Moving Beyond the

Yucca Mountain

Viability Assessment

U.S. Nuclear Waste Technical Review Board

April 1999

A Report to the U.S. Congress

and the Secretary of Energy
Nuclear Waste Technical Review Board

Dr. Jared L. Cohon, Chairman

Carnegie Mellon University

Pittsburgh, Pennsylvania

Mr. John W. Arendt

John W. Arendt Associates, Inc.

Oak Ridge, Tennessee

Dr. Daniel B. Bullen

Iowa State University

Ames, Iowa

Dr. Norman L. Christensen, Jr.

Duke University

Durham, North Carolina

Dr. Paul P. Craig

University of California, Davis, Emeritus

Davis, California

Dr. Debra S. Knopman

Progressive Policy Institute

Washington, D.C.

Dr. Priscilla P. Nelson

National Science Foundation

Arlington, Virginia

Dr. Richard R. Parizek

The Pennsylvania State University

State College, Pennsylvania

Dr. Donald D. Runnells

Shepherd Miller, Inc.

University of Colorado, Emeritus

Fort Collins, Colorado

Dr. Alberto A. Sagüés

University of South Florida

Tampa, Florida

Dr. Jeffrey J. Wong

California Environmental Protection Agency

Sacramento, California
Introduction

The Nuclear Waste Policy Act, as amended (U.S. Congress, 1987), designated

the Yucca Mountain site in Nevada as the sole location to be studied for possible development as a repository for the permanent disposal of spent nuclear

fuel and high-level radioactive waste. The act also established the U.S. Nuclear

Waste Technical Review Board and charged the Board with evaluating the

technical and scientific validity of activities undertaken by the Secretary of

Energy, including characterization of the Yucca Mountain site.

The U.S. Department of Energy (DOE) recently published Viability Assessment of a Repository at Yucca Mountain (DOE 1998). The purposes

of the viability assessment (VA) are to summarize the scientific information that has been collected at the site over the last 15 years, present a

conceptual design for the repository and its waste packages, estimate how

well such a repository would isolate wastes from the human environment, identify the additional studies (and their costs) needed to evaluate

the suitability of the site and prepare a license application, and estimate

the overall cost of waste disposal at the site. The VA is an evaluation of

progress on site characterization at Yucca Mountain and provides the

technical basis for deciding whether to continue studying the site.

The VA is not, and was not intended to be, a determination of whether

the Yucca Mountain site is suitable for development as a permanent geologic repository. Additional site studies, repository design work, and

analyses of repository system performance will be completed before the

DOE makes a suitability determination, currently planned for 2001. So

far, neither the Board’s review of the VA nor its other reviews of the program have identified any features or processes that would automatically

disqualify the site. However, the Board believes that the DOE should

give serious consideration to alternatives to the VA reference design, including changing from a high-temperature design to a ventilated

low-temperature design (e.g., below the local boiling point of water).

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Moving Beyond the Yucca Mountain Viability Assessment

Moving Beyond the Yucca Mountain

Viability Assessment
General Views on the VA

The VA is the most significant milestone thus far in the characterization

and evaluation of the Yucca Mountain site. Many parts of the VA present

cutting-edge scientific analyses in a comprehensible format. The Board

commends the DOE for the successful completion of this assessment. In

assembling the VA, the DOE integrated large amounts of data and analyses, established a preliminary repository design, and set priorities for

work to be completed before decisions are made about site recommendation and licensing. In this report, the Board presents its general views on

the site and on the design of a repository for the site, based on its review

of the VA.

The process of integration has had the salutary effect of focusing the objectives of the scientific investigations. In particular, the VA highlighted

the close connections between the repository design and the priority list

of key uncertainties about the natural system. For example, such site

characteristics as the movement of water in liquid and vapor forms at

temperatures above boiling and the effects of high temperatures on rock

stability are important only because of the VA’s high-temperature repository design. In a low-temperature design, uncertainties about these phenomena would be less significant and might not need resolving before

making a suitability determination.

The Board concurs with the DOE that the VA is simply a “snapshot” of

current knowledge about the Yucca Mountain site that the Congress can

use to make an informed decision on whether to continue funding studies

of the site. The Board concludes that Yucca Mountain continues to merit

study as the candidate site for a permanent geologic repository and that

work should proceed to support a decision on whether to recommend the

site to the President for repository development. The 2001 date anticipated for this decision is very ambitious, and much work remains to be

completed. At a minimum, significant progress in the work identified by

the Board in its November 1998 report (NWTRB 1998) and by the DOE

in volume 4 of the VA will be required to support a technically defensible decision. The Board supports continuing focused studies of both

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U.S. Nuclear Waste Technical Review Board

... Yucca Mountain

continues to merit

study as the candidate

site for a permanent

geologic repository ...
natural and engineered barriers at Yucca Mountain to attain a defense-in-depth repository design and to increase confidence in predictions of repository performance.

Uncertainties in Repository Performance

In its November 1998 report, the Board outlined its views on future research needed to address uncertainties about the performance of the

repository system, including both the engineered and the natural barriers.

The Board concluded in that report that although there are economic and

technical limits to reducing uncertainties about the performance of the

proposed repository system, some key uncertainties can be reduced further over the next few years through a focused research effort. The Board

realizes that there will be uncertainty about the performance of a repository far into the future and that eliminating all uncertainty is not possible

or necessary. However, the Board believes that identifying important

sources of uncertainty, estimating the magnitude of those uncertainties, reducing critical uncertainties, and evaluating the effects of residual uncertainties on expected repository performance are essential for supporting a

technically defensible site-suitability determination and license application.

The Board notes that the VA relies heavily in some cases on formal elicitation of expert opinion. This was necessary and extremely useful, given

the lack of field and laboratory data in certain areas and the equivocal nature of some of the data in other areas. However, as the experts themselves

pointed out, expert opinion should not be used as a substitute for data that

can be obtained directly from site, laboratory, and other investigations.

After reviewing the VA, the Board concludes that a significant amount of

additional scientific and engineering work will be needed to increase

confidence in a site-suitability determination and a license application.

The DOE should evaluate alternative repository designs that have the potential to reduce uncertainties in projected repository performance,

thereby reducing the scope of additional necessary scientific study.

Regardless of the design adopted, long-term scientific studies will be

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Moving Beyond the Yucca Mountain Viability Assessment

... additional scientific

and engineering work

will be needed to

increase confidence ...
needed to establish a solid foundation for projecting repository performance thousands of years into the future. The Board’s views are discussed in more detail below.

Additional Scientific and Engineering Work

The DOE has spent many years studying the Yucca Mountain site and

designing the engineered components of a repository system compatible

with the site. These efforts have produced a large amount of information,

but significant uncertainties remain about the ability of the VA reference

design to safely isolate radioactive waste. In part, this is a problem inherent in extrapolating repository performance for thousands of years from

data acquired over a much shorter period (years to decades). Uncertainties also are associated with specific characteristics of the Yucca

Mountain site, especially the nature of water movement through the fractured unsaturated rocks of the mountain and the possible entry of water

into repository tunnels and its contact with waste packages. Many of

these uncertainties likely would be exacerbated by the high temperatures

of the reference repository design, which may reduce tunnel stability, increase waste package corrosion, and perturb geochemical reactions and

water movement in ways that are difficult to predict.

Predicting the performance of the waste packages, which play a crucial

role in the performance of the VA reference repository design, is a critical area that needs more study. Candidate waste package materials rely

on the presence of a thin passive layer to protect the underlying metal

from the oxidizing environment that will be present in a Yucca Mountain

repository. Improving the basic understanding of long-term passivity is

essential because, at present, there seem to be no documented natural or

man-made analogs that can be used to demonstrate whether this mode of

protection would persist over the desired period of time. Research also

should be continued on the susceptibility of the passive layer to known

modes of corrosion, especially potentially catastrophic failure modes,

such as stress-corrosion cracking.

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U.S. Nuclear Waste Technical Review Board
The Board believes that the scientific and engineering work completed to

date should be supplemented to improve the technical foundation for

evaluating the suitability of the site or preparing a license application.

The Board agrees with a DOE-commissioned peer review panel

(Whipple et al. 1999) that two types of additional data are needed to improve the credibility of the total system performance assessment part of

the VA (TSPA-VA): (1) fundamental data that are essential to the development and implementation of the models and (2) data sets designed to

challenge conceptual models and test the coupled models used in the

TSPA-VA. The substantial uncertainties about the performance of a repository that is based on the VA reference design can be resolved only by

considering alternative repository and waste package designs and by collecting additional scientific data.

In volume 4 of the VA, the DOE has identified and set priorities for a

suite of additional studies to produce information that would be needed

for repository licensing, if the site is determined suitable for development

as a repository. The planned studies include data collection, analysis, and

engineering design, as appropriate, for the three major barriers discussed

by the Board in its November 1998 report (unsaturated zone, engineered

barrier system, and saturated zone). Among the most important are ongoing and proposed geologic, geochemical, and hydrologic studies, including those planned for the east-west cross drift. These studies are aimed at

understanding the magnitude and distribution of seepage into the repository under conditions similar to those of today and under conditions like

those that existed in the past when the climate was very different. They

include systematic analysis of the rock samples being collected, especially for chlorine-36 and other indicator isotopes; flow and seepage tests

at different locations along the drift; moisture-monitoring activities; tests

in the lower lithophysal zones that would host the majority of waste

packages; and studies of the Solitario Canyon fault, the active fault

bounding the repository on the west. Of equal importance are studies for

supporting projections of the performance of the engineered barrier system,

which, in the VA reference design, plays a critical role in isolating radioactive wastes for tens of thousands of years.

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Moving Beyond the Yucca Mountain Viability Assessment
The studies identified by the DOE in volume 4 of the VA appear to be

appropriate in the sense that they are technically feasible and are likely to

produce useful information that will improve the understanding of

long-term repository performance. Of course, there is no guarantee that

completion of these studies will lead to successful development of a repository at the site. The studies could show the site to be unsuitable, or

they could raise new questions about potential repository performance.

On the basis of current information, however, the Board is pleased that

volume 4 identifies an appropriate suite of studies to be pursued in the

years ahead.

The Board is concerned that some of the planned studies identified in volume 4 of the VA may be deferred because funds are not available to carry

them out in a timely manner. Deferring scientific and engineering studies

will delay the assembly of a more credible technical basis to support the

site suitability determination anticipated in 2001 and, if the site is found

suitable, a license application in 2002. For the current VA repository design, a credible basis does not yet exist.

Alternative Repository Designs

High temperatures in the VA repository design cause large uncertainties

about how the site would behave both before and after repository closure.

The Board believes that repository designs with lower waste package surface temperatures merit further detailed analyses. Such designs have the

potential to reduce uncertainty, simplify the analytical bases required for

site recommendation, and make licensing easier. Combined with improved

waste package shielding, such designs also could simplify preclosure performance confirmation by enhancing access to the tunnels, thus reducing or

eliminating the need for separate performance-confirmation drifts, and by

permitting direct access to performance-confirmation instrumentation near

the waste packages.

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U.S. Nuclear Waste Technical Review Board

... designs with lower

waste package surface

temperatures ... have

the potential to reduce

uncertainty ... and

make licensing easier.
The following factors influenced the Board’s thinking on repository design.

• Lower temperatures would significantly reduce the uncertainty associated with coupled thermal-hydrologic and thermal-geochemical processes. Maintaining near-field temperatures below the boiling point of

water after repository closure, by ventilation or through aging, could

reduce uncertainties about the movement of water and associated geochemical processes in the repository’s natural barriers. This could increase confidence in the analyses of repository performance required

for a site-suitability determination.

• For a given environment, the chances of degradation of corrosionresistant waste package materials would be reduced significantly if

peak waste package surface temperatures were reduced.

• High repository temperatures are expected to increase the mechanical

degradation of repository rocks. There is little, if any, relevant experience to draw on for predicting the long-term effects of repository heating and subsequent cooling on drift stability.

The DOE is evaluating alternative repository designs that might be appropriate as the basis for a license application, and the reference repository design presented in the VA is expected to change as the alternatives

are considered. The Board strongly urges that serious consideration be

given to alternatives that keep waste package surface temperatures below

the boiling point of water.

Long-Term Scientific Studies

If Yucca Mountain is found suitable and construction of a repository is authorized, the Board believes that a long-term science program will be

needed to improve understanding and reduce uncertainties about the performance of engineered barriers and the interactions between the repository

and natural processes. An important goal of these studies should be identification of currently unknown long-term failure modes or unexpected

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Moving Beyond the Yucca Mountain Viability Assessment
evolution of natural processes that could adversely affect the performance of the major barriers of the repository. Thus, the studies may be

more extensive than the performance confirmation activities now anticipated for a repository. For example, if the waste package design continues to rely strongly on corrosion-resistant metals protected from

corrosion by a passive layer, long-term scientific studies need to be carried out to improve the basic understanding of the processes that could

affect the passive layer and the susceptibility of the passive layer to

known modes of corrosion, especially potentially catastrophic failure

modes, such as stress-corrosion cracking.

Long-term studies of the natural barriers also will be needed, primarily to

verify projections of water movement within the unsaturated and saturated zones near the repository. For a high-temperature repository design,

fundamental studies of coupled thermal-hydrologic and thermalgeochemical processes will be needed. For a low-temperature design, a

less extensive program of studies of coupled thermal-hydrologic and

thermal-geochemical processes may be adequate because of the much

lower degree of coupling. Whether the long-term scientific studies are a

decade-long program or much longer will depend in part on how the repository design evolves. There is no doubt, however, that a program of

some sort will be needed to increase confidence in estimates of long-term

repository performance.

Postclosure Safety Case

The ultimate goal of the studies at Yucca Mountain is to demonstrate that

a repository at the site can safely isolate wastes from the human environment. As discussed in Chapter 4 of the VA, the DOE proposes to demonstrate safe waste isolation through a five-part postclosure safety case

consisting of the following:

• assessment of expected postclosure performance (i.e., TSPA)

• design margin and defense-in-depth

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U.S. Nuclear Waste Technical Review Board
• consideration of disruptive processes and events

• insights from natural and man-made analogs

• a performance-confirmation plan.

The Board believes that this proposed strategy is an appropriate way to

evaluate a Yucca Mountain repository, although each component, especially defense-in-depth and the performance-confirmation plan, requires

significant additional development. Multiple lines of evidence will provide a more convincing demonstration of repository safety than will any

individual component of the safety case. TSPA, including sensitivity and

uncertainty analyses, is the appropriate core analytical tool of the safety

case. TSPA is the analytical technique that pulls together relevant information about the performance of the repository system, determines which

features or parameters could strongly influence performance, and estimates the uncertainties in projections of performance. TSPA has its limits, however, and the DOE will need to aggressively pursue the other four

components of the safety case.

Judging the realism of the “bottom-line” TSPA estimates of repository performance in the VA is difficult because some of the underlying assumptions may be overly conservative and others may be nonconservative. This

is due, in large part, to a general lack of data that support many critical

assumptions in the mathematical models. Numerous examples of this are

presented in the recent report of the TSPA-VA peer review panel (Whipple

et al. 1999).

The DOE has acknowledged the limits of the TSPA-VA. In fact, a DOE

presentation to the Board (Van Luik 1999) stated that the VA’s performance assessment (TSPA-VA) cannot now be used to do the following:

• Assess compliance with regulatory criteria.

• Show defense-in-depth for the design of the repository system.

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Moving Beyond the Yucca Mountain Viability Assessment
• Assess the importance of small design changes.

• Determine the suitability of the overall repository system.

The DOE intends to improve future versions of TSPA so that these

objectives can be accomplished.

Assessing the realism (or, at least, verifying the conservatism) of TSPA

projections of repository performance is an important goal of the additional studies identified by the Board. The Board doubts, however, that

relying solely on TSPA to demonstrate repository safety will ever be possible. For that reason, the other four components of the postclosure safety

strategy should be developed aggressively as complements to TSPA.

A sixth component of the safety strategy also should be considered:

designing the waste packages and the repository to minimize uncertainties in projected repository performance.

Conclusion

The VA concludes, “... Yucca Mountain remains a promising site for a

geologic repository and ... work should proceed to support a decision in

2001 on whether to recommend the site to the President for development

as a repository” (DOE 1998, Vol. 1). The Board agrees that Yucca Mountain continues to merit study as the candidate site for a permanent geologic

repository and that work should proceed to support a decision on whether

to recommend the site to the President for development. The 2001 date anticipated for this decision is very ambitious, and much work remains to be

completed. At a minimum, progress on the work identified by the Board in

its November 1998 report and by the DOE in volume 4 of the VA will be

required to support a technically defensible decision. The Board supports

continuing focused studies of both natural and engineered barriers at Yucca

Mountain to attain a defense-in-depth repository design and to increase

confidence in predictions of repository performance.

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U.S. Nuclear Waste Technical Review Board
References

Nuclear Waste Technical Review Board (NWTRB). 1998. Report to The

U.S. Congress and The U.S. Secretary of Energy. Washington, D.C.

U.S. Department of Energy (DOE). Office of Civilian Radioactive Waste

Management. 1998. Viability Assessment of a Repository at Yucca

Mountain. Vols. 1-5. DOE/RW-0508. Washington, D.C.

Van Luik, Abraham. 1999. “Overview of Total System Performance Assessment - Viability Assessment.” Presentation to Nuclear Waste

Technical Review Board. January 27, 1999. Las Vegas, Nevada.

Whipple, C.; R. Budnitz; R. Ewing; D. Moeller; J. Payer; and P.

Witherspoon. 1999. Final Report: Total System Performance Assessment Peer Review Panel. February 11, 1999. Las Vegas, Nevada.

Laws, Legislation, and Hearings

U.S. Congress. 1987. Nuclear Waste Policy Amendments Act of 1987

(NWPAA). 100th Congress, 1st Session. Washington, D.C.:

Government Printing Office.

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Moving Beyond the Yucca Mountain Viability Assessment
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U.S. Nuclear Waste Technical Review Board

Additional copies of this report and previous NWTRB reports are

available from the Board’s office:

NWTRB, 2300 Clarendon Boulevard,

Suite 1300, Arlington, VA 22201-3367

All NWTRB reports also are available on the NWTRB web site at:

www.nwtrb.gov.