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U. S. NUCLEAR REGULATORY COMMISSION |
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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:
. 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.
. 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.
. 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.
. 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.
. 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.
. 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.
. 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.
. 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 con-
tainers, 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 regula-
tion 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
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