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Prepared for
Nuclear Regulatory CommissionContract NRC-02-88-005
Prepared by
Center for Nuclear Waste Regulatory AnalysesSan Antonio,
Texas
September 1993
RI
462. 2 -- - - T 1 973116500Ol'Cie R.Aie of Nat tiral Analog}il
Jeolc9gic Disposal ofHigh- ILrve1l Nuclear Waste
CIWrRA 9 3 -0 20
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CNWRA 93-020
THE ROLE OF NATURAL ANALOGS IN GEOLOGICDISPOSAL OF HIGH-LEVEL
NUCLEAR WASTE
Prepared for
U.S. Nuclear Regulatory CommissionContract NRC-02-88-005
Prepared by
J.A. Apps, J.W. Bradbury, R.E. Cady, R.C. Ewing, D.L.
Gustafson,R.B. Hofmann, D.T. Hoxie, L.A. Kovach, M.B. McNeil,
W.M. Murphy, W.R. Ott, E.C. Pearcy, B. Sagar, M.E. Shea,N.
Sridhar, G.W. Wittmeyer, J.R. Wood, S.R. Young
Edited by
William M. MurphyCenter for Nuclear Waste Regulatory AnalysesSan
Antonio, Texas 78228
Linda A. KovachU.S. Nuclear Regulatory CommissionWashington,
D.C. 20555
-
TABLE OF CONTENTS
Page
FOREWORD TO THE PROCEEDINGS OF THE WORKSHOP ON THE ROLE OF
NATURAL ANA-
LOGS IN GEOLOGIC DISPOSAL OF HIGH-LEVEL NUCLEAR WASTE
by William M. Murphy and Linda A. Kovach
.............................. 1
THE ROLE OF NATURAL ANALOGS IN THE REPOSITORY LICENSING
PROCESS
by William M. Murphy .........................................
3
U.S. NUCLEAR REGULATORY COMMISSION NATURAL ANALOGUE
RESEARCH PROGRAM by Linda A. Kovach and William R. Ott
.................. 7
NATURAL ANALOG STUDIES: LICENSING PERSPECTIVE by John W.
Bradbury ... ..... 15
ROLE OF NATURAL ANALOGS IN PERFORMANCE ASSESSMENT OF NUCLEAR
WASTE
REPOSITORIES by Budhi Sagar and Gordon W. Wittmeyer
..................... 21
LONG-TERM PREDICTIONS USING NATURAL ANALOGUES by Rodney C. Ewing
... .... 29
ANALOG EARTHQUAKES by Renner B. Hofmann
........................... 37
APPLICATION OF NATURAL ANALOG STUDIES TO EXPLORATION FOR
ORE DEPOSITS by Donald L. Gustafson
............................... 43
NATURAL ANALOGS IN THE PETROLEUM INDUSTRY by James R. Wood ...
........ 49
THE PO(COS DE CALDAS INTERNATIONAL PROJECT: AN EXAMPLE OF A
LARGE-SCALE
RADWASTE ISOLATION NATURAL ANALOGUE STUDY by Michael Shea ...
....... 61
NATURAL ANALOGUE STUDIES AS SUPPLEMENTS TO BIOMINERALIZATION
RESEARCH by M.B. Mc Neil .....................................
67
NATURAL GEOCHEMICAL ANALOGUES OF THE NEAR FIELD OF HIGH-LEVEL
NUCLEAR
WASTE REPOSITORIES by John A. Apps
.............................. 75
NATURAL ANALOGS FOR FAR-FIELD ENVIRONMENT/HYDROLOGY by Dwight T.
Hoxie . . 101
WASTE FORMS, PACKAGES, AND SEALS WORKING GROUP SUMMARY
by Narasi Sridhar and Michael B. McNeil
................................ 105
NEAR-FIELD ENVIRONMENT/PROCESSES WORKING GROUP SUMMARY
by William M. Murphy ...................... 107
FAR-FIELD ENVIRONMENT WORKING GROUP SUMMARY
by English C. Pearcy and Ralph E. Cady
................................ 111
VOLCANISM/TECTONICS WORKING GROUP SUMMARY
by Linda A. Kovach and Stephen R Young
............................... 115
iii
-
LIST OF FIGURES
Figure Page
3-1 Organizational structure of high- level waste research
program plan as it relates toregulatory requirements. 7
3-2 Hierarchy of goals and objectives requiring supporting data
and analyses. 8
3-3 Application of analogue studies to provide understanding for
time-temperature ranges forUnited States repository. 9
3-4 Applicability of near-field analogues to time-temperature
curves for United States repository . 10
3-5 Spent fuel analogues versus peak repository temperature
........ . .. .. . . . . . . .. 10
3-6 Breakdown of NRC analogue studies with respect to hydrologic
conditions . . . . . . . . . . . 11
3-7 Conceptual flow diagram of performance assessment showing
possible contributions fromnatural analogue studies .12
5-1 Performance requirements and steps for analysis .22
5-2 Simulation of Las Cruces Trench Validation Experiment .... .
. ... .. . . .. ... . .. 25
5-3 Evolution of the ore body at the Oklo (natural analog) site
(McKinley, 1989) . . . . . . . . . . 26
8-1 Geologic age of world class ore deposits .. ........... ....
.. .... .. . . . . 44
8-2 Hypothetical mercury-hot springs-gold model developed
utilizing theoretical and fieldobservations .47
8-3 Map showing location of the McLaughlin Gold Mine, Napa and
Yolo Counties, California . . 48
9-1 Anticlinal model for gas and oil accumulation showing
irregular distribution of gas, oil, andwater in an asymmetric
anticline . ........ . ..... ... ... . . . .. . . ... . . . 49
9-2 A series of natural analogs showing hydrocarbon trapping in
faulted strata with erosionaluncomformities (Stewart, 1951) .50
9-3 Example of Allan fault-plane for a faulted anticline
illustrating structural and stratigraphicgeometry ......... .. . ..
. .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . . 51
9-4 Porosity versus depth for a series of wells in the Texas
Gulf Coast ..... . . . . . . . . . . . 52
9-5 Typical geohistory plot for a well in the southern San
Joaquin Valley of California . . ... . . 53
9-6 Geohistory plot for reservoir interval of Stevens sand at N.
Coles Levee . . . . . . . . . . . . 54
9-7 Plot of 8 7Srt6Sr versus computed time of crystallization
for strontium data from N. ColesLevee .............. .
...................... 54
9-8 Variations in fluid pressure, rock stress, porosity,
viscosity, and permeability calculated usingMartin equations . .5.
. . . ......... ......... .. .. .. .. .. . . ... . .. 55
9-9 Initial model for seismic pumping based on concept of rock
dilation in vicinity of a fault . . . 56
V
-
I
LIST OF FIGURES (Cont'd)
Figure Page
9-10 Fault-valve model which replaces seismic pumping model
................... . 57
9-11 Model for basin compartments in which normally pressured
rocks overlie several compartmentsby supernormally pressured rocks
which are separated laterally by faults and vertically by shaleor
cement seals (Prowley, 1990) ..... . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 57
9-12 Model for pressure compartment at Ekofisk field in the
Cental Graben of the North Sea. . . . 58
12-1 Schematic diagram to show the decomposition paths of
rhyolitic glass when exposed to theaqueous phase . . . . . . . . ..
... ... ..... ... ... . ....... ...... .. 79
12-2 Schematic diagram to show the decomposition paths of
basaltic glass when exposed to theaqueous phase . . . . . . . .....
... ... ............ ............. 80
12-3 Solubility products of silica polymorphs as a function of
temperature (Apps, 1970) ... . . . 80
12-4 Schematic diagram to show thermodynamic and mass transfer
considerations duringirreversible dissolution and precipitation
(Dibble and Tiller, 1981) ... . . . . . . . . . . . . 80
12-5 Observed stability ranges of secondary minerals in
hydrothermally altered Icelandic basalts . 82
12-6 Activity ratios of major elements as a function of
temperature in well water from Icelandicgeothermal wells
(calculated from chemical analyses cited in Arnorsson et al., 1983)
.83
12-7 Activity ratio of [Nai]/[KI and saturation index of quartz
as a function of temperature in wellwaters from Icelandic
geothermal wells (calculated from chemical analyses cited
inArnorsson et al., 1983) .84
12-8 Variation in log f02 versus fS2, calculated from volatile
and noncondensible gasconcentrations in stream from various
geothermal fields (D'Amore and Gianelli, 1984) . . . . 85
12-9 Dispersion of yttrium and rubidium in relation to zirconium
in Icelandic basalts(W ood et al., 1976) . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 85
12-10 Saturation indices of quartz, calcite, pyrite, low albite,
potash feldspar, illite, laumonite,wairakite, and stilbite as a
function of temperature in Icelandic well waters ... . . . . . . .
88
12-11 Log [NaJ1I[K] calculated from compiled thermodynamic data
as a function of temperature,compared with the corresponding ion
activity products from geothermal well waters ... . . 90
12-12 Log K = [Na][WK]/[Ca 41 calculated from compiled
thermodynamic data as a function oftemperature, compared with the
corresponding ion activity products from geothermalwell waters
....... .. .. .. . .. .. .. .. .. .. . .. .. .. .. . .. .. .. . .
92
14-1 Classification of the engineered barrier systems .... . . .
. . . . . . . . . . . . . . . . . . 106
Vi
-
LIST OF TABLES
Table Page
15-1 Matrix of nuclear waste repository near-field issues and
analog systemsthat may be used to address the issues . . . . .....
... ...... ..... .. ... . .. 109
16-1 Speciation and solubility limits of key radionuclides:
present knowledge . . ... .. .... . 114
vii
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Foreword to the Proceedings of theWORKSHOP ON THE ROLE OF
NATURAL ANALOGS INGEOLOGIC DISPOSAL OF HIGH-LEVEL NUCLEAR WASTE
1
William M. MurphyCenterfor Nuclear Waste Regulatory AnalysesSan
Antonio, Texas 78228
A Workshop on the Role of Natural Analogs inGeologic Disposal of
High-Level Nuclear Waste(HLW) was held in San Antonio, Texas, on
July22-25, 1991. It was sponsored by the U.S. NuclearRegulatory
Commission (NRC) and the Center forNuclear Waste Regulatory
Analyses (CNWRA). In-vitations to the workshop were extended to a
largenumber of individuals with a variety of technical
andprofessional interests related to geologic disposal ofnuclear
waste and natural analog studies. Participa-tion by over 50
scientists and engineers included staffmembers of the NRC and CNWRA
and repre-sentatives from the U.S. Department of Energy(DOE); the
U.S. National Laboratories; the U.S. Geo-logical Survey; several
universities and private or-ganizations; the Nuclear Waste
Technical ReviewBoard; the Advisory Committee on Nuclear
Waste,Clark County, Nevada; and other organizations. Con-tributors
to the workshop are identified in the sum-maries of the working
groups proceedings (Chapters14 to 17). The objective of the
workshop was toexamine the role of natural analog studies in
perform-ance assessment, site characterization, and prioritiza-tion
of research related to geologic disposal of HLW.
Expert opinions were informally solicited frommembers of the
nuclear waste management commu-nity and from individuals outside
this field. Severalpresentations focused on natural analog
studiesmounted specifically in support of geologic disposalof
nuclear waste. In addition, contributions were pro-vided by
scientists and engineers from other fieldswho routinely construct
conceptual and computa-tional models for the evolution of geologic
systemsand who have experience in model validation usingdata from
natural systems. A theme of the meetingwas the generality of
reasoning by analogy in earthscience applications.
In an opening session, presentations focused onrelations between
natural analogs and nuclear wastemanagement and applications of
reasoning by anal-
Linda A. KovachU.S. Nuclear Regulatory CommissionWashington,
D.C. 20555
ogy in a variety of scientific and engineering endeav-ors.
Subsequently, separate working groups ad-dressed the use of natural
analogs in four technicalareas of nuclear waste management: waste
packageand waste form; near-field processes and environ-ment;
far-field processes and environment; and vol-canism and tectonics.
Working groups wereinstructed to define specific technical issues
to whichnatural analog studies can contribute, to evaluate
thestatus of studies on these issues, and to identify areasof
additional fruitful research. Conclusions reachedby the separate
working groups were reviewed in aclosing plenary session.
These proceedings comprise manuscripts writtenby plenary session
speakers, additional papers con-tributed by workshop participants,
and summaries ofresults from each working group. Five articles
(Chap-ters 2 through 6) address the relation of natural
analogstudies to the regulation, performance assessment,and
licensing of a geologic repository for HLW. Aseries of papers then
focuses on applications of rea-soning by analogy in other earth
science applications,including the effects of earthquakes on
engineeredstructures (Chapter 7) and exploration for ore depos-its
and petroleum (Chapters 8 and 9). In addition, anoral presentation
at the workshop addressed naturalanalogs studies in the prediction
of future volcanicactivity and volcanic risk assessment. In Chapter
10,an overview is provided of a recently completed,internationally
coordinated natural analog study atPoqos de Caldas, Brazil. Papers
are also presented onproblems and applications of natural analog
studiesin each of the four technical areas addressed by theworking
groups (except volcanism/tectonics) (Chap-ters 11 through 13).
Finally, the proceedings andconclusions of the working groups are
summarizedin Chapters 14 through 17.
Diverse subjects and points of view were encour-aged and freely
aired at the workshop. Both the utilityand limitations of natural
analog studies were
1
-
Foreword
stressed. Debate developed on many topics from thespecifics of
the thermodynamic properties of miner-als to the generality of the
range of systems for whichnatural analog studies are appropriate.
All workshopparticipants were invited to provide written
contribu-tions to these proceedings, and divergent views
arerespectfully represented here. After review and revi-sion, all
submitted manuscripts have been included.Although many insights
were gained and problemswere clarified at the workshop, the role of
naturalanalogs for geologic disposal of HLW continues tobe an issue
of debate and definition.
The organizers and editors desire that the work-shop activities
and these proceedings contribute to a
greater definition of the utility of natural analog stud-ies in
site characterization and performance assess-ment for geologic
disposal of HLW. The editorsexpress their sincere appreciation for
the insightfulcontributions made by workshop participants and
theserious and time-consuming efforts of speakers,authors, working
group coordinators, reviewers ofmanuscripts, and editorial
assistants.
The chapters of this document represent contribu-tions of the
individual authors or workshop partici-pants. They do not
necessarily reflect the views orregulatory positions of the NRC,
the DOE, or otherorganizations with which the authors are
affiliated.
2
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THE ROLE OF NATURAL ANALOGS IN THE REPOSITORYLICENSING
PROCESS
2
William M. MurphyCenterfor Nuclear Waste Regulatory AnalysesSan
Antonio, Texas 78228
2.1 BACKGROUND
The concept of a permanent geologic repositoryfor high-level
nuclear waste (HLW) is implicitlybased on analogy to natural
systems that have beenstable for millions or billions of years. The
time ofradioactive and chemical toxicity of HLW exceedsthe duration
of human civilization, and it is impossi-ble to demonstrate the
accuracy of predictions of thebehavior of engineered or social
systems over suchlong periods. In contrast, demonstrably stable
geo-logic environments can provide the required isola-tion. Only
geologic (or archaeologic) systems offerthe opportunity for direct
study of chemical isolationand transport phenomena over the time
scale appro-priate to nuclear waste isolation. Large time andspace
scales are prevalent themes in earth sciences,and reasoning by
analogy can aid the scientific evalu-ation of geologic phenomena.
For example, the his-tory of the early Earth, for which no
accessible recordexists, has been largely deduced through
investiga-tions of meteorites and the moon. Key uses of
naturalanalog studies are the identification and evaluation oflarge
space- and time-scale processes and mecha-nisms and testing of
qualitative and quantitative mod-els of system behavior, for
example, repositoryperformance.
2.2 REGULATORY BASIS
U.S. Environmental Protection Agency (EPA)regulations stipulate
that compliance with HLW cu-mulative release requirements for a
period of 10,000years is to be demonstrated by performance
assess-ments (40 CFR 191.13) (EPA, 1985). Performanceassessment
analyses are to identify all significantprocesses and events and to
examine their effects onthe performance of the disposal system [40
CFR191.12(q)] (EPA, 1985). EPA and U.S. NuclearRegulatory
Commission (NRC) rules acknowledgethat absolute confirnation of
repository performanceis impossible and that a finding of
reasonable assur-ance of environmental protection and public safety
is
the realistic requirement. NRC rules recognize thatevaluation of
waste isolation and identification of allsignificant processes and
events on the EPA-man-dated time scale are feasible only with
supportingstudies of analogous systems, and they specify
thatpredictive analyses and models given in the licenseapplicant's
safety analysis report shall be supportedby appropriate use of
field tests, in situ tests, andnatural analog studies [10 CFR
60.21(c)(1)(ii)(F)](NRC, 1983a). The demonstration of
compliancewith objectives and criteria for repository perform-ance
over long times in the future imposed by 10 CFRPart 60 is
stipulated to involve the use of predictivemodels that are
supported by such measures as naturalanalog studies (10 CFR 60.101)
(NRC, 1983a). TheNRC staff has elaborated that methods such as
naturalanalog studies will give confidence in the validity ofmodels
(response to comment No. 130 in NUREG-0804) (NRC, 1983b). These
explicit references inNRC documents to natural analog studies and
tostudies of separate but representative geologic areasconstitute
the formal regulatory basis for the NRCprogram on natural
analogs.
2.3 PROGRAMMATIC NEED
Although the logical basis and regulatory require-ments for
studies of natural analogs to support thelicensing process are well
established, practical ap-plications of natural analogs to
licensing of specificgeologic repository systems have not been well
de-veloped. Essential questions remain unanswered. Towhat extent
can data from analogous systems beextrapolated to assess processes,
events, and occur-rences at a particular repository site? In
practice, howcan reasonable assurance that performance
objectivesand technical criteria are satisfied be derived
fromnatural analog studies? To what degree can perform-ance
assessment models be validated using data fromnatural analogs?
Conduct of natural analog researchand analysis is required to
address questions such asthese.
3
-
The Role of Natural Analogs
2.4 ROLE IN THE REPOSITORYLICENSING PROCESS
The explicit role of natural analogs in the licensingprocess is
to support site characterization and predic-tive modeling of
repository performance. This sup-port will come in three forms as
described in thefollowing sections.
2.4.1 Identification of Processes andEvents
The geologic tenant of uniformitarianism, com-monly paraphrased
as "the present is the key to thepast," implies a regularity of
natural processes gov-erning the evolution of the Earth over time.
Theprinciple can be extended to indicate that the past isthe key to
the future. Observations and interpretationsassociated with
repository site characterization willidentify many important
aspects of the geology (hy-drology, geochemistry, etc.) that could
affect wasteisolation in the future. However, other
significantphenomena are not expected to be manifested in
theambient site, including chemical and hydrologicprocesses and
events associated with the introductionof foreign materials and
with radiation and thermaleffects. Studies of analogous systems
will identifyprocesses and events likely to influence the
evolutionof the perturbed geologic system, and they will
enableevaluation of the importance of these phenomena.For example,
rocks altered by natural hydrothermalsolutions are likely to record
changes analogous tothose that will occur in the hydrothermal zone
asso-ciated with the repository near field. The behaviorsof uranium
(and other) natural mineral deposits andof volcanic glasses in
environments analogous to therepository site demonstrate processes
and events thatwill affect nuclear waste forms over long periods
oftime. The performance of certain container materialscan also be
evaluated by analysis of analogs such asnative copper deposits or
archaeological metallic ar-tifacts. Occurrences of natural
resources in analogousgeologic sites will assist evaluation of
their likelihoodat the repository site. Furthermore, natural
analogstudies can provide evidence for potential effects
inscenarios for future disruptive events, such as seis-micity and
volcanism. Identification through analogstudies of important
interactions among processes inthe complex engineering and
geological environmentof a HLW repository will permit their
considerationin safety analyses. Conversely, identification of
proc-esses and events that would have insignificant effects
can help justify their omission from predictive mod-els. The
development of relatively complete and re-alistic conceptual models
and scenarios forperformance assessments will require data from
ana-log systems.
2.4.2 Calibration of Models
Performance assessment models used to support afinding of
reasonable assurance of repository safetywill depend in part on
theoretical and empirical rela-tions and parameters. Studies of
systems analogousto the repository system, in conjunction with
labora-tory and field tests, will provide sources of
empiricalrelations and parameters. For example, concentra-tions and
variations of solutes, colloids, and microbesin natural
groundwaters will help bound their likelyconcentrations in the
repository environment. Inversemodeling of natural hydrologic
systems can providehydrologic parameters pertinent to large-scale,
het-erogeneous systems. Iterative modeling of systemsthat can be
directly observed with progressive modelrefinement is a routine
method of calibration. Errorsand omissions in model parameters and
relations canbe identified in this process. Natural analogs will
bethe objects of exercises to calibrate models used inperformance
assessment to augment parametric val-ues and empirical relations
derived from laboratorystudies of limited space and time
scales.
2.4.3 Validation of Models
Strict validation of predictive models for reposi-tory
performance is impossible because of the largetime and space scales
and the geologic and engineer-ing complexity of the repository
system. Neverthe-less, for models to support a finding of
reasonableassurance of repository safety, a judgment must bemade of
their accuracy and applicability to the systemof interest. Both
qualities can be evaluated, at least inpart, using the degree of
correspondence betweenmodel results and observable features of
natural ana-log systems. Correct predictions of the
characteristicsof analog systems not involved in the calibration
ofthe models will help to demonstrate model accuracyover the range
of characteristics. Correct model pre-dictions of processes in an
analog system that arerepresentative of specific repository
processes willshow applicability of the model to the
repositorysystem. Aspects of validation derived from analogsmay be
largely qualitative, because the representationof the repository
system provided by analogs is ap-
4
-
The Role of Natural Analogs
proximate and because some quantitative features ofnatural
systems are difficult to obtain (e.g., initialconditions).
Nevertheless, evaluations of natural ana-logs will provide
information on the completeness ofperformance assessment models,
that is, the extent towhich the models account for all important
processesand events and their coupled effects.
Furthermore,performance assessments will generate predictions
ofprobabilistic distributions of consequences, and stud-ies of the
properties of a number of analogous systemswill assist in
validation of the predicted distributions.
2.5 CONCLUSIONSIn the United States, the licensing of a HLW
re-
pository will require reasonable assurance that thepublic
radiological health and safety will be protectedthrough compliance
with regulatory objectives forrepository performance. Reasonable
assurance in thebehavior of a unique and complex engineering
andgeologic system operating over a space scale of manycubic
kilometers and a time scale of 10,000 years orlonger is likely to
be provided only through converg-ing lines of evidence from a
variety of investigations.Studies of analogous natural systems can
supportthese investigations. Stable natural systems, analo-gous to
hypothetical repository systems, provide the
conceptual basis for permanent geologic disposal.Natural analogs
offer field evidence for processes andevents that could affect
repository performance overlarge time and space scales. The
usefulness of naturalanalog systems in calibration and validation
of mod-els required for predicting repository performance isan
issue of active research. In combination with siteinvestigations,
laboratory studies, and scientific andengineering analyses, studies
of natural analogs areexpected to contribute to the reasonable
assurancenecessary for repository licensing.
2.6 REFERENCES
EPA (1985) Environmental Radiation ProtectionStandards for the
Management and Disposal ofSpent Nuclear Fuel, High-Level and
TransuranicWastes: Final Rules. Title 40 Part 191. Code ofFederal
Regulations. Washington, D.C.
NRC (1983a) Disposal of High-Level NuclearWastes in Geologic
Repositories. Title 10 Part 60.Code of Federal Regulations.
Washington, D.C.
NRC (1983b) Staff Analysis of Public Comments onProposed Rule 10
CFR Part 60, "Disposal of High-Level Radioactive Wastes in Geologic
Reposito-ries." NUREG-0804, Nuclear RegulatoryCommission,
Washington, D.C.
5
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U.S. NUCLEAR REGULATORY COMMISSION NATURALANALOGUE RESEARCH
PROGRAM
3
Linda A. KovachU.S. Nuclear Regulatory CommissionWashington,
D.C. 20555
3.1 INTRODUCTION
The following words can be extracted from 10CFR
60.101(a)(2):
Proof ... is not to be had in the ordinary senseof the word.
To those involved in the safe disposal of nuclearwaste, these
words have a special meaning. The regu-latory periods, be they 300,
1,000, 10,000 or1,000,000 years, place a burden of proof with
regardto regulatory requirements which cannot be satisfiedin
ordinary engineering terms or even by heroic labo-ratory or
real-time field investigations. Beyond thisacknowledgement of the
difficulty and unique natureof the demonstration that must be made,
the scope andcomplexity of the problem have led many
technicalexperts in this field to observe, correctly, that
anoverall performance assessment model can never befully validated.
The study of natural analogues toprocesses that may affect
repository performance hasbeen advanced as a way to approach this
problem.
The performance assessment problem facing thewaste disposal
community is one that requires cred-ible conceptual models of
processes and events and aquantitative basis for exercising those
models to de-scribe the performance of real disposal systems
overthe period of interest. The U.S. Nuclear RegulatoryCommission
(NRC) has stated an approach in itsregulations that includes
natural analogue studiesamong "such measures" supporting predictive
mod-els [10 CFR 60. 101(a)(2)]. Adherence to criteria ad-vanced by
the Commission of the EuropeanCommunities Natural Analogue Working
Group(NAWG) to judge the potential value of a proposedanalogue
study allows investigators to focus moreclearly on ultimate
objectives in the context of per-formance assessment by drawing
specific attention toseparability of effects, a clear statement of
the analo-gous systems or processes being studied, independentmeans
to establish basic parameters, well-definedboundaries and boundary
conditions, and the ability
William R. OttU.S. Nuclear Regulatory CommissionWashington, D.C.
20555
to describe the temporal history quantitatively andwith
acceptable accuracy (Chapman et al., 1984).
3.2 REGULATORY CONTEXTThe primary responsibility of providing
data for
support of the license application rests with the U.S.Department
of Energy (DOE). However, for the NRCto have an independent basis
to evaluate the DOEwork, the NRC will selectively investigate
analoguework to support its evaluations. The NRC Office ofResearch
is attempting to develop a systematic ap-proach to natural analogue
research that will repre-sent a balanced approach to providing
"ReasonableAssurance" (consistent with NRC regulations and
asopposed to "proof' or "validation") that analyses arereasonable
approximations to reality and that per-formance objectives will be
achieved. Parallel to theefforts on natural analogues has been a
broader effortto define an overall high-level waste (HLW)
researchprogram plan. The plan is structured around the regu-latory
performance objectives of Part 60 and is di-rectly keyed to the
most recent officially announcedDOE schedule.
Figure 3-1 shows the regulatory structure as it isused to
provide the format for the Research ProgramPlan. Conceptually, this
simple approach organizesregulatory concerns into the engineered
system,
REGULATORY STRUCTURE FOR HLW
OBJECTIVE: SAFE HLW DISPOSAL
WHOLE SYSTEM PERFORMANCENRC AND EPA STANDARDS
ENGINEERED SYSTEM GEOLOGIC SYSTEM!A~~~~~~~~~~~------- ------;U
---T--------
RELEASE CONTAINMENT GROUNDWATERPERF. OBJ. PERF. OBJ. PERF.
OBJ.
DESIGN SITNGCRITERIA CRITERIA
Figure 3-1. Organizational structure of high-level waste
research program plan as it relatesto regulatory requirements
7
-
NRC Natural Analogue Research Program
within which the bulk of mechanical and thermalperturbation will
occur, and the geologic system,within which the fundamental
geologic processeswill be largely undisturbed. The interface
betweenthese two areas is the "near field' where
coupledthermal-chemical-hydrologic processes may signifi-cantly
affect groundwater flow and contaminanttransport and thus perturb
the boundary conditionsfor the far-field transport evaluation.
While the ther-mal perturbation may be fairly localized, its
influencecould extend a significant distance beyond the nearfield
and be displayed in such processes as mineraldissolution and/or
precipitation, moisture redistribu-tion, and gas flow. Waste
package/waste form andnear-field processes loosely fall within the
engi-neered system of Figure 3-1 and provide the sourceterm for the
far-field calculations. Far-field processesand volcanic/tectonic
events fall under the geologicsystem and provide the final link to
the overall per-formance assessment and consideration of
disruptivescenarios.
Figure 3-2 displays the hierarchy of goals andobjectives that
must be satisfied through the regula-tory framework, technical
evaluations, and develop-ment of independent capability to assess
DOE claimsof facility performance. The goal of the NRC HLWresearch
is to reduce uncertainty so that responsibleregulatory decisions
can be made in the HLW licens-ing program for the protection of
public health andsafety. The regulations and standards of the NRC
andthe U.S. Environmental Protection Agency (EPA)provide the
framework for this evaluation, but theycontain imprecisely defined
terms, such as "substan-tially complete containment" and the
"disturbed
RESEARCH GOAL: REDUCE UNCERTAINTY
zone," which create uncertainty in their implementa-tion.
Performance assessment is the process of system-atic,
quantitative evaluation of compliance with thestandards established
by the EPA and the NRC forboth overall and subsystem performance.
This evalu-ation is built on complex conceptual and computa-tional
models of the engineered and geologic systemsand thus carries the
technical uncertainties inherentin the evaluation of any complex
engineered or natu-ral system. Processes that are not well
understood cangenerate large uncertainties when they perturb a
sys-tem that may otherwise be well understood. Theentire evaluatory
framework is built on a combinationof (i) scientific theories
describing the systems, (ii)laboratory experiments testing
processes amenable tosmall-scale and short-time-frame studies,
(iii) fieldexperiment and testing programs designed to provideboth
large-scale and real-time confirmation of labo-ratory tests, and
(iv) the longer time frames attainablethrough the study of
analogous natural systems.Technical uncertainty and the propagation
of errorthrough complex evaluations must be estimated andconstantly
reassessed to further focus efforts to pro-vide credible
analyses.
3.3 CONDITIONS AND PROCESSESCONSTRAINING SELECTION OF
NATURAL ANALOGUES
Extensive progress in natural analogue researchhas been achieved
over the last 10 years by suchcountries as the United Kingdom,
Sweden, Switzer-land, Canada, Japan, and the United States.
Severallarge international projects have been sponsored bythe
Nuclear Energy Agency (NEA) and the Commis-sion of European
Communities (CEC), such as thePocos de Caldas Project, Cigar Lake
Analogue Pro-ject, the Alligator Rivers Analogue Project to name
afew. The completion of these projects and othersprovides extensive
literature on subjects of commoninterest to the various
international waste disposalprograms, such as corrosion of waste
packages, ra-dionuclide mobility and retention, importance of
col-loids and redox fronts in radionuclide mobility, etc.
Unique aspects of the United States repositoryprogram drive the
approach to the use of naturalanalogues. The United States
repository will poten-tially contain younger fuel and a higher
thermal load-ing than repository designs other countries
areconsidering. This design will lead to higher tempera-
I OBJECTIVE: HEALTH AND SAFETY II
I REGULATION: 10 CFR PART60'
REGULATORY UNCERTAINTY:|
I PIPERFORMANCE ASSESSMEN7T
TECHNICAL UNCERTAINTYI
NAT ANALOGUE|
N THEORY 10
Figure 3-2. Hierarchy of goals and objectivesrequiring
supporting data and analyses
8
-
NRC Natural Analogue Research Program
tures in the host rock and, perhaps, significant altera-tion due
to boiling of pore water in the unsaturatedzone. The thermal regime
for the repository host rockis graphically depicted in Figure 3-3,
which is anapproximation of the time-temperature curve antici-pated
for the United States repository (Pruess et al.,1990a and b). The
distances shown are from thecenterline of an individual waste
container. Interna-tionally, there is little interest in systems
over 100 'C,because disposal sites and wastes will be managed
tomaintain temperatures-below this figure. The currentUnited States
repository designs result in muchhigher temperatures over the first
1,000 years. Inaddition, the unsaturated, oxidizing environment
atYucca Mountain poses two other conditions uniqueto the United
States program that are considered inthe selection of appropriate
analogue studies.
eruption, largely only relate to low-temperature con-ditions.
Natural systems that may provide informa-tion over time periods
from 100 to 1,000,000 yearsinclude hydrothermal systems (which can
also repre-sent the thermal effects) and epithermal systems(which
are applicable to the lower-temperature, near-surface region).
APPLICABILITY OF CATEGORIES OF ANALOGUES
VERSUS TIME-TEMPERATURE OF REPOSITORY
300
8 250wE 200
I 150
2 100XwsI-
In order to reduce concerns over uncertainties inextrapolation
of laboratory data over large temporaland spatial scales, several
different natural systemscould be analyzed, which would bound the
criticalregions of regulatory or scientific uncertainty.
Mostnatural systems leave a footprint in time, providinginformation
regarding the final state of the system. Itis often difficult to
determine all the processes leadingto the final observed state.
Processes that are transientin nature cannot be examined at one
particular site. Inorder to understand processes and synergistic
condi-tions leading to the final state of a natural system,
awell-planned matrix of studies could be developed toexplore
different aspects of the problem. If one axisrepresents potential
variations in one parameter whileanother is held constant, a set of
analogues mightprovide points on the isopleth where in situ values
ofthe constant parameter are the same and the othertakes on values
unique to the system under study.Another example might be to test
variations withscale by looking at the same processes in
similarsystems but over larger distances. A matrix of studiesover a
range of temperature, distance, and time wouldhelp to reduce
uncertainty and bound parametric val-ues of concern in both the
robust performance assess-ment models, and, more specifically, in
the morecomplex subsystem conceptual and numerical mod-els.
TIME (THOUSANDS OF YEARS)
- 0.Im __ 0.4m - 20m ........ 1km
Figure 3-3. Application of analogue studies toprovide
understanding for time-temperatureranges for United States
repository
Superimposed on the temperature profiles of Fig-ure 3-3 are the
sources of information that can be usedto enhance our understanding
of system performancefor various parts of the time-temperature
history ex-pected for the United States repository. Laboratoryand
real-time field experiments yield information thatextends, at most,
to several years. With proper designand long-term funding, this
period might be extendedto tens of years. Modem industrial
experience withhigh-technology alloys or other engineered
materialsand state-of-the-art underground excavation and
con-struction techniques may span a period of 30-50years.
Archaeological analogues may span 100-5,000 years, but, except for
some artifacts that mayhave been subjected to brief heating from a
volcanic
3.4 INITIATIVES BY NRC
The structure of the NRC Natural Analogue Re-search Plan has
evolved, in part, from considerationof the time-temperature curves.
First, as noted above,the unique time-temperature aspects of the
UnitedStates program lead to consideration of higher tem-perature
regimes than other programs. Second, dis-ruptive scenarios take on
a greater importancebecause of tectonic and volcanic activity in
the vicin-ity of the Yucca Mountain repository site. Otherconcerns
of radionuclide mobility over long timesand the performance of
engineered containment sys-tems are common to most disposal
programs, and,
9
-
NRC Natural Analogue Research Program
hence, the NRC will make use of information fromother programs
as appropriate.
The high-temperature curve (Figure 3-4) is typicalof the
immediate vicinity of the waste canisters.Superimposed are
idealized representations of therange of temperature versus time
conditions of ana-logues currently in the NRC program designed
toaddress the uncertainties of near-field processes. Pro-jects at
the Valles Caldera and Pefia Blanca, Mexico,should provide
information regarding the thermalstability of the host rock and the
transport charac-teristics of the medium over a range of
temperatureand water saturation. The Akrotiri site on the isle
ofSantorini, Greece, may provide information on thecorrosion and
near-field transport of metallic arti-facts. The Akrotiri site
provides a well-constrainedtime of burial (3,600 years ago),
well-defined expo-sure temperatures, and a chemical environment
simi-lar to Yucca Mountain. Most difficult to determine atthe site
will be the hydrogeologic conditions to whichthe artifacts were
exposed.
NATURAL ANALOGUE FOR NEAR-FIELD:
TIME-TEMP. CURVE FOR HIGH-LEVEL WASTE
CD 200
S50 AK¶IR'f.....~.. .e....O '.;:7V.*.…b
8.loo o oi ''1 . 1. 10 'iooTIME (THOUSANDS OF YEARS)
-0.lm-ILm--20m .--- 1km
Figure 3-4. Applicability of near-field analoguesto
time-temperature curves for United Statesrepository
The response of the host rock to a thermal pulsefrom one
volcanic flow overlaying a second was thesubject of studies at the
Valles Caldera (see Figure3-4). This project was specifically aimed
at alterationobserved near the contact between the two forma-tions,
and results have been reported concerning thedifferential migration
of halogens away from thecontact via vapor phase transport. The
site was se-lected for study because many of the initial
andbounding conditions are well constrained. The datesof the events
are well established. The thermal historyof the event has been
reconstructed from direct physi-
cal evidence and knowledge of similar flows. Thesystem has
planar geometry and is well bounded. Thetuff host rock is similar
in chemical composition tothe Yucca Mountain tuffs. The goal of the
study wasnarrowly defined. While not as comprehensive nor
asglamorous as the larger ore body studies, the resultsmay be just
as significant as any individual piece ofthose larger programs.
Conditions and processes affecting the stability ofthe waste
form and source term will also be slightlydifferent at the Yucca
Mountain site than at otherproposed disposal sites. Therefore, an
attempt is be-ing made to investigate the role of the
unsaturatedzone and elevated temperatures on waste packagematerials
and waste form. Figure 3-5 identifies thenatural analogue projects
that are currently underconsideration by the NRC.
NATURAL ANALOGUES FOR SPENT FUELTIME-TEMP. CURVE FOR HIGH-LEVEL
WASTE
300
q 250
c 200
d 1500.Wm 100
0o
o.i1 '' -6oo0 I 0. 1 1 r r , , ,TIME (rHOUSANDS OF YEARS)
- 0.m
Figure 3-5. Spent fuel analogues versus peakrepository
temperature
The project at Nopal I, a tuff-hosted uranium orebody located in
the Sierra Pefia Blanca, Chihuahua,Mexico, is focused on source
term degradation andtransport processes. The alteration of
uraninite underunsaturated oxidizing conditions is one aspect of
thestudy. Transport processes of uranium in the unsatu-rated zone,
and low-temperature alteration of a tuf-faceous host rock are also
being considered at this siteas a natural analogue to processes
expected to occurin the proposed United States repository. A
prelimi-nary investigation of the site has been completed, anda
program of field investigations relevant to sourceterm and
contaminant transport is progressing (Mur-phy et al., 1990).
NRC work on the Oklo reactors [in cooperationwith the
Commissariat a l'Energie Atomique (CEA)investigations] is focused
on source term (uraninite
10
-
NRC Natural Analogue Research Program
alteration) and characterization of the organic phasesinvolved
with radionuclide retention in and aroundthe Oklo reactors. The
much more extensive CEA andCEC investigations are concerned with
the stabilityof uraninite and the mobility of fission products
inand around the reactors. The NRC also hopes tobenefit from
investigations by the CEA/CEC that willprovide data on the effect
of igneous intrusions in thevicinity of a natural reactor zone.
The International Alligator Rivers Analogue Pro-ject is an
international cooperative study of a uraniumore body that has been
under intense investigation forthe last 5 years, a program
supported by organizationsin the United States, Australia, the
United Kingdom,Sweden, and Japan. The objectives of this project
areto contribute to the development of realistic modelsfor
radionuclide migration within the geosphere, de-velop methods of
validation of models using labora-tory and field measurements, and
encourageinteraction between modelers and experimentalists
inachieving objectives. This is being accomplishedthrough six
subprojects: (i) modeling of radionuclidemigration (including the
study of primary and secon-dary uranium ore dispersions in the
matrix and soilsdetermining the nature of radionuclide
transportthrough the rock matrix and fractures), (ii) hydrogeol-ogy
of the Koongarra uranium deposit, (iii) investi-gation of U/Th
series disequilibria, (iv) the role ofgroundwater and colloids in
radionuclide transport,(v) the study of naturally occurring fission
products,and (vi) transuranic nuclide studies (Duerden, 1990).
Figure 3-6 shows another way to categorize infor-mation that may
be obtained from the various naturalanalogue projects currently
under investigation bythe NRC. This diagram indicates processes
affectingwaste form stability, waste package, source term
CURRENT NRC NATURAL ANALOGUE STUDIES
degradation, host rock stability, and transport of
ra-dionuclides under the two extreme hydrologic condi-tions.
Natural analogue studies will also address thenature of fluid flow
in an unsaturated fractured rockand transport under similar
conditions. The PeftaBlanca and Akrotiri sites will both address
theseissues.
3.5 THE ROLE OF ANALOGUES INTHE LICENSING PROCESS
The broad interpretation utilized in the NRC pro-gram for
natural analogue studies is used as theframework for Figure 3-7:
elevated temperaturesand/or results pertinent to evaluations over
timesgreater than 100 years. The emphasis on higher-tem-perature
processes and the inclusion of disruptivescenarios constitute the
expanded view taken by theNRC program.
The intransigence of the model validation problemhas brought us
to the point of no return. Neitherreal-time laboratory or field
studies nor natural ana-logues can truly "validate" a performance
assessmentmodel. However, "validation" in its purest sense isnot
what we are after. "Reasonable assurance" is theterm of choice, and
the path to it is a reasoned andsystematic approach building
credibility and confi-dence into the use of models to simulate
waste dis-posal system performance. Natural analogues canprovide an
important service in this role.
Scenario development is one of the ways in whichnatural
analogues can contribute to disposal facilityassessment. Not only
disruptive scenarios, but ex-pected normal and off normal
scenarios, such asabnormal rainfall or cycles of climatic change,
can bestudied in analogous systems. When scenarios areadvanced for
review, it would be appropriate to askif there are natural systems
available for study inwhich the same phenomena have been observed
andfrom which information might be extracted to quan-tify
expectations of frequency and magnitude forthese phenomena.
Sensitivity studies and coupled processes com-prise additional
areas where analogue studies can bevery productive. In instances
where coupled proc-esses are involved, this may be particularly
important,because the ability to vary parameters when studyingthe
evolution of a natural system only exists throughfinding another
system in which that parameter orseries of parameters is different.
Several analoguescan thus begin to provide the same perspective as
a
HYDROLOGICCONDITIONS
SATURATED
UNSATURATEC
WASTE FORM I WASTE I HOST ROCK I TRANSPORTSOURCE TERM PACKAGE
STABIUTY
ARAP PENA BLANCA ARAPPENA BLANCA
OKLO
PENA BLANCA AKROTIRI VALLES VALLES
ARAP CALDERA CALDERAPENA BLANCA ARAP
PENA BLANCAKROTIRI
ELEVATEDTEMPERATURES. TIME >100YEARS
Figure 3-6. Breakdown of NRC analoguestudies with respect to
hydrologic conditions
11
-
NRC Natural Analogue Research Program
SUPPORTING ANALOGUE STUDIES FORCOMPONENTS OF TOTAL SYSTEM
PERFORMANCE ASSESSMENT
MINED GEOLOGIC REPOSITORYSYSTEM DESCRIPTIONCOMPONENT STUDIES
ENGINEEREDSITE WASTE BARRIER
SYSTEMVALLES CALDERA PENA BLANCA AKROTIRIPENA BLANCA OKLO
ARCHEOLOGICALAKROTIRI ARAP
1
SCENARIOANALYSIS
DESCRIPTIONSCREENINGPROBABILITIES
VOLCANISMTECTONISMHYDROTHERMALISMCLIMATE CHANGE
COMPARISONTO REGULATORY
STANDARD
I
SYSTEM STUDIES
PERFORMANCECALCULATION
CCDFSYSTEM CODE
CONSEQUENCEANALYSIS
SOURCE TERMFLOW & TRANSPORTGEOCHEMISTRY
PENA BLANCAOKLOARAP
SENSITVITY &_ UNCERTAINTY
ANALYSIS
Figure 3-7. Conceptual flow diagram of performance assessment
showing possible contributions fromnatural analogue studies
laboratory experiment in which critical parameterscan be held
constant or varied. In this example, se-lected comparison to
laboratory experiments can bean informative complement to the
natural analogueinvestigations.
Model development is also amenable to input fromnatural analogue
studies. By testing both conceptualand computational models against
descriptions ofprocesses from natural analogues, weaknesses maybe
revealed that could be addressed by additionaltheoretical or
laboratory work and augmentation ofboth models. This iterative
process is now explicitlyincluded in the formal process since it is
now referredto as Iterative Performance Assessment. In the
finalanalysis the most significant result of an analoguestudy will
be to tell us under what circumstancesmodels do not work or are not
applicable. This, then,stimulates the next phase of model
development anda more credible product for use in the review
process.
Database validation is perhaps the most directapplication of
analogues. Comprehensive databases
developed in laboratories can be spot-checkedagainst field
measurements under similar conditions.The same is true of extended
databases developed bytheoretical extrapolation of laboratory
data.
A final area for consideration of natural analoguedata is in
site characterization. Insights gained fromattempting to both study
and model natural analoguesyield important information about the
type of dataneeded to support credible simulations of
repositoryprocesses. The value of such information cannot
beoverstated. The analogy is in the ability to test themethods of
data collection, treatment of samples, andeffects of destructive
and invasive techniquesweighed against nondestructive and
noninvasivemethods. The analogue results provide insights for amore
deliberate and, in the long run, more effectiveprogram of site
characterization.
3.6 CONCLUSIONS
The NRC is the government agency responsiblefor licensing the
HLW repository. Not only must the
12
-
I
performance assessment models be tested, but theNRC must ensure
that plausible scenarios of perform-ance are considered, both
favorable and unfavorable.The primary responsibility of providing
supportingdata for support of the license application rests withthe
DOE. However, for the NRC to have an inde-pendent basis to evaluate
the DOE work, the NRCwill selectively investigate analogue work to
supportour evaluations. To this end, natural analogue studiesmay
play a key role in providing the necessary con-fidence in support
of the DOE license application by(i) providing data to test the
ability of models toaddress potential future states of the disposal
systemand (ii) providing insight through development ofconceptual
models for designing site characterizationand data collection
programs. Figure 3-7 presents aconceptual flow diagram for an
iterative performanceassessment. Indicated on this figure are the
presentand planned components of the NRC Natural Ana-logue Research
Plan. It is a small start, as it must beconsistent with NRC
resources in this area, but it issystematic and focused to provide
information incritical areas related to the nature of the
problem.
3.7 REFERENCESChapman, N.A., I.G. McKinley, J.A.T. Smellie,
1984. The Potential of Natural Analogues in As-sessing Systems
for Deep Disposal of High-LevelRadioactive Waste. NAGRA, NTB
84-41.
Duerden, P., editor, Alligator Rivers Analogue Pro-ject 1st
Annual Report, 1990, ANSTO.
Murphy, W.M., E.C. Pearcy, and P.C. Goodell, 1990.Possible
analog research sites for the proposedhigh-level nuclear waste
repository in hydrologi-cally unsaturated tuff at Yucca Mountain,
Nevada.in "Fourth natural analogue working group meet-ing and Pocos
de Caldas project final workshop,"B. Come and N.A. Chapman, eds.,
EUR 13014,pp. 267-276.
Pruess, K, J.S.Y Wang, and Y.W. Tsang, 1990a. Onthermohydrologic
conditions near high-level nu-clear wastes emplaced in partially
saturated frac-tured tuff, 1, Simulation studies with
explicitconsiderations of fracture effects, Water ResourceResearch,
vol. 26, no.6, pp. 1235-1248
Pruess, K., J.S.Y Wang, and Y.W. Tsang, 1990b.
Onthermohydrologic conditions near high-level nu-clear wastes
emplaced in partially saturated frac-tured tuff, 2, Effective
Continuum Approximation,Water Resource Research, vol. 26, no.6, pp.
1249-1261.
13
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I
NATURAL ANALOG STUDIES: LICENSING PERSPECTIVE 4John W.
BradburyOffice of Nuclear Material Safety and SafeguardsU.S.
Nuclear Regulatory CommissionWashington, D.C. 20555
4.1 INTRODUCTION AND PURPOSE
This report is intended to describe the licensingperspective of
the term, "natural analog studies," asused in 10 CFR Part 60 (the
Rule), including a clari-fication of the range of topics to which
these studiescan apply. Evidence suggesting a misunderstandingof
this term comes from a discussion on its definitionat the U.S.
Nuclear Regulatory Commission (NRC)-Center for Nuclear Waste
Regulatory Analyses(CNWRA) Workshop on the Role of Natural
Analogsin Geologic Disposal of High-Level Nuclear Waste,July 22-25,
1991. That discussion concerned therange of topics to which natural
analog studies shouldapply. Since the workshop, an additional paper
andreport has come out recommending the range oftopics
appropriately covered by natural analog stud-ies. Chapman (1992)
states "a natural analog is anenvironment that has been perturbed
in some way bymaterials or processes analogous to those in or
arounda repository, and resulting from its presence." Fur-thermore,
he states, "the majority of analog studiesconcern geochemical
processes." These ideas arereflected also in a report written by a
panel of scien-tists [Natural Analogue Review Group (NARG)]
se-lected "to provide guidance and recommendations tothe U.S.
Department of Energy's (DOE) Office ofCivilian Radioactive Waste
Management for the im-plementation of natural analog studies in the
sitecharacterization program" (letter from C.P. Gertz toT.H.
Isaacs, 6/1/92). The report states that, "naturalanalog studies
should be process-oriented and shouldbasically address the issues
resulting from the pertur-bation of a natural system (the geologic
site) by theintroduction of a technological system (the
reposi-tory)." Furthermore, the report goes on to say that,"all
investigations normally part of site charac-terization, even when
considering comparisons withsimilar remote sites, such as
(paleo)hydrology, etc.,should not be considered as natural analog
studies."The range of issues suggested in the NARG report to
which natural analogs can apply is more restrictivethan required
by 10 CFR Part 60.
4.2 ANALYSIS
The term "natural analog studies" is used twice in10 CFR Part
60. The first occurrence of the term is inthe section describing
License Applications, §60.21Content of application, specifically
in§60.21(c)(1)(ii)(F). The second occurrence is in Sub-part
E-Technical Criteria §60.101 Purpose and na-ture of findings. The
text of 10 CFR Part 60 using theterm "natural analog studies" is
provided in the Ap-pendix.
In Paragraph (F) of §60.21(c)(1)(ii), "natural ana-log studies"
is one of the methods from an appropriatecombination of methods
that can be used to supportanalyses and models "to predict future
conditions andchanges in the geologic setting." The geologic
settingis defined in the Rule as "the geologic, hydrologic,and
geochemical systems of the region in which ageologic repository
operations area is or may belocated." The "changes in geologic
setting" wouldnecessarily result from various processes acting
inand on the systems. The systems are, thus, composedof both
conditions and processes, and the analysesand models are meant to
support prediction of thefuture states of the systems. In turn, it
is interpretedthat these analyses and models will be part of
the"explanation of measures used to support the assess-ments
required in paragraphs (A) through (D)."
Paragraph (A) requires "an analysis of the geol-ogy, geophysics,
hydrogeology, geochemistry, cli-matology, and meteorology of the
site." This analysisinvolves determining both conditions and
processespresent at the site and, thus, can be considered
sitecharacterization. Site characterization is defined inthe Rule
as "the program of exploration and research,both in the laboratory
and in the field, undertaken toestablish the geologic conditions
and the ranges ofthose parameters of a particular site relevant to
theprocedures under this part...."
15
-
Natural Analog Studies: Licensing Perspective
Paragraph (B) refers to "Analyses to determine thedegree to
which each of the favorable and potentiallyadverse conditions, if
present, has been charac-terized... ." Favorable and potentially
adverse condi-tions are described in §60.122 Siting Criteria,
wheresome of the analyses involving favorable and poten-tially
adverse conditions can be considered site char-acterization work
[e.g., §60.122(b) and (c)] and somecan be considered performance
assessment [e.g.,§60.122(a)].
A distinction can be made between site charac-terization and
performance assessment. Whereas sitecharacterization involves
collecting, describing, andanalyzing processes and conditions of
the site, per-formance assessment specifically evaluates the
siterelative to the performance objectives described inthe Rule.
The information from site characterizationthus feeds into
performance assessment.
Paragraph (C) requires an "evaluation of the per-formance of the
proposed geologic repository...."Paragraph (D) refers to "The
effectiveness of engi-neered and natural barriers... ." Both of
these para-graphs refer to performance assessment
The use of the term "appropriate combination ofsuch methods" in
paragraph (F) provides for flexibil-ity in the choice of the
methods used to support theanalyses and models predicting future
conditions andchanges in the geologic setting. As a result, there
isnot a one-to-one correlation between the methodslisted in the
Rule "such as field tests, in situ tests,laboratory tests which are
representative of field con-ditions, monitoring data, and natural
analog studies"and the required analyses of paragraphs (A)
through(D). However, the use of the term "conditions... .in
thegeologic setting" in paragraph (F) could include con-ditions
such as those listed as favorable and poten-tially adverse
conditions in the Siting Criteria. Thus,an "appropriate combination
of such methods" is tosupport site characterization work. The use
of theterm "changes in geologic setting" could refer eitherto
changes caused by the repository, which wouldnecessarily involve
performance assessment, or tochanges that occur naturally over the
period of regu-latory concern, which would involve site
charac-terization. Consequently, §60.21 requires that
an"appropriate combination of such methods" be usedto support site
characterization work and perform-ance assessment. However, the
Rule does not de-scribe what constitutes an appropriate combination
ofmethods to support the analyses.
The Staff Analysis of Public Comments on Pro-posed Rule 10 CFR
Part 60 "Disposal of High-LevelRadioactive Wastes in Geologic
Repositories"(NRC, 1983) states that the "support for the
modelsfrom an appropriate combination of methods con-cerns not only
the reliability of the codes themselves,but also the
representativeness of the models withrespect to the physical
conditions of the site." Thus,by referring to conditions, the
paragraph is addressingaspects of site characterization.
Since the Rule does not explicitly restrict the ap-plication of
natural analog studies to any one of thespecified analyses in
Paragraphs (A) through (D), itis prudent to assume that these
studies could beapplied to all of them. Consequently, a broad
defini-tion of the term "natural analog studies" would
beappropriate.
The second occurrence of the term "natural analogstudies" is in
§60.101, where it states that naturalanalog studies are used to
support predictive modelsto demonstrate compliance with objectives
and crite-ria. The term "objectives and criteria" in the
samesentence that contains "natural analog studies" canrefer to
either the phrases "performance objectivesand site and design
criteria" or "objectives and crite-ria for repository performance"
in preceding sen-tences of §60.101. Thus, natural analog studies
are,at least, to be used to support performance assessmentand
possibly to support analyses to demonstrate com-pliance with site
criteria.
Finally, both occurrences of the term "natural ana-log studies"
in the Rule relate it to describing thefuture, for example, "models
that will be used topredict future conditions" in §60.21 and
"predictivemodels" in §60.101. All information in a
licenseapplication, whether representing conditions andprocesses
from the past or the present, will be used topredict conditions and
processes in the future thatdescribe the ability of the site to
isolate radioactivewaste. Otherwise, the information would be
unneces-sary.
4.3 DISCUSSION
In the past the term "natural analog" has often beenconsidered
synonymous with a site with similar geo-chemistry. This is apparent
in the DOE Site Charac-terization Plan (1988), where the term is
found onlyin the chapter on geochemistry. There, the examplesof
natural analogs provided are warm and hot springsand uranium and
thorium ore deposits. "The study of
16
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I
Natural Analog Studies: Licensing Perspective
warm and hot springs in tuffaceous rocks providesinformation
about several important aspects of a re-pository environment in
tuffaceous rock including (1)the effect of the thermal pulse on the
chemistry ofgroundwater; (2) the effect of heated groundwater onthe
host rock including dissolution and precipitationreactions; (3) the
transport of certain elements (e.g.,strontium, cesium, uranium,
thorium, etc.) found inradioactive waste in a hydrothermal
environment;and (4) hydrothermal fluid flow in fractured tuff'(DOE,
1988). "Uranium and thorium ore deposits area source of data on the
following: (1) the long-termstability of radioactive solids; (2)
the long-term re-lease of radionuclides from these solids; (3) the
trans-port of radionuclides under various pH, Eh,temperature, and
pressure conditions, ground-waterand host rock compositions, and
hydrologic regimes;and (4) the long-term effects of radiolysis"
(DOE,1988).
These examples of natural analogs are consistentwith those
recommended in the NARG report. How-ever, they do not represent the
full range of issues towhich natural analog studies can apply as
indicatedby the Rule.
Consistent with 10 CFR Part 60, a natural analogcan be defined
as a condition, process, or event, or acombination of these, that
is similar to the same inanother environment and/or another time.
The use ofthe term "natural analog" in 10 CFR Part 60 can betaken
to mean that the other environment is theplanned site of a
high-level nuclear waste (HLW)repository, including the regions
around this reposi-tory site that may affect its performance or may
havehad an effect on its current or past characteristics.Likewise,
the other time is either the future whenpredicting the performance
of the repository or sitecharacteristics or the past or present
when describingthe site characteristics.
The term "condition" in the definition is meant tobe very
nonspecific. It can refer to a physical condi-tion, like
temperature or pressure, or a chemical con-dition, like a phase
assemblage or composition, or astructural, temporal, or spatial
condition, or condi-tions not yet considered or known. The term
"processand event" likewise is meant to encompass a widerange of
possibilities. For example, it could includedissolution,
precipitation, erosion, groundwater flow,diffusion, faulting,
volcanism, flocculation, or respi-ration.
The scope of natural analog studies is not specifiedby the Rule.
Thus, it is conceivable that these studiescould range from
full-blown international efforts atone extreme to simple literature
searches at the other.Given the broad definition of natural
analogs, numer-ous examples of the use of natural analog studies
existin the DOE Site Characterization Plan. These rangefrom
development of the appropriate use of potas-sium-argon methods of
age determination of volcanicrocks to the methods of measuring
stream flow in anarid environment. In fact, prior to the collection
ofsite-specific data, much of the information from natu-ral
examples used to develop the Site Charac-terization Plan can be
considered to have beenderived from natural analog studies.
As another example of the use of natural analogstudies, if the
elicitation of expert judgment is usedto provide information in the
license application,natural analog studies can support this
information.The elicitation of expert judgment is considered to bea
formal process where, when site-specific data arelacking, experts
are called upon to provide their bestestimates of the value of
certain parameters at the site(Bonano et al., 1990). However, it is
questionable ifthere is such a thing as an expert when there are
nodata. What, then, makes a person an expert? Theanswer must be an
expert is one who has collected,studied, and analyzed data from
analogous environ-ments, under analogous conditions, and/or on
analo-gous processes and events. This information, if fromnature,
comes from natural analog studies. It is ex-pected that in the
license application, in support of theexpert judgment, information
from natural analogstudies would be provided.
A good example of the use of natural analogs tohelp describe a
site condition is the characterizationefforts of the calcite and
opaline silica vein deposits.Vaniman et al. (1988) conducted a
preliminary com-parison of mineral deposits in faults near
YuccaMountain, Nevada, with possible analogs includinghydrothermal,
warm-spring, cold-spring, playa, andsoil deposits. Since then,
investigators from the U.S.Geological Survey (USGS) and Los Alamos
NationalLaboratory (LANL) have continued the analog workas part of
the study plan for Characterization of YuccaMountain Quaternary
Regional Hydrology. The pres-ence of these deposits could indicate
saturated condi-tions in the past as caused by an elevated water
tableor near-surface pedogenic conditions. The determi-nation of
the origin and age of these deposits could
17
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Natural Analog Studies: Licensing Perspective
provide information suggesting the likelihood of hy-drologic
conditions of the site in the future.
To provide further support to the notion that natu-ral analog
studies address site characterization activi-ties, one need only
consider the work done on theAlligator Rivers Analog Project
(ARAP).
The project has been active for several years andhas involved
numerous scientists from five countries.The final report of ARAP
consists of 17 volumesdescribing the various studies of the
Koongarra orebody in northern Australia. The bulk of the
materialdescribes the analog, its site characteristics. On theother
hand, only two pages are devoted to calculatingthe quantity of
uranium that has left the system sincethe deposit was formed. This
is performance assess-ment information. Another example of the use
ofnatural analogs is applied to determining the prob-abilities of
volcanism at the Yucca Mountain site.Here basaltic volcanic fields
that are most analogousto the Crater Flat volcanic field will be
studied todetermine their evolutionary cycles. This informationwill
then be used to estimate probabilities of mag-matic disruption of
the repository.
The draft Regulatory Guide DG-3003 (NRC,1990), Format and
Content Guide for the LicenseApplication for the High-Level Waste
Repository,can be used as an indication of the range of topics
towhich natural analog studies may be applied. In thatdocument, the
term "natural analog studies" is usednumerous times in supporting
predictive models ap-plied to topics as diverse as geology,
hydrology,geochemistry, climatology and meteorology, shaftsand
ramps, underground facility, and waste form andpackages.
Studies of natural analogs require that two sys-tems/sites have
to be adequately characterized toshow analogous behavior or
conditions. Conse-quently, the initial identification and selection
ofnatural analogs is often rudimentary. For example,uranium ore
deposits have been assumed to mimicrepository chemistry; the
unsaturated zone intrudedby magma has been assumed to mimic the
heatedhydrologic system of a HLW repository in the unsatu-rated
zone; redox fronts mimic corrosion product-radionuclide
interactions. In order to find out howgood the analogs are, one
must carry out a charac-terization program comparable to that of
the site.Only as the characterization progresses can the selec-tion
of analogs become more refined and specific.
Furthermore, many of the techniques and modelsused to
characterize a HLW repository are state-of-the-art. For example,
prior to selection of YuccaMountain as a possible site for a HLW
repository,there had been little interest in understanding the
flowof water in unsaturated fractured rock. Most hydrolo-gists had
been trained and focused their energies onsystems where the
presence of water was important,such as the saturated zone and
soils. Now hydrolo-gists are rushing to characterize a system where
theabsence of water is important. This is an area of
activeresearch, so techniques and models used to describethis
system have yet to be applied to many analogoussystems.
Consequently, literature surveys of analogsof flow in unsaturated
rock along with analogs ofother aspects of a HLW repository would
tend to bespotty or incomplete.
4.4 CONCLUSIONSThe Rule requires that an "appropriate
combina-
tion of methods," one of which is natural analogstudies, be used
to support predictive models of per-formance assessment and site
characterization activi-ties. These studies supply information
concerningconditions, processes, and events, both anticipatedand
unanticipated, at the site. Finally, it should berecognized that
"natural analog studies" can be ap-plied to aspects of the
repository site characterizationand performance assessment that are
not necessarilygeochemical in nature.
4.5 BIBLIOGRAPHY
Bonano, E.J., S.C. Hora, RL. Keeney, and D. vonWinterfeldt,
1990, "Elicitation and Use of ExpertJudgment in Performance
Assessment for High-Level Radioactive Waste
Repositories,"NUREG/CR-541 1, U.S. Nuclear RegulatoryCommission,
Washington, D. C.
Chapman, N.A., 1992, Natural Analogues: The Stateof Play in
1992, Proceedings of the Third Interna-tional Conference on
High-Level RadioactiveWaste Management. American Nuclear
Society,LaGrange Park, IL, p. 1695-1700.
Gertz, C.P., 1992, Letter to T.H. Isaacs entitled FinalReport of
the Natural Analogue Review Group(NARG).
U.S. Department of Energy, 1988, "Site Charac-terization Plan:
Yucca Mountain Site, Nevada Re-search and Development Area,
Nevada,"DOE/RW-0199.
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I
Natural Analog Studies: Licensing Perspective
U.S. Nuclear Regulatory Commission, 1983, StaffAnalysis of
Public Comments on Proposed Rule10 CFR Part 60 "Disposal of
High-Level Radioac-tive Wastes in Geologic Repositories,"
NUREG-0804.
U.S. Nuclear Regulatory Commission, 1990, DraftRegulatory Guide
DG-3003 (1990), Format andContent Guide for the License Application
for theHigh-Level Waste Repository.
Vaniman, D.T., D.L. Bish, and S. Chipera, 1988, "APreliminary
Comparison of Mineral Deposits inFaults near Yucca Mountain,
Nevada, with Possi-ble Analogs," LA-i 1289-MS, UC-70, Los
AlamosNational Laboratories, Los Alamos, New Mexico.
4.6 APPENDD(: SECTIONS OF 10 CFRPART 60 REFERRING TO NATURAL
ANALOG STUDIESIn §60.21 (a) Content of Application, the Rule
states that "An application shall consist of generalinformation
and a Safety Analysis Report."
Further, the Rule describes the information con-tained in a
Safety Analysis Report in §60.21(c) whereit states, "The Safety
Analysis Report shall include:(1) A description and assessment of
the site at whichthe proposed geologic repository operations area
is tobe located with appropriate attention to those featuresof the
site that might affect geologic repository op-erations area design
and performance."§60.21(c)(1)(ii) states that "the assessment shall
con-tain:
(A) An analysis of the geology, geophysics, hydro-geology,
geochemistry, climatology, and meteorol-ogy of the site,
(B) Analyses to determine the degree to whicheach of the
favorable and potentially adverse condi-tions, if present, has been
characterized, and the ex-tent to which it contributes to or
detracts fromisolation. For the purpose of determining the
presenceof the potentially adverse conditions, investigationsshall
extend from the surface to a depth sufficient todetermine critical
pathways for radionuclide migra-tion from the underground facility
to the accessibleenviromnent. Potentially adverse conditions shall
beinvestigated outside of the controlled area if theyaffect
isolation within the controlled area.
(C) An evaluation of the performance of the pro-posed geologic
repository for the period after perma-nent closure, assuming
anticipated processes andevents, giving the rates and quantities of
releases of
radionuclides to the accessible environment as afunction of
time; and a similar evaluation whichassumes the occurrence of
unanticipated processesand events.
(D) The effectiveness of engineered and naturalbarriers,
including barriers that may not be them-selves a part of the
geologic repository operationsarea, against the release of
radioactive material to theenvironment. The analysis shall also
include a com-parative evaluation of alternatives to the major
designfeatures that are important to waste isolation,
withparticular attention to the alternatives that would pro-vide
longer radionuclide containment and isolation.
(E) An analysis of the performance of the majordesign
structures, systems, and components, bothsurface and subsurface, to
identify those that areimportant to safety. For the purposes of
this analysis,it shall be assumed that operations at the
geologicrepository operations area will be carried out at
themaximum capacity and rate of radioactive wastestated in the
application.
(F) An explanation of measures used to support themodels used to
perform the assessments required inparagraphs (A) through (D).
Analyses and modelsthat will be used to predictfuture conditions
andchanges in the geologic setting shall be supported byusing an
appropriate combination of such methods asfield tests, in situ
tests, laboratory tests which arerepresentative of field
conditions, monitoring data,and natural analog studies."
In Subpart E-Technical Criteria, §60.101, Pur-pose and Nature of
Findings, requires "... a findingthat the issuance of a license
will not constitute anunreasonable risk to the health and safety of
thepublic. The purpose of this subpart is to set outperformance
objectives and site and design criteriawhich, if satisfied, will
support such a finding of nounreasonable risk."
Finally, §60.101(a)(2) reads, "While these per-formance
objectives and criteria are generally statedin unqualified terms,
it is not expected that completeassurance that they will be met can
be presented. Areasonable assurance, on the basis of the record
be-fore the Commission, that the objectives and criteriawill be met
is the general standard that is required.For §60.112, and other
portions of this subpart thatimpose objectives and criteria for
repository perform-ance over long times into the future, there will
inevi-tably be greater uncertainties. Proof of the
futureperformance of engineered barrier systems and the
19
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Natural Analog Studies: Licensing Perspective
geologic setting over time periods of many hundredsor many
thousands of years is not to be had in theordinary sense of the
word. For such long-term ob-jectives and criteria, what is required
is reasonableassurance, making allowance for the time
period,hazards, and uncertainties involved, that the outcome
will be in conformance with those objectives andcriteria.
Demonstration of compliance with such ob-jectives and criteria will
involve the use of data fromaccelerated tests and predictive models
that are sup-ported by such measures as field and laboratory
tests,monitoring data and natural analog studies."
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I
ROLE OF NATURAL ANALOGS IN PERFORMANCEASSESSMENT OF NUCLEAR
WASTE REPOSITORIES
5
Budhi Sagar and Gordon W. WittmeyerCenterfor Nuclear Waste
Regulatory AnalysesSan Antonio, Texas 78228
5.1 ABSTRACT
Mathematical models of the flow of water andtransport of
radionuclides in porous media will beused to assess the ability of
deep geologic repositoriesto safely contain nuclear waste. These
models must,in some sense, be validated to ensure that they
ade-quately describe the physical processes occurringwithin the
repository and its geologic setting. Inas-much as the spatial and
temporal scales over whichthese models must be applied in
performance assess-ment are very large, validation of these
modelsagainst laboratory and small-scale field experimentsmay be
considered inadequate. Natural analogs mayprovide validation data
that are representative ofphysico-chemical processes that occur
over spatialand temporal scales as large or larger than
thoserelevant to repository design. The authors discuss themanner
in which natural analog data may be used toincrease confidence in
performance assessment mod-els and conclude that, while these data
may be suit-able for testing the basic laws governing flow
andtransport, there is insufficient control of boundaryand initial
conditions and forcing functions to permitquantitative validation
of complex, spatially distrib-uted flow and transport models. The
authors alsoexpress their opinion that, for collecting adequatedata
from natural analogs, resources will have to bedevoted to them that
are much larger than are devotedto them at present.
5.2 INTRODUCTION
The unusually large spatial and temporal scalesassociated with
high-level nuclear waste (HLW) geo-logic repositories present a
major challenge to radi-ologic safety assessment. While the basic
scientificlaws applicable to geologic waste disposal are thesame as
for other engineering projects, there are twomain features that
reduce the level of confidence withwhich the future performance of
the repositories canbe determined. First, uncertainties in site and
designdata tend to grow larger with increasing spatial and
temporal scales. Second, the conditions under whichthe
repository is expected to perform long into thefuture are hard to
define and are, to some extent,speculative in nature. The first
uncertainty resultsprimarily form the fact that, with the current
technol-ogy requiring drilling or excavation, it is difficult
tofully characterize a heterogeneous site without seri-ously
impacting its waste isolation capability. Thesecond factor arises
from the difficulty of predictingnatural events and processes far
into the future, a taskthat becomes more daunting when the effects
ofhuman actions must be considered.
Assuming that natural analogs are selected basedon desirable
attributes, which include spatial andtemporal scales similar to
those of repositories [seePearcy and Murphy (1991) for further
discussion ofthis aspect], they present unique opportunities
forstudying phenomena important to repository per-formance at those
scales. A sampling of literature onnatural analogs [e.g., see
Commission of EuropeanCommunities report EUR 13014 EN (Alexander
andMcKinley, 1991), and the literature reviews byPearcy and Murphy
(1991)] suggests that study ofnatural analogs may be used to:
(i) identify processes that operate at large scales;(ii)
determine how processes are coupled so that
conceptual models can be developed;(iii) estimate rates at which
various processes op-
erate so that appropriate constitutive equa-tions can be
formulated;
(iv) validate performance assessment models; and(v) obtain
qualitative corroboration of repository
safety.
In some of the natural analogs literature, the termsvalidation
and verification are applied interchange-ably to models. For this
paper, a model is defined asan abstract concept representing the
complex physio-chemical processes-the abstraction being specificfor
the purpose for which the model will be used. Inpractice, the
concept will be described by an algo-rithm, for example,
complemented through computer
21
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Role of Natural Analogs in Performance Assessment
code or software. These computer codes are verifiedto assure
that the implementation of the underlyingmodel is correct. However,
the model itself is vali-dated to assure that the conceptual
abstraction of theprocesses is acceptable. Validation of
performanceassessment models with data from natural analogs isthe
most often cited reason for undertaking analogstudies. It is this
aspect of natural analog studies thatwill be examined in this
paper.
5.3 BRIEF OVERVIEW OFPERFORMANCE ASSESSMENT
We shall restrict the definition of performanceassessment for
the purpose of this paper to quantita-tive estimates of measures of
future repository per-formance. The performance measures is
usuallydefined in regulations that vary from country to coun-try.
Annual risk is probably the most common meas-ure of performance,
human dose is another. In the
United States, there are quantitative performancemeasures for
both the total system and the subsys-tems. This is shown in the
bottom line of the chart inFigure 5-1, where five regulatory
requirements appli-cable to HLW repositories are identified. The
threeleft-hand boxes identify the generally applicable
en-vironmental standards promulgated by the U.S. En-vironmental
Protection Agency (EPA) (EPA, 1985).The EPA standards apply to the
entire repositorysystem. In contrast, the remaining two boxes
identifythe rules developed by the U.S. Nuclear
RegulatoryCommission (NRC) (NRC, 1983), and these apply
toparticular subsystem (e.g., Groundwater Travel Timefor the Site
subsystem and the Package Life and theRelease Rate Rules for the
Engineered Barriers sub-system). It may be noted that the EPA
standard willeventually be integrated into the NRC rule for
thepurpose of its implementation.
+ .i1. DISPOSAL SYSTEM
AND REGIONALCHARACTERIZATION
MODELCONCEPTUAL-
IZATION
2. SCENARIODEVELOPMENT
3. CONSEQUENCEANALYSIS
4. SENSITIVITYUNCERTAINTYANALYSIS
S. REGULATORYCOMPLIANCEASSESSMENT
Figure 5-1. Performance requirements and steps for analysis
22
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I
Role of Natural Analogs in Performance Assessment
Figure 5-1 also summarizes various analyses thatare thought to
be required for assessing performanceusing the current methodology.
It may be seen fromFigure 5-1 that assessing performance requires
mod-eling of geologic, hydrologic, geochemical, thermal,and
mechanical processes. It is known that theseprocesses are coupled
in a complex way, although,with the current state-of-the-art, all
of the couplingscannot be fully described. The representation of
thesecouplings in the performance assessment models iscommonly
simplified to accomplish the calculations.One of the difficult
strategic decisions in performanceassessment is to balance the
complexity of modelsagainst available data on site and engineered
barriers.It is very tempting for the modeler to use an
overlycomplex model even if data to support such a modeldo not
exist. It is equally tempting for the data collec-tor to
over-sample one facet of the site or to empha-size one aspect of
the design and neglect others. Somebasic considerations in
selecting models for perform-ance assessment and their
implementation as com-puter codes are:
(i) Mechanistic (in contrast to empirical) repre-sentation of
processes is preferable. Mecha-nistic representation requires
explicit use ofthe basic principles of conservation of
mass,momentum, and energy at an appropriatescale. Constitutive laws
and state equationsused in the formulation of mechanistic mod-els
are normally derived from observations.
(ii) Models whose complexity is compatible withthe complexity of
site and design data arepreferable.
(iii) Flexible implementations (e.g., numerical so-lutions) are
preferable. Flexibility is essentialto analyze system behavior
under the widelyvarying conditions that may occur in the
fu-ture.
(iv) Fast and efficient implementation is preferredso that
sensitivity and uncertainty analysescan be performed.
Another strategic decision to be made in perform-ance assessment
modeling is whether to model "real-istically" or "conservatively."
Only a degree ofrealism or conservatism is implied here, since
theseterms cannot be defined in an absolute sense. Inclu-sion of
greater detail regarding space-time depend-ence of processes leads
to a higher degree of realismin the conceptual model. A higher
degree of realismis preferable from a scientific viewpoint, while
a
pragmatic view may tolerate a higher degree of con-servatism. In
any case, knowledge of the "degree ofconservatism" may be important
for regulatory deci-sion making. This is certainly the case in the
UnitedStates, where the regulations acknowledge that
strictmathematical proof of the future performance of therepository
is impossible and, hence, require "reason-able assurance" or
"reasonable expectation" in meet-ing the desired safety goals.
Currently, it is commonto conduct both detailed realistic
simulations for cer-tain individual processes or a critical part of
a systemand simplified conservative simulations for the
totalsystem.
We assume that the conceptual abstractions con-stituting a model
will eventually be translated intomathematical models. Generally
accepted mathe-matical forms suitable for simulating physical
sys-tems are parametric in nature. The uncertainties inthese model
parameters, and also in the form ofrelationships representing
constitutive and stateequations, can collectively be called
"technical un-certainties." Usually, these technical uncertainties
arerequired to be explicitly represented in performanceassessment
models. For example, the risk measure ofperformance incorporates in
it not only the conse-quence, but also the probability of the
causativeevent The United States HLW regulations (EPA,1985)
incorporate probabilities explicitly; that is,they specify not only
a level of performance, but alsothe probability level at which it
must be met. Thisdictates that the performance assessment models
beprobabilistic in nature.
The fact that performance assessment models canvary in their
degree of realism (or conservatism) andcan be either deterministic
or probabilistic can havea large impact on how model validation is
to bedefined and demonstrated.
5.4 BRIEF OVERVIEW OF MODELVALIDATION
The concept of validation is generally definedfrom the view of
realistic, deterministic models. Inthis context, model validation
requires corroborationthat, under site specific conditions, the
abstractedmodel represents "reality" and, therefore, the
modelestimates of the (unverifiable) future state of thesystem are
acceptable. Since the system states can beobserved only in the
present, there are no experimen-tal means to determine its future
states. Therefore, no
23
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Role of Natural Analogs in Perfornance Assessnent
means to compare model predictions to actual systemstates are
available.
Therefore, for practical purposes, model valida-tion is sought
by comparing model results to experi-ments conducted by design
(laboratory or fieldexperiments) or by nature (natural analogs in
the caseof the HLW repositories). Natural analogs are sys-tems
whose behavior, at least in certain well-definedaspects, is
analogous to the system under investiga-tion. In addition, the
analogous system has evolvedso that many of its states have been
observed. If amodel can be validated against the analog, then
thismodel may be assumed to apply to the system ofinterest.
We note that, based on Popper's (1959) philoso-phy, the very
idea that a theoretical model can bevalidated by any one experiment
on any spatial ortemporal scale has been criticized on logical
grounds.In the Popperian view, experiments may only refute(rather
than validate) models. Thus, simply becausemodel and experimental
results compare does notconstitute a proof of model validity. Only
when noexperiment can be found to refute a model may it bedeclared
validated. Therefore, model validation isimpossible in the strict
sense. These and other