United States Nuclear Regulatory Commission

Office of Public Affairs

Washington, DC 20555

Phone 301-415-8200 Fax 301-415-2234

Internet:opa@nrc.gov

No. 96-88 FOR IMMEDIATE RELEASE

(Thursday, June 20, 1996)

NOTE TO EDITORS:

The Nuclear Regulatory Commission has received the attached

report from its Advisory Committee on Nuclear Waste. The report,

in the form of a letter, provides comments on establishing a time

span for compliance of the proposed high-level waste repository

at Yucca Mountain, Nevada, with regulatory standards.

#

Attachment:

As stated
June 7, 1996

The Honorable Shirley Ann Jackson

Chairman

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555-0001

Dear Chairman Jackson:

SUBJECT: TIME SPAN FOR COMPLIANCE OF THE PROPOSED HIGH-LEVEL

WASTE REPOSITORY AT YUCCA MOUNTAIN, NEVADA

The purpose of this letter is to communicate the Advisory

Committee on Nuclear Waste's (ACNW) observations and suggestions

on the general principles for establishing the time span for

compliance of nuclear waste facilities and our recommendations

for specifying the regulatory time frame of compliance for the

proposed geologic high-level waste (HLW) repository site at Yucca

Mountain, Nevada. This letter follows up a letter from the ACNW

dated February 9, 1996, on "Issues and [U.S. Nuclear Regulatory

Commission] NRC Activities Associated with the National Research

Council's Report, Technical Bases for Yucca Mountain Standards. "

The time period for compliance of geologic HLW repositories is

established at 10,000 years in the Environmental Protection

Agency (EPA) standard 40 CFR Part 191 and the NRC regulation 10

CFR Part 60. Elements of the HLW standards and regulations were

scrutinized by a National Research Council/National Academy of

Sciences (NAS) Committee, which was prescribed by the Energy

Policy Act of 1992. The findings of the NAS Committee are

published in the Technical Bases for Yucca Mountain Standards

(National Research Council, 1995). The NAS Committee concluded

that there was no scientific justification or basis for

specifying a truncation of the analyses at 10,000 years or at any

other period of time. Instead, it recommended that the

compliance evaluation be conducted to peak risk within the limits

of the basic geologic stability of the Yucca Mountain region,

which it suggested was on the order of a million years. In

contrast to this recommendation, the ACNW has supported the

10,000-year time frame (e.g., letters to the Chairman of the NRC

of June 27, 1991, and February 9, 1996). Nonetheless, in our

most recent letter on this topic, the ACNW stated that further

deliberations on the subject were appropriate. This letter

reports on the results of our additional study. The ACNW will

report to you in the near future on our recommendations on the

time span for compliance of low-level nuclear waste facilities,

building upon the principles identified and discussed in this
letter. In addition, the ACNW plans to review the reference

biosphere and critical group issues.

Our recommendations are derived from a working group meeting on

"Regulatory Time of Compliance for Radioactive Waste Disposal"

held during the 82nd meeting of the ACNW on March 27, 1996, and

subsequent deliberations by the Committee. Three main topics

were discussed at the working group meeting: (1) background and

regulatory context for the existing HLW standard that specifies

10,000 years as a time frame for regulatory compliance, (2)

insights on time of compliance from performance assessments for

both high- and low-level nuclear waste, and (3)

scientific/technical issues and concerns. During the working

group meeting, presentations were made by personnel from the EPA;

the Division of Waste Management, Office of Nuclear Materials

Safety and Safeguards, NRC; the U.S. Department of Energy; the

National Research Council staff; the Electric Power Research

Institute; the Oak Ridge National Laboratory; as well as by

individuals from private industry and academia. The latter

individuals provided both national and international viewpoints

on the problem of compliance time in regulations.

Background of the Problem

A necessary element of a standard or regulation that ensures the

health and safety of the public is the compliance period -- the

time that the risk of adverse consequences is below a specified

level. This compliance period requires the integrity of the

facility over the stipulated time interval. In the case of an

HLW repository, the assessment of risk involves evaluation of the

repository source term, including inventory and waste form; the

performance of waste containers and engineered barriers; and the

geological, hydrological, and climatological attributes of the

site. If the risk of health effects is to be determined, this

assessment also involves the specification of the biosphere and

the critical population group in proximity to the repository.

In the existing generic standard for geologic HLW repositories,

40 CFR Part 191, EPA established a 10,000-year time of compliance

at a distance of no more than 5 km from the boundary of the

repository -- a time value that also was used in the NRC

regulation. This time period has no scientific or technical

justification but was based on an arbitrary compromise between

conflicting desirable characteristics. Long time periods have

attendant large uncertainties in the behavior of the geosphere

and the biosphere, while short time periods have lower

uncertainties but do not adequately address the time spans of

some of the critical processes that cause release of

radionuclides to the biosphere. This compromise was perhaps a

justifiable approach for comparative evaluation of the multiple

sites being considered when 40 CFR Part 191 was promulgated.

Although not considered a compelling technical basis, this time

period was roughly consistent with the period of glacial cycling
and the potential profound impact of continental glaciation upon

the geosphere and the biosphere. In providing a rationale for

the 10,000-year time limit, the EPA stated, "This is not to say

that times beyond 10,000 years are not important, but the Agency

feels that a disposal system capable of meeting the proposed

Containment Requirements for 10,000 years would continue to

protect people and the environment well beyond 10,000 years."

Although the standards of other nations differ in detail, the

international community largely accepts the 10,000-year time

frame, but also recognizes the need to evaluate site performance

beyond the 10,000-year period, which constitutes a two-part

approach.

In its appraisal of the technical bases for site-specific Yucca

Mountain standards, the NAS Committee rejected the 10,000-year

compliance period although it accepted that a transition to a

glacial climate with its cooler, wetter seasons is probable

during the next 10,000 years. Rather, the NAS Committee decided

that long-lived radioisotopes derived from the repository might

not reach the biosphere for more than 10,000 years, and thus it

is important to evaluate the repository for a longer time

interval. The NAS Committee chose to set this period of time at

the predicted time of peak risk to the population as a result of

leakage from the repository. It viewed this decision as

requiring a period of time possibly extending into hundreds of

thousands of years. In so doing, it did not accept the view

espoused in the EPA and NRC standards and regulations that the

uncertainties in predicting the repository performance at these

periods are so large that the results are of questionable

utility. The basis of the argument is that the subsurface

environment at the repository horizon of Yucca Mountain is

sufficiently stable that repository performance can be assessed

with an acceptable uncertainty over a period of roughly one

million years. The NAS Committee believes that inherent spatial

uncertainties in interpolation of site characteristics, which are

time independent, are a major contributor to assessment

uncertainty.

The dilemma faced in developing the time span of compliance is

that the period of time must be sufficiently long to include the

evaluation of potential processes leading to the loss of the

integrity of the repository and transport of radionuclides to the

biosphere. Yet the time span should not be so long that the

uncertainties in the process and events, and in the biosphere and

critical population group, lead to meaningless results. In the

case of a specific site, sufficient information should be

available so that reasonable assumptions can be made in order

that a defensible solution can be reached regarding the problem

of a regulatory period of compliance. This approach is based on

general principles and knowledge of the engineering and

scientific aspects of the repository and its site.

Considerations in Defining a Time of Regulatory Compliance
After reviewing the basis for establishing a time of regulatory

compliance, the ACNW has concluded that a series of premises and

assumptions are a necessary foundation for the decision making

process. These include general policy decisions that are generic

and a range of scientific and technical considerations that are

largely specific to the site and problem:

ˇ The HLW repository system -- waste, containers, engineered

barriers, and site geology -- must be capable of preventing

leakage of radionuclides to the biosphere for a minimum

period of time measured in several thousands of years.

ˇ Risk evaluation is based on characterization of the

repository site and investigations of the waste and its

container and engineered barriers using performance

assessment (PA). However, in the development of the

regulations, the marked limitations in using PA as a

predictive tool needs to be recognized. PA is primarily an

investigative tool that can be used to distinguish between

positive and negative attributes of the elements of the

repository and, in the best of conditions, the relative

range of risk under various assumed scenarios.

ˇ The standard for a nuclear waste repository should be based

on limiting risk to a critical group without the constraint

of a prescribed time period of compliance. A time period

should be defined in the regulations that implement the

standard and should be prepared in concert with the

characteristics of the waste, engineered barriers, and the

nature and vagaries of the geosphere and the biosphere of a

specific facility and site.

ˇ The reference biosphere and the critical group that are used

in assessing compliance should be defined in the

regulations. These definitions are necessarily based on

site characteristics and on the impact of climate and

predicted climate modifications. They are related to

predictions of the nature of society through time. Because

of the great uncertainties in the latter, the ACNW

recommends that the current societal state be used as the

base scenario in predictions of the future states of

society.

ˇ Uncertainties in assessing future risks associated with the

geologic/geographic setting and the repository design and

related engineered features will increase with time.

Factors that influence this increasing uncertainty include

the following: geologic conditions and events that may

disrupt the repository; climatic changes that could

drastically increase the flux of water through the disposal

system or change the regional hydrologic flow regime;

degradation of the waste containers or repository materials;

and synergistic effects of changing site conditions on the
degradation of repository features. Design features can be

implemented to preclude extreme variations in releases

(e.g., waste forms, containers, and near-field barriers may

be engineered to minimize transport out of the immediate

repository facility and thus minimize uncertainties in

transport for several thousand years).

Regulatory Principles for Establishing the Time Span for

Compliance

On the basis of the preceding considerations, the ACNW recommends

that a two-part approach to definition of the compliance period

be established for nuclear waste facilities. The first part

involves the following three elements:

(1) The time period for compliance should be based on the

estimated time for release and transport of the radionuclide

contaminants to reach the critical group. This time

estimate should be based on geologic, geochemical, and

hydrologic characterization of the site and its environs, as

well as regional study of geologic processes and their

potential effects on the site, and total systems performance

assessment. This estimate must confirm the ability of the

repository system to retain radionuclides for a minimum of

several thousand years. The selection of the time of

compliance must be evaluated along with the specification of

the reference biosphere and critical group.

(2) The reference biosphere and the lifestyles of the critical

group should be defined on the premise that no major changes

will occur in society that will significantly affect their

lifestyles as they relate to risk from the repository and

that the climate can be reasonably bounded. The minimum

distance from the boundary of the repository to the critical

group will be a major decision.

(3) The compliance time should be sufficiently short such that

extrapolations of significant processes and their rates can

be made robustly with reasonably modest uncertainties.

The second part of the compliance period regulations should be

based on assessments extending from the specific compliance

period to the calculated time of the peak risk to the critical

group. There is no definitive measure of compliance in the sense

of a numeric match between a standard and the calculated peak

risk, and this second part should not be allowed to become a de

facto regulation. A comparison between the standard used in the

first part and the calculated peak risk should lead to

identification of important performance factors that define risk

to the critical group. Depending upon the extent to which the

peak risk exceeds the standard, ameliorating actions to reduce

this difference should be initiated, such as increasing the

integrity of the engineered barriers, improving site
characterization to more closely bound uncertainties, or, in the

extreme, abandoning the candidate site.

Scientific and Technical Insights Into the Time Span for

Compliance of the Proposed Yucca Mountain Repository

Critical steps in the regulatory principles for establishing time

of compliance as specified above in element (1) are the

characterization of the proposed repository site and the relevant

processes acting upon it and assessing the total system.

Although site characterization is still in progress at Yucca

Mountain, extensive data have been acquired and information has

been derived from these data. The following scientific and

technical insights that have been gained at the site over the

past decade bear upon the definition of the compliance time in

the forthcoming regulations designed specifically for Yucca

Mountain.

ˇ The current climate in the Yucca Mountain region is arid,

with annual precipitation of roughly 15 cm. In the future,

the climate will change, depending upon the relative

importance of advancing cooler (glaciation) conditions and

possible greenhouse effects that may counteract the cooling

effect. Although the timing and precise amplitude of the

climate change cannot be predicted, the range of conditions

can be bounded in terms of timing and effect.

Paleoclimatological studies in the region of Yucca Mountain

suggest that during the last glacial period (14 to 20

thousand years ago) the precipitation may have been four

times the present and the average annual temperature 10 oC

cooler (Forester and Smith, 1995). Climatic conditions are

anticipated to change, but the region is likely to be at

least semiarid and will lie south of the glaciated area.

Thus, it is unlikely that climate change will have a marked

effect on the reference biosphere or the lifestyle of the

critical group. Infiltration is likely to significantly

increase as a result of the increased precipitation and

cooler temperatures, but the total flux through the

repository will still be limited. The maximum climatic

change is not predictable with our present science, but all

evidence from extrapolations indicates that the principal

effect will occur prior to ca. 20,000 years.

ˇ Results of recent site characterization activities at Yucca

Mountain indicate that matrix, fracture, and fault infiltration are present in the unsaturated zone. Matrix flow

results in long travel times, but fracture and fault flow

that may lead to relatively rapid travel times also occurs.

Ground water travel times within the saturated zone between

Yucca Mountain and the location of the critical group, which

is likely to reside in the Amargosa Valley several tens of

kilometers south of the proposed repository, are poorly

documented at this time. However, the low hydraulic
gradient indicates that travel times are likely to be long.

Further, the sorptive capacities of formations through which

the water will traverse are not presently known and the

degree of dilution of contaminants within the saturated

zone has not been ascertained. In view of the likely long

travel time of water in the saturated zone from the proposed

Yucca Mountain repository to the critical group, the

movement of contaminants may well take in excess of 10,000

years to reach the accessible environment, despite the

potential for relatively short travel time through the

fractures and faults of the unsaturated zone.

ˇ The relative uncertainties in predicting the time dependent

and spatial variations in the Yucca Mountain geosphere and

related geologic processes have come to the forefront as a

result of the NAS Committee’s report and their statements on

the confidence that can be placed on performance assessment

at distant future times. The NAS Committee concluded that

although ". . . the level of confidence for some

predictions might decrease with time . . . [m]any of the

uncertainties in parameters describing the geologic system

are due not to temporal extrapolation, but rather to

difficulties in spatial interpolation of site

characteristics." The ACNW acknowledges that the spatial

variations in the Yucca Mountain geosphere contribute to

uncertainty. Nonetheless, we believe that with the

completion of an adequate characterization of the site and

with consideration of the integration over the

heterogeneities for the operational scale of the pertinent

processes, the time-dependent uncertainties in events and

processes, such as climate change, will be more prominent

than those derived from spatial variations. Yucca Mountain

lies within a region of potentially high gradient tectonic

and climatic processes. As a result, the ACNW anticipates

that uncertainties will increase with time, although we

agree with the National Research Council/NAS report that it

should be possible to bound these uncertainties over a time

span on the order of one million years.

Recommendations for a Yucca Mountain Repository Compliance Period

On the basis of the previous discussion of both generic

principles and Yucca Mountain specific insights, the ACNW

recommends the following two-part approach to establishing the

time period for compliance for the proposed HLW repository site

at Yucca Mountain, Nevada:

The first part involves the following:

(A) The time period of compliance should not be specified in the

risk-based standard for Yucca Mountain being prepared by the

EPA. Rather, it should be defined in the regulations being

developed by the NRC to implement the EPA standard and
should use existing knowledge of the engineering and

scientific aspects of this proposed repository and its

environment.

(B) The time period should be defined in concert with specifying

the reference biosphere and the critical group. The definition of the biosphere and the critical group should take

advantage of known site characteristics and any other

long-term effects that can be technically supported.

(C) The time span for the compliance period should be no shorter

than an estimate of the anticipated time it takes for potential radionuclide contaminants to reach the nearest critical

group and no longer than a time period over which scientific

extrapolations can be convincingly made. Because of the

need to come to closure on this subject, the ACNW suggests

that the NMSS staff review the scientific and technical

components needed to make these decisions, identify critical

missing elements, and provide the necessary information in a

timely manner. On the basis of currently available

information, the ACNW anticipates that the appropriate

compliance period will be somewhat greater than the present

standard of 10,000 years. The increased distance from the

proposed site to the nearest probable location of the

critical group, the nature of the site and the likely

characteristics of the waste, the containers, the engineered

barriers, and the design of the repository, together with

consideration of the stability of the site, suggest a time

frame on the order of a few tens of thousands of years, but

specifying a precise value must await more comprehensive

assessments.

The second part of the compliance regulation should require

assessment extending from the specified compliance period to the

time of the calculated peak risk to the critical group. The

regulation for compliance during this intervening period should

be significantly less stringent than is used in the previous

period, considering the increasing scientific, technical, and

critical group uncertainties. Depending upon the extent to which

the peak risk exceeds the standard for the first part, steps

should be considered to ameliorate the potential risk. This

second part of the compliance regulations should not be allowed

to become the de facto regulation.

Summary

The regulatory time period for compliance is an important element

in regulations for nuclear waste facilities and remains a problem

in developing site-specific requirements for protecting the

health and safety of the Nation, as well as its environment. The

ACNW suggests a solution to this problem from a generic

standpoint, which employs two parts. Using scientific and

technical insights into the environment of the repository
proposed for Yucca Mountain, we recommend an approach that

establishes the time of compliance of the facility at this site,

which differs from the current regulation and the proposal on

this topic made by the National Research Council/NAS Committee in

its report, Technical Bases for Yucca Mountain Standards . We

believe that our recommendations will lead to a simple, robust,

and defensible regulation that can be readily implemented.

Sincerely,

/s/

Paul W. Pomeroy

Chairman, ACNW

References:

1. Report dated February 9, 1996, from Paul W. Pomeroy,

Chairman, ACNW, to Shirley Ann Jackson, Chairman, NRC,

Subject: Issues and NRC Activities Associated with the

National Research Council's Report, "Technical Bases for

Yucca Mountain Standards"

2. Report dated June 27, 1991, from Dade W. Moeller, Chairman,

ACNW, to Kenneth M. Carr, Chairman, NRC, Subject: "Response

to questions Accompanying Working Draft #3 of the EPA

Standards"

3. R. M. Forester and A. J. Smith, "Late Glacial Climate Estimates for Southern Nevada: The Ostracode Fossil Record," in

High-Level Radioactive Waste Management, Vol. 4, pp. 2553-

2561, 1994