U.S. GEOLOGICAL SURVEY CIRCULAR 779

Geologic Disposal of High-Level Radioactive WastesEarth-Science Perspectives
Geologic Disposal of High-Level Radioactive WastesEarth-Science Perspectives

By J. D. Bredehoeft, A. W. England, D. B. Stewart, 

N. J. Trask, and I. J. Winograd

U.S. GEOLOGICAL SURVEY CIRCULAR 779

A summary of factors and processes

that must be understood for the safe containment

of high-level radioactive waste

1978
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress catalog card No. 78-600019

First printing 1978

Second printing 1979

Third printing 1985

Free on application to Distribution Branch, Text Products Section, 

U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304
FOREWORD

Earth scientists have been concerned with the problem of disposal of 

radioactive wastes since 1955 when the National .Academy of Sciences 

gathered 65 scientists in Princeton, N.J., to consider the geological, biological, physical, and chemical aspects of waste containment. Investigations 

during the 1950's and early 1960's examined the feasibility of disposal of 

liquid high-level wastes in deep geologic basins and in salt mines. Scientists 

since then, sponsored largely by the U.S. Atomic Energy Commission and 

later by the U.S. Energy Research and Development Administration, carefully considered salt deposits as a repository for solidified high-level wastes. 

Within the past few years, in response to growing pressures for a resolution of the problem of disposing of the wastes, earth scientists at various 

universities and government laboratories, as well as at the U.S. Geological 

Survey (USGS), also began an intensive examination of the problem. As 

a result of this expanded examination, modified concepts of geologic disposal have evolved, and aspects of some older concepts have been questioned. Some of these changes in outlook and philosophy as perceived by 

USGS scientists are described in this Circular. Because the authors are 

confident that acceptable geologic repositories can be constructed, this 

paper should not be construed as an attempt to discredit the concept of 

geologic containment or the work done in the 1960's and early 1970's. 

However, the earth-science problems associated with disposal of radioactive wastes are not simple, nor are they completely understood. The many 

weaknesses in geologic knowledge noted in this report warrant a conservative approach to the development of geologic repositories in any medium. 

Increased participation in this problem by earth scientists of various disciplines appears necessary before final decisions are made to use repositories. Basic philosophical, as well as technological, issues remain to be 

resolved.

WILLIAM A. RADLINSKI

Acting Director

U.S. Geological Survey

in
CONTENTS

Page

Foreword __________________________________________ III

Introduction ___________________________________ 1

Acknowledgments __________________________ 3

The repository host rocks and site characterization ________ 3

Perturbations caused by the repository and the waste ____________ _____ _ 4

The ground-water system ___________________________ 7

Containment time frames and geologic prediction ______________________ _ 9

How long is necessary? _____________________________ __- ____ 9

Geologic prediction _______________________________ _________ _____ 10

Research needs and concluding statement __________________________ _____ 12

References cited __________________________________________ ______ ___ 13

ILLUSTRATION

Page

FIGURE 1. Ingestion hazard of selected radionuclides in high-level waste during

10 million years _______________________________________ 10

TABLES

Pago

TABLE 1. Properties of high-level wastes to be accumulated by the year 2000 __ 2 

2. World supply and volume of annually cycled water ________________ 2
Geologic Disposal of High-Level Radioactive Wastes 

Earth-Science Perspectives

By J. D. Bredehoeft, A. W. England, D. B. Stewart, N. J. Trask, and I. J. Winograd'

INTRODUCTION

An effective solution to the problem of longterm storage of radioactive waste is essential to 

the expanded use of nuclear power. Radioactive 

waste is the inevitable byproduct of the generation of electricity by nuclear reactors or of 

the production of plutonium for weapons. Fuel 

assemblies must be removed periodically from 

nuclear reactors as fission products accumulate. 

These spent and contaminated fuel assemblies 

contain large concentrations of fission products 

such as 90Sr and 137Cs, which decay to innocuous 

levels during several hundred years, and of 

longer lived transuranic elements and their 

daughters, elements such as plutonium, americium, neptunium, and radium. These elements 

persist for as long as several million years.

The spent fuel from a nuclear reactor must 

be isolated either permanently or temporarily 

from the biosphere. Spent fuel can be processed 

chemically to recover most of the plutonium and 

unfissioned uranium for possible reuse. The residuum from this process is a highly radioactive byproduct which must also be isolated from 

the biosphere. This byproduct is what is usually 

referred to as "high-level waste." The United 

States does not plan to reprocess spent fuel 

chemically; thus, the immediate problem of 

commercial radioactive waste is one of handling, storing, and ultimately disposing of spent 

fuel assemblies.

Radioactive wastes from military activities 

are somewhat different. These wastes are 

mainly the residue after plutonium has been 

extracted from radioactive materials that have

been irradiated in a reactor designed specifically to produce plutonium. Although military 

wastes contain many of the same fission products and transuranic elements as commercial 

waste and occupy significant volumes, their 

total radioactivity is relatively low at present 

about the same as that stored in spent-fuel assemblies from commercial reactors (Nuclear 

Energy Policy Study Group, 1977, p. 244).

The volume of commercial radioactive waste 

that will accumulate during the next few decades will depend upon the growth of the 

nuclear- power industry and on the form of the 

waste. King and Baker (1976, p. 206) estimated that by the year 2000, as many as 476,- 

000 spent fuel assemblies will be on hand if 

none has been reprocessed. If these spent fuel 

elements were to be reprocessed, the resulting 

solid high-level waste would occupy approximately 3,000 cubic meters (ma ) (Blomeke and 

Kee, 1976, p. 99), and accompanying intermediate-level wastes contaminated by transuranic 

elements would occupy a volume one order of 

magnitude larger than the volume of high-level 

wastes (Blomeke and Kee, 1976, p. 108).

Wastes generated by military activities, 81 

million gallons in liquid and solidified form, are 

held in temporary storage by the U.S. Department of Energy (DOE) at Hanford, Wash., 

Savannah River, Ga., and Idaho Falls, Idaho 

(Nuclear Energy Policy Study Group, 1977, p. 

250). Because of their chemistry, most of these 

wastes present uniquely difficult problems in

* Authors listed alphabetically.
handling (Nuclear Energy Policy Study Group, 

1977, p. 249).1

It is generally accepted that repositories in 

geologic media can provide the most certain 

safe containment of radioactive waste. DOE is 

developing, and intends to have licensed sometime in the 1980's, a Waste Isolation Pilot Plant 

(WIPP) in bedded salt near Carlsbad, N. Hex. 

The Nevada Test Site and the Hanford, Wash., 

facilities are being explored for suitable geologic repositories. Evaluations of sites in 

bedded salt of the Salina and Permian basins, 

salt domes of the gulf coast, and salt anticlines 

of the Paradox basin are also continuing. Reconnaissance work to assess the potential of a 

repository in shale or crystalline rock is underway. In the first repositories used, wastes would 

be retrievable for 10 to perhaps as many as 50 

years.

Radioactive wastes are highly toxic. A commonly cited measure of this toxicity is afforded 

by a computation of the quantity of water 

needed to dilute high-level wastes to levels specified in the Radiation Concentration Guides 

(RCG, see headnote to table 1). These volumes 

were tabulated by Blomeke and Bond (1976) ; 

they are reproduced in table 1 and are compared with the volume of the oceans and of 

fresh water in lakes, rivers, and glaciers in 

table 2. A volume of 5.2 xlO16 m3 of water, 

which is almost 4 percent of the oceanic volume

1 An additional 600,000 gallons of liquid high-level waste and 

sludge derived from both commercial and military spent fuel are 

in temporary storage at West Valley, N.Y. (Nuclear Energy Policy 

Study Group, 1977, p. 252).

TABLE 2. World supply and volume of annually cycled 

water (abbreviated from Nace, 1969)

INace's (1969) data were reported in km3 X10-3. We converted his 

data to cubic meters to permit a direct comparison with 

the data of table 1 (1 km:J =109 m3 ) ]

Source of water Global volume Percentage of 

(in m3 ) total water

Oceans ____ ..- ._

Rivers (average channel storage) __

Freshwater lakes _ .- .__

Saline lakes ; inland seas

Soil moisture; vadose water

Ground water (upper 1,000 m

only) _ _ _ _ _ 

Ice caps and glaciers - -

Annual river runoff .__ . _ .

1.4 XlO18

1.7 X 101 -

1.2 X 10U

1.0 XlO14

1.5 XlO14

7.0 X 101-"

2. 6 XlO1"

3.0 XlO13

97.6

0.0001

0.0094

0.0076

0.0108

0.5060

1.9250

0.0021

(1.4xlO18 nr), would be needed to dilute the 

wastes on hand at the year 2000 to levels specified in the RCG; this volume is almost double 

that of fresh water in global storage in lakes, 

rivers, ground water, and glaciers. Even after 

a million years, the volume of water needed to 

dilute these wastes to the levels specified in the 

RCG is significant in terms of water stored in 

individual major lakes and aquifers. Another 

crude measure of the potential waste hazard 

was provided by Cohen (1977, p. 28). He 

showed that the potential for millions to billions 

of fatal cancer doses is contained in the wastes 

provided by just 1 year of United States nuclear-power production if all the waste were to 

be ingested, an admittedly unlikely scenario. 

The above-cited tabulations are simplistic because they ignore the environmental pathways 

to man after production and burial of the 

wastes, yet such computations are a sobering 

reminder of the seriousness of the waste-disposal endeavor. Clearly, radioactive wastes conTABLE 1. Properties of high-level wastes to be accumulated by the year 2000 

(from Blomeke and Bond, 1976, p. 87)

[Toxicity is defined as the cubic meters of air or water that would be required to dilute the radioactive constituents to levels specified 

in the Radiation Concentration Guides (RCG; see column 2 of tables I and II, U.S. Code of Federal Regulations, Title 10, Pt. 20) 

as the maximum allowable for unrestricted use. MCi, megacurie; kW, kilowatt]

Radioactivity, MCi _

Fission products _ __ __

Actinides _ _ __ _

Thermal power, kW

Fission products _

Actinides _ __

Inhalation toxicity, 

m3 air ___ __ __ _

Fission products _ __ _

Actinides __ _ _ _ ___ _ _

Ingestion toxicity, 

m3 water _ _ _ __ _

Fission products _ _ _

Actinides __

Accumulated 

by year 

2000

158,000

155,000

-._ _ 3,400

_ .__ 770,000

__ 657,000

_ _ 113,000

__ ___ 1.2X1022 7.0X1020

______ i.ixio22

_ __ 5.2X1018

_ _ 5.2 XlO1"

____ 4.4x10"

Time elapsed following year 2000, in years

1,000

63.1

4.7 

58.4 

1,500 

5.3 

1,500

3.4 XlO20 

1.8X1016 

3.4X102"

i.ixio13

1.8 XlO10 i.ixio13

10,000

24.5 

4.5 

20.0 

470 

4.9 

470

1.4X1020 

1.8 XlO15 

1.4 xlO2"

3.3 XlO12 

1.8 xlO10 

3.3xl012

100,000

4.7 

3.4 

1.3 

36 

3.1 

33

l.OxlO10 

1.3xl015 

1.0 XlO1"

1.7X1012 

1.4 xlO10 

1.7X1012

1,000,000

1.6 

0.7 

0.9 

21 

0.2 

21

4.4x10" 

2.1 XlO11 

4.4 XlO18

6.4X1011 

2.4 xlO9 6.4X1011
stitute a major hazard. The magnitude of the 

associated risk depends upon the environmental 

pathways to man. An assessment of that risk 

depends upon our understanding of those pathways now and for thousands of years.

Wastes from a deep repository could be exposed to the biosphere either (1) by some geologic process such as tectonism, diapirism, or 

erosion that directly exposes the waste or (2) 

by some process that transports the wastes to 

the biosphere. Transport by ground water is 

acknowledged to be the most probable mechanism for moving the radioactive wastes the 

necessary distances from repository to the biosphere.

Development of acceptable waste repositories 

involves in the most general terms: (1) identification of sites that meet broad criteria for 

tectonic stability, slow ground-water movement, 

and long flow paths to the surface; (2) intensive subsurface exploration of such sites to 

determine the hydrologic and geologic conditions in and around the potential repository; 

(3) predictions of repository behavior based on 

the initial conditions and on various assumptions about the future; (4) evaluation of the 

risk associated with these predictions; and (5) 

a judgment whether these risks are acceptable. 

This Circular is not intended to address each of 

these steps in detail; we do discuss some of the 

key earth-science questions involved in the first 

four steps. Step 5, of course, requires a political 

decision and is beyond a technical discussion.

The authors of this Circular are confident 

that the steps outlined above can be carried out 

in such a way that the ultimate decision on the 

acceptability of a given site and waste-handling 

procedure will have a strong scientific and 

technical foundation. However, some key geologic questions are unanswered, and answers 

are needed before the risk associated with geologic containment can be confidently evaluated. 

In this Circular, we discuss briefly the various 

media that have been considered for geologic 

disposal of radioactive waste and point out the 

need for several relatively new techniques for 

characterizing the rocks in the immediate vicinity of a repository. We consider a variety of 

possible interactions among the mined opening 

of the repository, the waste, the host rock, and 

any water that the rock may contain. Many

of these interactions are not well understood, 

and this lack of understanding contributes considerable uncertainty to evaluations of the risk 

of geologic disposal of high-level waste. Because salt is being considered as a potential 

repository medium, we point out some geochemical uncertainties connected with its use. Knowledge of the ground-water flow path from a 

repository is important in providing a backup 

containment system if waste should leak from 

the repository, and we devote a section to considerations of describing and modeling such 

systems. In the next section, modeling and geologic prediction are discussed from the point of 

view of some philosophical aspects of prediction 

in the geosciences. A concluding section suggests research areas that we feel need emphasis. The discussion is qualitative throughout; quantitative treatment of several of the 

topics considered is underway in the Geological 

Survey and elsewhere.

ACKNOWLEDGMENTS

The authors of this report are listed alphabetically. The sections were authored as follows: Introduction by N. J. Trask, I. J. 

Winograd, and A. W. England; The repository 

host rocks and site characterization by A. W. 

England and N. J. Trask; Perturbation caused 

by the repository and waste by D. B. Stewart 

and J. D. Bredehoeft; The ground water system, 

by J. D. Bredehoeft; Containment time frames 

and geologic prediction, by I. J. Winograd and 

N. J. Trask; and Research needs and concluding statement by N. J. Trask.

Critical and helpful reviews of this Circular 

were provided by Jesse Cleveland, Gary L. 

Dixon, Edwin B. Eckel, Leonard M. Gard, 

Arthur H. Lachenbruch, Walter B. Langbein, 

Nicholas C. Matalas, Thomas Maddock, Jr., Zell 

E. Peterman, John B. Robertson, Jacob Rubin, 

Robert Schneider, Harry W. Smedes, Frank W. 

Trainer, William E. Twenhofel, Leonard A. 

Wood, Warren W. Wood, and E-an Zen.

THE REPOSITORY HOST ROCKS AND 

SITE CHARACTERIZATION

Different rock types have been considered as 

the host rocks for radioactive-waste repositories. Rock salt received early attention because of its low porosity and permeability, its
plasticity, and its high thermal conductivity 

(Natl. Acad. Sci., 1957). In recent years, shale, 

limestone, granitic rocks, and basalt have been 

considered (Schneider and Platt, 1974). These 

rock types provide low primary permeability 

and, in many areas, a high degree of structural 

stability. Parts of the Precambrian shield are 

known to be extremely stable (Yardley and 

Goldich, 1975). The clay minerals in shales 

have a potential for significant radionuclide 

sorption (Brindley, 1977), although measurements of this property are still incomplete. Another rock type having a high sorptive potential 

is zeolitized tuff, which is widespread in the 

Basin and Range province of the Western 

United States. The overall desirable properties 

of these rock units, such as thickness, depth, 

homogeneity, and tectonic stability, have been 

discussed at length previously (ERDA, U.S. 

Energy Research and Devel. Adm., 1976) and 

are not reviewed here.

The identification of potential host rocks for 

radioactive-waste repositories and of broad 

geographical areas having generally acceptable 

properties is followed by more detailed studies 

of smaller specific areas of interest. The volume 

of rock that will be needed to accommodate an 

actual waste repository will probably be relatively small approximately 10 km3 (ERDA, 

1976). However, that volume must be intensively explored by all appropriate methods.

Planning a waste repository, like planning 

subsurface mining and tunneling, requires a 

full understanding of the subsurface geology, 

but the consequences of an undetected error in 

planning the waste repository are more severe. 

Finding unexpected ground-water conduits, 

shear zones, fault-displaced or pinched-out beds, 

abandoned excavations, or variable lithology 

could be hazardous and costly. The discovery of 

such anomalies during the excavation of a nuclear-waste repository could force abandonment 

of the site.

Confidence that a prospective site is likely to 

provide adequate containment derives from an 

understanding of structural and lithologic features apparent at scales from a few meters to 

tens of kilometers. Many critical features are 

subtle. For example, small faults, particularly 

strike-slip faults, can be extremely difficult to 

detect and to date. Fracture systems, which

could act as short-circuiting conduits for 

ground water, might be revealed from regional 

structure features clearly evident only at the 

scale of a Landsat image (Pickering and Jones, 

1974; Drahovzal and others, 1974) or perhaps 

from buried structural features indicated by 

the distribution of soil moisture (Johnson and 

England, 1977). Furthermore, the necessary 

structural integrity and lithologic homogeneity 

cannot be assured simply through detailed surface geologic mapping and coring, particularly 

if the number of core holes is limited by the 

risk to the potential repository caused by extensive local drilling.

Techniques for remotely exploring a volume 

of rock are in various stages of development 

and should be applied to potential waste-repository sites as soon as practical. These methods 

are used from the surface, between boreholes, 

and at the head of an excavation. They include 

high-resolution seismic and acoustic techniques, 

which can detect fine-scale structural and lithologic variations (Laine and others, 1977), and 

electric and electromagnetic methods, which are 

primarily sensitive to the distribution of water 

(Scott and others, 1975; Daniels, 1977). Some 

of the most recently devised methods use shortpulse radar (Stewart and Unterberger, 1976; 

Watts and England, 1976) and continuous-wave 

interferometry (Rossiter, 1977); their use is 

limited to high-resistivity materials such as salt 

and perhaps dry crystalline rock.

The techniques cited provide a nondestructive 

way of characterizing the site in detail. Some 

of the borehole-to-borehole electrical techniques 

need to be further refined, but the critical need 

to map the original subsurface structure, lithology, and ground-water regime warrants theaggressive utilization of such techniques where 

appropriate.

PERTURBATIONS CAUSED BY THE 

REPOSITORY AND THE WASTE

The original host rock and any included 

water will be perturbed for substantial periods 

of time by the waste and by excavation of the 

repository. Three kinds of perturbations can be 

caused: (1) mechanical disturbances initiated 

by mining of the repository; (2) chemical disturbances caused by the introduction into the
subsurface of wastes and waste-generated fluids 

that are not in chemical equilibrium with the 

rock mass; and (3) thermal disturbances of 

some sort, very large if hot wastes are introduced; this stress compounds the mechanical 

and chemical perturbations.

A major engineering task in any repository 

will be to prevent water from entering the 

workings through fractures caused by the mining process. All excavations produce a disturbance in the in situ stress field. This disturbance 

can produce fracturing in the vicinity of the 

underground working. Most estimates suggest 

that this mechanical disturbance extends for 

distances that are about the same as the size 

of the underground opening (Cook, 1977). The 

mining disturbance can only increase the permeability of the rock in the vicinity of the 

repository. Moreover, current estimates (Union 

Carbide Corp., Nuclear Div., Office of Waste 

Isolation, 1977) suggest that the backfilling 

process will fill the underground working to a 

density of perhaps 80 percent of what it was 

before mining. Plastic deformation in shale and 

salt may, however, ultimately return these rocks 

to approximately their initial densities. In more 

brittle rocks, permeability may continue to increase even after backfilling of the repository 

as stresses are relieved. Shafts to the workings 

and exploratory holes in the vicinity of the 

workings will, of course, be carefully plugged 

but will still provide potential conduits for the 

entry and exit of water during substantial time 

periods.

Chemical effects brought on by the introduction of heat from the waste are discussed below. 

Chemical reactions that may take place at low 

temperatures are related to the incursion of 

fluids into the repository volume. These fluids 

will react with the waste components during 

long timespans, either dissolving them or producing a new waste form having changed solubility. For repositories in bedded or domed salt, 

the fluids at low temperatures will be saturated 

brines. The effects of these brines on the containment properties of salt are being studied at 

several laboratories. Interstitial brine is known 

to reduce the mechanical strength of salt. Although scientists usually assume that water will 

never enter the workings of a repository in salt 

in amounts of more than 1 percent of the mass

of solid salt and waste, the potentially serious 

consequences of greater amounts of water need 

to be explored. In addition to possibly entering 

through shafts and boreholes, water may enter 

a salt repository from brine pockets such as 

those that were found (unexpectedly) in one 

salt deposit (Boffey, 1975). The consequences 

of appreciable weakening of salt could be serious. One consequence could be an accelerated 

collapse of workings, which might facilitate 

sealing of a closed repository but would, at the 

same time, severely jeopardize retrievability 

and integrity in an active repository that was 

accidentally flooded. Another possibility is that 

the canisters or fuel assemblies could be moved 

in a direction determined by density contrasts 

and unrelieved stresses near them.

The presence of brine in a salt repository adjacent to waste canisters or fuel assemblies 

would create additional problems concerning 

the ultimate fate of the hazardous nuclides. The 

protective metal canisters and sleeves will certainly degrade quickly in the strong brine environment, and leaching of the wastes will be 

enhanced. Ultimate containment will therefore 

depend on the enclosing salt mass itself. Elements from the waste will be partitioned into 

the brines depending on such factors as complex formation, ion exchange, particle suspension in the dense brine, and colloid dispersion. 

The meager data available on this problem suggest that many elements will be partitioned into 

the liquid phase and that few elements will be 

partitioned into the solids. In other words, the 

capacity of salt to "fix" or adsorb the nuclides 

from the waste in insoluble form is apparently 

low. This is a widely recognized serious drawback of salt as the repository medium; the 

mechanical integrity of the salt medium thus 

becomes of extreme importance. A possible way 

around this difficulty is to wrap the wastes in 

materials that do have a strong tendency to fix 

the nuclides from the wastes; research on possible materials is beginning.

The amount of heat introduced into a host 

rock by the waste depends on the form of the 

waste. Canisters of high-level waste may produce as much as 5 kW 10 years after reprocessing (ERDA, 1976) ; this output declines to 

one-tenth of this value within 100 years. Spent 

fuel assemblies have lower heat output, approx-
imately 0.5 kW 10 years after removal from a 

reactor (Jenks, 1977), although the decay is 

less rapid.

Thermal energy introduced into the rocks 

surrounding the repository may cause complex 

mechanical and chemical changes. Increased 

temperatures in salt would further decrease 

mechanical strength of the salt-brine mixtures 

discussed above and would increase the creep 

rate of dry salt. Thermal expansion and later 

contraction will cause compressional and tensional stresses in confined media and may lead 

to widespread fracturing in brittle rocks. Preliminary calculations of thermal effects in brittle media have been carried out (for examples, 

see Ratigan, 1976). Usually such calculations 

use finite-element or finite-difference methods 

and deal with relatively close-in effects. A difficulty in such calculations for shale is lack of 

knowledge of the thermal properties of shale 

in three dimensions.

One of the more significant chemical changes 

caused by the introduction of thermal energy 

to a rock system will be the breakdown of hydrated minerals and consequent release of 

water. For example, mixed-layer montmorillonite-illite, a common constituent of shale, undergoes a phase transition involving increase 

in illite layers and the liberation of free water 

starting at temperatures as low as 100 °C 

(Perry and Hower, 1972). At elevated temperatures in shale, this water will probably be in 

the form of steam. Oxygen, carbon dioxide, sulfur, and halogen gases may also be released. 

Interaction among preexisting minerals, volatile components, and waste will be promoted by 

high temperatures. Experience from the Geological Survey's geothermal studies indicates 

that hot moving fluids may alter existing minerals and form new ones, thus causing significant changes in permeability. Some of these 

changes could reduce permeability and fluid 

flow (Summers and others, 1978). These mineralogical changes may be accompanied by 

volume changes and attendant stress on the 

confining medium.

Thermal energy can also drastically modify 

hydrologic flow regimes by: (1) creating and 

driving convection cells; (2) inducing pressure 

gradients in fluids by their expansion, including 

the formation of systems containing both liquid

and gaseous phases; (3) decreasing viscosity of 

fluids; (4) changing the bulk permeability; (5) 

changing the rock-stress field in a manner to 

strongly affect the fluid-flow field; and (6) 

changing solution chemical characteristics. Participants in a symposium on the movement of 

fluids in largely impermeable rocks (Union Carbide Corp., Nuclear Div., Office of Waste Isolation, 1977) concluded that no adequate model 

now exists to describe fluid flow, energy or 

chemical species transport, or source kinetics 

during the time interval of the thermal pulse, 

namely, the first several hundred years after 

burial.

Conservative limits may need to be placed 

initially on the thermal load to be borne by a 

radioactive-waste repository until many of the 

problems outlined above have been resolved. 

We are not saying that it is not feasible to store 

hot wastes in a deep geologic repository. We 

are saying, however, that given the current 

state of our knowledge, the uncertainties associated with hot wastes that interact chemically and mechanically with the rock and fluid 

system appear very high.

A general rule might be to restrain the permissive maximum temperature to a level lower 

than that which will cause a gas (steam) phase 

to appear. The gaseous phase is to be avoided, 

if possible, because, among other reasons, its 

lower viscosity and larger volume than those of 

solid or liquid phases permit more rapid movement through porous media. The stress field 

and the presence of steam may interact to concentrate solution processes or chemical reactions in mechanically stressed regions near 

repositories; such interactions are still poorly 

evaluated, though they are potentially sources 

of breaches in containment.

Allowing the wastes to dissipate heat before 

burial also widens the spectrum of usable 

media. Montmorillonitic shales, for example, 

undergo a phase transition already mentioned 

involving reordering of mineral structure and 

release of free water at temperatures as low as 

100°C. The fact that such mineralogic transitions yield large volumes of fluids is a possible 

deterrent for placement of hot wastes in such 

shale. Massive zeolitized ash-fall tuffs, which 

are common over large areas in the Basin and
Range province of the United States and which 

locally are as much as hundreds of meters thick, 

are also attractive as potential media for waste 

disposal. They have most of the same desirable 

properties as the montmorillonitic shales; they 

are low in permeability and very high in sorptive capacity (Ramspott and Borg, 1977). Unfortunately, phase changes starting at about 

150°-200°C cause the release of free water and 

may preclude use of zeolitic tuffs for the disposal of hot wastes. Crystalline rocks have few 

of the hydrous phases present in shales and 

tuffs; their thermomechanical response to hot 

waste is unknown, however.

THE GROUND-WATER SYSTEM

The acknowledged most likely process for 

moving radioactive waste from repository to 

biosphere is transport by ground water. Model 

predictions of the fate of the emplaced waste 

must therefore take into account factors that 

affect transport by ground water. These include: (1) convective transport by the flow, (2) 

dispersive and diffusive transport, (3) chemical 

interactions with the rocks along the flow path, 

and (4) rates at which the wastes come into 

solution.

In the simplest one-dimensional case in which 

ground water flows through a porous medium, 

the velocity of flow is given by Darcy's law, 

which states:

v=-

-k Sh 

¥

where v is the average velocity of the fluid, k 

is the permeability, e is the porosity, and 8h/8l 

is the gradient of the hydraulic head. The average distance moved by water during an interval 

of time is a crude measure of how far long-lived 

radionuclides might be transported, assuming 

that the velocity field is uniform both in time 

and space, and that, as a worst case, the radionuclides are not sorbed by the porous media 

traversed. If, for example, we wished to restrict 

transport to 1 km in 105 years, the flow velocity 

should be no more than about 3xlO~8 cm/s. 

Further, assuming a head gradient of 1/100 (1 

percent) and a medium having a porosity of 10 

percent, we compute a permeability along the 

flow path on the order of 3x10 ~ 7 cm/s (about 

0.3 millidarcies). This permeability is not particularly low for natural rock materials. Rocks 

that have "intergranular" permeabilities at 

least this low include "tight" sandstones, shaly 

siltstones, a variety of shales, and many crystalline rocks, including most granites. The current DOE program to establish waste 

repositories is directed toward salt, shale, and 

crystalline rock in part because these media 

have low permeabilities. As the porosities of 

such media decrease, or as the head gradient 

increases, the permeability must be lower in 

order to maintain the same velocity of water 

flow. Several other factors in ground-water 

transport act to speed up or slow down the flow 

of contaminants.

Contaminants disperse and diffuse at the 

front of an advancing wave of ground water 

bearing such contaminants and tend to spread 

out their concentrations. These effects cause 

some contaminants to arrive faster than would 

be predicted by the average velocity of water 

movement (streamline convection) alone.

Adsorption and chemical reactions, including 

adsorption, ion exchange, and precipitation, 

retard or restrain the movement of some contaminants. Adsorption and ion exchange are 

strongly dependent on the chemical form of the 

contaminants, including their oxidation state 

and extent of complexing. Numerical measures 

of the sorptive properties of various chemical 

species vary through many orders of magnitude. 

They have yet to be determined for many of 

the actinide elements under repository conditions. However, the basic principles of these 

reactions are well known.

In order to construct a complete model of 

contaminant flow, the availability of the waste 

to percolating ground water must be known. 

This availability will depend in part on the 

form of the wastes. High-level wastes from reprocessing may be cast in a glass form that 

would have very low leachability (Marsily and 

others, 1977; ERDA, 1976) ; the leach rate of 

spent-fuel material appears to be comparable 

(Katayama, 1976). Even if the wastes have 

very low leachability, some contaminants will 

be released, and the rate .of leaching may 

change significantly with time as the waste undergoes radioactive decay, interacts with the 

enclosing medium, or comes in contact with 

ground water of changing composition.
In summary, combinations of hydrologlc factors can slow transport between the repository 

and the biosphere. Factors tending to lengthen 

the transport time include: (1) low permeability of the media, (2) high effective porosity of 

the media, (3) low gradient of hydraulic head, 

(4) high sorption capacity of the media, (5) 

long flow path, and (6) low rate of release of 

solutes by the wastes to the transporting fluids.

From the preceding general outline of factors 

influencing ground-water transport of radionuclides, preparing a predictive model of contaminant transport might appear relatively 

simple. Unfortunately, several large gaps exist 

in knowledge of the details of natural transport 

systems.2

Media in which ground-water flow is largely 

through fractures are apt to be present in any 

flow system and pose problems in predicting 

radionuclide transport. The presence of fractures increases the velocity of flow under the 

same gradient. Fracture flow velocity may be 

a few orders of magnitude faster than intergranular velocity. In addition, the sorptive 

capacity of fractured media is likely to be lower 

than that of a porous medium because the fluid 

is in contact with much less of the medium for 

shorter times.

In theory, fluid flow through fractured media 

can be predicted in two ways:

1. The flow through a network of fractures 

can be analyzed. The geometry of the network 

of fractures must be known, and thus the orientation of fractures and the size of fracture 

apertures must be known. Although the geometry of the fracture system may be known in 

the vicinity of an underground working, it 

seems difficult, if not unfeasible to know this 

in sufficient detail at any distance from the few 

boreholes or workings likely to be permitted at 

or near a repository.

2. The fracture system can be analyzed 

stochastically. This analysis uses variables such 

as permeability and porosity that are analogous 

to those for a porous medium. Unfortunately, 

this method of analysis is relatively recent and, 

as yet, is untested on a field scale.

In addition to natural fractures, manmade 

boreholes in the vicinity of the repository as

2 These uncertainties were the subject of a recent symposium 

(Union Carbide Corp., Nuclear Div., Office of Waste Isolation, 1977).

well as in the repository itself present problems. They must, even during hundreds of 

years, be considered as potential short-circuit 

pathways that could permit water flow from 

the repository horizon upward to shallow aquifers that may be utilized by man.

In addition to the problem of ground-water 

flow through fractures, a second major hydrologic problem involves measurements of fluid 

head and permeability in porous media of low 

permeability. These values are needed to characterize the hydrology around a repository, but 

meaningful in situ measurements of groundwater flow in low-permeability porous media 

have not been obtained routinely in the past. 

Although laboratory measurements of relevant 

variables are possible, experience in hydrology 

suggests that such measurements commonly 

vary by an order of magnitude or more from 

those obtained through in situ tests. The problem is compounded by the large number of variables that must be measured to describe the 

transport process through fractured porous 

media of low permeability.

The carbon-14 dating method provides a 

quasi-independent check on hydraulic computations of ground-water velocity, but only for 

waters that have been removed from the biosphere for less than 40,000 years, the practical 

limit of 14C dating of ground water. In all likelihood, the waters in the vicinity of most potential repositories will be at least tens of thousands of years old. Radioactive disequilibrium 

studies using the systems ^u/^u and ^o^/ 

234U offer some potential for dating ground 

water 10,000 to 1 million years old; these methods need considerable refinement before they 

can be routinely applied, however. Another 

potential technique for determining rates of 

ground-water flow in some rocks involves measuring the amount of dissolved helium in the 

water and comparing this measure to the 

amounts of uranium and thorium that yield 

helium through radioactive decay in the rock 

traversed (Marine, 1977).

In order to predict contaminant movement, 

the physical and chemical properties of the 

media that control transport must be known for 

a length of flow path sufficient to describe the 

movement for the requisite radionuclide containment time (Apps, 1977). Although measur8
ing these properties seems feasible, the manpower and observation time needed are not 

trival. We need, as a minimum, the permeability and porosity of the media and the hydraulic 

head gradients all in three dimensions. In addition, we need to know the sorptive characteristics of the media along all paths, and we need 

to estimate the variable rates at which the 

solidified wastes will enter the transporting 

fluids. Needed, in particular, is information on 

the distribution and extent of major heterogeneities. The need for such data severely 

taxes both the available data base and the technology for generating it. Most of the requisite 

data are presently unavailable; most of the 

available data have such large error limits that 

their usefulness in predictive models is limited 

(Philip, 1975). Indeed, in a recent review of 

ground-water models in general, Bugliarello 

and Gunther (1974, p. 136) stated that "the 

complete description of the quality and quantity 

aspects of groundwater flow under complex 

* * * conditions * * * is still in the future." 

Some basic problems of geologic models in general are discussed below.

CONTAINMENT TIME FRAMES AND 

GEOLOGIC PREDICTION

The objective of geologic disposal of radioactive wastes in the ground is to preclude their 

reaching the surface or near-surface hydrosphere, and thereby the biosphere, until they 

have decayed to a point at which their concentrations no longer constitute a health hazard. 3 

Common geologic processes capable of transporting wastes to the surface or near surface 

after deep burial include ground-water movement, tectonism, diapirism, erosion, and volcanism. Equally important, tectonism, erosion, 

seismicity, glaciation, and climatic change may 

alter the initial conditions of ground-water 

flow described in the preceding section. Fracture permeability, hydraulic gradients, and 

flow-path length are all subject to change, and 

predictive models must take into account the 

probabilities of such changes (Burkholder and 

others, 1977).

3 We recognize that definition of what might constitute a health 

hazard upon release of selected radionuclides to the hydrosphere 

involves consideration of many biologic and surface-hydrologic 

processes capable of reconcentratimg the wastes. In this report, we 

are concerned exclusively with pathways in the lithosphere.

HOW LONG IS NECESSARY?

How long must the wastes be sequestered and 

the individual and interactive effects of the 

processes discussed above be predicted? Different radionuclides require different containment 

periods ranging from several hundred to several 

million years. Six hundred years is the period 

commonly cited as mandatory for containment 

of 90Sr and 137Cs, which are expected to constitute 99 percent of the projected curie (Ci) 

accumulation in reprocessed waste by the year 

2020 (Gera and Jacobs, 1972, table 2.5). These 

fission-product nuclides have half-lives of 29 

and 30 years, respectively. Every 10 half-lives 

of a nuclide correspond roughly to a 1,000-fold 

reduction in radioactivity; hence, after 600 

years, or roughly 20 half-lives, the activity of 

these nuclides is reduced to 10~ 3 xlO- 3 or 

10~ (i (one millionth) of its initial value.

Other longer lived fission products will also 

be present in the waste, depending in part on 

its form. Spent fuel assemblies, for example, 

contain significant quantities of 129 I that has a 

very long half-life of 1.6 xlO7 years and that 

poses a potential hazard at least this long.

The toxicity during periods as long as 107 

years computed for radium and selected actinide elements and actinide daughters in highlevel waste from reprocessing are shown in 

figure 1; the toxicity computation is based on 

the hazard from ingestion. Such a measure of 

toxicity takes no account of pathways to the 

biosphere and to man (Pigford and Choi, 1976) 

but does help to pose the question of how long 

a time must be considered in geologic predictions to assure safe containment of waste. The 

critical feature of figure 1 is the very slow dropoff in total potential toxicity during the time 

indicated. As Gera (1975, p. 14) stated, "The 

variation in potential hazard from 100,000 to 

10 million years is so limited and slow that it 

is virtually impossible to make a rational case 

for any specific length of required containment 

falling within that time interval. Either containment failure can be considered acceptable 

after a period on the order of 100,000 years, or 

containment must be assured for periods of 

time exceeding at least 5 million years." Gera 

(1975, p. 14) considered that assuring containment for longer than 5 million years is "clearly 

impracticable since totally reliable geologic
10'

STORAGE TIME, YEARS

FIGURE 1. Ingestion hazard of selected radionuclides in 

high-level waste during 10 million years. The radiotoxic hazard measure is obtained by dividing the 

number of curies present for a given nuclide by the 

number of curies produced by the maximum permissible concentration of that nuclide in a cubic meter of 

drinking water. Data are normalized for 1 metric ton 

of light-water reactor fuel. The nuclide curves are 

plotted from data in table 2 of Hamstra (1975); the 

curve for uranium mill tailings was derived from 

figure 2 of Hamstra (1975) and figure 7 of Pigford 

and Choi (1976).

predictions of the detail required over such long 

time frames are beyond present capability."

Figure 1 also calls attention to the hazard 

from uranium mill tailings, which, although 

more than an order of magnitude more toxic 

than high-level waste, have customarily been 

treated in much more cursory fashion. Whether 

the hazards from high-level waste and uranium 

ore bodies and their derivative tailings are

comparable is not clear. The high-level wastes 

are in highly concentrated forms. This means 

that a breach of a repository and a direct short 

circuit to the biosphere could have serious consequences. On the other hand, the relatively low 

volumes of high-level waste should help to insure that it can be more easily handled and 

placed in a more secure geologic environment 

than the more voluminous and dispersed ore 

and mill tailings (Pigford and Choi, 1976).

GEOLOGIC PREDICTION

Although the hazards from geologically isolated radioactive wastes during long periods of 

time are difficult to specify exactly, it is pertinent to ask, "How credible are geologic predictions ranging from 1,000 to 10 million years 

into the future?"

We have not had, of course, any working experience with geologic containment for such 

long periods; thus, predictions must be based 

on conceptual models of what will happen to the 

waste after emplacement. Such models exist, 

and their complexity and sophistication have 

been increasing steadily in recent years (for example, see Logan and Berbano, 1977; Burkholder and others, 1977; Tierney, 1977). In 

theory, the models provide a compendium of 

all possible events and processes likely to influence release of radioactivity from a repository to the biosphere and eventually to man. 

Connected with each event or process is an estimate of its future probability or rate. Potential 

interactions among the events and processes 

are also identified, and estimated functional 

relationships are incorporated into the models. 

The starting point for the models is a set of 

initial conditions, including emplaced radioactivity, rock geometry, and rock properties 

around the repository and along potential flow 

paths. The end point for the models is a set of 

risks usually expressed in radiation exposures 

to humans for a series of times after final disposal.

Application of these models to policy making 

in the field of radioactive-waste management 

requires that their limitations be clearly understood. Models of natural systems that have 

come into use in recent years fall into four 

categories (Holcomb Research Inst., 1976): (1) 

simple and predictable, such as agricultural

10
crop patterns; (2) complex and predictable, 

such as river hydrology or short-term weather 

patterns; (3) simple and unpredictable, such 

as ecosystem response to natural disasters; and 

(4) highly complex and unpredictable, such as 

interrelations among the species of an ecosystem or, we believe, the fate of radioactive 

wastes in geologic repositories. The unpredictability of radioactive-waste models stems from 

the lack of a method for determining the future 

rates of many events and processes, such as 

tectonism, and from the current lack of adequate data needed to allow the model to function from start to finish for example, the data 

needed to characterize ground-water-flow systems.

Past geologic events such as faulting, seismicity, or climate change probably have not 

been random, but deterministic explanations for 

their frequency, place of occurrence, magnitude, 

and rate of change are difficult to establish. Regardless of whether deterministic or probabilistic models are favored to explain particular 

past geologic events, use of the geologic record 

to predict future events is a formidable task. 

Certainly, an assumption of the constancy of 

rates of geologic processes, namely substantive 

uniformitarianism,4 is open to question. The 

past rates of occurrence of geologic events and 

processes have varied widely over time and 

there appears to be no clear philosophical basis 

for determining rates for these events or processes in the future. Geology is basically a retrodictive rather than a predictive science (Kitts, 

1976). A related problem is that the observational record of the past, on which estimates of 

the rates of occurrence of geologic events are 

made, is invariably incomplete (Ager, 1973). 

The seismic record for most of the United 

States, for example, extends back only about 

200 years. For some parts of the stable interior, 

clusters of relatively few low-level events suggest that larger events have a finite probability 

of occurrence, but estimates of this probability 

must be assigned relatively high errors (S. T. 

Algermissen, oral commun., 1977). These considerations are not meant to suggest that the 

geologic record of the past can be ignored in

4 Substantive uniformitarianism contrasts with methodologic uniformitarianism that asserts merely the spatial and temporal invariance of the natural laws such as gravity. These concepts were 

discussed extensively by Gould (1965a, b), Hubbert (1967), Albritton 

(1967), Hooykaas (1963), and Simpson (1963, 1970).

siting radioactive-waste repositories, but only 

that estimates of probabilities of future events 

be recognized for what they are estimates only. Although many of the current gaps in needed 

data and theory will be filled eventually, Burkholder and others (1977) noted that many processes probably can never be modeled precisely. 

For example, the exact form the waste will 

have thousands of years after disposal probably 

cannot be determined with certainty from 

either laboratory experiments or theory.

Of particular concern in assessing the uncertainty of geologic prediction is the effect of 

complex interactions among events and processes whose individual effects may be simple. 

Natural events such as earthquakes may affect 

a backfilled repository containing fluids very 

differently from the way in which they affect 

undisturbed rock; seismicity may itself be induced by fluid pressures. Potential interactions 

such as these must be analyzed in any systems 

analysis approach to waste isolation. However, 

unanticipated interactions have taken place in 

many engineering efforts whose components 

were presumably well characterized; the Apollo 

13 near-tragic explosion is a conspicuous example (Cooper, 1973). Simpson (1963, 1970), 

Scriven (1959), and Mayr (1961) argued that 

long-term prediction in the biological and earth 

sciences is unreliable and impossible to perform 

with high confidence limits because of the great 

complexity of possible interactions among processes, both identified and unidentified.

Although validating a waste-management 

model for the timespans of concern will never 

be possible, we can ask the pragmatic question, 

"How good has the track record been in predicting the geologic response to man's disturbances for a shorter time, say 100 years?" 

Briefly, predictions in "routine" engineering 

geologic endeavors ranging from soil consolidation to tunnel geology have varied from good, 

in particularly favorable circumstances, to poor 

in some others where data seemed adequate and 

reliable (Lambe, 1973; Dowding and Miller, 

1975). Two major difficulties revealed in studies 

of incorrect predictions have been the geologic 

definition of the medium and the choice of 

boundary conditions, rather than the mathematical modeling. The record may be even 

worse than it appears, for as Lambe (1973)

11
pointed out, many published and apparently 

successful predictions were made after the 

fact; he cautioned against overreliance on 

such predictions as opposed to those documented before the events took place. The interested reader is referred to the highly 

pertinent papers by Lambe (1973), Dowding 

and Miller (1975), Rowe (1975), and Meigh 

(1976) for details.

In summary, predictive models are an essential step in the selection and implementation of 

a radioactive-waste repository and a radioactive-waste management system. They are invaluable tools for analyzing the problem and 

for identifying factors that are likely to have 

the greatest effect on radionuclide migration. 

However, some components of the models are 

inherently unpredictable at present and are 

likely to change at different times. In no sense, 

therefore, will these models give a single answer to the question of the fate of radioactive 

waste in geologic repositories; rather they will 

provide a spectrum of alternative outcomes, 

each based on a set of uncertain assumptions 

about the future. Decision makers outside the 

earth sciences will have to evaluate these uncertain predictions in the light of pressing social and economic concerns.

RESEARCH NEEDS AND CONCLUDING 

STATEMENT

This Circular has dealt largely with the difficulties and uncertainties connected with the 

geologic disposal of high-level radioactive waste 

because, from our viewpoint, these are significant potential stumbling blocks that need critical attention. In emphasizing these problems, 

we do not intend to slight the extensive effort 

currently going forward to find safe repositories. Significant progress is being made. We 

offer the following suggestions for research efforts and emphasis in the hope that they will 

prove constructive.

First, the many questions concerning the 

behavior of rock salt must be resolved questions centering on its high solubility. If relatively small amounts of brine can cause substantial decrease of mechanical strength and 

possible movement of waste during a relatively 

short time, special efforts will surely be necessary to insure retrievability from a salt repository for periods as short as 10-25 years. The 

question of whether the workings of a mine in 

salt can be predicted to stay dry will have to 

be faced.

Second, systematic examination of media 

other than salt should continue. Some of the 

advantages of shale, crystalline rocks, and zeolitic tuff were mentioned earlier. The suggestion of Winograd (1974) that waste be placed 

at relatively shallow depths (30 to several 

hundred meters) in the thick (as thick as 600 

m) unsaturated zones of the arid Western 

United States deserves consideration. The uncertainties connected with all these media are 

greatly reduced if the media are used, at least 

initially, only for relatively cool waste (surface 

temperatures <100°C).

Third, the complex task of characterizing 

ground-water transport systems around potential repositories should continue, and this task 

should be recognized as costly and time consuming. Empirical data on flow through fractured media and on the detailed chemistry of 

sorption-desorption phenomena for the actinide 

elements need to be collected and used in conceptual models.

Fourth, more tools to evaluate potential repositories should be aggressively developed. 

Although it may seem unduly cautious to call 

for still more technology and more types of 

data, we see the need to devise methocls of 

dating old (greater than 40,000 years) ground 

waters and of exploring the volume of rocks 

around the repository as amply justified by the 

seriousness of an inadvertent breech of a repository.

Fifth, more research emphasis is needed on 

the relatively short and long term effects of the 

repository structure and the waste on the environment around the repository. Such research 

should address the question of the extent to 

which the repository itself can localize escape of 

radionuclides to the environment.

A concluding statement concerns the uncertainties involved in geologic prediction for long 

timespans, discussed earlier. These uncertainties need to be faced candidly in public discussions of radioactive-waste disposal. Earth 

scientists can indicate which sites have been 

relatively stable in the geologic past, but they 

cannot guarantee future stability. Construction

12
of a repository and emplacement of waste will 

initiate complex processes that cannot, at present, be predicted with certainty. The inability 

to predict can be offset in part by adoption of 

a multiple-barrier or "defense-in-depth" philosophy for radionuclide containment. Such a philosophy provides a succession of independent 

barriers to nuclide migration. The waste form, 

the host rock, and the ground-water flow path 

all provide potential barriers. Continuing research is needed to measure the efficacy of these 

barriers and to obtain a better understanding 

of the processes involved.

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