Nuclear Regulatory Commission Contract NRC-02-88-005 · 2013. 4. 10. · Prepared for Nuclear Regulatory Commission Contract NRC-02-88-005 Prepared by Center for Nuclear Waste Regulatory

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

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

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-


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

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

U.S. NUCLEAR REGULATORY COMMISSION NATURALANALOGUE RESEARCH PROGRAM

3


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


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


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


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


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

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

I


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


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.

18

I


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


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."

20

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

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

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

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

Documents

Nuclear Regulatory Commission Contract NRC-02-88-005 · 2013. 4. 10. · Prepared for Nuclear Regulatory Commission Contract NRC-02-88-005 Prepared by Center for Nuclear Waste Regulatory