with these figures are Tables 4.1, 4.2, and 4.3,
which provide the most important geometric and hydrologic parameters
chat affect the results. These figures and tables of parameters are
based on Appendix A to this report, which is a detailed discussion or
how region-specific data from the North Central region and the
Northeastern region have been used to construct the generic
cross-sections for Granite A and Granite B. The cross-section shown
in Figure 4.3 Is based on the earlier analyses carried out in
connection with the Draft High Level Waste Standard. It too is
discussed tn Appendix A, but for more elaboration one should consult
the Population Risk Report and the references cited therein.
-------
Important observations about the similarity. ^ *<**
"iwiiarities and differences among
these sites are summarized below.
Both ,lt.. havc che repo$ltory
approximat.ly 460 m.ters, whlch la .
offer a reasonable balance between the "tightness" of greater depth,
*nd the .ngin.ering practicalities of .halter depths. While th.
repository is th. s«e d.pth below th. surface in both instances, in
Granite A, the surface deposits and weathered rock that constitute the
overlying aquifer are very thin and include ponds, bogs, or other
surface waters. Thus, for this setting, the groundwater in the
•quifer t. in intimate contact with surface water so that the
•ce.seible environment is at th. sit., rather than some distance away.
Cranit. B, on the other hand, is modeled with , considerably thicker
aquifer, but without surface wat.re. For this sit., th. accessible
•nvirorunent is two kilonet.rs away and a delay is implicit in the
«roundwat.r's having to traverse that two kilom.ter distance.
suggested ..rli.r in this chapter, ther. is no underlying
in .lthtr Granlte A o
i. .xpected t
-tur.ted with water. Under natural conditions, ther. would be no
h..d differential b.tw..n th. wat.r at the r.pos-ory d.pth and in the
overlying .qulftr, .nd chufl( no VBftlcal ^^^ o£ ^^ ^ ^
rock. However, a vertical gradient may develop a. . result of water
-------
gradient is dependent upon the^ temperature rise due to the HLU «nd on
the height of the water column that is .affected. The buoyancy effect
is a factor not only for normal.leakage along existing flow pathways
but also for n.w pathways that might develop or be introduced during
the life of the repository.
The release mechanisms included in the present analyses arc the
same for both granite sites, but differ in come aspects from those
pr.viously considered for salt. For granite, the release mechanisms
examined included: routine releases, faulting, volcanoes, drilling
without hitting a canister, and drilling and hitting a canister.
Routine releases, which involve groundwater migration through bulk
rock and engineered openings, such as shafts, are analyzed using rock
properties cited in the literature and assumptions about material
degradation which are discussed in the Population Risk Report. In the
case of faulting, the probability of occurrence has been changed from
.earlier values and is based on correlation between fault movement and
selsmicity in a recently jablished report of the U.S. Geological
Survey.
Volcanoes have been included in the analyses in order to
represent what may be, the limiting low probability but relatively high
eoiu.quence event. Since suitable granite sites may be found in many
pares of the country where the probability of volcanism is vary low,
both Granite A and Granite B sites are expected to have the same very
low probability of volcanisra. The parameters characterizing volcanoes
are identical to those used in the original generic analyses and are
diacussed in the references.
-------
„,
"" """^ " 1" 1B Mlt. due «. th.
HO..V., th.r. „.
u.t.r
.re
fer
th.t after th.t
o£ hum.n
^it ^ ^
P-rt .f . c.nut.r on
.u... th« r.t.
u gr.ntt.. r.th.r th.n
only P.rtuny ...u* by ltthelt«te
..... "t"11' ""'""«- - «"-...ling. Th.
th. fact
throughout
ef
of
ac
-------
tn th.
and the original gcntric r.poaitory.
-------
CHAPTER 5 •' REPOSITORIES IN BASALT
^
5.1 Introduction
Basalt deposits in the Pacific Northwest have been under
investigation for a number of years as potential host formations for a
nuclear waste repository. Basalt is a dense, dark, fine-grained rock
formed by the solidification of volcanic lava. The basalt deposits in
tho Northwest are flood basalts. They were extruded over extremely
large areas and formed a layered structure of individual flows tens to
hundreds of feet thick, separated by relatively minor sedimentary
deposits and fractured or highly porous zones at the tops and bottoms
of che basalt flows. The dense interiors of the basalt flows are the
potential repository host rocks under consideration. Similar to the
case of granite, basalt deposits are permeated by fractures, but at
the depths being considered for a * repository, these fractures are
expected to be quite tightly closed, thereby restricting the volume
and the velocities of any groundwater movement. Nevertheless, there
is expected to be some groundwater migration through a basalt
repository and it is possible this might be accelerated by repository
induced effects on the host rock. Unlike granite, the layered
structure of the basalt deposits provides for horizontal groundwater
movement through relatively permeable zones between flows. In
addition, the fracturing in a basalt deposit is expected to be
somewhat greater than that in a well-chosen repository site in
granite. This does not mean that such fracturing would lead to
unacceptable repository performance, but only that it must be an
important consideration in choosing a site and in estimating the
performance of a repository At that site.
-------
Tha Department of Energy has been actively investigating the
possibility of siting a repository in basalt at the Hanford
Reservation in southeastern Washington State. Tha Agency has paid
particular attention to tha possibility of a repository in basalt both
*
during the development of Che Draft High Level Waste Standard, as well
as in conjunction with the developnent of the Final Standard. The
relatively advanced stags of the Department of Energy investigations
at Hanford have provided considerable data en the characteristics of
potential cites and repository host flows. However, much of the work
*S
carried out at Hanford has been the subject of severe criticism by the
Nuclear Regulatory Commission and others, and therefore the Agency has
devoted special attention to incorporating in its analyses input not
only from the Department of Energy and its contractors, but also from
technical professionals from other organizations. Baaed on such data,
the Agency believes that it is possible to define conceptual models of
"» " •
a basalt repository that should be adequate to make rough first
approximations of the potential performance that might be expected
from such repositories and to identify some of the parameters that are
most critical in determining that performance.
Section 5.2
summarizes the important input parameters that have been used in the
Agency's risk analyses for basalt. These parameters are presented
both in the form of a generic basalt cross -section, which defines the
geometry And general hydrologic structure for the modeling effort, as
veil as an accompanying table which includes the detailed input
parameters that have been used in the analyses. Following these there
la a sequence of tables characterizing the release mechanisms that
-------
have b.en included in th. riik .n*ly,ea {ot th. site. Slnce ^ of
those data can beat be presented In Che fern, of such t.ble£ and
figures, cher. i. uinimm textual discussion of additional details.
Hov,v,r, a. noted previously, there is a more extend discussion of
the actual mathematical models used for th. analyses in th. Population
Ri.k Report> as well as in the mod.l documentation packages
themselves.
-------
5.2 Innut Parameters
Tho input parameters for the risk analyses carried out by the
Agency fall into several categories. These are listed below along
with a reference to the section ' of this and the accompanying
documentation in which wore elaborate discussion may be found.
o 51 U P«™*«t«rs. These are discussed in thi» section for
the current, «« veil »* the original generic basalt sites.
o Repository Parameters. These are the same for all
lithologies and are presented in Section 2.7, with a summary
table given as Table 2.2.
o u.-t«. P*ck*«e Poster.. The.e hav. been discussed in
Section 2.8 and summarized in Table 2.3. Radionuclide
inventory is the same for all lithologies and leach rate is
a generic parameter that is given the sane value (and the
same range of values for sensitivity analysis) for all
lithologies. Canister lifetime has also been varied for
sensitivity analysis. The solubility limits for different
radionuclides do depend on the site and are listed for all
lithologies in Table 2.3.
o Pflflflj-r Maehanisma. The input parameters to characterize
specific release mechanisms for the basalt repository are
discussed in this section.
o *nn rental and BMUh EffftCti Famine Wi. These are the
•aae for all lithologies and are discussed in the
accompanying report on Environmental Pathways Analysis.
Thus the focus of this section will be on the parameters used to
characterize the site and the most important release mechanisms.
-------
Figures 5.1 and 5.2 show the geologic cross-sections that have
b»en ue«d to defin* th« simplified modal* in the Agency's analysis.
Associated with these figures are Tables 5.1 and 5.2, which provide
the most Important geometric end hydrologic parameters that affect the
t
results. The figures and tables of parameters are based on Appendix A
to this report, which is a detailed discussion of how site-specific
data from the Hanford Reservation have been used to construct generic
cross-sections for a basalt repository. The cross-section shown in
Figure 5.3 is based on the earlier analyses carried out in connection
with the Draft High Level Waste Standard. It too is discussed in
Appendix A, but for more elaboration one should consult the Population
Risk Report and the Task D Report.
A comparison of Figures 5.1 and 5.2 indicates that the original
model used In connection with the development with the draft standard
assumed a repository at a shallower depth than is now the reference
plan of the Department of Energy. In addition, the earlier model
grouped together a number of basalt flows into a single unit for
modeling purposes, with the result that the upper and lower aquifer
vere further removed fron the repository. More recent data suggest
s
that it is important to consider as possible migration pathways the
upper and lower boundaries of the actual individual basalt flow in
which th« repository is located. As shown in Figure 5.1 this puts the
repository much closer to the model aquifers. Other modifications to
the site parameters incorporated in the more recent modeling Include a
lowar natural vertical gradient from the repository to the upper
aquifer and some modifications in other hydrologic parameters, as can
be seen in the tables. In addition, the temperature profile used for
-------
Che earlier calculations wltfi the generic basalt model .re based on
higher heat loading, than are now being conalder.d by the Department
of Energy. A. . result of thi. change in the Department', apparent
plan., the thermal buoyancy effects in the earlier ri.k analyses
reported, for example in the Population Ri.k Report, are much greater
than those presently believed to be the case.
Essentially the same release mechanisms have b«tn maintained
throughout the Agency's considerations of a basalt repository. The
so-called routin. release mechanl.m described in Table 5.3 refers to
th« fact that there Is expected to be some amount of groundwater
movement through a basalt repository due to the presence of fractures.
This flow may be accelerated by two factor.: an increa.e in hydraulic
conductivity after the thermal peak, due to slippage and movement of
the basalt along fracture planes, and a reduction in the water
viscosity due to higher temperature. Table 5.4 summarizes the
parameters used to characterize the faulting release mechani.m. The
probability of occurrence of faulting is somewhat higher than used in
che earlier analyses and i. based on a more recent faulting model
developed by the United States Geological Survey. The effect of
faulting Is to create (either by the development of a new fault or
reactivation of the old fault) a preferential groundwater pathway
which would have higher potential for flow than the intact rock and
would connect the repository with the adjacent aquifers, Table 5.5
summarize, the parameters associated with the volcanism release
»*chani«m which essentially provides a limbing low probability high
consequence event and suggests the general shape of the CCDF near the
lower extremes of probability. The parameters used here are identical
-------
with thos* used In the original generic analyse* and are discussed in
the references.
The possibility of future human intrusion leading to some
*
disruption of the repository has been an important concern for the
Agency in developing the High Level Waste Standard. While
institutional control may be successfully in preventing such intrusion
for a. limited period of time and long-term markers and other means of
communication may preserve knowledge of the sites still further, the
Agency has adopted for its risk analysis the underlying assumption
that such measures are of limited effectiveness and hence cannot be
relied on completely to eliminate the possibility of tuture intrusion.
Therefore, in the origin*! generic analyses described in the
Population Risk Report and in the references cited therein, certain
reference future drilling rates for each lithology were adopted.
These same values have been used in the present analyses. Some of the
future drillholes might even intersect actual waste canisters,
bringing a portion of their contents to the surface. Other drillholes
will not intersect the canisters themselves, but may bring
contaminated groundwater to the surface. It is also possible that in
abandoning such future drillholes, a more permeable pathway might be
established between the repository and overlying or underlying
aquifers. The calculations the Agency has carried out suggest that
this is of negligible importance compared with the direct release to
tha surface. The drilling parameters included in the risk 'analyses
are summarized in Tables 5.6 and 5.7.
-------
CHAPTER 6 - REPOSITORY IN TUFF
<
6.1 Introduction
Welded tuff has recently received increased attention as a
potential host rock for a high level waste repository. It is unique
among the geologic media considered for analysis by the Agency because
the deposits of tuff apparently most appropriate for a repository
occur in southern Nevada, where the water table is »ome distance below
potential repository horizons. .e welded tuff« that are the
candidate host rocks for this analysis consist of airborne volcanic
d«bri,« that fused into a mass with high porosity and low permeability.
They appear to have the necessary engineering properties for
repository construction. Because the tuff is composed of fragments of
porous volcanic rock, the residence time of water moving through it is
relatively long, and the mineral assemblages can be expected to
provide favorable retardation. Tuff shares with granite and basalt a
relatively low occurrence of fuels ^r valuable minerals that might be
exploited by future drilling to any considerable depth. Similarly,
the depth of the water table is a discouragement against the sinking
of wells or the development of underlying aquifers.
Two distinctive and important features emerge from Sandia
National Laboratory's (SAND84-1492) and the Agency's analyses of a
repository above the water table in tuff. First, unlike any other
»«dium, upward flow from the repository is not possible so long as the
rock remains unsaturated, and groundwater could not accumulate in the
repository. Second, as long as precipitation at the surface is low
-------
enough to maintain an unsaturated condition, ws'er •*". a flowpath, such
<*.
as a faulc zone, will preferentially move into the tuff by capillary
attraction, rather than downward along the fault trace (SAND84-1A92).
This helps to maintain long travel times.
•
The Department of Energy is currently investigating the area
including Yucca Mountain in southern Nevada as a possible candidate
site. Other tuff sites nay be found, but Che relative abundance of
hydrogeologic data for this location, coupled with the very low
preci-
-------
* '• ?^rut Parameters
The input parameters for the risk analyses carried out by the
Agency fall into several categories. These are listed below along
with a reference to the section of this or the companion documentation
•
in which more elaborate discussion may be found.
o Site Parameters. These are discussed in this section for
tuff.
o Repository Parameters. These are the same for ell
lithologies and have been presented in Section 2.7, with a
summary table given as Table 2.2.
o Waste Package Parameters. These have been discussed in
Section 2.8 and summarized in Table 2.3. Radionuclide
inventory is the same for all lithologies, and leach rate is
a .generic parameter that Is given the same values and the
same range of values (for sensitivity analyses) for all
lithologies. Canister lifetime has the same baseline values
for all lithologies, but for certain sensitivity analyses
different variations on canister lifetime have been
considered, based on the Agency's estimation of the
harshness of the environment in which the canister would be
placed. The solubility limits for different radionuclides
do depend on the site, but are listed for all lithologies in
Table 2.3.
o Raleafe Mechanisms. The input parameters to characterize
specific release mechanisms for the fmgejs^pPHslBHssV* are
discussed in this section.
-------
Section 6.2 summarizes the importer, input parameters that have
^
been used in the Agency's risk analyses for tuff, including a figure
Chat schematically illustrates the cross-section of the site and a
tabla that summarizes the input parameters used in the risk analyses.
Following this, there is a sequence of tables summarizing the release
mechanism* that have been included in the site's risk analyses. Since
most of these data can best be presented in the form of tables and
figures, there is minimal discussion of additional details. However,
the actual mathematical models used for analyses are discussed at
length in the Population Risk Report and in the Sandia report
previously referenced.
-------
o Envlronmentpl and Heplfh Effects Parameters. These are the
• arao for all litholosleo and are discussed in the companion
report on Environmental Pathways Analysis.
Thus the focus of this section will be on the parameters used to
«
characterize the tuff site as well as those used to represent the most
important release mechanisms.
A^ J
Figure 6.1 shows the geologic cross-section that^PW been used
to define the simplified tuff model used in the \gency's analyses.
Associated with this figure is Tables 6.1, which presents geometric
and hydrologic parameters that affect the results. This figure and
table are based on Sandia National Laboratory's report on the Yucca
Mountain sice (SAND84-1492), which includes detailed discussions of
parameter values and.derivations and representative cross-sections.
Important observations contrasting and comparing the tuff
repository site with the other lithologies may be summarized as
follows. Tuff contains a repository at an approximate depth of 400
meters, which is 100 meters above the regional groundwater table. The
repository horizon and overlying formations are unsaturated. Unlike
other lithologiea that have been studied, where the host rocks are
essentially saturated (though generally highly impermeable), flow
through tuff is limited by the availability of water more than by the
impermeability of the rock. In order to analyze this source-limited
condition, the Agency used an effective hydraulic conductivity that
had been back-calculated by Sandia Laboratories (SAND84-1492) from
data on groundwater flux. When used as the conductivity in darcy flow
calculations, Sandia1s value yields a flow rate that approximates the
actual measured flow.
-------
Another important difference .between the hydrology of the tuff
eito and the other iite« is the pronounced downward hydraulic gradient
found in tuff. Because o£ the unsaturated condition, this gradient.
which is simply the direct force of gravity, ii essentially unaffected
• . '
by temperature changes in the rock due to the heat output of the
waste*. Thu*. the vertical gradient is downward and constant and for
«ny groundwater release mechanism, and the only flowpath is towards
the water table.
The horizontal gradient in the underlying aquifer is quite small
around the tuff site. Although the conductivity of the rock at the
level of the water table is relatively high, compared to deep aquifers
in other lithologies, the very low gradient delays the migration to
the accessible environment of material in the aquifer.
The release mechanisms included in the analysis of tuff are:
routine releases, faulting, volcanoes, drilling and not hitting a
canister, and drilling and hitting a canister. Tables 6.2 through 6.6
sunmariza the important parameters associated with these release
mechanismsv For faulting, the probability, based on U.S. Geological
Survey estimates, is higher than for areas examined for moat other
lithologies, but the hydraulic conductivity of the flowpath produced
by A fault is co isiderably smaller. This is because water movement
down the fault trace would be effectively limited by the availability
of the water, much of which would be absorbed by the rock adjacent to
the fault. Thus, the effective hydraulic conductivity of a fault is
quite low.
-------
Volcanoes were analyzed in a- merrier analogous to the o:her
lithologiet, adjusting only the probability of a volcanic vent
intersecting a repository to conform to volcanic probability estimates
for the Nevada. Teat Site area determined by the U.S.C.S. (U.S.
Geological Survey, open file report 80-357). The consequences of such
an event are determined by the geometry of the repoaieory and the
\
volcanic vent, and are the tame for all lithologiea analyzed.
The potential for human Intrusion was Judged as likely in tuff as
In granite. While the Agency has addressed thia concern In developing
the High-Level Waste Standard, it recognize* that it is probably
impossible to perpetually maintain control over access to a site, or
to rely on markers or other means to prevent drilling And exploration
in the future. Therefore, in accordance with the generic analysis
described 'in the Population Risk Report, and references cited there,
'future drilling rates for various litologiea were adopted. Since tuff
was not one of the lithologies originally considered, a drilling rate
is not explicitly discussed in that Report. The Agency subsequently'
estimated, however, that the rate determined for granite would be an
appropriate surrogate for a rate in tuff.
Routine releases from the repository are limited by the amount of
water available. Thermal effects on vertical gradient and hydraulic
properties of the rock are have been ignored in the analysis because
Chay ara negligible, compared to the restraints imposed by the
relative lack of water. As modeled, there is a general flow of
migrating radionuclidea toward the underlying aquifer, and thence
horizontally to the accessible environment.
-------
SCENARIOS
ACCESSIBLE
ENVIRONMENT
DIRECT
PATHWAYS
TO SURFACE
WASTE
FORM
CANISTER
REPOSITORY
LAND
SURFACE
HUMAN
%
POPULATION
PATHWAYS
TO AQUIFER
(GROUNDWATER)
AO 1 IIP F R
SURFACE
WATER
1
FIGURE 2.1
Structure of Risk Analyses
for High Level Waste Repository
-------
10'2
<
CD
O
tr
10
'6
10'8
(C,P)
Probability is P of
consequence at least
as large as C
I
I
0.1 1
CONSEQUENCE, C
10
100
FIGURE 2.2
Example of Complementary Cumulative Distribution
Function (CCDF)
-------
Surface Deposits
Upper Aquifer
Upper Confining Beds
Hose Rock
. . Repository
Lower Confining Beds _ ''r
Lower Aquiter
T_-
Basement Rock£
FIGURE 2.3
General Cross-Sectional Structure Used in Risk Analysis
-------
Assembly Structure
and Components
10
Decay Time from Discharge
(vears)
FIGURE 2.4
10-
10
10
Total Radioactivity in Reference Repository
(from 100,000 MTHM)
-------
Upper
Aquifer
DEPTH (m)
0
305
r^rir_-.Upper-
Solt
Repository
550
595
— — Contirung
•-•."••^•••2000
FIGURE 3.1
Cross-Sectional Stucture Assumed for Salt A
-------
DEPTH (m)
0
— Surfoce Deposits
Upper .
314
rt_n_Confining Beds :j£3£H3^Lr^r£r!H?irtj£?^riJ'ir^"iri^^
~^^^^^^^^^^^^^^^^^^^^^^^^^^^^-^-^~-^~-^ 945
Solt 6 ——^—. -»— Repository
1021
ririiC onfi m ng _B e_d£ f^—HHrlHHHHHHHKriHHHHHHHH:^
•"-•;-^:^-:--::.:c^:'-\;"-^:-'-',v^S :^'^V^v'>/;-v-7:v;r.'.-?;•;•;:.;.--;:;;:-^ "~'.£-V;vv'.
Aquifer •;^>=^^^:^v..>;,;.,^.^^>v^^;^:^vx.^vV'/••.••"•/^'•>^::^•^':^vv^;'H•:.
.y..:.^i:. ^^^•.••^>.;-^^--;-y;..;:.:;.:..:;^.-.v.:vr.••:.-.:•:•-.::••••..•V:-.-:/~x ,890
FIGURE 3.2
Cross-Sectional Structure Assumed for Salt B
-------
DEPTH
(m)
Surface Deposits
Upper \
Aquifer!
Salt
•^•^^^^-•-ij^r: • 350
330
•— 410
Repository
^KKP: Confining Beds gr-r-r->Jb-z->J
!V"^V-^T-VV?>MriMir^ 560
510
590
FIGURE 3.3
Cross-Sectional Structure in Original
Salt Analysis
-------
DEPTH
(m)
Surface Deposits
FIGURE 4.1.
Cross-Sectional Structure Assumed for Granite A
-------
Surface Deposits
Aquifer
FIGURE 4.2
Cross-Sectional Structure Assumed for Granite B
-------
Aquifer
460
FIGURE 4.3
Cross-Sectional Structure Assumed in Original Granite Analysis
-------
Depth
(meters)
Surface
> . • _ .
1S5
FIGURE 5.1
Cross-Sectional Structure Assumed for Basalt Repository
-------
Depth
(meters)
0
Surface _
Deposits.
FIGURE 5.2
Cross-Sectional Structure Assumed in Original Basalt Analysis
-------
Depth (m)
0
Welded Tuff »ZZ"
!f™!^...r:.v. jlep o s ijt o r v£"S:"''"'
•••••*•**** •••••••*** •••••••**** „•*•••••*"
o a
*
<* «
0 « o
0 4 °
0
»*•.- •?,
••
" ° * ° 0 °6
* «• 0-
«' 0 * • .
° 4 4 •
0 . Aquifer « » p
» ° Welded and Non-U'elded Tuff
M
<* *
0
0
4 *
0 4
300
400
500
1500
FIGURE 6.1
Cross-Sectional Structure Assumed for Repository in Tuff
-------
TABLE 2.1
Site Parameters Considered in Risk Analysis
Distance between repository and "upper aquifer" and/or
"lower
aquifer"
Thickness of aquifers
Hydraulic conductivity of aquifers
Porosity of aquifers
Horizontal gradient in aquifers
Retardation values for individual nuclides in aquifer
Average hydraulic gradient along a hypothetical vertical
pathway between the upper aquifer and lower aquifer
Hydraulic conductivity of the rock between the upper aquifer
and the .lower aquifer
Porosity of the rock between the upper aquifer and the lower
aquifer
H( :zontal distance along aquifer to point regarded as
release point to accessible environment
-------
Table 2.2
Repository Parameters Considered In Risk Analysis
PARAMETER VALUE SOURCE
Dimensions of repository EPA 520/3-60-006
Length 4,000 meters
Width 2,000 meters
Height 5 meters
Total mined-out volume l.OxlO7 m3 EPA 520/3-80-006
Average porosity of backfilled 0.2 EPA 520/3-80-006
repository
Time to maximum backfill compaction 200 yr EPA 520/3-80-006
due to creep (salt only)
-------
Table 2.2 (continued)
Repository Parameters Considered In Risk Analysis
PARAMETER
Number of canisters of HLW
Number of waste drifts
Canisters per drift
Length of waste drift
Canister spacing
.VALUE
35,000
350
100
500 meters
5 meters
SOURCE
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
-------
Table 2.3
Waste Package Parameters Considered In Risk Analysis
PARAMETER VALUE SOURCE
Canister lifetime 300 yr* Agency Decision
Fractional leach rate (fraction per 1.0x10"** EPA 520/3-80-006
year)
*These values have been varied in the sensitivity analysis.
-------
Table 2.3 (continued)
Waste Package Parameters Considered In Riak Analysis
Solubility Limits for Each
Radionuclide by Mediua (Ci/ml)
Isotope
Anr-241
Am- 24 3
C-l'.
Cs-137
1-129
Np-237
Pu-238
Pu-240
Pu-242
Sr-90
Tc-99
Sn-126
Zr-93
Half-Life
458
7.650
5,730
30
16xl07
2.1xl07
89
6,260
380,000
29
210,000
100,000
950.000
Initial
Inventory
(per canister)
4.9xl05
4.9xl03
S.OxlO"1
2.5xlO"5
l.lxlO"1
9.4xlO~l
9.4xl02
1.4xl03
4.86
1.7xl05
4.0
1.6
5.4
Salt
3.2X10"1
1.9xlO"2
. none
none
none
7.1xlO"7
1.7xlO"2
2.2xlO"4
3.9xlO~6
none
1.7xlO~5
2.8xlO~5
4.0xlO~7
Granite
3.2x210"3
1.9xlO~4
none
none
none
7.0xlO~5
1.7xlO"2
2.2xlO~A
3.9xlO~6
none
none
2.8xlO~6
4.0xlO~9
Basalt
3.2xlO"3
1.9x10"*
none
none
• none
7.1xlO~8
1.7xlO"4
2.2xlO~6
3.9xlO~8
none
1.7xlO~5
2.8xlO"6
4.0xlO~7
Tuff
3.2xlO~3
2.0xlO"4
none
none
none
7.1xlO"5
1.7xlO"2
2,2xIO"A
3.9xlO"6
none
none
2,8xlO"6
4. 0x10" 7
-------
Table 2.4
Initial Radionuclide Inventory in Reference Repository
Radionuclide Curies Half-Life
Acr-241 1.7 x 108 458
Am-243 1.7 x 106 7.650
C- 14 2.8 x 104 5,730
Cs-137 8.6 x 109 -- 30
1-129 3.8 x 103 1.6 x 107
Np-237 3.3 x 104 89
Pu-238 3.3 x 107 24.400
Pu-240 4.9 x 107 6,260
Pu-242 1.7 x 105 3.8 x 105
Sr- 90 6.0 x 109 29
Tc- 99 1.4 x 106 2.1 x 105
Sn-126 5.6 x 10* 1.0 x 105
Zr- 93 1.9 x 105 9.5 x 105
-------
Table 2.5
Structure of Environmental Pathways Analysis
Release Mode
Release to River
Release to Ocean
Release Directly
to Land Surface
Release to Air Over Land
Resuspended Material
Release to Air Over Ocean
Pathways Analyzed
Drinking Water Ingeition
Freshwater Pish Ingestion
Food Crops Ingestion
Milk Ingestion
Beef Ingestion
Inhalation of Resuspended Material
External Dose-Ground Contamination
External Dose-Air Submersion
Ocean Fish Ingestion
Ocean Shellfish Ingestion
Food Crops Ingestion
Milk Ingestion
Beef Ingestion
Inhalation of Resuspended Material
External Dose-Ground Contamination
External Dose-Air Submersion
Food Crops Ingestion
Milk Ingestion
Beef Ingestion
Inhalation of Dispersed and
External Dose-Ground Contamination
External Dose-Air Submersion
Ocean Fish Ingestion
Ocean Shellfish Ingestion
-------
Table 3.1
Site Parameters Considered In Risk Analysis
SALT A: Palo Duro Basin
Parameter
Distance from repository to aquifer
(for Salt A, lower aquifer)
Thickness of aquifer
Hydraulic conductivity of aquifer
Porosity of aquifer
Horizontal gradient in aquifer
Retardation values for individual
nuclides
Natural hydraulic gradient between
repository and aquifer
Hydraulic conductivity of the rock
between the repository and the
aquifer, after thermal effects
Porosity of rock between the
repository and the aquifer
Horizontal distance along the aquifer
to point regarded as release point to
accessible environment
Value
NA
2,000 meters
Source
1105 meters
300 m
1.6 m/yr
0.05
0.005
Pu - 200
Am - 1,000
Np - 50
Cs " 10
I - 1
Tc • 5
Sn - 100
Zr - 1,000
Sr " 10
C - 1
0.26
0.0 m/yr
Appendix A
Appendix A
Appendix A
Appendix A
Appendix A
Americium value: Appendix B
All others: NRC/NAS WISP
Report, 1983
/
Appendix A
Appendix A
EPA-520/3-80-006
Agency Decision
-------
Table 3.2
Site Parameters Considered In Risk Analysis
SALT B: Paradox Basin
Parameter
Distance from repoaitory to aquifer
(for Salt B, upper aquifer)
Thickness of aquifer
Hydraulic conductivity of aquifer
Porosity of aquifer
Horizontal gradient in aquifer
Retardation vt!ucs for individual
nuclidea
Natural hydraulic gradient between
repository and'aquifer
Hydraulic conductivity of the rock
between the repoaitory and the
aquifer, after thermal effects
Porosity of rock between the
repository and the aquifer
Horizontal distance along the aquifer
to point regarded as release point to
accessible environment
Value
666 meters
18 m
7.6 m/yr
0.2
0.02
Pu - 200
Am • 1,000
Np - 50
Cs • 10
I " 1
Tc - 5
Sn • 100
Zr - 1,000
Sr • 10
C - 1
0.0
0.0 m/yr
Source
Appendix A
Appendix A
Appendix A
Appendix A
Appendix A
Americium value: Appendix B
All others: KRC/NAS WISP
Report, 1983
Appendix A
Appendix A
NA
2,000 meters
EPA-520/3-80-006
Agency Decision
-------
Table 3.3
Site Parameters Considered In Risk Analysis
Original Generic Salt
Parameter
Distance from repository to aquifer
Thickness of aquifer
Hydraulic conductivity of aquifer
Porosity of aquifer
Horizontal gradient in aquifer
Retardation values for individual
nuclides
Natural hydraulic gradient between
repository and aquifer
Hydraulic conductivity of the rock
between the repository and the
aquifer, after thermal effects
Porosity of rock between the
repository and the aquifer
Horizontal diatance along the aquifer
to point regarded aa release point to
accessible en 'ironmeni
Value
100 meters
30 m
31.5 n/yr
0.15
0.0
Pu • 100
Am - 100
Np - 100
Cs - 1
I - 1
Tc - 1
Sn • 10
Zr " 100
Sr - 1
C • 1
Source
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA 5/0/3-80-006
E A 520/3-80-006
0.1
0.0 n/yr
0.01
1,600 meters
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
-------
Table 3.4
Release Mechanism Parameters Considered In Salt Repository Risk Analysis
Faulting
Values by Repository Site
Parameter
Fraction of the repository
intersected by the release
mechanism
Hydraulic conductivity along
pathway created by release
mechanism
Porosity in flow path created by
release mechanism
Cross-sectional area of flow
EPA 520/3-80-006
path
Annual probability or frequency
Salt A Salt B Source
.003 .003 EPA 520/3-80-006
31.5 m/yr 31.5 m/yr EPA 520/3-80-006
0.1 0.1 .EPA 520/3-80-006
4.0xl03 sq m 4.0xl03 aq m
1.0xlO~6 l.OxlO"5 USGS Open File Report 82-972
-------
Table 3.5
Release Mechanism Parameters Considered In Salt Repository Risk Analysis
Breccia Pipes
Values by Repository Site
Parameter
Hydraulic conductivity in flow
EPA 520/3-80-006
path created by release
mechanism
Porosity in flow path created by
release mechanism
Cross-sectional area of flow
EPA 520/3-80-006
path
Probability or frequency
(after 1,000 years)
Salt A
3.2xl03 m/yr
0.2
3.0x10* sq m
-8
Salt B
0.2
1.0x10
,-8
Source
3.2xl03 m/yr
EPA 520/3-80-006
3.0x10* sq m
EPA 520/3-80-006
-------
Table 3.6
Release Mechanism Parameters Considered In Salt Repository Risk Analysis
l«
Volcanoes
Values by Repository Site
Parameter Salt A Salt B Source
Fraction of the repository 4.0xlO~4 4.0xlO~4 EPA 520/3-80-006
intersected by the release
mechanism
Annual probability or frequency l.OxlO""10 l.OxlO"10 EPA 520/3-80-006
-------
Table 3.7
Release Mechanism Parameters Considered In Salt Repository Risk Analysis
Drilling and Not Hitting a Canister
Values by Repository Site
Parameter Salt A Salt B Source
Volume of water in the repository 1.14 m3 1.14 m3 EPA 520/3-80-006
which can reach the surface.
Annual probability or frequency 2.0xlO~2 2.0xlO~2 EPA 520/3-80-006
(after control period)
-------
Table 3.8
Release Mechanism Parameters Considered In Salt Repository Risk Analysis
Drilling and Hitting a Canister
Values by Repository Site
Parameter Salt A Salt B Source
Fraction of canister brought to 0.15 0.15 £p^ 520/3-80-006
surface.
Annual probability or frequency 2.5x10"' 2.0xlO~5 EPA 520/3-80-006
(after control period)
-------
Table 4.1
Site Parameters Considered In Risk Analysis
GRANITE A: North Central
Parameter
Distance from repository to aquifer
Value
448 meters
Source
Appendix A
Thickness of aquifer
10 Deters
Appendix A
Hydraulic conductivity of aquifer
5.7 m/yr
Appendix A
Porosity of aquifer
Horizontal gradient in aquifer
Retardation values for individual
nuclides
Report,
Natural hydraulic gradient between
the repository and the aquifer
0.016
0.005
Pu • 200
An - 3000
Np
Cs
I
Tc
Sn
Zr
Sr
C
0.0
• 100
- 1,000
• 1
- 5
" 1,000
" 5,000
• 200
• 1
Appendix A
Appendix A
Americiun value: Appendix B
All others: NRC/NAS WISP
1983
Appendix A
Hydraulic conductivity of the rock
between the repository and the
effects
Porosity of rock between the
repository and the aquifer
Horizontal distance along the
aquifer to point regarded aa release
environment
site)
3.2xlO~2 m/yr
.0001
0 meters
Append ix A
aquifer, after thermal
Appendix A
Agency Decision
point to accessible
(aasunes surface water on
-------
Parameter
Distance from repository to aquifer
Table 4.2
Site Parameters Considered In Risk Analysis
GRANITE B: North East
Value
370 meters
Thickness of aquifer
80
eters
Appendix A
Hydraulic conductivity of aquifer
315 m/yr
Appendix A
Porosity of aquifer
Horizontal gradient in aquifer
Retardation values for individual
nuclides
Report,
Natural hydraulic gradient between
the repository and the aquifer
0.039
0.01
Pu - 200
An • 3000
Np
Cs
I
Tc
Sn
Zr
Sr
C
0.0
• 100
• 1.000
• 1
- 5
• 1,000
• 5,000
• 200
- 1
Appendix A
Appendix A
Americiuo value: Appendix B
All others: NRC/NAS WISP
1983
Appendix A
Hydraulic conductivity of the rock
between the repository and the
effects
Porosity of rock between the
repository and the aquifer
Horitontal distance along the
aquifer to point regarded as release
environment
3.2xlO~2 m/yr
.0001
2,000 meters
Append ix A
aquifer, after thermal
Appendix A
Agency Decision
point to accessible
-------
Parameter
Table 4.3
Site Parameters Considered In Riak Analysis
I' Original Generic Granite
Value
Distance from repository to aquifer
230 meters
Source
EPA 520/3-80-006
Thickness of aquifer
30 meters
EPA 520/3-80-006
Hydraulic conductivity of aquifer
31.5 m/yr
EPA 520/3-80-006
Porosity of aquifer .
Horizontal gradient in aquifer
Retardation values for individual
nuclides
Natural hydraulic gradient between
the repository and the aquifer
0.15
0.01
Pu • 100
Am - 100
Np - 100
Cs - 1
I - 1
Tc • 1
Sn - 10
Zr
Sr
C
0.1
100
1
1
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA.520/3-80-006
Hydraulic conductivity of the rock
between the repository and the
effects
Porosity of rock between the
repository and the aquifer
Horizontal distance along the
aquifer to point regarded as release
environment
m/yr
.0001
1,600 meters
EPA 520/3-80-006
aquifer, after thermal
EPA 520/3-80-006
EPA 520/3-80-006
point to accessible
-------
Table 4.A
Release Mechanism Parameters Considered In Granite Repository Risk Analysis
Routine Releases
Values by Repository Site
Parameter
Source
Granite A
Granite B
Fraction of the repository
with which groundwater can
communicate
Hydraulic conductivity of flow
path created by release
mechanism
1.0
3.2x10
,-2
1.0
3.2x10
,-2
EPA 520/3-80-006
Append ix A
Porosity in flow path created by
release mechanism
Cross-sectional are* of flow
path (vertical flow)
0.0001
0.0001
Appendix A
S.OxlO6 m2 S.OxlO6 m2 depository Area
Probability or frequency
1.0
1.0
EPA 520/3-80-006
-------
Table 4.5
Release Mechanism Parameters Considered In Granite Repository Risk Analysis
Faulting
Values by Repository Site
Parameter
Source
Granite A
Granite B
Fraction of the repository
intersected by release
mechanism
Hydraulic conductivity along
pathway created by release
mechanism
0.003
0.003
EPA 520/3-80-006
3.2xl03 m/yr 3.2xl03m/yr EPA. 520/3-80-006
Porosity in flow path 0.1
created by release mechanism
Cross-sectional area of flow 4.0xl03
path
0.1
.0x10
3 _2
EPA 520/3-80-006
EPA 520/3-80-006
Annual probability or frequency l.OxlO"6 l.OxlO"6 VSGS Open File Report 82-972
-------
Table 4.6
Release Mechanism Parameters Considered In Granite Repository Risk Analysis
Volcanoes
Values by Repository Site
Parameter Granite A
Source
Fraction of the repository 4.0xlO~4 '4.0xlO~^ EPA 520/3-80-006
intersected by the release
mechanism <
Annual probability or frequency l.OxlO"10 l.OxlO"10 EPA 520/3-80-006
-------
Table A. 7
Release Mechanism Parameters Considered In Granite Repository Risk Analysis
Drilling and Not Hitting a Canister
Values by Repository Site
Granite A (o P(-*j\~r^ _} J
Parameter Granite A (o P(-*j\~r^ _} Jo »'
Volume of water in the 2.0xl02 m3 2.0xl03 m3 EPA 520/3-80-006
repository which can reach
the surface.
Annual probability or frequency 2.5xlO~3 2.5xlO~3 EPA 520/3-80-006
(after control period)
-------
Table A.8
Release Mechanism Parameters Considered In Granite Repository Risk Analysis
Drilling and Hitting a Canister
Values by repository site
Parameter Granite A Granite B
Fraction of canister brought to 0.15 0.15 EPA 520/3-80-006
surface
Annual probability or frequency 2.5xlO~6 2.5xlO~6 EPA 520/3-80-006
(after control period)
-------
Table 5.1
Site Parameters Considered In Basalt Repository Risk Aoalyei*
Parameter
Distsnce fro* repository to aquifer
Will- J
Thickness of aquifer .'•' ''< !
Hydraulic conductivity iaf aquifer
Porosity of squ{
Horisontal
letsrdstioa v«
nuclides
(•port,
Jjf:P -ft
'W""
Natural hydraulic cradle** betveen
the repository aad the aquifer
Hydraulic conductivity of the book
between the repAfUary ao4 the
aquifer, after f4jif>J
Poroaity of rock ''Wf^MB' the
repository e*4 fo* «<|«4f*r
HoriconCal dis
aqvifer to poU
f«Ut to Mqaaar
the
•a ret
20 meters '^
30 alters
0.9093
Mp
Cs
I
Tc
3n
Zr
8r
100
1,000
1,000
5,000
' f-
3.2««r* B/yi
.0001
2,POQ Meters
*«*•, '''If?;
f'V.1
,« A • ''* '
'W$
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Appendix A
'»PA->»/3-80-006
,;!-.•
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ili-
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-------
Table 5.2
Site Parameters Considered In Risk Analysis
Original Generic Basalt
Parameter
Distance from repository to aquifer
Thickness of aquifer
Hydraulic conductivity of aquifer
Porosity of aquifer
Horizontal gradient in aquifer
Retardation values for individual
nuclides
Natural hydraulic gradient between
the repository and the aquifer
Hydraulic conductivity of the book
between the repository and the
aquifer, after thermal effects
Porosity of rock between the
repository and the aquifer
Horizontal distance along the
aquifer to point regarded as release
point to accessible environment
Value
100 meters
30 meters
31.5 a/yr
0.15
0.01
Pu - 100
Am - 100
Np - 100
Cs - 1
I - 1
Tc • 1
Sp - 10
Zr • 100
Sr - 1
C - 1
Source
EPA 520/3-80-006
EPA 570/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
0.025
3.2xlO~4 m/yr
.0001
1,600 meters
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
EPA 520/3-80-006
-------
Table 5.3
Release Mechanism Parameters Considered In Basalt Repository Risk Analysis
Routine Release
Parameter Value Source
Fraction of the Repository 1.0 . EPA 520/3-80-006
with which groundwater can
communicate
Hydraulic conductivity of flow 3.2xlO~?m/yr EPA 520/3-80-006
path created by release
mechanism .
Porosity in flow path created by 0.001 EPA 520/3-80-006
release mechanism
Cross-sectional area of flow 8.0x10* sq m Repository Area
path (vertical flow)
Annual probability or frequency 1.0 EPA 520/3-80-006
-------
Table 5.4
Release Mechanisn Parameters Considered In Basalt Repository Risk Analysis
l»
Faulting
Parameter Value Source
Fraction of the Repository .003 EPA 520/3-80-006
with which groundwater can
communicate
Hydraulic conductivity of flow 3.2xl03 m/yr EPA 520/3-80-006
path created by release SAND84-1492
mechanise .
Porosity in flow path created by 0.1 EPA 520/3-80-006
release •echaniso
Cross-sectional area of flow 4.0x10-* sq o EPA 520/3-80-006
path
Annual probability or frequency 3.0xlO~5 EPA 520/3-80-006
-------
Table 5.5
Release Mechanism Parameters Considered In Basalt Repository Risk Analye is
Volcanoes
Parameter Value Source
Fraction of the repository A.OxlO"4 EPA 520/3-80-006
intersected by the release
mechanism
Annual probability or frequency 6.0x10-1° EPA 520/3-80-006
USGS Open File
80-357^ J?)
-------
Table 3.6
Release Mechanism Parameters Considered In Basalt.Repository Risk Analysis
Drilling and Not Hitting a Canister
Parameter
Volume of water in the
repository that can reach
the surface
Annual probability or frequency
(after control period)
Value
Z.OxlO2 a3
1.0x10
,-2
Source
EPA 520/3-80-006
EPA 520/3-80-006
-------
Table 5.7
Release Mechanism Parameters Considered In Basalt Repository Risk Analysis
I*
Drilling and Hitting a Canister
Parameter Value Source
Fraction of canister brought to 0.15 EPA 520/3-80-006
the surface
Annual probability or frequency l.OxlO"5 EPA 520/3-80-006
(aftei control period)
-------
Table 6.1
Site Parameters Considered In Tuff Repository Risk Analysis
Parameter
Distance from repository to aquifer
i •
Thickness of aquifer
Hydraulic conductivity of aquifer
Porosity of aquifer
Horizontal gradient in aquifer
Retardation value* for individual
nuclides
Report,
Natural hydrajlic gradient between
the repository and the aquifer
Hydraulic conductivity of the rock
between the repository and the
aquifer, after thermal effecta
Porosity of rock between the
repository and the aquifer
Horizontal distance along the
aquifer to point regarded as release
point to accessible environment
Value
100 meters
1000 meters
30 m/yr
0.002
0.00034
Pu - 200
An " 1,000
Np - 100
Cs " 500
I " 1
Tc • 5
Sn " 1,000
Zr " 5,000
Sr - 200
C - 1
1.0
l.OxlO'3 m/yr
0.1
2,000 meters
Source
SAND84-1 ,92
SAND84-1492
SAND84-1492
SAND84-1492
SAKD84-1492
Americium value: Appendix B
All others: NRC/NAS WISP
1983
SAND84-1492
SAND84-1492
SAND84-1492
Agency Decision
-------
Table 6.2
Release Mechanism Parameter* Considered In Tuff Repository Risk Analysis
Routine Releases
Parameter Value Source
Hydraulic conductivity of flow 0.001 n/yr SAND84-1492
path created by release
mechanism
Porosity in flow path created by 0.1 EPA 520/3-80-006
release mechanism SAND84-1492
Cross-sectional area of flow 8.0x10** m? Repository Area
path
Probability or frequency 1.0 EPA 520/3-80-006
-------
Table 6.4
Release Mechanism Parameters Considered In Tuff Repository Risk Analysis
<• Volcanoes
Parameter Value Source
Fraction of the repository 4.0x10"* EPA 520/3-80-006
intersected by the release
mechanism
Annual probability or frequency 2.9xlO~" USGS Open File
Report 80-357
-------
Table 6.3
Release Mechanism Parameters Considered In Tuff Repository Risk Analysis
Faulting
Parameter Value Source
Fraction of the repository 0.003 EPA 520/3-80-006
intersected by the release
mechanism
Hydraulic conductivity in-flow 0.02 m/yr SAND84-1492
path created by release
mechanism
Porosity in flow path created by 0.1 EPA 520/3-80-006
release mechanism
Cross-sectional area of flow 4.0X103 sq o EPA 520/3-80-006
path
Annual probability or frequency 8.0x10"^ • VSGS Open File
Report 82-972
-------
Table 6.5
Release Mechanism Parameters-Considered In Tuff Repository Risk Analysis
Drilling and Not Hitting a Canister
Parameter Value Source
Volune of water in the 140 m3 EPA 520/3-80-006
repository which can reach ' SAND84-1492
the surface
Annual probability or frequency 2.5xlO~3 EPA 520/3-80-006
(after control period)
-------
Table 6.6
Release Mechanism Parameters Considered In Tuff Repository Risk Analysis
l»
Drilling and Hitting a Canister
Parameter Value Source
Fraction of canister brought 0.1S EPA 520/3-80-006
to the surface
Annual probability or frequency 2.5xlO~6 EPA 520/3-80-006
(after control period)
------- |