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EXECUTIVE SUMMARY
Proper management of high-level radioactive wastes, including
those resulting from the production of nuclear weapons ant} the operation
of nuclear electric power plants, is vital for the protection of the public
health ant] safety. It has been longstanding fecleral policy to (lispose of
these wastes underground in a miner! geologic repository. The U.S.
Department of Energy (DOE) is charged with the development anti
eventual operation of a repository. The U.S. Environmental Protection
Agency (EPA) and the U.S. Nuclear Regulatory Commission (USNRC)
share the responsibility for regulating the clisposal program to ensure
adequate protection of the health and safety of the public.
EPA promulgatecl its first stan(iar(l for deep geologic disposal of
high-level radioactive waste in 1985; this standard was challengecl,
litigatecI, anti ultimately reissued in 40 CFR 191 in December ~ 993. Before
EPA promulgates! the new stanciarci, however, Congress enacted the Energy
Policy Act of 1992, which mandated a separate process for setting a
standard specifically for the proposed repository at Yucca Mountain,
Nevada. In Section 801 ofthe Act, Congress requires] EPA to arrange for
an analysis by the National Academy of Sciences of the scientific basis for
a standard} to be applied at the Yucca Mountain site ant] directed] EPA,"
based upon ant! consistent with the finding and recommendations of the
National Academy of Sciences, [to] promulgate, by rule, public health ant}
safety standards for protection of the public from releases from radioactive
materials stored in or disposed of in the repository at the Yucca Mountain
site." This report responds to the charge of Section 801.
Implicit in setting a Yucca Mountain standarcl, is the assumption
that EPA, USNRC, and DOE can, with some degree of confidence, assess
the future performance of a repository system for time scales that are so
long that experimental methods cannot be used to confirm directly
predictions of the behavior of the system or even of its components. This
premise raises the basic issue of whether scientifically justifiable analyses
of repository behavior over many thousands of years in the future can be
made. We conclude that such analyses are possible, within restrictions
noted in this report. Nevertheless, these assessments of repository
performance must content] with substantial uncertainties, and some areas
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2
YUCCA MOUNTAIN STANDA=S
projecting the behavior of human society over very long periods, for
example-are beyond the limits of scientific analysis. We have macle
explicit those instances, and have also pointer! out where we believe it is
appropriate to rely on informed judgments and reasonable assumptions to
supplement scientific analysis.
In attempting to make the best use of the scientific understanding
that is available, we have arrived at recommendations that differ in
important ways from the approach follower] by EPA in 40 CFR 191. In
particular, we recommend:
The use of a stanciard that sets a limit on the risk to
individuals of adverse health effects from releases from the
repository. 40 CFR 191 contains an in~iividual-close
standard ant] it continues to rely on a containment
requirement that limits the releases of radionuclicles to the
accessible environment. The states! goal of the containment
requirement was to limit the number of health effects to the
global population to 1,000 incremental fatalities over 10,000
years. We do not recommenc] that a release limit be adopted
That compliance with the standard be measured at the time
of peak risk, whenever it occurs. ~ The standard in 40 CFR
191 applies for a period] of ~ 0,000 years. Based on
performance assessment calculations provicled to us, it
appears that peak risks might occur tens to hundreds of
thousands of years or even farther into the future.
Against a risk-based calculation of the adverse effect of
human intrusion into the repository. Under 40 CFR 191, an
assessment must be made of the frequency ant! consequences
of human intrusion for purposes of demonstrating
compliance with containment requirements. In contrast, we
conclucle that it is not possible to assess the frequency of
intrusion far into the future. We do recommenc! that the
consequences of an intrusion be calculated to assess the
resilience of the repository to intrusion.
Within the limits imposed by the long-term stability of the geologic
environment, which is on the order of one million years.
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EXECUTIVE SUMAt4RY
3
Finally, we have identified several instances where science cannot
provide all of the guidance necessary to resolve an issue. This is
particularly true in developing procedures for compliance assessment.
Setting the standard, therefore, requires addressing policy questions as well
as scientific ones. We recommend that resolution of policy issues be done
through a rulemaking process that allows opportunity for wide-ranging
input from all interested parties. In these cases, we have tried to suggest
positions that could be used by the responsible agency in formulating a
proposed rule. Other starting positions are possible, and of course the final
rule could differ markedly from any of them.
Although we have taken a broad view of the scientific basis for the
standard, we have not addressed the social, political, and economic issues
that might have more effect on the repository program than the health
standard. In particular, we have not recommended what levels of risk are
acceptable; we have not considered whether the development of a
permanent repository should proceed at this time; nor have we made a
judgment about the potential for the Yucca Mountain site to comply with
the standard eventually adopted.
PROTECTING HUMAN HEALTH
In Section 801, Congress directs that EPA set a standard for Yucca
Mountain by specifying the maximum annual effective dose equivalent to
individual members of the public. The first question posed in Section 801
is whether such a standard will provide a reasonable basis for protecting
the health and safety of the general public. We recommend the use of a
standard designed to limit individual risk, and describe how a standard
might be structured on this basis. We then address the specific question of
protection of public health in the context of the individual-risk standard
and compare this standard to the one currently used by EPA. Based on this
analysis, we conclude not only that the individual risk standard would
protect the health of the general public, but also that it is a particularly
appropriate standard for the Yucca Mountain site in light of the
characteristics of this site.
The risks to humans from exposures to low levels of radiation have
been assessed in detail by national anti international organizations. These
assessments are fraught with uncertainty, but it has been possible to reach
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4
YUCCA MOUNTAINSTANDARDS
a reasonable consensus within the scientific community on the relationship
of dose ant! health effects, which is generally considered to provide an
acceptable basis for evaluating the risks attributable to a given dose or the
degree of protection afforded by a given limitation of exposure.
Additionally, a general consensus exists among national and International
bodies on a framework for protecting the public health that provides a limit
of ~ milliSievert (mSv) (100 millirem (mrem)) per year effective dose for
continuous or frequent exposures from all anthropogenic sources of
ionizing radiation other than medical exposures. A general consensus also
appears to exist among national authorities in various countries to accept
and use the principle of apportioning this total radiation dose limit among
the respective anthropogenic sources of exposure, typically allocating to
high-level waste disposal a range of 0.1 to 0.3 mSv (10 to 30 mrem) per
year.
Elements of the Standard
A standard is a societally acceptable limit on some aspect of
repository performance that should not be exceeded if the repository is to
be judged safe. We recommend the use of a standard that sets a limit on the
risk to inclividuals of adverse health effects from releases from the
repository. A risk-based standard] would not have to be revised In
subsequent rulemaking if advances in scientific knowledge reveal that the
dose-response relationship is different from that envisaged today. Such
changes have occurred frequently in the past, and can be expected to occur
in the future. For example, ongoing revisions in estimates of the radiation
doses received by atomic bomb survivors of Hiroshima and Nagasaki might
significantly modify the apparent dose-response relationships for
carcinogenic effects in this population, as have previous revisions in
dos~meby (see Straume et al., 19921. Moreover, risks to human health from
different sources, such as nuclear power plants and toxic chemicals can be
compared in reasonably understandable terms.
It is essential to define specifically how to calculate risk, however,
for otherwise it will not be clear what number to use to compare to the risk
limit established in the standard. We define risk as the expected value of
a probabilistic distribution of health effects. The first step in calculating
risk is to develop a distribution of doses received by Individuals. A
_ _ =,
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EXECUTIVE SUMMARY
s
probabilistic distribution of health effects can be developed as the product
of each value of dose received and the health effect per unit dose.
Structuring ofthe individual-risk standard requires specifying what
level of protection is to be afforded, who is to be protected, and for how
long. We acknowledge that determining what risk level is acceptable is not
ultimately a question of science but of public policy. We note, however,
that EPA has already used a dose limit equivalent to a risk level of 5x10-4
health effects in an average lifetime, or a little less than 10-s effects per
year assuming an average lifetime of 70 years, as an acceptable risk limit
in its recently published 40 CFR 191. This limit is consistent with limits
established by other federal nuclear regulations. In addition, the risk
equivalent of the dose limits set by authorities outside the United States is
also in the range of 10-5 to 1~6 per year (except for exposure to radon
indoors or releases from mill tailings). This range is a reasonable starting
point for EPA's rulemaking.
To determine whether a repository complies with the standard, it
is necessary to calculate the risk to some individual or representative group
of individuals and then to compare the result to the risk limit established
in the standard. Therefore, the standard must specify the individual or
individuals for whom the risk calculation is to be made. Although not
strictly a scientific issue, we believe that the appropriate objective is to
protect the vast majority of members of the public while also ensuring that
the decision on the acceptability of a repository is not unduly influenced by
the risks imposed on a very small number of individuals with unusual
habits or sensitivities. The situation to be avoided, therefore, is an extreme
case defined by unreasonable assumptions regarding the factors affecting
dose and risk, while meeting the objectives of protecting the vast majority
of the public. An approach that is consistent with this objective' and is
used extensively elsewhere in the world, is the critical-group approach.
We recommend that the critical-group approach be used in the Yucca
Mountain standards.
The critical group has been defined by the International
Commission on Radiological Protection (ICRP) as a relatively
homogeneous group of people whose location and habits are such that they
are representative of those individuals expected to receive the highest
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6
YUCCA MOUNTAIN STANDARDS
doses2 as a result of the discharges of radionuclicles. Therefore, as the
ICRP notes, "because the actual doses in the entire population will
constitute a distribution for which the critical group represents the extreme,
this procedure is intended} to ensure that no individual doses are
unacceptably high." (ICRP' 1985a, at paragraph 46~. In the context of an
inciiviclual-risk stanciard, ant] using cautious, but reasonable, assumptions,
the group would inclucle the persons expected to be at highest risk, would
be homogeneous in risks, and would be small in number. The critical-
group risk calculateci for purposes of comparison with the risk limit
established in the stanciarc} would be the mean of the risks to the members
of the group.
This definition requires specifying the persons who are likely to be
at highest risk. In the present and near future, these persons are real; that
is, they are the persons now living in the near vicinity of the repository ant]
in the direction of the postulated flow of the plume of radionuclides. For
the far future, however, it will be necessary to ciefine hypothetical persons
by making assumptions about lifestyle, location, eating habits, and other
factors. The ICRP recommends use of present knowledge and cautious,
but reasonable, assumptions.
The current EPA standard contains a time limit of 10,000 years for
the purpose of assessing compliance. We find that there is no scientific
basis for limiting the time period of an individual-risk standard in this way.
We believe that compliance assessment is feasible for most physical and
geologic aspects of repository performance on the time scale of the long-
term stability of the fundamental geologic regime a time scale that is on
the order of 106 years at Yucca Mountain ant} that at least some
potentially important exposures might not occur until after several hundrecl
thousand years. For these reasons. we recommend that compliance
The ICRP defines critical group in dose terms. We use the ICRP terminology
here to describe the concept as developed by the ICRP, and later adapt the
concept to the risk framework.
3 That is, the difference between the highest and lowest risk faced by individuals
in the group should be relatively small. Should a radiation dose occur, however,
it may affect only a few members of the group. This is the difference between
risk (the chance of an adverse health effect) and outcome (a cancer that actually
develops). Risk can be homogeneous, even when outcomes are quite diverse.
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EXECUTIVE SUMMARY
7
assessment be conducted for the time when the greatest risk occurs within
the limits imposed by long-term stability of the geologic environment.
Another time-related regulatory concern, based on ethical
principles, is that of intergenerational equity. A health-based risk standard
could be specified to apply uniformly over time and generations. Such an
approach would be consistent with the principle of intergenerational equity
that requires that the risks to future generations be no greater than the risks
that would be accepted today. Whether to adopt this or some other
expression of the principle of intergenerational equity is a matter for social
judgment.
Protection of the General Public
Congress has asked whether a standard intended to protect
individuals would also protect the general public in the case of Yucca
Mountain. We conclude that an individual-risk standard would protect
public health. given the particular characteristics of the site. provided that
policy makers and the public are prepared to accept that very low radiation
doses pose a negligibly small risk.
The individual risk-standard that we recommend is intended to
protect a critical group. In this context, the general public includes both
global populations as well as local populations that lie outside the critical
group. Global populations might be affected because radionuGIide releases
from a repository can in theory be diffused throughout a very large and
dispersed population. In the case of Yucca Mountain, the likely pathway
leading to widely dispersed radionuclides is via the atmosphere beginning
with release of carbon dioxide gas containing the carbon-14 (TIC)
radioactive isotope which might escape from the waste canisters.
The risks of radiation produced by such wide, dispersion are likely
to be several orders of magnitude below those of a local critical group.
Great uncertainty exists about the number of health effects that would be
imposed on the global population because of the difficulties in interpreting
the risks associated with very small incremental doses of radiation. As
noted in the BEIR V report (NRC, 1 990a), the lower limit of the range of
uncertainty in such risk estimates extends to zero (no effects). To address
scenarios of widespread but extremely low-level doses, the radiation
protection community has introduced the concept of negligible incremental
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8
YUCCA MOUNTAIN STANDARDS
dose (above background levels). For example, the National Council on
Radiation Protection ant] Measurements (NCRP) has recommended a value
of 0.01 mSv/yr (Imrem/yr) per radiation source or practice (NCRP 1993),
which currently would correspond! to a projected risk of about 5 x 1 0~7/yr
for fatal cancers, assuming the linear hypothesis. We believe that this
concept can be extended to risk ant! can be applied to the establishment of
a radiation standard at Yucca Mountain. Defining the level of incremental
risk that is negligible is a policy judgment. We suggest the risk equivalent
of the negligible individual incremental close recommended by the NCRP
as a reasonable starting point for developing consensus.
Persons in some population outside the critical group may,
however, still be exposed to risks in excess of the level of the negligible
incremental risk but below the level of the critical group risk. The risks to
these persons as indivicluals are, by definition, acceptable, but whether the
effects on this population as a whole are acceptable remains a matter of
jucigment. Based on our review, we conclude that there is no technical
basis for a population risk stan`darc! by which to make such a judgment.
ASSESSING COl\~IlP6LIANCE
Any standard to protect individuals ant} the public after the
proposed repository is closet! will require assessments of performance at
times so far in the future that a direct demonstration of compliance is out
of the question. The only way to evaluate the risks of adverse health
effects ant! to compare them with the standard is to assess the estimates}
potential future behavior of the entire repository system and its potential
effects on humans. This procedure, involving mocleling of processes anti
events that might lead to releases anti exposures, is called performance
assessment.
l he technical teas~b~l~ty of developing performance assessment
calculations to evaluate compliance with a risk standard at Yucca Mountain
clepends on the feasibility of modeling the relevant events and processes
(including their probabilities) specific to that site. By soliciting technical
appraisals at our open meetings, reviewing solicited ant! unsolicited written
contributions, and drawing on the available literature and our own
experience and expertise, we have assessed the types, magnitudes, ant]
time-dependencies of the uncertainties associates] with potential
_ - . . .. ... ~. .
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EXECUTIVE SUMMERY
9
radionuclide transport from a Yucca Mountain repository, the effects of
potential natural and human modifiers of repository performance, and the
pathways through the biosphere.
Physical and Geologic Processes
The properties and processes leading to transport of radionuclides
away from the repository include release from the waste form, transport to
the near-f~eld zone, gas phase transport to the atmosphere above Yucca
Mountain and its dispersal in the world atmosphere, and transport from the
unsaturated zone to the water table and from the aquifer beneath the
repository to other locations from which water might be extracted by
humans. We conclude that these physical and geologic processes are
sufficiently quantifiable and the related uncertainties sufficiently
boundable that the performance can be assessed over time frames during
which the geologic system is relatively stable or varies in a boundable
manner. The geologic record suggests that this time frame is on the order
of 106 years. We further conclude that the probabilities and consequences
of modifications by climate change, seismic activity, and volcanic
eruptions at Yucca Mountain are sufficiently Soundable that these factors
can be included in performance assessments that extend over this time
frame.
Exposure Scenarios
Performance assessment of physical and geologic processes will
produce estimates of potential concentrations of radionuclides in ground
water or air at different locations and times in the future. To proceed from
these concentrations to calculations of risks to a critical group requires the
development of an exposure scenario that specifies the pathways by which
persons would be exposed to radionuclides released from the repository.
Once an exposure scenario has been adopted, performance assessment
calculations can be carried out with a degree of uncertainty comparable to
the uncertainty associated with geologic processes and engineered systems.
Based upon our review of the literature. we conclude. however
that it is not possible to predict on the basis of scientific analyses the
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10
YUCCA MOUNTAIN STANDARDS
societal factors required for an exposure scenario. Specifving exposure
scenarios therefore requires a policy decision that is appropriately made in
a rulemaking process conducted by EPA. We recommend against placing
the burden of postulating and defending an exposure scenario on the
applicant for the license.
As with other aspects of defining standards and demonstrating
compliance that involve scientific knowledge but must ultimately rest on
policy judgments, we consiciered what to suggest to EPA as a useful
starting point for rulemaking on exposure scenarios. Reflecting the
disagreement inherent in the literature, we have not reached complete
consensus on this question. It is essential that the scenario that is
ultimately selected be consistent with the critical-group concept that we
have advanced. Additionally, EPA should rely on the guidance of ICRP
that the critical group be defined using present-day knowledge with
cautious, but reasonable, assumptions.
We considered two illustrative approaches to the design of an
exposure scenario that EPA might propose to initiate the rulemakin~
A,
process. The approaches have many elements in common but differ in
their treatment of assumptions about the location and lifestyle of persons
who might be exposed to releases from the repository, and in the method
of calculating the average risk of the members of the critical group. A
substantial majority of the committee members, but not all, considers one
of the approaches to be more consistent with the foregoing criteria. This
particular approach explicitly accounts for how the physical characteristics
of the site might influence population distribution and identifies the
makeup of the critical group probabilistically.
HUMAN INTRUSION
Human activity that penetrates the repository (by drilling directly
into it from the surface, for example) can cause or accelerate the release of
radionuclides. Waste material could be brought to the surface and expose
the intruder to high radiation doses, or the material could disperse into the
biosphere. The second and third questions asked in Section X01 of the
Energy Policy Act of 1992 concern the potential that at some time people
might intrude into the repository.
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EXECUTIVE SUMMARY
With respect to the second question of Section
11
we concl~
that it is not reasonable to assume that a system for post-closure oversight
of the repository can be developed. based on active institutional controls.
that will prevent an unreasonable risk of breaching the repositorY's
engineered barriers or increasing the exposure of individual members of
the public to radiation beyond allowable limits. This conclusion is founded
on the absence of any scientific basis for making projections over the long
term of the social, institutional, or technological status of future societies.
Additionally, there is no technical basis for making forecasts about the
long-term reliability of passive institutional controls, such as markers,
monuments, and records.
With respect to the third question in Section 801~ we conclude that
it is not possible to make scientifically supportable predictions of the
probability that a repository's engineered or geologic barriers will be
breached as a result of human intrusion over a period of 10~000 Years. We
reach this conclusion because we cannot predict the probability that a
future intrusion would occur in a given future time period or the probability
that a future intrusion would be detected and remediated, either when it
occurs or later. In addition, we cannot predict which resources will be
discovered or will become valuable enough to be the objective of an
intruders activity. We cannot predict the characteristics of future
technologies for resource exploration and extraction, although continued
developments in current noninvasive geophysical techniques could
substantially reduce the frequency of exploratory boreholes.
Although there is no scientific basis for judging whether active
institutional controls can prevent an unreasonable risk of human intrusion,
we think that, if the repository is built, such controls and other activities
might be helpful in reducing the risk of intrusion, at least for some initial
period of time after a repository is closed. Therefore, we believe that a
collection of prescriptive requirements, including active institutional
controls, record-keeping, and passive barriers and markers would help to
reduce the risk of human intrusion, at least in the near term.
Moreover, because it is not technically feasible to assess the
probability of human intrusion into a repository over the long term, we do
not believe that it is scientifically justified to incorporate alternative
scenarios of human intrusion into a fully risk-based compliance
assessment. We do, however, conclude that it is possible to carry out
calculations of the consequences for particular types of intrusion events.
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YUCCA MOUNTAIN STANDARDS
The key performance issue is whether repository performance would be
substantially degraded as a consequence of an intrusion of the type
postulated. For this purpose, we have focused on the particular class of
cases in which the intrusion is inadvertent and the intruder does not
recognize that a hazardous situation has been created.
To provide for the broadest consideration of what human intrusion
scenario or scenarios might be most appropriate, we recommend that EPA
make this determination in its rulemaking to adopt a standard. For
simplicity, we considered a stylized intrusion scenario consisting of one
borehole of a specified diameter drilled from the surface through a canister
of waste to the underlying aquifer. In our view, the performance of the
repository, having been intruded upon, should be assessed using the same
analytical methods and assumptions, including those about the biosphere
ant! critical groups, used in the assessment of performance for the
undisturbed case. We recommend that EPA require that the estimated risk
calculated from the assumed intrusion scenario be no greater than the risk
limit adopted for the undisturbed-repository case because a repository that
is suitable for safe long-term disposal should be able to continue to provide
acceptable waste isolation after some type of intrusion. As with other
policy-related aspects of our recommendations, we note that EPA might
decide that some other risk level is appropriate.
IMPLICATIONS OF OUR CONCLUSIONS
Limits of the Scientific Basis
It might be possible that some of the current gaps in scientific
knowledge and uncertainties that we have identified might be reduced by
future research. It seems reasonable, therefore, to ask what gaps could be
closed by taking time to obtain more scientific and technical knowledge on
such matters as the nature of the waste, its potential use, the health effects
of radionuclicles, the value of waste products for later generations, and the
security of retrievable storage containers. New information in these and
other areas could improve the basis for setting the standards.
Whether the benefit of new information would be worth the
additional time and resources required to obtain it is a matter of judgment.
This judgment would be strengthened by a careful appraisal of the probable
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EXECUTIVE SUMMARY
13
costs ant] risks of continuing the present temporary waste disposal practices
and storage facilities as compared to those attaching to the proposer!
repository. No such comprehensive appraisal is now available.
Conducting such an appraisal, however, shouIc] not be seen as a reason to
slow (town ongoing research ant] clevelopment programs, including
geologic site characterization, or the process of establishing a standard to
protect public health.
Technology-Based Standards
Technology-based stanciarcis play an important role in regulations
clesigne~i to protect the public health from the risks associates} with nuclear
facilities. We have examined three technological approaches in our study.
The "as low as reasonably achievable" (ALARA) principle is
intenclec! to be applier! after threshold regulatory requirements have been
met, and calls for additional measures to be taken to achieve further
reduction in the calculated health effects. While ALARA continues to be
widely recommended as a philosophically clesirable goal, its applicability
to geologic disposal of high-level waste is limitecl at best because the
technological alternatives available for designing a geologic repository are
quitelimited. Furthers the difficulties ofclemonstratingtechnicalorIegal
compliance with any such requirement for the post-closure phase couIci
well prove insuperable even if it were restricted to engineering and design
issues. We conclude that there is no scientific basis for incorporating the
ALARA principle into the EPA standard or USNRC regulations for the
repository.
If EPA issues stanciards based on individual risk, the USNRC
would be required to revise its current regulations embodies} in 10 CFR 60
to be consistent with such standards. One purpose of 10 CFR 60, which
contains technology specifications, is to help ensure multiple barriers
within the repository system. We conclude that because it is the
performance of the total system in light of the risk-basec! standard that is
crucial, imposing subsystem performance requirements might result in
suboptimal repository design.
Finally, several persons suggested to our committee the use of a
technology-basect standard that would specify a strict release limit from an
engineereci barrier system during the early life of the repository. We find
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YUCCA MOUNTAIN STANDARDS
that such a limitation on early releases would have no effect on the results
of compliance analysis over the long-term. Nonetheless some members of
the committee believe that such a limitation might provide abided assurance
of safety in the near-term, and EPA might wish to consider this as a policy
matter.
Administrative Consequences
Our recommendations, if aclopteci, imply the development of
regulatory and analytical approaches for Yucca Mountain that are different
from those employed in the past and from some approaches currently user]
elsewhere by EPA. The change in approach ant! the time required to
develop a thorough ant! consistent regulatory proposal and to provide for
full public participation in the rulemaking process will require considerable
effort by EPA. This process probably will take more than the year,
currently provided in statute, for EPA to complete clevelopment of a Yucca
Mountain standard in a technically competent way. This floes not mean
that DOE's Yucca Mountain Site Characterization Project cannot proceed
usefully in the interim.
Representative terms from entire chapter:
human intrusion
