United States Office of EPA/520/4-78-004
Environmental Protection Radiation Programs
Agency Washington DC 20460
Radiation
&EPA State of Geological
Knowledge Regarding
Potential Transport of
High-level Radioactive
Continental Repositories
Report of An Ad Hoc Panel
of Earth Scientists
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This report was prepared as an account of work sponsored
by the Environmental Protection Agency of the United States
Government under Contract No. 68-01-4470, Modification
No. 2. Neither the United States nor the United States
Environmental Protection Agency makes any warranty, express
or implied, or assumes any legal liability or responsibility for
for the accuracy, completeness or usefulness of any information,
apparatus, produce or process disclosed, or represents that its
use would not infringe privately owned rights.
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vvEPA State of Geological
Knowledge Regarding
Potential Transport of
High-level Radioactive
Waste From Deep
Continental Repositories
Report of An Ad Hoc Panel
of Earth Scientists
JUN 1878
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TABLE OF CONTENTS
Foreword
Preface
PART I: INTRODUCTION AND STATEMENT OF THE PROBLEM
PART II: THE CONTENTS AND PROPERTIES OF THE CANISTER
PART III: MECHANICAL PROPERTIES OF ROCKS
PART IV: CANDIDATE LITHOLOGIES
PART V: SOLUTION TRANSPORT THROUGH ROCKS
PART VI: RADIONUCLIDE TRANSPORT FORECASTING
PART VII: GEOLOGIC AND OTHER ACCIDENTAL HAZARDS
PART VIII: MONITORING THE REPOSITORY
PART IX: CONCLUSIONS
References
List of Abbreviations
Glossary
Page
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vii
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5
13
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25
29
35
41
43
47
49
51
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FOREWORD
A major Federal effort is underway to develop methods of disposal of high-
level radioactive waste. An important element of this program, scheduled
for early completion, is the development and promulgation by the Environ-
mental Protection Agency (EPA) of environmental standards for such materials.
In conjunction with its efforts to develop these standards, EPA is using
state-of-the-art techniques to estimate the expected and potential environ-
mental impacts from potential waste repositories, including repositories in
deep geologic formations. It recognizes, however, that there may be signi-
ficant uncertainties and controversy regarding knowledge of rock properties,
hydrogeology, and other factors, especially as they relate to the ability to
provide long-term containment of radioactive wastes.
Therefore, in order to assure the protection of public health, EPA believed
that an expert panel was needed to evaluate objectively the adequacy of
basic knowledge in the pertinent earth sciences for reliably estimating
environmental impacts.
In March 1977, the Environmental Protection Agency contracted with Arthur D.
Little, Inc., Cambridge, Massachusetts for technical studies of high-level
radioactive waste disposal using deep geological repositories. Directive
of Work No. One under this contract, issued in June 1977, required that the
contractor, in conjunction with the EPA Project Officer, select the Ad Hoc
Panel of Earth Scientists comprised of nationally recognized experts in the
fields of geology, geochemistry, geohydrology, rock mechanics and other
applicable disciplines in the earth sciences. The Panel's basic charge was
to perform an independent evaluation of the adequacy of the state of know-
ledge in the earth sciences for reliably estimating the environmental impacts
to be expected from the disposal of radioactive waste in deep geologic forma-
tions, to provide guidance to EPA regarding the uncertainties inherent in
estimates based upon such knowledge. This report was prepared by the Panel
in response to its charge.
The Panel conducted its work and formed its conclusions independently of
Arthur D. Little, Inc., EPA, or any other agency. EPA and its contractor
provided only administrative services and such procedural assistance as the
Panel requested.
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MEMBERS OF AD HOC PANEL OF EARTH SCIENTISTS
Dr. Bruno Giletti, Co-Chairman
Department of Geological Sciences
Brown University
Providence, Rhode Island
Dr. Raymond Siever, Co-Chairman
Department of Geological Sciences
Harvard University
Cambridge, Massachusetts
Dr. John Handin, Director
Center for Tectonophysics
Texas A & M University
College Station, Texas
Dr. John Lyons
Department of Earth Sciences
Dartmouth College
Hanover, New Hampshire
Dr. George Finder
Department of Civil Engineering
Princeton University
Princeton, New Jersey
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PREFACE
In August of 1977, a panel of geologists, geochemists, and geophysicists
was brought together by Arthur D. Little, Inc., under assignment by the
Environmental Protection Agency (EPA) to evaluate the state of knowledge
in the earth sciences relevant to environmental aspects of the disposal
of high-level radioactive wastes (HLW), by deep burial on the continents.
This report presents the results of the panel's work in 1977.
This report contains our evaluation, based on the available written reports
of individuals and research groups in government, industry, and the uni-
versities; as well as meetings of the panel, subgroups of it, or individual
members with several individuals currently active in this field. We have
met as a group with members of the Arthur D. Little Staff and the Office of
Waste Isolation (OWI) at Oak Ridge.
We have tried to find out the nature of the pertinent efforts at other
government agencies and their contractors, including groups of the Depart-
ment of Energy (DOE), formerly the Energy Research and Development Agency
(ERDA) and the Nuclear Regulatory Commission (NRC), Arthur D. Little,
Battelle Northwest Laboratories (BNWL), the United States Geological Survey
(USGS), Sandia Laboratories, Los Alamos Scientific Laboratory, and Lawrence
Berkeley and Lawrence Livermore Laboratories (LBL and LLL). We do not claim
to have made a detailed and comprehensive survey of all of this work; neither
time nor manpower allowed that.
We have attempted to assess the research that has been or is being done, or
that is planned, so as to identify what needs most to be known in the earth
sciences in order to permit reliable prediction of the behavior of HLW
following deep burial. We do not make judgments on the merit of any specific
research effort of any organization or individual. Our aim is simply to find
out what is known and what is not.
We have taken as our charge an assessment of the state of the art of the
geological sciences that relate to the disposal of radioactive wastes at some
depth under the land surface. How well do we know the physical and chemical
characteristics of the rock materials at depths comparable to those en-
visioned for waste disposal? How reliably can we calculate such diverse
quantities as rates of chemical reactions around waste containers and hydro-
logic flow rates in low-permeability rocks? Can we predict the likelihood
of changes in the current geologic regime that might affect the storage
site in the future?
We include in our charge judgement of the confidence with which we can
predict the extent of transport of radioactive materials and how this
relates to certain kinds of geologic formations in various tectonic
and surface environmental settings, but we do not assess the merits
of any particular site. We consider three phases of the problem:
(1) what we can do now,
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(2) what we can find out within the short term—a year or two—
that will allow us to do much better, and
C3) what we can determine only after several years of well-
planned, careful research.
Implicit are the questions: do we know how to formulate the right specific
questions and can we expect to obtain answers of sufficient accuracy to
suffice for our needs?
Increasingly earth scientists are being called upon to predict future geo-
logic phenomena specifically. Earthquake prediction is relatively advanced
because of the important research of the past few years; prediction of
climatic change is an actively researched subject. Short-term prediction
of volcanic eruptions is possible now. On a less spectacular level, hydrolo-
gists can predict with fair accuracy the behavior of ground water flow under
various pumping regimes. Yet most of these predictions are valid over time
scales of days, or a few tens of years at most. In contrast, our ability
to predict in any but the most general way over times of thousands of years
is poor, yet this is the interval over which we must forecast if an under-
ground HLW repository is to be safe for future generations.
Our study is intended to inform the EPA of the nature of the advice that it
is getting and can expect to get from competent earth scientists who are
knowledgeable about the various aspects of, and current developments in, the
background science of the problem. From such sources the EPA should expect
assessments and predictions that are solidly based upon current knowledge,
not hypothesis and extrapolation from inadequate or inapplicable field or'
laboratory data. Above all it should be remembered that the Earth—or any
small part of it—is an extraordinarily complex and heterogeneous assemblage
of materials, constantly subject to slowly acting forces. Attempts to model
its behavior :in this case must rely not simply on the best data that are
readily available but on sufficiently good data that are truly pertinent
and on a consideration of all of the important factors that may affect the
outcome. In any case, we must know the reliability of any estimates before
we can use them as a basis for practical decisions.
In assessing the possibility of loss of integrity of a disposal site, we
have considered that total containment is desirable. Realistically, however,
some loss of radioactive material, albeit small, appears likely. We have
addressed the question of estimating the amount of contamination to the
biosphere. It is worth noting that estimates of (and regulations concerning)
maximum permissible levels of radioactivity in different phases of the biosphere
have been lowered over the years since World War II, and we must assume that
this trend will continue into the next few centuries.
In preparing this report we have been assisted by the many individuals who
have talked with us, sent us reports of research in progress, and informed
us of where work is going on. We have consistently found a high degree of
cooperation, which has made this task easier, and for which we are grateful.
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We acknowledge the important contribution of Arthur D. Little, Inc., in
bringing us together and informing us of their work, facilitating our task,
and seeing our report through publication and submission. We alone, however,
are responsible for the contents and conclusions of this report.
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PART I; INTRODUCTION AND STATEMENT OF THE PROBLEM
The impetus for addressing the questions we seek to answer is the necessity
for long-term storage or disposal of various kinds of long-lived radioactive
wastes that already have been generated by nuclear processing plants and
reactors and that will be produced by current and future nuclear power
reactors. We consider high-level wastes of two categories:
(1) those generated by the reprocessing of spent fuel rods, and
(2) the unprocessed spent fuel rods themselves.
Wastes of both kinds must be stored in such a manner that leakage from the
deeply buried repository to the biosphere will be inconsequential to public
health over the times during which the stored materials remain sufficiently
radioactive to be hazardous. Thus we consider a maximum time scale of
hundreds of thousands (10^) to a few million (10^) years, but pay special
attention to the dangers of leakage over shorter terms up to a thousand
years, when the levels of radioactivity are highest.
Unlike ordinary engineering problems, there is no experience with long-
term, sealed underground storage of such materials, and thus no foundation
of empirical knowledge upon which to build. Almost all of our experience
with underground openings relates to technologies designed for removal and
extraction, such as mining and pumping for water, oil, or gas. There is
some limited knowledge from the underground storage of gas, oil, and water.
However, small leaks of these materials would not be significant and,
therefore, are not studied.
The engineering design of underground openings such as those envisioned
under various HLW storage plans is like nothing ever developed previously.
This basic difference stems mainly from the addition of radioactive heat
to the normal geothermal temperatures at the relevant depths on the order
of 500 m.
We assume that a safe underground repository requires a volume of rock in
a stable geologic environment. We envision storage in an area that is
subject neither to frequent, high-energy earthquakes nor to volcanic eruptions,
Variations in the hydrologic regime due to climatic variations of rainfall
and evapo-transpiration must not jeopardize the safety of the biosphere
from HLW contamination. Above all, we assume that a rock environment is
required that is scalable and has a minimal permeability for fluids and
the radionuclides that might become dissolved in them.
The rocks that now seem to be most suitable are salt, shale, basalt, and
certain rocks of the granitic clan, but anhydrite and some impermeable
tuffs may be worth considering. Other common rock types, owing to their
permeability or chemical reactivity, are not worth serious investigation
at this time.
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A central consideration for a great many questions is the type of waste
to be stored. Until recently storage schemes have been predicated on the
reprocessing of spent fuel to HLW and its subsequent combination in con-
centrated form to produce a calcine, ceramic5 or glass cylinder encased in.
a. metal canister. Small in volume, these canisters develop high temperatures
in the first few years, when short-lived radioactivity is at its highest level.
For the first 10 years or so the material would have to be stored above ground
in order to allow rapid heat loss. After this time, during the initial period
of underground storage, the canisters may reach temperatures of 300°C or more,
depending on the thermal conductivity of the surrounding rock. Neverthe-
less, these canisters enclose the HLW in a form that is designed to be
far more resistant to corrosion and leaching by formation waters than
unprotected spent fuel rods would be.
Storage of spent fuel rods presents fewer heat problems: the radioactive
materials are in less concentrated form and would not be expected to reach
temperatures greater than 100'C. However, the rate of corrosion of the fuel
rods would be much greater, and great reliance would have to be placed on
the impermeability of the surrounding rock to contain the HLW. Although
relatively little work has been done on the feasibility of storing spent
fuel rods, current federal policy favors this option.
Reprocessed HLW or unprocessed spent fuel rods both contain a mixture of
short- and long-lived radioactive elements of two kinds, in addition to
the unconsumed 238u an
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years they become marginally reliable; and for times greater than a million
years they become so poor that they become little better than guesses. Of
course the reliability also depends on where the repository is with respect
to the tectonic framework of the continent. We can be much more confident
of the stability of old granites on a Precambrian shield than of young
basalts near a tectonically active continental margin such as the Pacific
coastal regions of the U.S.
Finally we must confront the issue of retrievability of the HLW over a
specified time period. Because high-grade uranium ore resources are limited,
it may be desirable at some future time to recover unprocessed fuel rods and
reprocess them for what may have become much needed Pu and U. Indeed, it
would appear that a breach of the repository in order to mine the U and Pu
could be a serious problem for the future.
The geological site selection criteria for and engineering design of re-
positories will be strongly affected if retrievability is deemed to be de-
sirable. It is unlikely, for example, that spent fuel rods could be safely
recovered from a repository in salt more than a few tens of years after emplace-
ment and backfilling, for by then the salt would have completely sealed the open-
ings. Storage in granite or shale might be more desirable if long-term mechanical
stability is required. As yet there is no firm policy. We must consider
the state of knowledge concerning the reopening of underground workings in
the rock types proposed for storage and how such reopenings—or maintenance
of a tightly scalable, open shaft—affect the choice of storage repositories.
Here again we are faced with lack of experience, for in ordinary mining
operations, openings are usually abandoned after working, with no thought
of returning. Though there has been some reworking of mines in a few places,
these are highly hazardous because of the danger of roof-falls in a deterio-
rated mine. It is well known that keeping any underground mine open and
clean requires constant maintenance and checking of rooms and entries. The
deeper the mine the greater the danger of rock bursts and floor heaving,
the more so because accumulated strain in surrounding rock may build up over
a long period of time and then suddenly give way by failure.
Retrieval may only be feasible so long as an active crew is kept at the
repository site, perhaps then for only a relatively short number of years,
5 to 10, while the repository is being filled. We assume that constant
underground human surveillance for significantly longer periods of time is
unlikely.
In the sections that follow we discuss specific details of the integrity
of the storage assembly itself, the nature of the various rocks in relation
to their suitability for storage, the transport of radionuclides through
rock barriers, the hydrologic regimes, and the probability of disruptive
geological events.
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PART II: THE CONTENTS AND PROPERTIES OF THE CANISTER
Most planning to date for possible designs of HLW storage and disposal
systems deep underground has included several stages of containment in
case parts of the system were breached. These stages usually include:
the resistance of the waste, in whatever form, to leaching and transport;
the resistance of the metal canister to corrosion or other loss processes;
possible special materials placed immediately around the canister; the
geological formation into which the repository is to be placed; and the
nearby geological formations surrounding the host formation itself. These
containment stages have permitted computation of a variety of time delays
prior to possible release of the HLW to the biosphere. The various model
computations utilized have differed in the nature of the materials employed
and their geometries.
The nature of the HL¥-containing "vessel" cannot be defined clearly because
no firm decisions have been made concerning a number of parameters to be
listed below. By "vessel" is meant the canister and its contents, plus any
packing placed immediately around the canister on out to the undisturbed
rock wall. The parameters that have not been defined include:
• The composition of the canister waste contents. That is,
Olfi_ *) *\ ^
the relative proportions of fission products ("°U + •J-3U)
and TRU elements are not defined because it is not known
if the materials will have been reprocessed, or even if
spent fuel rods will be placed in the canisters directly.
• The composition and phases of the canister contents. This
includes the inert (radioactively) materials used as "binders."
Glasses, calcines, and ceramics have all been suggested as
possible candidate materials into which the HLW could be
incorporated.
• The "mix" and concentration of the HLW in the canister.
This affects the heat production rate and its variation
with time.
• The nature of the canisters. It is generally assumed that
the canisters will have approximately a 1-ft inside diameter
and an approximate 8-ft length. It is not clear what mate-
rial (s) will be used to make the canisters. Steel, stain-
less steel, and molybdenum have been named as candidates,
but this is clearly not an exclusive list. Wall thicknesses
appear to have been considered for purposes of transport of
the contents to the repository only.
• The nature of the packing around, the canisters. This has
included as candidates: metal sleeves useful if recovery
at a later date is desired; zeolites or clays with high
ion-exchange capacity; crushed rock of the type in -which
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the repository is placed; this same rock or other material
cemented together; some combination of these.
It should be clear that much of the uncertainty concerning the basic
characteristics of the repository resides in two areas: policy decisions
have not yet been made and accepted generally, concerning whether or not
the materials will have been reprocessed, and whether or not to provide
for retrievability of the HLW for time scales of decades. The consequences
of these policy decisions concerning the mode of HLW disposal might, in
fact, eventually result in revision of the choices made with respect to the
form of the waste, storage, rock type, and site.
II.1 THE INTEGRITY OF THE HLW AND ITS MATRIX
Several materials have been proposed for mixing with the HLW so as to pro-
vide a solid composite that would resist leaching or other loss of the HLW
in the event of canister breaching. Included are calcines, ceramics, and
glasses. The possibility that the leaching solutions would be acid has
argued against the use of calcines, since they are not very resistant to
such attack.
The use of ceramics may be viable, but not enough work has been done to
demonstrate that. In view of some of the difficulties with glasses to be
discussed shortly, ceramics might well be worth more intensive study. They
have the advantage that they are already crystallized so that devitrifica-
tion is not a problem. Questions concerning their stability and resistance
to leaching under the local pressure and temperature conditions of the solu-
tions that are likely to exist need to be explored.
Various glasses have been suggested as suitable products to be placed into
the canisters. The ability to make a homogeneous phase that will fully
occupy the canister shape, the good compressive strength, and the moderate
resistance to solution, all are positive arguments for the use of glass.
There are two phenomena that can generate serious problems in the use of
glasses, however. These are that glasses are not stable, but will tend to
devitrify; and that it is possible that solutions of high ionic strength
will leach the glass at the ambient temperatures around the canister.
Two aspects affecting the possible devitrification of glasses have not been
adequately investigated. The first is the high radiation flux from alpha
particles. These can create considerably more recoils and damage than beta
particles and gamma rays. It is primarily the latter that have been used in
tests for devitrification. With the new possibility that unprocessed fuel
rods will be used, or that the HLW might be made into a glass without prior
separation of the U and Pu, the dose of alpha particles will increase
markedly. The devitrification will also be enhanced by the presence of
water, particularly high ionic strength solutions, at somewhat elevated
temperatures. We are unaware of any tests conducted on any of the proposed
types of glasses that show the effects of high dosages of alpha particles
imposed on glass during immersion, in likely corrosive solutions, at the
various temperatures from 300°C on down. It would not be at all surprising
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to find that the integrity of the glass was lost over time scales of a
decade, instead of the millenia that are now computed on the basis of the
results of dry tests with gamma and beta radiation.
The consequences of devitrification are the formation of one or more
crystalline phases. Such processes commonly result in the formation of
materials with simpler chemical compositions than the glass. The new
phase excludes impurities. If the impurities include the class of nuclides
that are the fission products or TRU's, then these will end up in inter-
granular boundary areas. Leaching of these would then be relatively easy,
and the bulk of the original glass need not even go into solution, but could
remain as relatively non-radioactive crystals.
The other area that requires demonstration is the leaching of glasses, even
if no devitrification is assumed to occur. Experiments need to be conducted
with solutions at temperatures as high as those that may occur in the can-
ister, clearly higher than 300°C in some cases and possibly in excess of
500°C. These solutions should include the likely ones for the proposed
setting: such as concentrated brines, including the variety of bitterns
that exist as fluid inclusions in some rocks.
At this point, the Panel believes that there is no evidence that incorpora-
tion into a glass will ensure resistance to significant leaching over time
scales of a decade. We wish to make clear that this is an area in which
experiments can be done. If carefully controlled, such studies should be
able to answer the question reasonably well.
In the event that containment by the glass was necessarily a short-term
phenomenon, the primary defense against migration of radionuclides to the
biosphere would clearly have to reside in the other containment stages
referred to at the beginning of this section. We assume that it is of
little importance whether the glass releases some radioactive waste in times
of a decade or hour, since the effect would be essentially instantaneous
compared with the time scales of interest for HLW. We shall assume, there-
fore, that the glass presents no significant barrier to transport by leach-
ing solutions.
If the spent fuel rods are placed directly into the canisters, or are not
changed chemically (that is, they may be crushed and pelletized), we know
of no data concerning the ease with which such materials will be leached.
It would be expected that the material is so far out of chemical equili-
brium with the hot solutions that can reach it, however, that a significant
amount of solution would occur. We see no safe alternative at this point
but to assume instantaneous access to leaching solutions. The limiting
constraint we see here, as with the glasses, is the degree to which the
individual ions are soluble, either as those ions or as complexes.
In dealing with questions of devitrification kinetics, solution of the
various compounds and glasses, or solution of the spent fuel rod mate-
rials themselves, some items should be noted. It is the repeated ex-
perience of those of us who work in this area:
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• that rates measured in dry systems often are orders of
magnitude slower than those measured in wet systems;
• that rates measured with pure water or dilute solutions
are often far slower than those in concentrated solutions,
particularly if the latter are at high or low pH; and
• that complexing of ions (by making chloride complexes, as
an example) may permit considerably higher concentrations
of the cation in the solution.
Transport of ions in such cases would clearly be faster for the same
volume of solution.
The use of some data in this area in the mathematical models that have
been constructed for HLW is justified as the only information then available.
However, we should not lose sight of the major discrepancies possible between
available data and those that are needed to bear directly on what the models
are trying to determine.
Until it is demonstrated otherwise, we conclude tentatively that a loss of
integrity of the HLW matrix in a repository would result within a very short
time of the emplacement. This short time could well be less than a decade.
In view of the need to maintain isolation for time scales of centuries to
one million years, it appears safest to consider the materials in the canis-
ter primarily from the aspect of what is being dissolved and transported
rather than as barriers of any consequence.
Some comments are in order concerning the nature of the solutions that might
attack the canisters and their contents. It is clear that the rock type
will affect to some extent the nature of the solutions.
A frequently discussed candidate lithology for a repository is salt. Ob-
servation in salt mines has shown that chambers cut into some salt deposits
remain dry indefinitely, suggesting the long-term safety from water that is
desired for HLW. Closer inspection of salt, however, reveals that the
crystals do contain significant amounts of water as fluid inclusions and
along intergranular boundaries. Work done by Roedder and by Stewart (to
be discussed in Part IV) suggests that a potential hazard exists here. The
fluid inclusions in the salt decrepitate (burst on heating) at comparatively
low temperatures, which means that they are reasonably certain to do so in
the vicinity of the canister as the temperature rises following emplacement.
Decrepitation is quite likely to occur at temperatures in the general
vicinity of 150°C. If we assume that the wall temperature of the canister
will reach 300°C, a significant amount of water might be available. This is
most likely in bedded salts where water may be in excess of 1% of the rock.
It becomes imperative to determine if similar amounts of water exist in the
salt of salt domes.
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The process of migration, of water up thermal gradients in salt has been
described numerous times. It is cited here to make the point that the
canister is likely to be bathed in water soon after emplacement. This
water would, of course, be saturated with salt. At the temperatures en-
countered at the wall, the concentration is considerable (as will be
discussed).
The availability of water in shale deposits is considerable. This is
water normally tied up as water of hydration in the clays or as OH ions
in the structures of the clays and micas. The high temperatures at the
canister wall will suffice to release some of the water. Again, the water
will contain a variety of salts in solution. These will derive from the
cations in and on the clay particles. As a result, the composition will
differ from that of an evaporite unit in which the salt unit itself will
tend to dominate the composition. If the salt is largely halite, so will
be the solution. In the case of the shale, the solution may derive from
salts that were present during deposition and diagenesis, plus the con-
tribution from the particular clays in the shale and their degree of dia-
genetic change.
The granite clan rocks and other igneous rocks will contribute water from
two sources. The rock itself is slightly permeable, and there will have
been long times available for the water to enter the rock as it would in
any other permeable rock. Most crystalline rocks of this sort are not
totally massive, but have joints and shear zones on numerous scales. These
will again permit water to enter. In this connection, it should be noted
that much of the water obtained in New England from "artesian" wells drilled
in these crystalline rocks is actually from fissures in the rock. The rock
itself is relatively impermeable if a particular specimen is tested, but
the unit of rock is much more permeable owing to these fissures. In the
event that HLW is stored or disposed of in such rocks, it is not clear what
the maximum surface temperature of the canister would be. The thermal con-
ductivity of both these rock types is less than that of salt by a factor of
about 3, a difference suggesting some higher temperature unless different
concentrations of the fission component of the HLW were used.
A source of water from minerals themselves would not be a problem in the
"anhydrous" rocks such as basalt or the granulite facies rocks such as
charnockites or anorthosites. These may contain quartz, feldspars, and
pyroxenes, in various proportions. Basalts are common, but the granulite
facies rocks are more rare.
Any of these crystalline rocks, when heated in the presence of water, will
contribute ions to form a solution, the composition of which will depend
on the rock composition, including its fluid inclusions.
II.2 THE INTEGRITY OF THE CANISTER
The canister into which the HLW would be placed is usually described as a
cylinder of approximately 1-ft inside diameter and 8-10 ft in length. It
is assumed that it would be made of some metal, that it would be sealed by
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welding, and that it would be able to withstand both the high temperatures
that would result when the molten glass or other molten material was poured
into it, and the stresses during transport to the burial site. The wall
thickness is not known to us, but would presumably be on the order of
1/4 inch to perhaps 1 inch.
In all cases, a pure metal or an alloy appears to be the choice for the
material. Suggestions include: steel, stainless steel, molybdenum,
titanium, and copper. Each has different structural advantages and dis-
advantages, which are not part of our concern. The resistance to corrosion
does, however, concern us. We know of no tests that have shown that any of
the candidate metals will resist corrosion by the salt solutions that are
likely to be at the canister surface for a significantly long time. Again
we stress that the solutions will be at temperatures of approximately 300°C,
will have a high ionic strength, may have a very low PH, but probably will
have very low starting concentrations of the metals of the canister. Under
these circumstances, it is likely that the canister could be breached within
time scales of a decade or less.
For this reason, we do not consider the canister to be a significant barrier
to the solutions, at least for the time scales of centuries to a million
years with which we are dealing. In passing, we might note that it is not
essential that all components of the canister be resistant to leaching or
attack. The optimum canister may prove to be one that is highly effective
as a container and transport vessel and one that does not enhance attack
by solutions on the canister contents by adding corrosive materials to the
solution that will ultimately breach the canister.
II.3 THE INTEGRITY OF THE CANISTER SURROUNDINGS
According to our nomenclature, the immediate surroundings of the canister
are part of the "vessel." That is, the vessel extends from the canister,
through the packing around the canister, and out to the undisturbed rock.
If the canister is to be retrievable, the packing would include a metal
sleeve to permit easy withdrawal of the canister. Around this might be
packed crushed rock similar to the host rock of the repository or some
other material.
The metal sleeve would doubtless behave in a manner similar to the canister
metal. It is assumed that the same metal would be used, since the use of
dissimilar metals could well set up an electrolytic cell, with the con-
sequent possibility of H2 production and rapid corrosion of the metal walls.
The comments above for the canister apply to the sleeve material as well.
Any packing would have a porosity and permeability that are much greater
than would the undisturbed rock. If grouting materials were used to cement
the packing materials, the result would be a less porous and permeable
packing, but the grouting would still be susceptible to attack by the
solutions.
10
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The variety of possible packing materials is infinite, and it remains to
be shown that the selected material would enhance the resistance of the
"vessel" to attack by solutions. The use of zeolites or other ion ex-
changers might help in the chemisorption of the HLW as it moved through
the packing, thus delaying the migration of the radioactivity. The amount
of such packing might not be sufficient to make a significant difference,
however, because zeolites lose their water (and so their cation-exchange
capacity) at temperatures well below 300°C. To be effective, the zeolite
packing would have to be far enough from the canister to remain cool.
Based on the above considerations, we assume that the near-in stages of
containment cannot be relied upon to effect any significant retardation
of the release of the HLW. We include in the near-in stages all the com-
ponents of what we call the "vessel," that is, the HLW in its matrix, the
canister, any sleeve around the canister, and the packing around the
canister or sleeve. By significant retardation, we mean for times longer
than a decade.
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PART III; MECHANICAL PROPERTIES OF ROCKS
The introduction of a repository into a geologic unit poses a number of
mechanical requirements on the rock: the need for sufficient strength
to allow safe excavation and occupancy until the repository has been
sealed; mechanical integrity despite the subsequent high temperatures;
low permeability; and absence of discontinuities like jointing or bedding;
or a very small number of these. Knowledge of the mechanical properties
for the various candidate lithologies varies considerably and some un-
certainties remain for all rock types.
The Arthur D. Little report on "Assessment of Geologic Site-Selection
Factors1^1' outlines problem areas fairly well, but it neither assesses
nor criticizes the state of the art that is currently available to re-
solve these problems. Nor for that matter does any other recent document
known to us except for the NAS (National Academy of Sciences) draft report
on "Rock-Mechanics Limitations to Energy-Resource Recovery and Development,"
which should be published in early 1978. (2) ye concur with the conclusion
of its Subpanel on Nuclear-Waste Disposal that "present rock-mechanics
technology is not sufficient to predict the full range of thermal effects
on site containment caused by the emplacement of heat-generating radioactive
waste. The Subpanel believes, however, that the improvements in technology
needed to make such predictions within defined and acceptable bounds of
accuracy can be acquired with the proper research and development effort."
The engineering design of mines and other underground works is largely
empirical, but it is firmly based on many hundreds of years of practical
experience. Except for rock bursts in deep mines in strong rock in which
residual stresses can be very high (as in South Africa), accidents that
threaten human life rarely occur these days because of failure of the rock
itself. (The risks of flooding and explosion of accumulated gas remain
all too high.)
The expectable configuration of a repository is essentially that of a room-
and-pillar mine. In practice, excavation is designed for the maximum rate
of extraction that is allowed by the short-term stability of the roof. In
salt mines the rooms may close eventually, but gradually and non-cata-
strophically, long after they have been worked out and abandoned. In
principle, long-term stability can be achieved by increasing support—
reducing the open area relative to that of the pillars and, if necessary,
providing lining and other reinforcement. Thus, the state of the art is
such that a mechanically safe cavity could ordinarily be placed in virtually
any rock, provided only that cost effectiveness be judged not by the value
of the rock extracted but by the critical need to isolate high-level waste
from the biosphere.
The facts are, however, that the design of an HLW underground repository is
not ordinary, and there is no fund of previous experience because the wall
rock will be subjected to temperatures greatly exceeding those due to the
average geothermal gradient. The temperature will depend principally on
the thermal conductivity of the rock and the thermal load imposed, which
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depends in turn on the time allowed for heat dissipation during previous
surface storage and the size and distribution of canisters in the floor
of the repository. Temperatures as high as 300°C should be expected in
a highly conductive rock like salt. Heating to as much as 500°C might
well occur in a relatively poor conductor like dry granite. Our knowledge
of the mechanical properties of rocks under long-term loading in the labo-
ratory at the relevant temperatures (300-500°C) and confining pressures
(100-200 bars) is fairly good for rock salt. It is practically nil for
all other rock types.
Much experimental work has been done on the high-temperature creep of
single crystals and artificial aggregates of salt, and work on natural
samples from the Waste Isolation Pilot Plant site is progressing at RE/SPEC
(a consulting firm in Rapid City, S.D.) and Sandia Laboratories. The labo-
ratory data on flow laws are being incorporated into non-linear numerical
models in order to predict the behavior of cavities in salt. Although
extrapolation of laboratory data must still be tested and the codes must be
validated by careful measurements in the field, our ability to predict the
formation of salt should soon be adequate, given the current level of re-
search effort throughout the world.
We are far indeed from the capability to predict, with sufficient accuracy,
the behaviors of any other rocks because:
(1) data on mechanical properties under relevant conditions
are lacking and
(2) most rocks cannot be treated as continua because dis-
continuities like jointing and bedding are nearly
ubiquitous.
Because the need for underground isolation of HLW has been recognized for
some 30 years, the long postponement of pertinent research on rock other
than salt is unfortunate. The problems will not be .solved quickly. The
research is inherently time-consuming because the critical data are attain-
able only from creep tests of months-long duration. Furthermore, the required
testing machines (to accommodate 10-cm specimens, at temperatures of 500°C,
under pressure of 200 bars, and for a duration of several thousand hours) do
not even exist. It may take a major research effort of 5 years to build the
necessary laboratory facilities; to collect adequate data; to develop real-
istic, three-dimensional, non-linear, large deformation codes; and to vali-
date predictions in the field.
In addition to the strength of rock, which relates to long-term mechanical
stability, we must know a lot more than we do about thermo-elastic expansion,
thermal conductivity, and permeability, particularly as they are affected by
thermal cracking. These parameters relate not only to mechanical but also to
hydrologic stability. A modest experimental effort is under way at Terra Tek,
the U.S. Geological Survey, and a few universities and DOE laboratories.
Several heater tests in the field are in progress or planned for the immediate
future (Avery Island salt dome, RE/SPEC; granite, Stripa mine, Sweden, LBL;
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granite, Climax stock, Nevada Test Site, LLL; basalt, Hanford, LBL;
Conestauga shale, Oak Ridge National Laboratory, Sandia; Eleana shale,
Nevada Test Site, Sandia; norite, Sudbury, Canadian Government). Valuable
data on thermo-elastic deformation and permeability may be obtained in situ
where they count. However, serious questions can be asked about the choice
of sites and the scale of these tests, relative to the natural joint spacing.
Often site selection seems to have been either geologically naive or directed
politically rather than technically.
We feel compelled to emphasize, once again, the significantly different con-
sequences of the alternatives of disposal and storage of HLW. Must the
facility remain stable only for a decade while it is being filled? Can the
cavity then be back-filled with rock and the access shafts permanently sealed?
Must it stay open for several decades, perhaps indefinitely, so that spent
fuel rods can be retrieved and ultimately reprocessed so as not to lose a
valuable store of energy; so that mistakes could be corrected should isola-
tion be incomplete; or so that canisters could be moved should the site fail
altogether? A weak, ductile medium like salt should be mechanically ideal
for permanent disposal but might be poor indeed for long-term storage with
the option of retrievability. Salt will certainly flow readily at 200°C
under expectable in situ stresses. Canisters will have to be isolated from
the surrounding rock, and artificial support of the cavity would have to be
provided.
Excavating a strong rock like granite is much more expensive, and because it
is brittle and always jointed to some degree, it may not be as good a candi-
date for permanent disposal. On the other hand, granite, basalt, or other
strong rocks seem preferable for storage, at least with regard to long-term
mechanical stability.
In summary, we shall know enough in a year or two to compute the consequences
of HLW disposal in dry, homogeneous, ductile salt that would be mechanically
metastable for at least a decade, perhaps indefinitely if we are to pay the
price for artificial support. On the other hand, we know very little indeed
about either the long-term strengths of jointed, brittle rock masses at
temperatures on the order of 500°C, or the effects of thermal cracking on
the flow of fluids through them.
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PART IV; CANDIDATE LITHOLOGIES
Several different rock types have been discussed as possible host lithologies
for the HLW repositories. Important candidates are: salt, shale, granitic
clan rocks, and basalt. Because of the undemonstrated security of the assem-
blages of materials in and immediately around the canisters, the containment
by the rock that is the host of the repository becomes of paramount importance.
This is probably the last stage of containment of the HLW prior to entry into
circulating ground waters and contact with the biosphere.
We do not attempt to review the properties of the different lithologies.
Rather, we focus on the physical and chemical properties that bear directly
on the question of HLW transport.
The first category discussed is salt. This occurs in two ways in nature,
as bedded salt deposits in a sedimentary sequence, and as salt domes. Be-
cause their characteristics are quite different, they are treated in dif-
ferent subsections.
IV.I SALT BEDS
Bedded salt deposits are commonly believed by those unfamiliar with them
to be massive, thick, pure beds of the mineral halite (NaCl), and this is
the usual basis for model studies. Though there are a few local places
where such beds can be found, most salt beds are more typically somewhat
thin-bedded, may alternate with other evaporite deposits such as anhydrite
(CaS04) or other salts (of the KCl-MgCl2 groups), and are frequently
separated by thin beds of shale. Salt is commonly interbedded with lime-
stones or dolomites or may interfinger with them laterally. Salt beds may
also grade laterally into sandstones.
On a microscopic scale, it can be seen that many halite crystals contain
liquid (brine) or solid (anhydrite) inclusions. A recent report by Roedder
and Belkin estimates the volume of these inclusions to be less than 1% but
"an additional possibly even greater volume % fluid is present ^.n situ,
filling intergranular pores."W
There is abundant evidence for post-depositional recrystallization, dif-
ferential solution, redeposition in cross-cutting veins, and mass flowage
in evaporite deposits. The textures produced in salt beds by these processes
increase the heterogeneity produced by depositional processes. There is also
ample evidence of solution channels being established along veins and litho-
logic breaks.
Because salt is so soluble it rarely crops out at the surface; thus its
stratigraphy and detailed lithology are mainly known from drill hole records.
Though individual distinctive laminae have been traced for long distances
through the study of drill cores, most of our information comes from geo-
physical logs of drill holes or shallow seismic profiles. Neither is ade-
quate to show the amount and number of thin shale, anhydrite, or limestone
breaks in salt beds. For this reason, site testing needs a carefully de-
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signed drilling program that includes provision for continuous coring that
would detect all lithologic breaks. No geologist can predict the presence
of a truly homogeneous bed of salt 30 m thick without such a drilling cam-
paign plus detailed, including microscopic, study of the core samples to
determine their brine and solid inclusion content.
Realistic estimates of water and brine content of salt beds are critical
to proper estimation of the fate of canisters sealed in a salt repository,
for, as indicated earlier, such hot briny solutions may effectively corrode
the metal canister, leach the HLW matrix, and transport dissolved radio-
nuclides. Water content is also important in affecting the creep rate of
salt.
Recent experiments in the Experimental Geochemistry and Mineralogy Branch
of USGS by D. B. Stewart and others(4) have shown that at 200°C, water
in a rock salt including potassium and magnesium components can contain
approximately 70 wt % of dissolved salts. This implies that as little as
1 wt % H20 in a salt bed may yield fluid that is 3 wt % because of the high
concentration of the solution. The dissolving power of such brines should
not be underestimated. We know from many hydrothermal experiments that be-
cause of changes in hydrolysis constants and the association of ion pair
complexes, highly saline solutions at elevated temperatures are highly
acidic—the pH can be as low as 2.
It should be noted that the pressure-temperature conditions near the canister
may result in the brine becoming two phases (liquid and vapor), which could
retard brine access to the canisters. The activity (in a chemical sense) of
the brine components would still be the same in the two phases, however, and
the liquids would still act as buffers to maintain the corrosive attack on
the canisters.
These solutions would strongly attack any metal and effectively leach glass.
There is also the possibility that permeation of these solutions along glass
grain boundaries and incipient fractures would speed up devitrification
which would then feed back positively to enhance leaching. '
Because of the high density of the canister, it might sink in any salt bed
that contained the above quantities of saline water solutions, being corroded
and leached as it moved downward. Many beds of salt overlie permeable lime-
stone formations and the canisters might end up there, in the path of ground
water movement. Any lithologic breaks in the salt might impede that movement
depending on the thickness and the rock strength. Bu? solution movem™y
also be enhanced along those very breaks. luvcmcuu uiay
We emphasize the importance of knowing the water content of salt beds proposed
for repositories, particularly with the background and the experience at Lyons,
Kansas, where considerable volumes of water migrated in an unpredicted manner.
That problem arose as a consequence of dissolution of salt by ground water
seeping into the repository. Seepage was along an abandoned drill hole that,
like most, had not been cased and plugged. This puts a premium on picking a
^^ dr±U ^S °* °ld -S.rg.oSnd
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Though remote sensing techniques seem likely to be able to spot some sur-
face manifestations of these underground penetrations to supplement state
and company records, it remains to be proven by ordinary double-blind
experiments that such detection is highly accurate. In this connection
it is necessary to prove not only that holes identified from aerial photo-
graphs are in fact there, but also that all holes actually have been identi-
fied from the photograph—no mean task. If this assurance cannot be made,
then modellers of repository behavior will need to take into account the
probability of leakage of water into salt through such old holes.
Any abandoned drill holes discovered may disqualify a site if they cannot
be properly plugged and sealed. If the well has penetrated the salt there
may be a cavernous dissolved region around the hole. In addition, most such
wells will show extensive caving of weak rocks in the section, which might
provide an efficient vertical transfer path among various aquifers. Since
metal casing cannot be guaranteed against corrosion over the time scales
we have to consider, some impermeable and chemically inert cement to grout
and plug the hole must be developed. This indeed may be an important re-
quirement for any drill holes designed to explore a site for a future
repository.
IV.2 SALT DOMES
Of all the geologic media suggested for the underground isolation of HLW
from the biosphere, the rock salt in the dry domes of the Gulf Coastal
Plain seems mechanically best for disposal (permanent isolation by back-
filling and sealing) but probably not for long-term storage and ready
retrievability. Room-and-pillar mining in salt is cheap relative to that
in hard rock.
Given the time frame of a million years, one cannot claim that the proba-
bility of any geologic event is absolutely zero. However, the risks of
catastrophic events like destructive earthquakes and volcanism, or of
sudden, unpredictable changes in the secular rate of erosion, or of con-
tinental glaciation are evidently extremely small in the coastal regions
of Texas and Louisiana, certainly as unlikely as they would be anywhere
in North America.
Once a critical thickness of overburden had been achieved, Gulf Coast domes
grew more or less concurrently with the deposition and loading of the sedi-
ments above them. Since the rock over onshore domes is now being eroded,
further growth ("diapirism") should not be expected. There is no obvious
reason why the coastal plain should not remain tectonically stable for at
least another million years.
Below the superjacent cap rock, the lithology is remarkably uniform. Al-
though bedding may have been contorted (it is not always so), the rock is
typically free of open fractures. Although its lateral extent is limited,
the salt is several kilometers thick, and it should be possible to identify
a repository site at least 1000 m from the closest water-bearing strata.
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Some (though not all) salt domes appear to be remarkably dry and to have
been so for some millions of years. Professor Robert R. Unterberger of
Texas A&M University has been probing domes with 40-cm radar. In a dry
salt mass like that of Grand Saline in northeast Texas, he is able to map
reflections at the boundaries with surrounding water-bearing strata, as
much as 2 km distant from the transmitting antenna. He could not do so
if the average water content exceeded on the order of 0.1%. (In the "wet"
Weeks Island dome, on the contrary, attenuation of the signal is so high
that no reflections are recorded, and he turns to sonar.) In a dry dome
no lateral migration of ground water appears to have occurred over several
millions of years. It is hard to imagine how any ground water could reach
the repository and, hence, how any radioactive elements could reach the
biosphere.
In a dry dome, the bulk density of the encapsulated HLW would presumably
exceed that of the salt, so that containers would tend to migrate downward,
all to the good, even if the high-temperature viscosity of the salt were
low enough to permit significant movements over the relevant span of time.
Thus disposal in salt domes is likely to be permanent.
We do not know whether the sinking will tend to "focus" the canisters, al-
though this effect should be amenable to computation. By focusing we mean
that the originally horizontal array will have not only a temperature
gradient that decreases away from each canister site, but also a higher-
than-expected local temperature at the center of the condensed array. As
the canisters sink in the salt, will they be "refracted" inward toward each
other? If they are, the central temperature would rise, leading to still
faster sinking and focusing until very high temperatures indeed might be
reached. This possibility should be studied. There is also an important
corollary: additional production of HLW by sub-critical or even, conceivably,
critical activity with the U, TRU, and water. We do not suggest that this
will happen, but rather that the question be properly addressed. Enough is
now known to permit a computational assessment.
Salt is a valuable mineral resource, but worldwide reserves are virtually
limitless, so there is no logical reason why a single dome should not be
perpetually withdrawn and dedicated to HLW disposal. The possibility of
human intrusion at a far future time cannot, however, be discounted.
IV.3 SHALES
Just as most salt beds are not pure and monolithic, shales are rarely homo-
geneous and uniform. A typical shale is a rock that is more or less silty
and contains occasional interbeds or laminae of permeable siltstone or
sandstone. Other shales may be interbedded with limestone or dolomite or
grade into a calcareous shale and then into a shaly limestone.
To varying degrees shales may be thinly bedded or show fissility, the
ability to break into thin sheets. They may also be cut by systems of
joints and fractures, and, where the shale is traced into areas where the
rocks are metamorphic, may show incipient fracture cleavage. It is just
20
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now becoming known that fracture cleavage in many shales is related to
original conditions of sedimentation and early diagenesis that make it
susceptible to breaking along certain zones. In general, the older and
more physically and mineralogically altered the original mud becomes, the
more brittle the resulting shale becomes.
Since approximately 70% of the sedimentary rocks of the earth are shales,
it is not surprising to find a great many thick shale sections. Yet it
is extraordinarily difficult to find one that is uninterrupted by inter-
beds of other, generally more permeable, lithologies. As in the case for
salt beds, it is difficult if not impossible to get an accurate detailed
log of a vertical section of most shales from outcrops because of their
rapid weathering in most climates and topographies. At the same time,
geophysical methods are inadequate to pick up fine interbeds and inter-
laminations. As thin as these may be, frequently on the order of a few
millimeters in thickness, there is good geological evidence that there
has been sufficient water flow through these laminae to alter the clay
mineralogy in the sandstone whereas the more impermeable shale is unaltered.
Hence a requirement for a shale repository must be the acquisition and care-
ful study of drill cores.
Shales are particularly susceptible to mineralogical alteration that pro-
motes physical change if they come in contact with solutions of a different
kind than they were naturally bathed in at depth. Shales are complex mix-
tures of quartz, feldspar, several varieties of clay minerals, zeolites,
carbonates, sulfides and oxides. Any significant chemical reactions of
these minerals are likely to weaken the physical structure and promote
cracking and disintegration at the relatively low confining pressures of
a repository. All of these reactions are promoted by increase in tem-
perature.
Before any detailed plans for a shale repository can be drawn, the strati-
graphic and mineralogical-geochemical work has to be done. The detailed
stratigraphy and petrography we know how to do, but the study of how altered
mineralogy affects physical structure is in its infancy. Long-term heating
to 300-500°C is likely to induce mineralogical changes including dehydration
of smectites. This dehydration could liberate significant quantities of
free water and induce textural changes that might be reflected in fracture
patterns, which in turn affect the fluid permeability of the formation. We
cannot as yet predict such behavior of a specific complex mineral assemblage
from general rules.
The USGS has done a fair amount of work on the merits of the Pierre
shale of the Dakotas as a possible repository. The stratigraphic and
mineralogical-chemical knowledge of the Pierre shale is possibly greater
than that of any other shale in the world. We know that there are thick
shale sections in it and that it has a generally stable mineralogy. On
the other hand, as much as we know, we still are unable to predict with
any high probability what the fluid permeability is of such a section after
long-term heating by buried HLW. And we are not able at this time to de-
fine how rare and how thin occasional interbedded silt laminae would have
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to be to prevent significant fluid migration in or out of the formation.
At this time we could not rule out extensive fluid migration via fracture
permeability following a 10-20-year period of moderate heating.
IV . 4 GRANITE
Because they are widespread, underlie large regions of the mountain belts
and shield areas of North America, are relatively homogeneous, have high
crushing strengths (up to a few kilobars at low confining stress) , low
porosities and, therefore, low pore-water contents (0.01 to 0.03) 'and low
permeabilities (10-7 to 10-17 Cm2), granitic rocks have good potential for
development as high-level waste repositories. Large granite plutons range
in thickness from 2 km to 15 km and, unlike sedimentary rocks are not
likely to have aquifers at depth. Retrieval of HLW canisters would be
easier in granite than in most other rocks. However, granites are brittle,
and we currently know neither their behavior under thermal stress nor their
sorptive properties.
Many of the important physical properties of granite are site-specific, such
as joint density and state of stress. Other site-specific features such as
hydrothermal alteration zones, dikes, quartz veins, the irregular three-
dimensional geometry of the granite body itself, and faults will all con-
tribute, along with the joint pattern, to the hydrologic regime. It is
the Panel s opinion, and apparently that of several foreign countries as
well, that a sizable body of granite underlying a hydrologic basin of
appropriate dimensions may prove, in the long run, to be an excellent under-
ground repository. We know of no reasons, as yet, to ^ it out. Research
on granites should be pushed vigorously, particularly because there may be
either socio-political or geological reasons why burial in salt may be ruled
" ^
isn obvious ""
IV. 5 BASALT
The presence of tens of thousands of square miles of up to a 5,000-f t-thick
succession of plateau basalts in the northwestern United States and their
nearness to the Hanford, Washington and Idaho National Eng^lnTLaSrrt
Fresh massive basalt will have crushing strengths as high or higher than
those of granite, and porosities, permeabilities, and linear thlrmal expan-
sion coefficients that may be as low or lower Thp nnlT^ ! thermal expan
conductivity for basalt and granite lies the rfx 10-3 8 x
10-3 cal/[cm][sec2n°C]), but basalt is likely to be thThigher or the two
Although the laboratory-measured physical properties of granite and basalt
thus seem to be roughly equivalent, there are geological differences in their
mode of occurrence that favor the former rock over the latter. The typical
basalt flow of the Columbia and Snake River plateaus ranges from 10 m to 45 ni
in thickness, and is often separated from the overlying and underlying flows '
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by an aquifer. Lower columnar and upper fan-type jointing of each indi-
vidual flow is characteristic, and most lavas have a 5-m thick vesicular
zone at the top, and a 1-m thick vesicular zone at the base of each flow.
Thus it seems that, on the average, basalt should be far more porous and
permeable than granite, and that it would also offer a higher risk of
contaminating the ground water if used as an HLW repository. Advantages
of some basalts would be their lateral continuity over hundreds of square
miles, and the fact that some of the rocks are slightly altered and contain
clays or zeolites, which may enhance the sorptive properties of the basalt.
Unfortunately, though there are some data on clay minerals and zeolites,
there are no published data on the sorptive properties of well-described
altered basalts.
The general feeling of the Panel is that because of geologic constraints,
establishing a safe repository will be more difficult in basalt than it will
be in granite. Finding a thick unfractured or unjointed basalt flow that
can be opened up and resealed without the development of fractures that will
communicate with an actual or potential interflow aquifer may be a require-
ment that is difficult to meet. Again, we know enough about the areal
geology, stratigraphy, and general lithologic character of basalt, but would
need means to determine the site-specific fracture permeability or bulk
retentivity. At a minimum, a strong laboratory reconnaissance program would
be needed in order to demonstrate the desirability of further consideration
of basalt.
IV.6 OTHER ROCKS
It is appropriate at this point to comment briefly on other rock types that
may have excellent potential as disposal hosts but have been largely ignored
up to the present.
Analysis of thousands of water well records in places such as New England
demonstrates that the flow of ground water in bedrock in these areas is
entirely fracture-dependent, and that metamorphic rocks have very low po-
rosities and negligible permeabilities. With increasing degree of meta-
morphism, such rocks become increasingly anhydrous and granular textured,
approaching the igneous rocks in their general physical properties. High-
grade metamorphic terrains are common in many mountain belts, as well as
in areas such as the Minnesota Shield, the South Dakota Black Hills, and
the Adirondacks. In the latter region they are associated with igneous
rocks such as anorthosite, gabbro, syenite, and charnockite, all of which
are composed of anhydrous silicates and all of which are potentially usable
depository hosts.
Dunite, a relatively uncommon igneous rock occurring in parts of the
Appalachians and the Coast Ranges, has the unique property of being po-
tentially self-healing. Access of water to an HLW canister in these rocks
would result under the pressure and temperature conditions of burial, in
the development of serpentine; the large volume increase accompanying this
reaction would help to make the repository impermeable. It would need to
be shown, however, that the process itself would not induce fracturing.
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In a similar way, a proposal by Professor W.S. Fyfe that canisters be
surrounded by a packing of MgO powder (which would hydrate to brucite-
Mg(OH)2—upon contact with water) is also worth considering as a safe-
guard against possible waste leakage.
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PART V; SOLUTION TRANSPORT THROUGH ROCKS
The principal barrier to contamination of the biosphere by HLW is the time
delay expected during the migration of the nuclides through the rock unit
that contains the repository. This delay can be predicted from experimental
data plus some assumptions regarding the particular setting, or from evidence
of previously completed "experiments."
The first approach is based on data concerning three key factors: the actual
rock permeability in situ, whether this be true permeability or flow through
fractures; the flow rate; and the retardation, if any, of the dissolved
nuclide relative to the flow of the water. Part VI will consider the first
two factors. In this section, the third factor will be discussed and then
the previously completed "experiments."
V.I SORPTION OF RADIONUCLIDES ONTO ROCK MATERIALS
Critical to the operation of a secondary barrier to radionuclide migration
is the ability of a rock through which fluids may slowly flow to adsorb or
react with dissolved radionuclides. There is now an abundance of experimental
data on the extent of adsorption of various fission products and transuranics
by soil and rock materials. Unfortunately these experimental data are useful
only in a general way and even then may be misleading.
The reasons are that for any such surface chemical process, the nature of the
mineral surface must be specified precisely (mineralogical and chemical com-
position, surface area per unit of mass) and the experiment must be carried
out under conditions similar to those expected underground. In addition, for
experimental results to be meaningful a uniform methodology would have to be
employed so that various materials and different elements could be compared
on a sound common basis.
The experiments thus far performed have been on materials described variously
as "desert soil", "tuff", "basalt", or "roontmorillonite", to mention a few.
None of these terms is a precise description that would allow anyone to infer
what the chemical behavior of the material might be. "Montmorillonite" is a
term used in many different ways by clay mineralogists and rarely to describe
a specific mineral with a fixed composition and crystal structure. Specific
standard clay minerals (American Petroleum Institute Standards) have been used,
but only for the sake of comparison and not to give the impression that they
are standard mineral types. Adequate description of any clay is made only by
chemical analysis (either bulk or electron microprobe) coupled with crystal
structure analysis by X-ray diffraction. Since the methods of sample prepara-
tion strongly influence surface area, these too must be specified. The ion-
exchange capacity and selectivity of the clays used must also be determined.
The same comments apply to the use of rock terms such as "basalt" or "tuff",
which are just field terms of gross lithology used for the convenience of
the geologist mapping in the field and not in any sense the same as quotation
of the precise chemical formula of a reagent.
25
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Results of experiments conducted at 25°C and 1 atm. pressure are not a good
guide to what might happen at 100°, 200°, or 300°C. We know of temperature-
induced reversals in cation selectivity, exchange capacity, and sorption
characteristics that are a result of surface configurational changes. In
addition, all of these data are almost meaningless unless some realistic
choice of solution is made. Formation waters of deeply buried rocks tend
to be rich in dissolved solids, and this ionic strength can greatly alter
sorptive properties of mineral surfaces. Cation adsorption is generally
competitive, and radionuclides may be passed unsorbed if competing cations
of formation waters have already been sorbed strongly onto surfaces.
What we can say with the information at hand is that we would expect cer-
tain classes of materials to be more retentive than others at room tempera-
tures. Thus desert soils would have high retentivity and granites and salt
low retentivity; with tuffs, basalts and shales lying between, we would ex-
pect notable cation exchangers, such as smectites and zeolites, to be highly
retentive. But this is no quantitative basis for estimation of what will
really occur in a specific formation at depth and at elevated temperatures.
Before that can be done we need a carefully planned and executed program of
testing by experimentation.
V.2 FIELD "EXPERIMENTS"
An. alternative method to predict behavior of HLW transport through rocks is
to examine natural settings where such transport might have occurred. Two
such oases have been studied in some detail and can shed some light on the
process. These are the Oklo natural fission reactor discovered in Gabon,
and the underground nuclear explosion test, Cambric, carried out in Nevada.
Approximately 1.8 billion years ago a uranium ore deposit experienced the
appropriate conditions of U concentration, 235^ concentration, and H/U ratio
for it to go critical and yield fissions and energy. This is known as the
Oklo phenomenon. The four to six local zones where this occurred are thought
to have produced 15,000 megawatt years (MW yr) of energy, over a period of
approximately 0.5 to 1 million years. The site has been studied in order to
determine the extent of loss of the various fission products and TRU's that
were produced during and subsequent to the time fission was occurring.
The results indicate that the losses were largely dependent on the chemistry
of the nuclides in question. Alkalies and alkaline earths were lost either
completely or in very significant amounts. These would include the majo
fission product nuclides of concern, 90Sr and 137cs [see Lancelot et a
On the other hand, the radioactive rare earth elements and the TRU's largely
remained in or near the reactor zones.(6' A very precise material balance
is not possible. It would not be surprising if 10% or 20% of the products
in a given reactor zone had been lost.
The difficulty with the interpretation of the Oklo event is the lack of
precise geological control. It is not clear to what depth these deposits
were buried at the time of the fission reaction. Some believe that the
process was going on under fairly shallow burial, because the water that
26
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acted as a moderator had to be vaporized and this could only happen close
to the surface. The sediments have been altered by heat and pressure to
a state suggestive of burial to approximately 4,000 ft.^'J although there
is a suggestion that the depth may be as much as 6 or 7 km. The mineral
assemblages, their compositions, and their fluid inclusions all suggest a
variety of possible depths of burial so that no sure conclusions can be
drawn. It is likely, however, that the depth was considerably more than
the 1,500 ft envisioned for HLW burial.
The uncertainty concerning the depth of burial means that it is not possi-
ble to make a direct comparison of the Oklo phenomenon with an HLW disposal
site. This is so both because of the dependence of the flow of fluids on
depth (and the consequent variation in degree of openness of cracks) and
because the distance that the nuclides would have traveled to reach the
surface is not known.
Whatever the depth, however, there must be serious concern over the loss
of the major fission products, 90sr and 137Cs.
The second "experiment" is the Cambric nuclear test in Nevada and the subse-
quent water pumping tests conducted using drill holes at the site. The massivt
pumping tests conducted in the decade since the explosion have shown that the
radioactivity that has entered the water is in very low concentrations. This
is very encouraging.
Two factors that play a role here, however, make the test less relevant to
HLW containment. The first is that much of the radioactivity is in a glass
produced by the extreme heat of the explosion. This glass has had ten years
to devitrify and be leached. Since little radioactivity is detected in the
pumped water, it can be inferred that the nuclides are being retained. The
problem is that the water is cold, and the pumping continues to bring cold
water to the site. This test, therefore, is not using waters that are com-
parable to those anticipated in an HLW repository: hotter than 300°C and with
high ionic strengths. As a result, the channels through which the water is
flowing are behaving like pipes. Further, the very low concentrations of
nuclides are partly due to the rapid flow rates, which serve to dilute the
nuclides as they are released.
27
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PART VI; RADIONUCLIDE TRANSPORT FORECASTING
The objective of the transport modeling considered in this analysis is to
forecast the subsurface movement and evolution of radionuclides emanating
from a radioactive waste repository under various hypothetical situations.
Perhaps the most challenging aspect of this problem is the necessity to
forecast over long time periods (250,000 years) (°' with uncertain informa-
tion. In addition to the elements of uncertainty concerning the transport
listed previously, (9,10) the following also play a role:
• hydrologic characteristics of the site (primary versus
secondary permeability and porosity, hydrodynamic dis-
persion) ,
• mathematical representation of subsurface transport, and
• solution of the resulting equations for realistic physical
conditions.
Many of these uncertainties have been encountered in petroleum engineering
and subsurface hydrology. In these instances, uncertainty is reduced
primarily through the "calibration" process whereby the mathematical model
and its input parameters are modified until a historical record (pressure
history, concentration trends) is reproduced within a range of error deemed
to be "acceptable." Obviously, the simulation of radioactive waste trans-
port does not, in general, lend itself to a "calibration" form of uncertainty
reduction. In summary, the problem is one of either minimizing the uncer-
tainty outlined above such that a deterministic analysis (inclusive of
sensitivity analysis) can be meaningful, or alternatively, incorporating
this uncertainty directly into the analysis so that the decision-maker is
cognizant of the degree of uncertainty inherent in the forecasts.
VI. 1 STATE OF THE ART OF TRANSPORT MODELING
Two operational models are currently capable of simulating the movement of
multiple radionuclides in the subsurface. These models have been developed
for the Nuclear Regulatory Commission and the Energy Research and Develop-
ment Administration (now Department of Energy) by Intera Environmental
Consultants and Battelle Northwest Laboratories, respectively. (Note that
we have not included the model employed by the American Physical Society
Study Group on Nuclear Fuel Cycles and Waste Management because of the
physical and mathematical simplifications inherent in their approach.)
The salient features of the two models are summarized in Table 1. Examina-
tion of this information reveals that the Intera numerical model, is the
most physically realistic and flexible. The BNWL models, however, have
the advantage of analytical simplicity and associated computational
efficiency.
Burkholder et al. have argued because of the non-reducible uncertain-
ties inherent in some elements of the analysis of radionuclide transport,
"that highly sophisticated transport models should be used with caution or
29
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TABLE 1
CHARACTERISTICS OF TRANSPORT MODELS
Model
Characteristic
Dimensionality
Mathemat ical
Formulation
Dispersion
Representation
Convection
Representation
Species
Transported
Transporting
Medium
Sorption
Source
Dissolution
Chemical
Form
Discrete Fracture
Representation
Stochastic
Capability
Geologic Strata
Temperature
Dependence
Solubility
Model
INTERA
3D Cartesian
2D Radial
Numerical Finite
Difference
Variable, Function
of Velocity,
Tensor
General Velocity
Field
Multiple
Radionuclides
Saturated
Porous Medium
Linear
Variable
Single
None
None
BNWL I
ID Cartesian
Analytical
Scalar, Constant
Constant Velocity
Field
Multiple
.Radionuclides
Saturated
Porous Medium
Linear
Constant
Single
None
None
Non-Homogeneous Homogeneous
Density, Viscosity None
Limited ?
BNWL II
2D Cartesian
Analytical-
Numerical
Scalar, Constant
Spacially Varying
Velocity Field
Multiple
Radionuclides
Saturated
Porous Medium
Linear
Constant
Multiple*
None
None
Homogeneous
None
Under development
30
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perhaps not be used at all." They further state that such models "could
create unwarranted illusions of certainty." The significance and im-
portance of this point of view depends, of course, on the interpretation
of the phrase "highly sophisticated."
In other branches of science and engineering, the evolutionary trend has
been to minimize forecasting uncertainty through improved mathematical
representation of the physical system and careful evaluation of associated
physical parameters (see, for example, work in structures and mechanics,
hydrology, meteorology, etc.). The use of a simpler model, however, is
advantageous when numerous alternative scenarios must be examined, such as
in a sensitivity analysis or an optimization algorithm. A combination of
both approaches, such as utilized by Intera in their report to EPA,(12)
represents a cost-effective approach to investigating the problem of
radionuclide transport.
VI.2 THE QUESTION OF UNCERTAINTY
There is little question that the models presented in Table 1 represent the
state of the art of engineering capability in transport modeling. Certain
degrees of sophistication could be included, such as a better representation
of the fracture system, but these modifications are not likely to enhance
the accuracy of the model forecasts materially, because of the uncertainties
that will still remain.
It is more important to consider the possibility of quantifying the uncer-
tainty in the model so that decision-makers are aware of the degree of un-
certainty associated with the analysis. The simplest approach to quantifying
uncertainty in simulation modeling is through a sensitivity analysis. Such
an analysis was performed by Intera for EPA. t1^
In this approach, a range of parameter values is used in the predictions.
These are selected so as to encompass the entire range of physically mean-
ingful parameter values. Such an analysis provides an insight into the
role of each model parameter on the calculated radionuclide distribution.
To conduct such an analysis generally requires a tremendous amount of com-
putational effort. To analyze two levels of the 12 parameters considered
by Intera unambiguously would require 21 independent simulations. Because
this is prohibitive, even with use of analytical models, a subset of the
general problem is normally considered and the results, of necessity, are
indicative rather than definitive.
While a sensitivity analysis provides the decision-maker with a range of
possible radionuclide distributions, it does not give any insight into
the probability of occurrence of any particular forecast. In simulations
wherein large residual uncertainties in input are unavoidable, the uncer-
tainty in the predicted radionuclide concentrations should be recognized
and quantified. In other, words, in an analysis of the radionuclide trans-
port problem, the solution should appear as a confidence region (i.e., we
are 95% certain the real concentration of 90Sr lies between x and y, given
the probability distributions of the input parameters). Thus, the re-
31
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liability and importance of the forecast can be directly related to the
reliability of the input parameters. To conduct such an analysis requires
estimates of parameters and their uncertainty, and a suitable mathematical
model.
Standard mathematical models such as presented in Table 1 can be modified
for simulation under uncertainty. The procedure, known as Monte Carlo
analysis, involves conducting a series of simulations in which parameter
values are selected at random from their respective probability distribu-
tions. These values are used in simulation runs and the output variables,
in this case radionuclide distributions, are assembled into probability
distributions. From these distributions, confidence intervals can be com-
puted. Such analyses, however, require excessive computational effort,
particularly when several different parameter distributions are involved.
Alternative schemes are available to circumvent these computational diffi-
culties, but these cannot be considered at this time to be state-of-the-art
engineering methodologies.
While uncertainty in the partial differential equations may be accounted
for by use of new mathematical methodology, one must still consider the
problem of estimating these uncertain parameters. Whether intentional
or by default, these parameters will be subjective probability estimates.
Tools are available and currently being developed to compute these estimates
by use of objective and subjective information (see, for example, Bayesian
methods). The credibility of forecasts made under condition of uncertainty
will depend upon a reasonable representation of the probabilistic parameters,
The impact of parameter uncertainty, that is the resultant uncertainty in
the projections, is dependent on problem and boundary conditions. In ex-
ample problems that have been considered/13) situations were encountered
in which the uncertainty increased, reached a maximum, and asymptotically
decreased to zero as a function of time. Another case resulted in an
exponential increase in uncertainty throughout the simulation period.
Thus it is apparent that generalizations regarding the impact of parameter
uncertainty on forecasts of radionuclide concentrations are not normally
possible.
It should also be noted that uncertainties associated with parameters in
the governing equations interact in such a manner that their individual
contributions to the spread in the confidence interval of the projections
cannot be discerned. Thus it is not possible, in general, to delineate
unambiguously the importance of the uncertainty associated with, for ex-
ample, the source term.
VI.3 CREDIBILITY OF THE ANALYSIS
At this point it is evident that uncertainty is the distinctive element
of radionuclide transport analysis. While uncertainty is associated to
some degree with forecasts emanating from all mathematical models of sub-
surface phenomena, this uncertainty is generally minimized through cali-
bration with known historical data. Even where historical data are scarce
generally reserves the option of measuring in situ hydrologic and trans-
one
32
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port parameters. Because the disposal site has not yet been selected,
such measurements are not currently available. Moreover, even after site
selection, the in situ measurement of parameters must be minimized to
ensure the integrity of the site. Because of the apparent inability to
decrease significantly the uncertainty associated with the transport
parameters, the only viable alternative is to incorporate as much of the
uncertainty as possible directly into the analysis so that the decision-
maker is aware of at least the minimal uncertainty inherent in the fore-
casts .
In evaluating the suitability of available radionuclide transport analyses
as a basis for establishing radiation standards for high-level radioactive
waste management, EPA should particularly consider the following:
• The existing simulations of radionuclide transport emanating
from high-level waste are limited to a small number of hypo-
thetical problems associated with selected waste-disposal
scenarios. The results represent qualitatively the movement
of the waste under these conditions. Although limited in
number relative to the spectrum of hydrological settings
possible for a repository, the problems selected represent
a reasonable and relatively unbiased choice.
• The problems examined should be considered indicative rather
than representative of situations to be encountered in the
storage of radioactive waste.
• The relationship between the parameters and processes
employed in the model and those to be encountered in a
disposal site is very uncertain.
• The Intera model incorporates directly or indirectly (as
in the case of fractures) those attributes of the proto-
type physical system most likely to play an important
role in radionuclide transport from a repository. A
possible exception is unsaturated transport.
• The credibility of the model and the associated analysis
would be enhanced by a satisfactory simulation of radio-
nuclide transport at the Nevada Test Site.
• Current models do not incorporate parameter or process
uncertainty in their forecasts.
• The scenarios considered in the analyses are, in large
part, arbitrary and may or may not encompass the hydro-
geological systems to be encountered at a specific site.
• The general approach currently used by some consulting en-
gineers for forecasting radionuclide transport, though strongly
limited by the inherent uncertainty in the hydrogeologic, geo-
33
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chemical and chemical data input, nevertheless represents the
best means currently available for establishing the movement
of radionuclides if and when the proposed containment is
breached and water comes in contact with the waste. We
have considered the quality and applicability of the simu-
lation methods but not their adequacy. The adequacy of the
forecasts depends on the sensitivity of the decision-making
process to the state of the art of transport modeling.
VI.4 PROPOSED RESEARCH
We have been asked to suggest techniques that would result in an improved
capability for risk assessment regarding the HLW repository concept. In
transport simulation, three areas appear to be prime candidates for further
research:
• In order to incorporate the uncertainty associated with
parameter identification, a program should be initiated
that is directed toward the development of an analytical
capability for forecasting under uncertainty. This would
not involve great sums of money inasmuch as the primary
goal is software development, which is not usually capital
intensive. In the short term, it would appear reasonable
and appropriate to employ existing techniques for simula-
tion under uncertainty in order to analyze one or more
mathematically simple scenarios. This would at least pro-
vide a ball-park estimate of the uncertainty associated with
the forecasts.
• Before the methodology developed in the preceding step can
be utilized, estimates of parameter uncertainty must be
available. These estimates can be obtained through an
effective utilization of subjective and measured information.
The analytical apparatus for this type of analysis is incom-
pletely developed and should be considered a priority ele-
ment of the research effort.
• Once uncertainty can be quantified and incorporated into
transport forecasts, the next major objective is to mini-
mize this uncertainty. This can be achieved primarily
through additional field observation. Thus, a primary
goal of a research program should be the in situ measure-
ment of hydrological and geochemical parameters. Because
of the nature of the repository, this methodology must be
compatible with the maintenance of repository integrity.
The liberal use of subjective information will minimize the
cost and negative impact to the repository of data collec-
tion and, it would seem, provide a cost-effective program
of parameter identification.
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PART VII; GEOLOGIC AND OTHER ACCIDENTAL HAZARDS
Assuming that physical and hydrologic requirements for a safe HLW reposi-
tory have been established, one must still consider whether all other
factors that may disrupt the integrity of the site have been taken into
consideration, and whether these factors are tractable to numerical risk
analysis. As one report states, "At some point in the repository site
and emplacement method selection and safety assessment, whether willing
or not, we will quantify in order to make decisions."(14) Without attempt-
ing to assess all non-geologic and geologic hazards, we shall comment
briefly on some of them, primarily to illustrate the extreme numerical
uncertainties attached to most.
VII.1 NON-GEOLOGIC ACCIDENTS
VII.1.1 Intrusion by Man
The development of an underground waste repository involves the breaching
of what was probably initially an intact geologic container. After the
repository has been backfilled, heating combined with the possible access
of ground water will set up a convective hydrological system in which
ionic transport rates will also be greatly accelerated. These features,
as well as uncertainty about the long-term effectiveness of having sealed
shafts in bedrock, are matters on which there are very few bases for judge-
ment.
Intrusion of a repository at some future date in the search for mineral
materials, including the uranium and TRU elements that were buried, or to
satisfy archeological or other curiosities appears to us also to be a risk
for which no trustworthy probability estimates may be applied for the time
span over which the integrity of the repository is to be assured. Material
demands shift with technology. The most successful civilizations last for
only a few millenia at best, and even these are not without their disastrous
interludes. Man's unpredictability far outstrips most of the imagined geo-
logic hazards we can foresee, and we doubt that it is amenable to meaningful
probability analysis.
VII.1.2 Meteorites
An astronomic risk that is generally cited is meteorite impact. The earth
annually receives an infall of approximately 10^ tons of cosmic dust, but
meteorites large enough to produce craters the size of Meteor Crater in
Arizona (1.2-km diameter), or larger, are estimated to have a frequency on
the order of 1 per 20,000 years or less. The bedrock in a meteorite crater
is pulverized to a depth of approximately one-tenth of the crater diameter,
but the surrounding and underlying rock will be highly fractured. If one
assumes conservatively that a repository at a depth of 0.5 to 1 km might be
breached if it lay within a radius of 10 km from the point of meteorite
impact, and also that the probability for impact is equally distributed
over the earth's surface (5.1 x 10^ km^), the likelihood of meteorite
damage is approximately 3.3 x 10~H/yr, or 1/30,000 averaged over a
35
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million-year time span. Meteorites, therefore, should not be regarded as
significant factors in evaluating repository safety. A more detailed
treatment by Arthur D. Little, Inc.' ' yields similar results except for
secondary effects such as stream diversion by impact. Given the need to
make provision in the repository for major water table variation, this
problem can also be given low significance.
VII.2 GEOLOGIC ACCIDENTS
The structure of the earth and energy sources that drive its physical
processes are such that stochastic models are not widely applicable, unless
one closely restricts the area of. inquiry, and probably the time as well.
We are aware that fault tree analysis is the numerical basis for risk
assessment in the Rasmussen report on nuclear reactor safety.(l^) However
the question of assessing engineering failure by a surface facility over
a 50-year time span is not the same problem as assuring geologic and
hydrologic integrity at an underground site over a million-year time span,
especially because there is information on reactor performance in the past.
VII.2.1 Extrusive and Intrusive Magmas
Aside from gradual subsidence of the Gulf Coast and the Mississippi Embay-
ment commencing approximately 70 million years (m.y.) ago, and the isostatic
depression and rebound of the northeastern and north-central part of the
country in response to continental glaciation, the United States east of
the Rocky Mountains has been geologically stable for approximately 100 m.y.
In this vast region the youngest dated igneous rocks (dikes) have an age
of approximately 70 m.y. For this entire area, then, the probability for
a magmatic incursion into a waste repository during the next million years
has to be taken as asttonomically low, but numerically uncertain.
Along the entire Cascades chain, by contrast, the probability for a volcanic
eruption may be on the order of 10~2/yr. If there are an estimated 65,000 km2
of potential volcanic terrane and a repository within a 10-km radius of a
volcano might be adversely affected by an eruption, the average rate of risk
per 300 km2 is on the order of 10~Vyr. However, it obviously makes a differ-
ence whether we designate one of these 300 km2 areas as lying close to Lassen
Peak, Mt. Baker, Mt. St. Helens or Mt. Ranier, or remote from them. Even for
the remote areas, it is well to recollect that no one predicted or could have
predicted the May, 1943 eruption of a new volcano at Paricutin, in the western
Mexico volcanic belt.
For the area of intermediate magmatic risk lying between the Rocky Mountain
Front and the Cascade-Coast Range Belt, there is no very good basis for specu-
lating on numerical risk probabilities, nor is there any ready answer to why
areas such as Sunset Crater, Arizona, and the Craters of the Moon, Idaho, have
been active within the past few thousand years.
The situation with respect to magmatic hazard typifies what the Panel believes
to be true of most other geologic risks. Within different specified regions
we may estimate a relative order of danger, but for very few areas can we have
36
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any confidence in orders-of-magnitude estimates of risk probabilities.
Of all the world's volcanoes now under surveillance, Kilauea is one of
the few whose short-term behavior is predictive. Even here, projections
for a million years in the future are well beyond the current state of
the art.
VII.2.2 Earthquakes and Faults
Even in regions of transform faulting where great earthquakes occur re-
latively frequently on the geologic time scale, like the active, western
continental margin of North America, we cannot yet predict specific events
of magnitude 7 or more with precisions better than about 100 years and
100 km. We do know, however, that the recurrence rate of events on the
San Andreas fault has been on the order of 200 years over the last few
thousand years, that geologically rapid displacements are still going on,
and hence that California is obviously a high-risk region where we would
not place a repository if we fear the hazards of earthquakes.
On the other hand, we cannot claim that the probability of an earthquake
is identically zero anywhere in the world because the origins of earth-
quakes in the interior of the continent (New Madrid, 1811) or at its
passive margin (Charleston, 1886) remain pretty much a mystery. Had ,
these events occurred a few generations earlier, before written records
became available, we would not now be worried about destructive earth-
quakes in the central and eastern United States. About all that we can
confidently say is that the earthquake risk is much less in some regions,
say the Gulf Coastal Plain, than it is elsewhere, say along the trace of
the San Andreas fault and its subsidiaries.
The hazards of earthquakes may well be overly exaggerated. Recent sur-
veys by Professor H. Dowding of Northwestern University and Dr. Howard R.
Pratt of Terra Tek reveal that damage to underground openings is very
much less than that to surface structures immediately above. Disruption
would occur if a seismogenic fault actually transected an opening, but
adequate site characterization should have eliminated this eventuality.
Thus, once a repository has been sealed and its surface facilities
abandoned, the effects even of large earthquakes are likely to be negli-
gible. , .
It is well to recall that the earthquake history of the United States is
based chiefly upon diaries or newspaper accounts, which may cover a time
span varying from 300 years to a few decades, depending upon the area.
The development of a widespread instrumented seismic network with sophis-
ticated equipment and data processing is partly an outgrowth, during the
past three decades, of the interest in detecting nuclear bomb blasts.
Instrumental seismology has existed'for decades, but systematic data com-
pilation is relatively recent. On either basis, whether from an incomplete
historical record stretching backward for up to three centuries, or from a
compilation of seismic record data, perhaps adequate for only three decades,
there is little reason for confidence in forward predictions covering a
million years.
37
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During the past three decades a series of qualitative seismic risk maps
has been produced by the Coast and Geodetic Survey, the most recent of
these in 1969. Responsibility for the next set of risk maps lies with
the U.S. Geological Survey, but their own estimate is that a reliable
seismic probability risk map lies a decade or more in the future. Even
this assessment may be optimistic.
VII.2.3 Glaciation, and Climatic Changes
Geologic records of glaciation in Alaska extend backwards for at least 10
million years and in the coterminous United States for more than 2 million
years. Despite the generally adopted statement that there were only four
major cycles of glaciation, it has now been demonstrated that there have
been numerous glacial epochs, that they follow a cyclicity on the order
of 1C)5 years, and that they are predominantly controlled by the earth's
orbital eccentricity, as proposed long ago by Milankovitch. Both theory
and the now established record show that the bulk of these lO^-year cycles
represents glaciation for the Northern Hemisphere, with interglacial
periods like the present one lasting on the order of 10,000-20,000 years.
The current interglacial was established between 16,000 and 11,000 years
ago. It is now predicted that the long-term trend for the next 20,000
years is toward extensive glaciation in the Northern Hemisphere. Although
this forecast is in accord with climatic analyses that show a consistent
cooling trend for the past three decades, the latter trend is too short to
permit interpretation that the two are coupled.
The near certainty that re-glaciation will occur long before any disposal
site will have fulfilled its assigned role implies that there will be other
complications. The depth of bedrock scouring by ice sheets averages only
20 ft. However, re-routed streams have incised bedrock for depths of up to
200 ft; Niagara Falls, the best known example, has excavated a 170-ft-deep
trench for a length of 7 mi during the past 10,000 years. Waxing icecaps
cause isostatic depression of bedrock amounting to hundreds of feet, po-
tentially producing faulting; waning icecaps allow isostatic rebound, more
potential faulting and, with extreme melting, a good possibility for flooding
continental margins to depths of 100 ft or more above present sea level.
Mean precipitation and evaporation differ markedly between glacial and inter-
glacial times. We cannot assume that an area that is arid today will remain
so during a glacial cycle. This is most obviously the case in Utah and
Nevada where, during the recent glacial epochs, there was a pronounced plu-
vial environment. What such a pluvial environment might do to the hydrologic
regime in the site of an HLW repository seems to have been largely ignored in
current risk assessments of repositories such as Hanford and the Nevada Test
Site.
The Panel believes that continental re-glaciation has a very high probability
of occurring within the time period of concern for HLW, and that the associ-
ated climatic changes will have differing effects on all waste repositories,
depending on location. There may, however, be certain advantages to locating
repositories in areas likely to be re-glaciated, because this would effec-
tively seal them from intrusion several thousands of years hence.
38
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A different problem arises from the potential combustion of coal and
petroleum products. This, the so-called greenhouse effect, would be a
warmer world-wide climate created by the C02 resulting from fuel burning.
Even if the effect were not total melting of the Antarctic and Greenland
ice-sheets, sea level would rise significantly. The evaluation of salt
domes as repository sites should include consideration that such an
eventuality is possible. If the top of the dome were accessible to sea-
water, extensive dissolution of the salt could occur, possibly exposing
the HLW. Further, even if the greenhouse effect did not play a role, it
is known that sea level has varied for different interglacial epochs and
could be higher next time than it is now.
VII.3 FUTURE RESOURCE EXPLORATION
Although the energy supply appears to loom as the major mineral resource
problem for the next few decades, this is only because its present urgency
has diverted attention from the far more important question of how the
world's rapidly expanding population can maintain the raw material supplies
necessary for an industrialized society. Current annual consumption of
newly mined mineral products is 3.75 MT per person, and by 1985 this figure
will have doubled. Geochemical and economic arguments have been adduced to
show that when the average ore grade of a metal slips below a critical level,
the amount of energy needed to extract that mineral will far exceed any
costs that society can reasonably pay for it, so the mineral will no longer
be available. Mercury, silver, gold, copper, and lead are candidates for
such early scarcity. The 1975 National Academy of Sciences Report of the
Committee on Mineral Resources and the Environment has concluded that
technology cannot always close the gap between rising demands, and available
resources, so we shall be inexorably driven to lower ore grades, larger
tonnages, and other materials.
As an illustration of the enforced shift to increasing tonnages of decreasing
grade we may cite uranium, for which the 1990 projected demand of UaOg is
700,000 MT; production of this amount of UaOs will be possible only if the
present ore (economically mineable) grade (0.22% ^03) falls to 0.075% in 1990.
From what is known of the world's mineral resource picture, it is 'difficult
to be optimistic that industrialized society can sustain itself at anything
approaching its current levels in the next millennium. A steady decline in
the quality of life appears inevitable, as well as an increasingly desperate
exploitation of raw materials. What the mineral exploitation might be like
a thousand years from now is impossible to predict.
Of the rock types currently under consideration as hosts for HLW repositories,
shale and basalt seem unlikely to assume an important economic role. Granite
is somewhat different because there are some granites that potentially are
mineable because they contain relatively large amounts of thorium, uranium,
and rare earth. Several other types of ores are also associated with granites.
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The most likely targets for near-term exploitation, however, are salt domes
because of the potential productivity of petroleum, halite, and sulfur; and
bedded salt deposits because of their potash, halite, and gypsum. The
United States has only 4% of the world's total proven potash reserves, and
most of these are concentrated in the New Mexico area now being evaluated
as an HLW repository. Future conflicts between the demand for HLW reposi-
tories in bedded salt and the needs of agriculture for potash seem in-
evitable, and may even now constitute an important negative socio-economic
factor in the development of some repositories. Such conflicts could be
minimized in the case of salt domes by selection of domes that are barren
of petroleum and sulfur.
VII.4 OTHER GEOLOGIC HAZARDS
Some data on rates of uplift and denudation may be used to underscore the
unpredictable nature of so-called steady-state geologic processes. The
average rate of regional denudation in the United States (Q.063 m/103 yr)
is such that it would require approximately 15 million years to uncover an
HLW repository at the burial depths now planned. This seems to be such a
large safety factor that we could easily relegate denudation to the status
of a relatively minor risk. However, denudation rates vary by factors of
up to 75, depending on climatic and topographic factors, so there is a
large uncertainty in applying averages that are not site-specific.
Assuming that a repository is planned for a stable area of the continent,
we may now ask how certain we are of this stability. The Adirondacks,
long regarded as an ancient stable shield area, are now known to be rising
at the rate of 3.7 mm/yr in the center of the mountains. The beginning of
the doming of the Adirondacks is not well dated, but at the present rate
of uplift the 1,600 meters of maximum uplift, and subsequent deep erosion,
could have taken place within a half-million years; but it could also, of
course, have taken millions of years longer. Prior to the initiation of
this recent uplift, there would have been no reason to predict that this
large area of the United States would be rapidly uplifted and eroded.
There is also no basis for predicting where, or when, other such uplifts
may occur on the stable shield of the continent. The point is that there
is a finite but very low probability that a repository buried at a depth
of 0.5 km almost anywhere might, within a relatively short time, unpredict-
ably reappear at the earth's surface. How one can make an orders-of-
magnitude conjecture concerning these probabilities is beyond the Panel's
ability to resolve.
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PART VIII; MONITORING THE REPOSITORY
Part of the responsible establishment of an HLW repository is the provision
for a warning system in the event of loss of some of the HLW to the biosphere.
If we assume that some form of monitoring for radioactivity will be mandatory,
there are two aspects to this problem: early warning to permit some attempt
to repair the site or confine the spread of the HLW, and longer range sur-
veillance to provide warning in sufficient time to reduce exposure to the con-
tamination. The first type of monitoring would be done close-in to detect
early signs of the breach of the repository. For the second, more widely
spaced monitors would be used.
Any close-in monitoring would, of necessity, include some connection with
a surface-recording device, a means for replacement or repair of the sensor,
and due consideration of the possibility that the sensing scheme itself could
provide an additional pathway for migration of radionuclides. That monitor-
ing be continued for a period of time on the order of at least several cen-
turies for the fission products and one million years for the TRU wastes
is a commitment that it is not reasonable to expect future inhabitants to
fulfill.
There is no known instrumentation that has demonstrable reliability for dec-
ades, much less for centuries. Beyond those times, the radioactivity is
much less, will have a large alpha component, and the sensitivity of any
currently available instrumentation to the losses would decrease markedly
just when the sensitivity needed for detection would increase. The commit-
ment must be to a repository that may be monitored remotely for some period
of time, but we shall not know what is going on within the general confines
of the repository once the monitors left in place malfunction.
Remote monitoring can be effected both from the surface and from the drill
holes made to help define the site to begin with. However, such monitoring
cannot define any activity such as migration that is occurring in the
repository. A new technology would be needed to achieve this, and such a
technology would first have to be shown to be possible.
An analysis of the nature of'remote monitoring arrays pinpoints the very
difficulty that predicting the direction and rate of migration of the HLW
poses in the first place. Even if the overall hydrologic regime were well
determined, small deviations from the average flow might still result in
the arrival of some HLW at the surface. Such short-circuits could contain
only a small fraction of the total HLW, but they could still have serious
local effects. It is rarely possible to identify where such effects might
occur, thus they are potentially hazardous even beyond the confines of the
fenced-off reservation of a repository. We see no way to predict such
occurrences either in time or space. It is not part of our charge to con-
sider risk tradeoffs that might arise in the case of small numbers of people
being at potentially serious risk in such cases. Consideration of this
problem should be included in any monitoring scheme.
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If our prediction of the behavior of a repository were quite accurate,
we could allow for small losses that take sufficiently long times to
arrive at a biosphere reservoir so as to be below presently specified
maximum safe levels. However, owing to the variable hydrologic regimes
that would almost surely occur over the next million years, the leaks of
the HLW might occur significantly earlier than predicted, but still much
beyond any time when continuous monitoring can be anticipated. The TRU
wastes that enter the biosphere undetected at such time in the far future
might impose serious risks on our descendants.
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PART IX; CONCLUSIONS
It should be emphasized that our intention is not to recommend what may
be the best host rock or geographic location for HLW storage or disposal.
Our intention is to identify gaps in the state of geological knowledge
that may prohibit reasonable predictions to be made about events and
processes that might occur in certain host lithologies. In some in-
stances we feel compelled to indicate potential problems.
(1) Knowledge concerning the properties of the salt in dry domes
is good. If the HLW canisters are not to be retrieved after
the 5 or 10 years needed to complete the filling of the
repository, then the short-time deformations in dry salt domes
around the canisters can be computed and reasonable predictions
are possible. Disposal would be permanent because at the ex-
pectable high temperatures, the high-density canisters would
probably sink into thetsalt. The usual assumption seems to
be that the salt is indeed dry. This is clearly not so in
certain bedded deposits or in all domes. Careful measure-
ments of water content will be most important. Further,
even dry salt contains some water, so that the sinking of
canisters computations, with the possibility of "focusing,"
should be done with this in mind.
(2) Retrievability of HLW in other rock types is not so much
a question of locating the canisters because they have
bodily moved elsewhere, but being able to collect all
of the waste because corrosion and leaching might so
disintegrate the canisters that much of it is dispersed.
This problem is further exacerbated by the possibility
of dispersal by fluids far away from the site. We be-
lieve that the appropriate computational methods for
assessing such possibilities are known, but our knowledge
of the access of the repository system to fluids via
transport through cracks is not now adequate. Research
is needed on how to determine the extent to which a
repository is an "open" system. As a consequence, while
several lithologies are potentially viable candidates
for repositories that permit retrieval, not enough is
known to permit a selection or priority assignment.
(3) There is a fundamental paradox to be encountered in the
design and construction of a "closed" repository. It is
desirable to avoid disturbance of the rock "mass by ex-
ploration drilling as this provides extra pathways for the
HLW to reach the surface. However, one must determine very
precisely the geometric distribution of rock properties
throughout the future repository site and its immediate
surroundings. Prior to excavation, only careful examination
of many drill cores can possibly delineate these properties.
These two contradictory demands must somehow be resolved.
Proper assessment may have to await excavation of shafts
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and adits, despite the high risk of the capital investment
should the site then be found to be unsuitable.
(4) If the HLW is not reprocessed, U and Pu will pose long-
term radiation hazards, as well as a different chemistry
of transport. It is unlikely, however, that the integri-
ties of the canister, its contents, and its immediate
surroundings will last very long, whether or not repro-
cessing is carried out. We have seen no evidence of
survivals longer than a decade. The question of repro-
cessing does relate to the mix produced and thus the
temperatures to be expected in the canisters as a result
of the heat production from that mix.
(5) We are surprised and dismayed to discover how few rele-
vant data are available on most of the candidate rock
types even 30 years after wastes began to accumulate
from weapons development. These rocks include granitic
types, basalts, and shales. Furthermore, we are only
just now learning about the problem of water in bedded
salts, and the need for careful measurements of water
content In domes.
(6) The state of knowledge concerning total permeability in
jointed shales, granites, and basalts is still inadequate
for meaningful forecasting. Total permeability includes
all paths for fluid migration through the rock, including
cracks, joints, faults, and inhomogeneities of lithology.
The lack of such knowledge does not necessarily imply
that a rock is unsuitable. Comparatively anhydrous rocks
in the granitic clan or basalts might well be the best
rock types for storage,-and perhaps disposal, even though
they are jointed and salt is not. Transport properties
depend on both the chemical and mechanical nature of the
rock.
(7) Two principal questions arise for all lithologic types
other than the salt in domes:
1. How does one determine the real "permeability"
of the rock mass surrounding the repository
and extending to the surface, and
2. How*does the permeability of the fissures
affect solute-retardation factors relative
to the flux of water throughout the rock mass?
Development of-methods to answer the first question will
be very hard, but must be undertaken. The plethora of
distribution coefficient and retardation coefficient data
now available provide a good start, but they are not
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adequate to address the critical problems of retardation
under conditions of high ionic strength.
(8) Existing computational methods for assessing transport
may well be adequate, and the accuracies of these models
can be specified in principle. However, until the un-
certainties of the input parameters are included in the
numerical models, the reliability of any model remains
undemonstrable. further, while mathematical simulation
of radioactive ion transport from the repository is an
important and very useful tool in the risk analysis of
an HLW repository, such simulations can be dangerously
misleading unless careful attention is directed toward
inherent physical and mathematical assumptions. Of
particular concern here is the relatively superficial
attempt to date to incorporate parameter uncertainty
in the contaminant transport forecasts. It is of para-
mount importance that the analyst present unambiguous
estimates of uncertainty in such forecasts so that
decision-makers are not misled into believing that such
forecasts are independent of parameter uncertainty.
(9) It seems clear that the uncertainties of forecasting
the behaviors of conceptual HLW repositories are due
principally to inadequate knowledge of the relevant
mechanical, radiochemical, and hydrologic properties
of the candidate rock types. Most of these can be
measured by well established methods, but times required
even for adequately funded research efforts are likely
to vary widely—from a year or so to a decade or more.
As noted in the text, there are also several questions, notably the de-
termination of real permeabilities and porosities in the rocks at a site,
or the nature of the long-term monitoring systems, answers to which must
await the invention of new technology. The time scale for such research
is much less readily determined.
Except for the modest effort on salt, the geological aspect of the HLW
repository problem had largely been neglected by our generation until a
year or so ago. It will not be solved without a strong commitment of
money and manpower, lasting beyond 1985.,
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REFERENCES
1. Arthur D. Little, Inc. Technical Support for Radiation Standards for
High-Level Radioactive Waste Management. Sub-task C-l. Assessment
of Site Selection Factors for Deep Geologic Disposal of High-Level
Radioactive Waste. Draft Report. Performed by D'Appolonia Consulting
Engineers, Inc. Contract 68-01-4470, Office of Radiation Programs,
U.S. Environmental Protection Agency, 1977.
2. National Academy of Sciences. Rock-Mechanics Limitations to Energy-
Resource Recovery and Development. Draft to be published.
3. Roedder, E., and H.D. Belkin. Fluids Present During the Diagenetic
History of the Salado Formation, Delaware Basin, Southeastern New
Mexico, As Recorded by Fluid Inclusions. Abstracts of Midwest
American Geophysical Union Meetings, 1977.
4. Bredehoft, J.D., et al. Geologic Disposal of High-Level Radioactive
Waste-Earth Science Perspectives. Draft U.S. Geological Survey
Circular, 1978.
5. Lancelot, J.F., A. Vitrac, and C.J. Allegre, The Oklo Natural Reactor:
Age and Evolution Studies by U-Pb and Rb-Sr Systematics. Earth & Planet.
Sci. Letters 25:189-196, 1975.
6. Loubet, M., and C.J. Allegre. Behavior of the Rare Earth Elements in the
Okla Natural Reactor: Geochimica et Cosmochimica Acta 41:1539-1548, 1977.
7. Bryant, E.A., G.A. Cowan, W.R. Daniels, and W.J. Maeck. Oklo, An Exper-
ment in Long-Term Geologic Storage. LA-UR-76-701, Los Alamos Scientific
Laboratory, 1976.
8. Logan, S.E. Workshop on Geologic Data Requirements for Radioactive Waste
Management Models: University of New Mexico, Report No. NE-27(76),
Union Carbide Report No. 297-1, 40, 1976.
9. Merritt, W.F. High-Level Waste Glass: Field Leach Test, Nuclear
Technology 32, 1977.
10. Arthur D. Little, Inc. Technical Support for Radiation Standards for
High-Level Radioactive Waste Management. Task B. Effectiveness of
Engineering Controls. Draft Report. Contract 68-01-4470, Office
of Radiation Programs, U.S. Environmental Protection Agency, 1978.
11. Burkholder, H.C., J.A. Stottletnyre, and J.R. Raymond. Safety Assess-
ment and Geosphere Transport Methodology for the Geologic Isolation
of Nuclear Waste Materials, OECD/NEA Sponsored Workshop on Risk
Assessment and Geologic Modeling, Ispra, Italy, 1977.
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12. Arthur D. Little, Inc. Technical Support for Radiation Standards
for High-Level Radioactive Waste Management. Sub-task C-2. Analysis
of Migration Potential. Draft Report. Performed by Intera Environ-
mental Consultants, Inc. Contract 68-01-4470, Office of Radiation
Programs, U.S. Environmental Protection Agency, 1977.
13. Tang, D.H., and G.F. Finder. Simulation of Groundwater Flow and Mass
Transport Under Uncertainty; Advances in Water Resources, 1 (1), 1977,
14. Logan, S.E. Workshop on Geologic Data Requirements for Radioactive
Waste Management Assessment Models. University of New Mexico Report
#NE-27 (76) Union Carbide 297-1, 30, 1976.
15. Arthur D. Little, Inc. Technical Support for Radiation Standards.
Task D. Assessment of Accidental Pathways. Draft Report. Contract
No. 68-01-4470, Office of Radiation Programs, U.S. Environmental
Protection Agency, August 1977.
16. Reactor Safety Study. An Assessment of Accident Risks in U.S.
Commercial Nuclear Power Plants. WASH-1400, U.S. Atomic Energy
Commission, Washington, D.C., August 1974.
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LIST OF ABBREVIATIONS
EPA U.S. Environmental Protection Agency
HLW High-level radioactive wastes
OWI Office of Waste Isolation
DOE U.S. Department of Energy
ERDA Energy Research and Development Administration
NRG Nuclear Regulatory Commission
BNWL Battelle Northwest Laboratories
USGS U.S. Geological Survey
LBL Lawrence Berkeley Laboratory
LLL Lawrence Livermore Laboratory
TRU Transuranic
m.y. Million years
m Meter
MX Metric ton
ft Foot
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GLOSSARY
Adit: A nearly horizontal opening by which a mine is entered, drained,
or ventilated.
Alpha particle: A positively charged particle emitted by certain
radioactive materials. It is made up of two neutrons and two
protons bound together, hence it is identical to the nucleus of
a helium atom. It is the least penetrating of the three
common types of radiation (alpha, beta, gamma) emitted by
radioactive material, being stopped by a sheet of paper.
Alpha particles are dangerous to plants, animals, or man only if
the alpha-emitting substance has entered the body.
Anhydrous; Free from water, especially water of crystallization.
Aquifer; A subsurface formation or geological unit containing
sufficient saturated permeable material to yield significant
quantities of water.
Bars (pressure); Absolute unit of pressure in the cgjs system
equal to 1 dyne per square centimeter.
Beta particle: An elementary particle emitted from a nucleus
during radioactive decay, with a single electrical charge and
a mass equal to 1/1837 that of a proton. A negatively
charged beta particle is identical to an electron. A
positively charged beta particle is called a positron.
Biosphere; Zone at and adjacent to the earth's, surface where all
life exists.
Bittern: The residual liquids left after crystallization is complete.
Calcine; Product of calcination wherein materials are heated to a
higher temperature under oxidizing conditions but without
fusing.
Diagenesis; The physical and changes that occur to sedimentary
rocks. In the history of a sedimentary rock there is no
point at which change stops, such as occurs in the solidification
of a molten igneous rock. Thus are changes that occur with
sedimentary accumulations are known collectively as diagenesis.
Diapirism; The forceful intrusion of one geologic material into
another; overlying geologic material.
Devitrification; Crystallization from a glassy phase. Glass is an
unstable substance. Because its components are spaced without
plan, at unequal distances,, the forces of attraction that surround
a "particle" are unbalanced. Ultimately, the components respond
to the unequal stresses and are drawn together to form crystals.
This process is known as devitrification.
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Fission products; The nuclei (fission fragments) formed by the
fission of heavy elements, plus the nuclides formed by the
fission fragments' radioactive decay.
Formation waters; Water that exists in the intergranular or fracture
space in rock formations.
Gamma rays; High energy, short wavelength electromagnetic radiation.
Gamma radiation frequently accompanies alpha and beta emissions
and always accompanies fission. Gamma rays are essentially
similar to X-rays but are usually more energetic and are nuclear
in origin.
Half-life^ ^radioactive); The time required for one-half of an initial
radioactive material to undergo nuclear transformation, the
half-life is a measure of the persistence of a radioactive
material and is unique to each radionuclide.
Halite; Impure common salt, NaCl.
High-level waste; The highly radioactive waste resulting from the
reprocessing of spent fuel to separate uranium and plutonium
from the fission products. The term includes the high-level-
liquid-wastes (HLLW) produced directly in reprocessing, and
the solid high-level-wastes (HLW) which can be made therefrom.
Laminae; Thin geologic layers, plates, or scales disposed in
layers like the leaves of a book.
Lithologic; Pertaining to the characteristics of rock formations such
as layering.
Magma; Molten igneous rock.
Montmorillonite; A mineral resembling clay consisting of an
hydrous aluminum silicate with considerable capacity for
exchanging part of the aluminum for magnesium, alkalies
and other bases.
Pluton; Very large masses of igneous rocks that extend along the
cores of most major mountain ranges and underlie vast areas of
the ancient shield or central stable areas; sometimes called
batholiths.
Salt dome; A type of geologic structure resulting from the upward
thrust of a great salt mass through overlying rock layers. The
resulting salt form is roughly cylindrical and in some cases
has resulted in observable uplift at the earth's surface.
Smectite minerals; Clay minerals.
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Tecjtonic: Of or pertaining to the formation of the earth1 s crust;
the forces involved in or producing such deformation and the
resulting forms.
Transuranic elements^: Elements with atomic number greater than 92.
They include neptunium, plutonium, americium, curium, and others.
Zeolite: Any of a family of hydrous silicates, which have capacity
to act as ion exchangers.
720-335/6105
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