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. 97-059 FOR IMMEDIATE RELEASE

(Tuesday, April 8, 1997)

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

reference biosphere and critical group issues and their application to the proposed high-level

waste repository at Yucca Mountain, Nevada.

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Attachments:

As stated
ACNWR-0125

April 3, 1997

The Honorable Shirley Ann Jackson

Chairman

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Dear Chairman Jackson:

SUBJECT: REFERENCE BIOSPHERE AND CRITICAL GROUP ISSUES AND THEIR

APPLICATION TO THE PROPOSED HIGH-LEVEL WASTE REPOSITORY AT

YUCCA MOUNTAIN, NEVADA

The purpose of this letter is to convey the observations and recommendations of the Advisory

Committee on Nuclear Waste (ACNW) regarding the issues associated with defining the

reference biosphere and the critical group in site-specific regulations for the proposed high-level

waste (HLW) repository at Yucca Mountain. This letter supplements the ACNW letter dated

June 7, 1996, subject, "Time Span for Compliance of the Proposed High-Level Waste

Repository at Yucca Mountain, Nevada," in which we indicated the dependence of the time of

compliance on the specification of the reference biosphere and critical population group. In that

letter, the Committee indicated that it would provide additional comments on these topics after

review and discussion. We believe this letter is appropriate and timely in view of the

Commission's need to respond to the Environmental Protection Agency's (EPA’s) efforts to

develop a Yucca Mountain specific standard (40 CFR Part 197) and make appropriate

modifications to the Nuclear Regulatory Commission's (NRC’s) HLW regulations.

Our observations and conclusions are based on discussions during a working group meeting on

"Specification of the Critical Group and Reference Biosphere" held during the 84th meeting of

the ACNW on June 25, 1996, and presentations on biosphere modeling at the 90th meeting of

the ACNW on March 21, 1997. During the working group sessions, both oral and written

presentations were made by members of the National Academy of Sciences/National Research

Council Panel that authored the report, "Technical Bases for Yucca Mountain Standards," and

by representatives of the NRC staff, the EPA, the Department of Energy (DOE), the Electric

Power Research Institute, and the State of Nevada.

In this letter we provide definitions and assumptions that generally apply to the issues of the

biosphere and critical group, and offer suggestions on how these general considerations and

definitions can be applied to the Yucca Mountain site. In the course of providing advice on the
treatment of biosphere and critical group issues, as well as all other issues, the Committee has

been supportive of the agency's desire to move in the direction of risk-informed, performancebased regulations and standards. Important attributes of adopting a risk-informed approach are

the focus on the health and safety of the public and the quantification of uncertainties.

THE PROBLEM

It is necessary to establish a rational basis for determining the radionuclide exposure

scenario(s) for the proposed Yucca Mountain repository. Key to solving this problem is

specification of a reference biosphere and a critical group. Specification of a reference

biosphere sets the conditions for all pathways by which radionuclides from the repository may

reach humans, including movement through the food chain as well as direct ingestion of ground

water. Specification of a critical group is required because regulations will likely be based on

acceptable doses to this group, a surrogate for the public. This approach is in contrast to the

traditional NRC dose-based regulatory approach that considers the maximally exposed

individual at a prescribed distance from the facility for determining compliance. Also, because

of the long time periods involved and the desire to simplify the performance assessment

wherever possible, an appropriate definition of the time of compliance is an important

consideration.

The definitions for time of compliance, critical group, and reference biosphere should enable a

license applicant to identify clearly the assumptions and calculations used in performance

assessment. The specification of these elements should be based as much as possible on

scientific and technical evidence. In the absence of such evidence, a decision based on policy

will have to be made by the appropriate Government unit on the basis of current conditions and

reasonably bounded uncertainties. For example, the specification of the critical group, as well

as certain factors of the reference biosphere, will require decisions about appropriate estimates

of future human demographics and behavior. The overall objective in defining these terms for

Yucca Mountain should be protection of the public health and safety and the environment for

future generations, using policies and procedures rooted in available science.

GENERAL DEFINITIONS AND ASSUMPTIONS

The following definitions and assumptions form a basis for providing guidance on the important,

interconnected issues of the reference biosphere and the critical group necessary for the

performance assessment of a geologic repository for radioactive waste.

1 . The reference biosphere is the environment (biologic, geologic, hydrologic, and

atmospheric) in the vicinity of the repository in which the biota and the critical group may

come in contact with radionuclides. The biosphere defines the portion of the

environment in which radionuclides can reach human populations. Exposure pathways

in the biosphere include ground water, surface water, soils, plants, animals, and air.

2. The critical group is a relatively homogeneous group of people whose location in the

general vicinity of the repository and whose habits are such that they include individuals

expected to receive the highest doses from radionuclides discharged from the

repository. Estimates of exposures to the critical group may be extended to risk through

the appropriate dose-response relationship.

3. The biosphere and the critical group should be based on known site characteristics.
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Site characteristics include the geologic and climatic setting, engineered safety features,

and demographics. The policy set forth in the standard and regulations must allow sitespecific characteristics to be considered; that is, they must not be overly prescriptive in a

generic sense.

4. The societal state of the region around the repository cannot be predicted with

confidence for thousands of years into the future. Large uncertainties exist in the

description of the societal states. Decisions regarding demographics, human behavior,

and land use thousands of years into the future should be a matter of policy derived

from available science. A reasonable policy is that the current societal state in the

vicinity of a site will be the basis for analyzing the expected safety of the public unless

there is scientific evidence to indicate that other climatic, demographic, or biological

conditions are more appropriate.

These four definitions and assumptions are believed to be a reasonable foundation for

addressing the issues of biospheres and critical groups for any nuclear waste repository. We

now apply these definitions and assumptions to the proposed Yucca Mountain repository and

the surrounding area with respect to the specific issues of the biosphere and the critical group.

APPLICATION OF THE REFERENCE BIOSPHERE AND THE CRITICAL GROUP TO THE

PROPOSED YUCCA MOUNTAIN REPOSITORY

Reference Biosphere

The description of the biosphere is an important element of defining repository exposure

scenarios for calculating public health risk. The primary source of risk is likely to be the

ingestion of food and water. Site-specific attributes are critical to the description of transport

pathways that lead to calculated exposures of the critical group. Thus, site characterization is a

key source of information for describing the reference biosphere.

Future changes in the biosphere that can be reasonably characterized should be considered in

this definition. For example, climatic components of the reference biosphere will change with

time. Paleoclimatic and paleohydrologic studies of the Quaternary Period, and especially the

Holocene, indicate (and can provide bounds on) climate change to a cooler, wetter regime

associated with the onset of the next glacial cycle. The anticipated changes in climate over

time can be used to refine the reference biosphere when supported by scientific evidence. For

example, a potential rise in the water table as a result of a cooler, wetter climate could make

ground water more accessible in the currently arid regions in the vicinity of Yucca Mountain.

Should evidence to the contrary arise as a result of site characterization studies, that too should

be part of the input to the definition of the reference biosphere.

Critical Group

As indicated in definition 2 above, the critical group is a relatively homogeneous group of

people whose location and habits are such that they are representative of those individuals

expected to receive the highest doses as a result of the discharge of radionuclides. The

suggested policy assumption of item 4 above leads to the definition of the critical group. For

example, the density, distribution, and habits of the population in the Amargosa Valley could be
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the basis for defining the critical group. Although the definition of a Yucca Mountain specific

critical group will likely be decided on the basis of policy, the need remains to develop a

repository performance assessment model that is convincing, clear, and justifiable.

The purpose of this letter is not to prescribe a calculation procedure but to identify some

principles that the Committee believes should guide the critical group risk calculations for the

proposed Yucca Mountain site. These principles are as follows:

1. The principle of consistency should be applied in the use of probabilistic methods.

The "principle of consistency" as used here is the consistent application of

probabilistic methods to different aspects of the Yucca Mountain repository

performance assessment, including waste container degradation, radionuclide

transport, and the human exposure scenario.

2. The critical group should include those individuals who are at greatest risk.

3. The group should be relatively homogeneous in terms of the exposure of its

individuals to radiation.

4. Even if the regulation prescribes a maximum acceptable dose to only the critical

group, risk-based calculations should be performed that clearly display the

probability and health effects to the public in the vicinity of the repository.

Calculations of the type specified in item 4 above are consistent with the adoption of a riskinformed approach to regulation. In particular, it is important to understand how the risk is

distributed. Thus, calculations that indicate the likelihood that "N" or more people will receive

"D" or more dose will provide a meaningful indication of the risk to the entire population being

studied. Such calculations can be especially constructive in revealing site peculiarities that may

result in highly localized doses. Furthermore, the results may reveal interdiction options that

could turn an otherwise questionable site into an acceptable one at a fraction of the resources

that might be required to consider an alternate site.

The Committee believes that the definitions, assumptions, and principles outlined herein

provide a basis for the NRC staff to develop guidance on the licensing of the proposed Yucca

Mountain HLW repository. We wish to emphasize an important message in this letter. Even

though a policy solution is required for some of the issues associated with the Yucca Mountain

site, the Committee believes that scientific evidence should prevail when it exists. For example,

both the regulations and the staff should encourage the DOE to consider temporal changes,

such as climate, and to utilize those changes that can be scientifically supported.

SUMMARY

Specification of the reference biosphere and the critical group is an important element in the

regulation of nuclear waste disposal facilities. The ACNW proposes a set of definitions and

assumptions to guide the specification of the biosphere and the critical group. For the

proposed Yucca Mountain repository, we recommend using the results of the site

characterization program as the principal basis for defining the reference biosphere, the

environment through which the critical group may be exposed to radionuclides. The Committee
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believes that specification of the critical group, a small relatively homogeneous assembly of the

most highly exposed people that serve as a surrogate for the public, requires the establishment

of policy. The policy suggested is that the reference biosphere and the critical group could be

based on present conditions in the Amargosa Valley unless a scientific basis can be given for

using other assumptions.

We believe that our proposed approach can result in a robust and defensible set of regulations.

Sincerely,

/s/

Paul W. Pomeroy

Chairman