referred to as ``the difficulties in spatial interpolation of site
characteristics'' which ``will be present at all times'' (NAS Report
p. 72). Similarly, ``model'' uncertainty includes not only whether
the processes acting on the site have been correctly represented
mathematically and coupled with each other, but also whether the
basic understanding of which processes operate, whether there are
competing mechanisms that must be considered (e.g., for corrosion or
ground-water flow), and the extent to which and conditions under
which one mechanism is dominant.
---------------------------------------------------------------------------

    NAS supported the use of probabilistic modeling as one way to
address the effects of uncertainty. However, NAS noted that this
process itself can involve significant uncertainties in defining the
parameter value distributions from which the probabilistic selections
would be made. (NAS Report pp. 78-79) As a result, interpretation of
probabilistic results, which illustrate uncertainty through the
distribution of calculated values, may

also be affected by this underlying uncertainty, which may not be fully
appreciated or understood.
    Selecting an appropriate dose limit for periods up to 1 million
years must also consider the ability of performance assessments, and
those who interpret them, to distinguish between differing repository
designs, as well as different conceptualizations of total system
performance over very long time frames. We have described the general
view that the predictive capabilities of performance assessments
diminish as the time periods covered by the assessments increase. It is
also important to understand that, while mathematical calculations can
result in very precise estimates of dose (to multiple significant
digits), this precision is misleading in its presentation of the
approximate outcomes of multiple interacting processes. We believe it
is not appropriate to imply that there is a clear and immutable
difference between two projections of dose, when it is understood that
neither on its own is an unqualified representation of reality. Such
representations may promise more than can be delivered by the model's
ability to ``slice it thin.'' \23\ In our view, it makes little sense
to assert that a 15 mrem/yr dose limit for the period within 10,000
years is more ``protective'' than a higher limit much later in time if,
in the time frame of hundreds of thousands of years, the uncertainties
in projecting disposal system performance cannot easily make
distinctions at such incremental levels.\24\
---------------------------------------------------------------------------

    \23\ This problem is not specific to quantitative performance
assessment. Similar issues have been identified in analysis of
different policy options for energy or other areas associated with
technological risk. It has been noted that ``The results of
individual risk assessment studies are often reported with
formidable precision, expressed as discrete numbers (rather than
ranges) and presented to two, three and even four significant
figures. Yet * * * such precision seems entirely to misrepresent the
accuracy of this style of appraisal taken as a whole * * * the
problem does not tend to be driven by any single factor in analysis,
nor is it a simple matter of some studies being more `accurate' or
`reasonable' than others in any definitive sense. The manifest
variability * * * is rather a simple reflection of * * * the
adoption of different (but equally scientifically valid) assumptions
and priorities concerning the multitude of different dimensions of
risk. Where [different options cannot be clearly distinguished] in
any absolute sense, then the value of appraisal lies in exposing the
relationships between different assumptions in analysis and the
associate pictures of the relative importance of different options.
It is better to be roughly accurate in this task of mapping the
social and methodological context-dependencies than it is to be
precisely wrong in spurious aspirations to a one-dimensional
quantitative expression of technological risk.'' (``On Science and
Precaution in the Management of Technological Risk,'' Volume 1,
Institute for Prospective Technical Studies, 1999, Docket No. EPA-
HQ-OAR-2005-0083-0413, pp 13-16, emphasis in original)
    \24\ One might compare this situation to finding two proximate,
but distinct, locations on a road map. In the first instance, the
scale on the map is such that all individual roads and landmarks
(e.g., schools, churches, libraries) can be seen. One can easily
locate each site and circle it. Now consider a map of the same size,
in which the scale is much smaller, showing only major thoroughfares
and main local roads. One would still be able to approximate the
desired location(s), but any attempt to circle them would likely
encompass both (and may be deliberately larger to ensure that both
are captured). Thus, the ability to distinguish the two locations
hinges on the scale and detail of the map in question. The change in
``scale'' for our rulemaking is the extension of the compliance
period to 1 million years.
---------------------------------------------------------------------------

    In responding to comments on this issue, we considered how it might
be possible to demonstrate the increase in projected uncertainties and
provide a quantitative estimate of the degree of increased uncertainty
that might be encountered as a result of variation in parameter values.
To examine the long-term propagation of uncertainty in dose
projections, we used a simplified Yucca Mountain site performance
assessment model and constructed a hypothetical disposal system that
would produce a mean dose to the RMEI of 15 mrem/yr at 10,000 years.
That is, we estimated the number of waste package failures that would
be necessary to produce a disposal system operating at the ``edge of
compliance'' at 10,000 years. This disposal system, which would still
meet the performance standard at 10,000 years, was the reference base
case for our uncertainty analyses. The number of ``failed'' waste
packages needed to produce the reference case dose (a mean of 15 mrem/
yr at 10,000 years) was calculated using the simplified site model and
parameters used in the DOE model, and assumed some components of the
engineered barrier did not function to provide containment (i.e., the
titanium drip shields designed to divert water from the waste packages,
as well as other components of the engineered barrier system, were
removed from the model).\25\ Further, upon ``failure'' of a waste
package, the entire inventory of that package was assumed to be
available for dissolution and transport, subject to solubility limits
applied to each radionuclide.
    To assess the progressive effects of uncertainty, the number of
``failed'' packages was limited to the number necessary to produce 15
mrem/yr at 10,000 years, and the hypothetical site model was used to
make dose projections from 10,000 years (the reference base case)
through the period of peak dose within the period of geologic
stability. Thus, the system established as a starting point for the
peak dose projections was one in which some degree of release and
transport to the RMEI had already taken place within the initial 10,000
years, providing a basis for judging how the continuation of these
processes would change the results over time. These analyses examined
the effects of uncertainties from the natural barrier portion of the
disposal system, since additional waste package failures were not
considered.\26\ It should be recognized that the base case was
determined using probabilistic methods, so the results at 10,000 years
already showed some effects of uncertainty, as indicated by the range
of projected doses with the mean at 15 mrem/yr.
---------------------------------------------------------------------------

    \25\ Although it employed site parameter value distributions
used by DOE, the model used in this analysis was simplified and
``forced'' to the boundary condition of a 15 mrem/yr mean dose at
10,000 years. This analysis should in no way be compared to the
modeling conducted to support DOE's license application.
    \26\ We considered release of radionuclides from the waste form
as a natural process dependent on solubility parameters. The waste
form itself (spent fuel assemblies or vitrified HLW) is often
considered part of the engineered barrier system.
---------------------------------------------------------------------------

    We found that the uncertainty in dose projections, from the base
case (at 10,000 years) to peak dose (as measured by the spread in dose
estimates between the 5th and 95th percentiles at these times),
increased by approximately two orders of magnitude. These results
showed quantitatively that uncertainty in performance projections does
increase with time for the Yucca Mountain system, and supports the
premise that increasing uncertainty reduces the degree of confidence
that can be assumed for very long-term performance assessments. We
believe this supports the premise, discussed earlier, that increasing
uncertainty in dose projections over very long time periods lessens the
ability of performance assessment modeling to meaningfully distinguish
among alternative (and equally ``likely'') ``futures'' represented by
individual model simulations, and ultimately to distinguish among
alternate models and assumptions for site performance assessments. More
detail on the site model we used, parameter databases, sensitivity
analyses and discussion of the results, is provided in the technical
reports describing this work (Docket No. EPA-HQ-OAR-2005-0083-0386).
    NRC must reach a determination of compliance based on the specific
case presented by DOE. In order to conclude that there is a reasonable
expectation that the Yucca Mountain disposal system will comply with
our standard of 100 mrem/yr, NRC must understand the technical basis
for DOE's projections,

including the inherent uncertainties. We believe it is appropriate for
NRC to examine uncertainty in its licensing review in order to achieve
the necessary level of confidence in DOE's understanding and depiction
of the disposal system. Ultimately, in reaching its compliance
determination, it is incumbent upon NRC to clearly state what it can or
cannot conclude from the performance assessment results, within the
limits of science.
5. How Did We Consider Background Radiation In Developing the Peak Dose
Standard?
    We are not adopting the proposed 3.5 mSv/yr (350 mrem/yr) level as
the compliance standard for the period beyond 10,000 years, nor have we
adopted the reasoning used to support the proposed standard (i.e.,
considerations of specific background radiation estimates) to the
selection of the 100 mrem/yr level. We received significant comment on
this aspect of our proposal, much of it taking issue with the concept
of using background radiation as an indicator of ``safe'' levels of
exposure from an engineered facility. We also received additional
information that provided insights into and refined our consideration
of background radiation. For example, commenters referred to monitoring
data collected by the Desert Research Institute indicating that the
unshielded (outdoor) background radiation from cosmic and terrestrial
sources in Amargosa Valley is roughly 110 mrem/yr. Commenters also
informed us that roughly 90% of the population in Amargosa Valley lives
in mobile homes, which has implications for indoor radon exposures.
Other commenters supported the use of a different factor for converting
radon concentrations into dose.
    In considering these comments, as well as those taking issue with
the overall premise described in the proposal, we found the relatively
simple approach used in the proposal evolving into a more complex
undertaking requiring numerous decisions where science did not provide
a definitive answer. Addressing indoor radon estimates presented the
greatest challenge, as indoor radon represented the highest proportion
of overall background radiation. Complicating factors included multiple
ways of calculating radon dose, the prevalence of mobile homes in
Amargosa Valley, limited data sets primarily from the early 1990s, and
data for individual counties in a different format than state-wide
data. We concluded that there was no generally agreed-upon approach in
the context of Amargosa Valley for incorporating indoor radon exposures
into an analysis of background radiation that would lead to a
regulatory standard, particularly given the fact that many commenters
viewed the entire concept as arbitrary. Accordingly, we have decided
not to adopt a standard derived from an analysis of background
radiation estimates at specific locations or the differences between
background radiation estimates at different locations.
    We continue to believe that references to natural sources of
radiation can provide useful insights. IAEA has observed that ``[i]n
very long time frames * * * uncertainties could become much larger and
calculated doses may exceed the dose constraint. Comparison of the
doses with doses from naturally occurring radionuclides may provide a
useful indication of the significance of such cases''. (IAEA WS-R-4,
Docket No. EPA-HQ-OAR-2005-0083-0383, Paragraph A.8) We note that the
100 mrem/yr level reasonably comports with such an analysis as well.
For example, as noted above, 100 mrem/yr is roughly the value reported
by the Desert Research Institute for cosmic and terrestrial radiation
at Amargosa Valley (unshielded). When shielding from buildings is
considered and indoor radon doses are estimated using a more
conservative conversion factor suggested by some commenters, 100 mrem/
yr is at the low end of overall background radiation estimates in
Amargosa Valley and nationally. Within the State of Nevada, the
difference in average estimates of background radiation for counties is
greater than 100 mrem/yr. (Docket No. EPA-HQ-OAR-2005-0083-0387) As
previously stated, this suggests that 100 mrem/yr can be considered to
be a level such that the total potential doses incurred by the RMEI
from the combination of background radiation and releases from Yucca
Mountain will remain below doses incurred by residents of other parts
of the country from natural sources alone.\27\ It may also be noted
that the 100 mrem/yr public dose limit recommended by ICRP is itself
related to background radiation, so indirectly our peak dose standard
does incorporate the concept of variations in background radiation.\28\
However, in the absence of compelling reasons for selecting specific
background radiation estimates and points of comparison, we conclude
that comparing background radiation estimates from specific locations
does not provide a clear or sufficient basis for a regulatory
compliance standard applicable to the Yucca Mountain disposal system.
Discussion of specific issues raised in public comments is in Section 3
of the Response to Comments document.
---------------------------------------------------------------------------

    \27\ It could also be considered consistent with the NEA
statement that ``[w]hat can be aimed at, however, is to leave future
generations an environment that is protected to a degree acceptable
to our own generation. It is also relevant to observe that this
level of protection will ensure that any radiological impacts due to
disposal will not raise levels of radiation above the range that
typically occurs naturally.'' (``The Handling of Timescales in
Assessing Post-Closure Safety: Lessons Learnt from the April 2002
Workshop in Paris, France,'' p. 9, Docket No. EPA-HQ-OAR-2005-0083-
0046)
    \28\ ``This natural background may not be harmless * * * but the
variations from place to place (excluding the large variations in
the dose from radon in dwellings) can hardly be called unacceptable
* * * Excluding the very variable exposures to radon, the annual
effective dose from natural sources is about 1 mSv, with values at
high altitudes above sea level and in some geological areas of at
least twice this. On the basis of all these considerations, the
Commission recommends an annual limit on effective dose of 1 mSv.''
(ICRP Publication 60, Docket No. EPA-HQ-OAR-2005-0083-0421,
Paragraphs 190-191)
---------------------------------------------------------------------------

6. How Does Our Rule Protect Future Generations?
    Because of its long lifetime, high hazard, and potential for
misuse, SNF and HLW present special challenges to those charged with
protecting the health, safety, and security of the public and the
environment. Geologic disposal has long been viewed by policymakers as
the management option that best addresses all of these concerns.\29\ In
the United States, geologic disposal was first endorsed by the NAS in
1957 (``The Disposal of Radioactive Waste on Land'') and established as
national policy in the Nuclear Waste Policy Act of 1982.
---------------------------------------------------------------------------

    \29\ In its 1995 Collective Opinion, the NEA Radioactive Waste
Management Committee concludes that ``from an ethical standpoint,
including long-term safety considerations, our responsibilities to
future generations are better discharged by a strategy of final
disposal than by reliance on stores which require surveillance,
bequeath long-term responsibilities of care, and may in due course
be neglected by future societies whose structural stability should
not be presumed'' and ``after consideration of the options for
achieving the required degree of isolation of such wastes from the
biosphere, geological disposal is currently the most favoured
strategy,'' whereby ``it is justified, both environmentally and
ethically, to continue development of geological repositories for
those long-lived radioactive wastes which should be isolated from
the biosphere for more than a few hundred years.'' (``The
Environmental and Ethical Basis of Geological Disposal of Long-Lived
Radioactive Wastes,'' Docket No. EPA-HQ-OAR-2005-0083-0412, pp. 5-6)
Similarly, the NAS Board on Radioactive Waste Management stated:
``There is a strong worldwide consensus that the best, safest long-
term option for dealing with HLW is geological isolation.''
(``Rethinking High-Level Radioactive Waste Disposal: A Position
Statement of the Board on Radioactive Waste Management,'' 1990,
Docket No. EPA-HQ-OAR-2005-0083-0420, p. 2)
---------------------------------------------------------------------------

    However, the fact that geologic disposal has potentially
significant

impacts over times far in excess of recorded human history naturally
raises concerns as to how the welfare of people living far in the
future can and should be taken into account when societal institutions
may no longer exist to provide oversight of a disposal facility.\30\
---------------------------------------------------------------------------

    \30\ NEA states: ``The design and implementation of a repository
involves balancing of risks and responsibilities between
generations. The obligations of the present generation toward the
future are complex, involving not only issues of safety and
protection but also of freedom of choice and of the accompanying
burden of responsibility, and of the need to transfer knowledge and
resources. Our capacity to deliver these obligations diminishes with
distance in time, which complicates the setting of criteria to be
used today in order to demonstrate that obligations to the future
will be met.'' NEA-6182, Docket No. EPA-HQ-OAR-2005-0083-0408, p.
25)
---------------------------------------------------------------------------

    In considering how our standards reflect these intergenerational
issues, we considered the guidance offered by the NAS committee. (See
70 FR 49036) In citing NRC and IAEA sources on the question of
intergenerational equity, NAS wrote:

    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.

NAS Report pp. 56-57, emphasis added.

    We generally agree with the NAS statement. A single dose standard
applicable at all times would typically be consistent with a close
reading of the principle of intergenerational equity as stated by NAS.
However, NAS clearly acknowledges that ``some other'' approach could
also be consistent with that principle. We believe it is reasonable to
conclude that ``some other'' approach must include situations where it
may not be reasonable to apply the same dose standard at all times
because of the extremely long compliance period. We believe
establishing a peak dose standard for the Yucca Mountain disposal
system is a situation in which ``some other expression of
intergenerational equity'' is more appropriate than is applying a
single dose standard of 15 mrem/yr throughout the compliance period.
The rulemaking process we are following is the accepted way for
``social judgment'' to be incorporated into regulations.
    NAS made no recommendation regarding the appropriate expression of
intergenerational equity, just as it made no recommendation regarding
the level of the final peak compliance standard. Rather, NAS
acknowledged EPA's wide latitude to exercise its policy judgment.
    We emphasize that we do not question whether there is an obligation
to future generations, but we believe there is no consensus regarding
the nature of that obligation, for how long it applies, whether it
changes over time, or how it can be discharged. Regarding radioactive
waste management and geologic disposal, there is general agreement that
assurances can be provided that the protections offered will be similar
to those applied to current activities for periods approximating 10,000
years, which is a very long time. It also is generally accepted that
engineered barriers cannot be relied upon indefinitely, and that
projected doses may eventually exceed the initial regulatory levels.
The question of equity is also raised by the fact that the repository
is part of a passive disposal system that may provide complete
containment for hundreds of generations without their knowledge, but
present the greatest risks to equally unsuspecting generations beyond
that time. However, it is unclear as to exactly how such long-term
projected doses should be factored into a judgment of facility safety,
if we are not confident they can be interpreted in the same way at all
times.\31\ We are establishing today a standard consistent with a
public dose limit of 100 mrem/yr that is deemed protective today as a
matter of international consensus, which would not affect the quality
of life for future generations, even those hundreds of thousands of
years distant. We believe this is a reasonable level of commitment for
such long times, given the complexities of the situation and what we
see as our responsibility to establish a level of compliance, not a
soft target or reference level that could be exceeded for unspecified
reasons and by unspecified amounts.
---------------------------------------------------------------------------

    \31\ NEA-6182: ``National programmes which have already
established such criteria have generally found it possible to make
cautious, but reasonable assumptions to extend the use of
radiological limits already applied to contemporary activities for
several thousands of years. The greater challenge lies in setting
criteria for very long time frames, extending to a million years and
beyond, for which safety analyses must account for high uncertainty
and for which the understanding of the needs and impacts on future
generations become increasingly speculative.'' (Docket No. EPA-HQ-
OAR-2005-0083-0408, pp. 20-21).
---------------------------------------------------------------------------

    In conclusion, EPA acknowledges and remains committed to the
principles of intergenerational equity. However, we do not interpret
these principles as requiring that the same compliance standard must
apply at all times. Such an approach is overly simplistic in the
circumstances and ignores the complexities involved in establishing
radiological protection standards for periods approaching 1 million
years. We believe that peak dose limits over such periods should be
viewed as qualitatively different from limits applied at earlier times;
in other words, the basis for judgment at different times is not the
same. As a matter of public policy, a commitment to protect future
generations over the next 10,000 years at levels consistent with
standards applied for the current generation, and to protect more
distant generations at levels consistent with the overall public dose
limits deemed protective today and adopted nationally and
internationally, protects public health and the environment across
generations in a manner that comports with the objective of
intergenerational equity. Under this approach, future generations will
not face undue burdens or the irreversible loss of reasonable options
arising from a decision by the current generation to pursue a policy of
geologic disposal at Yucca Mountain, nor will the compliance
demonstration demand more than can be provided by scientific analysis.
The standards applicable to both time frames are protective of public
health and safety and will offer comparable, if not identical,
protections to the affected generations. See section 9 of the Response
to Comments document for more detailed discussion of these issues.
7. What is Geologic Stability and Why is it Important?
    Underlying the NAS recommendation to assess compliance at the time
of maximum risk is the concept of geologic stability (i.e., peak dose
should be assessed ``within the limits imposed by the long-term
stability of the geologic environment,'' NAS Report p. 2). NAS viewed
this as an important consideration in assessing performance, both
analytically and in regulatory review. Indeed, NAS discussed two
important kinds of uncertainty in describing this concept, which are
spatial and temporal uncertainty. The committee concluded that spatial
uncertainties will always exist no matter what time frame is used for
the performance assessments. Temporal uncertainties, on the other hand,
will vary over different time frames, and the presence of such
uncertainties indicates the advisability of defining a ``period of
geologic stability,'' during which performance projections can be made
with some degree of confidence. For time periods where conditions at
the site would change dramatically in a relatively short time,
projections of site

conditions would be highly speculative, and consequently performance
assessments would have very limited if any validity. It is important to
understand that ``stable'' in this context is not synonymous with
``static and unchanging.'' Rather, NAS recognized that many ``physical
and geologic processes'' are characteristic of any site and have the
potential to affect performance of the disposal system. NAS concluded
that these processes could be evaluated as long as ``the geologic
system is relatively stable and varies in a boundable manner'' (NAS
Report p. 9). Thus, the site itself could be anticipated to change over
time, but in relatively narrow ways that can be defined (``bounded'').
Implicit in the NAS recommendation is the idea that the maximum risk
might occur outside the period of geologic stability, but assessments
performed at that time would have little credibility and would not be a
legitimate basis for regulatory decisions: ``After the geologic
environment has changed, of course, the scientific basis for
performance assessment is substantially eroded and little useful
information can be developed.'' (NAS Report p. 72)
    NAS judged this period of ``long-term stability'' to be ``on the
order of one million years.'' (NAS Report p. 2) We describe in section
III.A.8 (``Why is the Period of Geologic Stability 1 Million Years?'')
the policy judgment on our part to explicitly equate the period of
geologic stability with 1 million years. More important, however, is to
understand the relationship among the regulatory definition, the
physical reality of the site, and the performance assessment models. In
reaching its conclusion, NAS considered information available on the
site properties and the processes as they currently operate. This
provides a basis for understanding how the site functions today, but
would not be sufficient to project that understanding for periods of
millions of years into the future. To do that, NAS also considered
information obtained through studies of the geologic record at the
site, to see if evidence existed for times when processes were either
fundamentally different or they operated at different rates. This is
similar to our recommendation that DOE consider at least the last two
million years (the Quaternary period) in characterizing FEPs. In fact,
examination of the Quaternary geologic record is an important component
in understanding the evolution of the geologic setting over time. NAS
expressed confidence that neither the processes active at the site, nor
the site itself, had changed in fundamental ways over the Quaternary
Period and longer, and probably would continue to behave much as it
does today for the next million years. NAS therefore suggested that
geologic conditions could be bounded with reasonable confidence for
periods ``on the order of one million years.'' (NAS Report p. 2)
    Models used to assess performance need to incorporate a description
of the bounds under which the model can be considered valid, so as to
avoid physically impossible situations, as well as assure that the
conceptual models upon which the performance assessments are based
reasonably represent the way the site is expected to behave over the
period of stability. They must be defined so that significant changes
to the properties of the site and physical and geologic processes are
not projected inadvertently to create conditions of ``geologic
instability.'' That is, they must avoid crossing over into sets of
conditions that would in reality not be a geologically stable
situation, or are outside the bounds under which the model can be
considered valid. Here again the examination of the geologic record at
the site provides the means of constructing the models to adequately
make simulations of future performance that reflect the range of
potential expected conditions at the site over the regulatory
compliance period. Parameter value distributions used in the
simulations, which are the fundamental input information used to make
the dose assessments, should not be limited only to data collected for
the present situation at the site, but should consider how those
parameter values could change over the period of stability. Expert
judgment, where appropriate, based upon site-specific information and
broader understanding of how these processes operate in general, plays
an important role in defining such modeling input data.
    The geologic record is the primary source of information on the
question of geologic stability and was considered by NAS in reaching
its conclusions about the geologic stability period. We believe that
the geologic record at the site clearly supports the position that the
site will be stable over the course of the next million years.
Conclusions based on extrapolation beyond what can be supported in the
geologic record should be avoided.
8. Why is the Period of Geologic Stability 1 Million Years?
    Today's final rule includes a compliance period of 1 million years,
over which DOE must project performance and demonstrate compliance with
the individual-protection and human-intrusion standards. As discussed
at length in our proposal and more briefly in Sections I and II of this
document, our rulemaking is in response to the DC Circuit decision
vacating the 10,000-year compliance period in our 2001 rule. The Court
concluded that the 10,000-year compliance period was not based upon and
consistent with the NAS recommendations, as the EnPA required. NAS
recommended ``that compliance with the standard be assessed at the time
of peak risk, whenever it occurs, within the limits imposed by the
long-term stability of the geologic environment, which is on the order
of one million years.'' (NAS Report p. 2) NAS found 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,'' and accordingly ``there is no
scientific basis for limiting the time period of an individual-risk
standard.'' (NAS Report p. 6) As a matter of policy, we believe it is
appropriate and necessary to define a compliance period within which
our standards apply. This section discusses the considerations that led
us to conclude that a compliance period of 1 million years is
appropriate from a policy perspective and consistent with NAS
statements regarding geologic stability at Yucca Mountain.
    As discussed in section III.A.7 (``What is Geologic Stability and
Why is it Important?''), the NAS introduced the concept of geologic
stability in its report and referred to it repeatedly in its
discussions (NAS Report, e.g., pp. 9, 55, 69, 71, and 72). In
discussing the physical properties and geologic processes leading to
the transport of radionuclides away from the repository, the NAS
committee concluded ``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.'' (NAS Report p. 9) While variation of site characteristics
over time produces some uncertainty, NAS believed that such changes
could be bounded during the period of geologic stability of the site,
i.e., as long as the conditions do not change significantly. (NAS
Report pp. 72, 77) NAS also noted that ``[a]fter the geologic
environment has changed, of course, the scientific basis for

performance assessment is substantially eroded and little useful
information can be developed.'' (NAS Report p. 72) While NAS made no
additional qualification on what constituted ``significant'' changes,
it made numerous references in its report to a stability period for the
site ``on the order of one million years.'' The committee concluded
that during this period it would be feasible to make projections of
repository site conditions. We concur and believe that assessments can
be made and bounded where uncertainty exists, and consequently
performance assessments can be developed with adequate confidence for
regulatory decision-making within the context of the requirements
adopted in today's final rule. We discuss some additional
qualifications to this proposition in the remainder of this section.
    While the NAS characterized the length of the geologic stability
period in loose terms (``on the order of''), we believe it is
appropriate to fix the stability period duration as a matter of
regulatory policy. We find support on this point from NAS: ``It is
important, therefore, that the `rules' for the compliance assessment be
established in advance of the licensing process.'' (NAS Report p. 73).
We believe, therefore, as a matter of regulatory philosophy and policy,
that a relatively loosely defined stability period ``on the order of''
one million years is not sufficiently specific for regulatory purposes,
i.e., implementing our standards and reaching a compliance decision.
Indeed, NAS clearly considered that the compliance period could be one
of the ``rules'' that should be established for compliance assessments.
(NAS Report p. 56) Some commenters suggested that the period of
geologic stability could be longer (or interpreted ``on the order of
one million years'' as possibly as long as ten million years), and said
our rule should allow consideration of longer timescales if justified
by considerations of geologic stability. The actual period of geologic
stability at Yucca Mountain is unknowable, and we disagree that an
open-ended compliance standard is justified over such time frames. We
believe that the applicant (DOE) and the compliance decision-maker
(NRC) must have definitive markers to judge when compliance is
demonstrated, and that a loosely defined time frame does not provide
such a marker for implementation of our standards in a licensing
process. We believe that the geologic stability period of 1 million
years that we have defined provides the necessary marker, and is within
our discretion to set as a matter of policy. (See generally NAS Report
p. 3) To do otherwise we believe would leave the licensing process in a
potentially untenable situation of dealing with possibly endless debate
over exactly when a peak dose occurs in relation to a compliance period
time limit. Such debate can arise because of the inherent uncertainty
that exists in characterizing the complex processes and variables
involved in projecting performance of the disposal system over very
long periods of time. As the NAS explained, ``although the selection of
a time period of applicability has scientific elements, it also has
policy aspects we have not addressed.'' (NAS Report p. 56)
    As commenters have pointed out, the rate of waste package failure
is a dominant factor in determining when the peak dose for a
probabilistic assessment will occur. With all the parameters (and the
uncertainty in their values over time) involved in a total system
performance assessment, as well as the assumptions necessary to select
processes involved in projecting performance, it is quite possible that
significant debate could result in the licensing process over selection
of the parameter values and the resulting timing of the peak dose
results. We do not believe such debate is constructive because it would
not advance the goal of providing a reasonable test of the disposal
system. We also believe that the 1 million year stability period
provides the needed definitive marker for judging the time over which
the standards apply and is an appropriate exercise of our policy
discretion.
    Throughout our proposal and in this final rule we have cited a
significant number of international references to support policy
judgments such as the one discussed here. Readers may recall that we
cited such references suggesting that dose projections beyond 1 million
years have little credibility and believe that we used those arguments
to justify proposing the 1 million-year compliance period (70 FR 49036,
August 22, 2005). We did not explicitly discuss in the proposal our
reasons for selecting 1 million years as the compliance period and
equating it to the period of geologic stability, other than references
to the NAS language that it is ``on the order of'' 1 million years.
However, these sources do generally reflect widespread acceptance of
the proposition that quantitative performance projections at very long
time frames have limited utility for regulatory decision-making, and
that 1 million years may be a reasonable reference point beyond which
such projections either should not be required or should be considered
only in their broadest sense.\32\ Further, while it should be clear
that we agree with the thrust of those international sources regarding
the effects of uncertainty on long-term dose projections and the
relative level of confidence that can be placed in them for decision-
making, we believe the post-10,000-year peak dose standard in today's
final rule appropriately accommodates those considerations and is
protective of public health, meaningful, implementable, and provides a
reasonable test of the disposal system that is consistent with the NAS
Report, DC Circuit decision, and the principles of reasonable
expectation.
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    \32\ For example, in general guidance documents, the IAEA has
stated that ``little credibility can be attached to assessments
beyond 10\6\ years.'' (``Safety Indicators in Different Time Frames
for the Safety Assessment of Underground Radioactive Waste
Repositories,'' IAEA-TECDOC-767, p. 19, 1994, Docket No. EPA-HQ-OAR-
2005-0083-0044) In its final 2006 Safety Requirements for Geological
Disposal of Radioactive Waste, IAEA also states, ``Care needs to be
exercised in using the criteria beyond the time where the
uncertainties become so large that the criteria may no longer serve
as a reasonable basis for decision making.'' (Docket No. EPA-HQ-OAR-
2005-0083-0383, page 11, paragraph 2.12) As a country-specific
example, final guidelines from the Swedish Radiation Protection
Authority state that ``the risk analysis should be extended in time
as long as it provides important information about the possibility
of improving the protective capability of the repository, although
at the longest for a time period of one million years.'' (Docket No.
EPA-HQ-OAR-2005-0083-0388) Also, in an example where the official
guidelines specify a risk target that is of undefined duration, the
United Kingdom's National Radiological Protection Board has stated
that ``[o]ne million years is * * * the timescale over which stable
geological formations can be expected to remain relatively
unchanged,'' while concluding that the scientific basis for risk
calculations past one million years is ``highly questionable.''
(``Board Statement on Radiological Protection Objectives for the
Land-based Disposal of Solid Radioactive Wastes,'' 1992 Documents of
the NRPB, Volume 3, No. 3, p. 15, Docket No. EPA-HQ-OAR-2005-0083-
0416)
---------------------------------------------------------------------------

    To support these general policy arguments, which would lead us to
consider a time period of approximately 1 million years as an
appropriate regulatory time frame, it is necessary to address NAS's
scientific judgments. While NAS did not define with precision the
period of time that the geologic environment likely would remain
stable, for purposes of our regulation we believe scientific
information can be relied upon to support a firm definition of that
period as ending at 1 million years after disposal. Further, we believe
that equating a specific time period with the ``period of geologic
stability'' is a site-specific decision, as NAS's statements regarding
geologic stability were wholly in the context of Yucca Mountain. (See,
for example, NAS Report p. 69: ``The time scales of long term geologic

processes at Yucca Mountain are on the order of 10\6\ years''; and NAS
Report p. 85: ``The geologic record suggests this time frame is on the
order of about 10\6\ years.'') Therefore, we have considered how the
natural processes and characteristics at the Yucca Mountain site would
support defining the period of geologic stability as ending at a
specified time after disposal. In considering the natural setting, many
comments expressed the view that the site's natural characteristics are
so conducive to rapid release and transport of radionuclides, only the
waste packages and other engineered barriers would make it possible for
significant doses to be delayed much beyond 10,000 years. We believe it
is therefore also appropriate to consider the geologic stability period
from the perspective of a reasonable length of time for significant
events to act on the waste packages and engineered barriers, and
ultimately lead to release of radionuclides. Natural processes and
events would contribute to both the package failures and to the
subsequent transport of radionuclides, even if such failures occur
relatively late in the period under consideration.
    A consideration of the geologic history of the site, in the areas
of igneous and seismic activity, also supports a 1 million year
stability period. Information compiled by NRC (Docket No. EPA-HQ-OAR-
2005-0083-0373) concerning basaltic igneous activity around the site
shows that this type of activity has been the only activity around the
site through the Pliocene (beginning roughly 5.4 million years ago),
and that the volume of eruptive activity (both tuff and basaltic
material) has decreased continually over the last 10 million years
(Coleman et al., 2004, Docket No. EPA-HQ-OAR-2005-0083-0378). From the
identification of surface features as well as indicators of buried
remnants of past volcanic activity, the episodes of basaltic activity
around the site can be shown to have occurred in clusters of events
around 1 million and 4 million years ago (Hill, 2004, Docket No. EPA-
HQ-OAR-2005-0083-0373). The occurrence of these clusters indicates that
the nature and extent of past volcanic activity can be reasonably well
characterized and that annual probabilities for such events can be
reasonably estimated from the geologic record around the site. Annual
probabilities of volcanic disruptions to the repository have been
estimated by various investigators, and range from as high as
10-6 to as low as 5.4 x 10-10 (Coleman et al.,
2004, Docket No. EPA-HQ-OAR-2005-0083-0378).
    Further, while geologic stability may be viewed as being affected
primarily by large-scale events, accumulations of small-scale changes
over very long time periods also have the potential to alter the
geologic setting and affect the technical basis for performance
assessments. Tectonic events have such a potential at Yucca Mountain.
Rates of displacement on the nearest potentially significant fault in
the region average about 0.02 mm/yr. (DOE, Science & Engineering
Report, 2002, p. 4-409, Docket No. EPA-HQ-OAR-2005-0083-0069) This
means that in 10,000 years, there could be 20 cm (0.65 ft) of
displacement, a relatively small change not likely to affect
performance of the geologic system. However, in 1 million years, the
same rate of movement results in 20 m (65 ft) of displacement on the
fault. Using the larger estimates of movement within the range of
potential movement, displacement could be as much as 30 m (100 ft) over
1 million years. Such changes in the geologic setting at Yucca Mountain
have the potential to erode the scientific basis for performance
assessment and possibly to affect the quality of the information the
assessment can provide to decision-makers.
    NAS also stated that ``we see no technical basis for limiting the
period of concern to a period that is short compared to the time of
peak risk or the anticipated travel time.'' (NAS Report p. 56) This
statement suggests that the stability period must be long enough to
allow FEPs that pass the probability and significance screens to
demonstrate their effects, if any, on the results of the performance
assessments, even from waste package failures occurring relatively late
in the period. In contrast to the accumulated small-scale changes
discussed above, larger-scale seismic events are more likely to
contribute directly to radionuclide releases through the effects of
ground motion. Strong seismic events could damage waste package
integrity by causing emplacement drift collapse or vigorous shaking of
the packages themselves. Earthquake recurrence intervals for the site
indicate that strong events could reasonably be assumed to test waste
package integrity at various times within the 1 million-year period
(Docket No. EPA-HQ-OAR-2005-0083-0374 and 0379). In addition, we note
that estimates of ground water travel time from the repository to the
RMEI location are on the order of thousands of years (see the BID for
the 2001 final rule, Docket No. EPA-HQ-OAR-2005-0083-0050). At these
rates, the effects of disruptive volcanic and seismic effects on
releases would not be delayed from reaching the RMEI location during
the stability period, e.g., added releases from a low probability
seismic event at 800,000 years would have ample time to be captured by
the performance assessments. Based on these considerations, the 1
million-year period is a sufficiently long time frame to evaluate the
potential consequences of both gradual processes and disruptive events
on disposal system performance.
    In summary, for regulatory policy as well as site-specific
scientific considerations, we believe that fixing the period of
geologic stability for compliance assessments at 1 million years
provides a reasonable test for the disposal system performance. We
believe a fixed time period is necessary both to provide a definitive
marker for compliance decision-making and to prevent unbounded
speculation surrounding the factors affecting engineered barrier
performance and the ultimate timing of peak dose projections.
Examination of site characteristics indicates that the influences of
natural processes and events on release and transport of radionuclides
would be demonstrated even for waste package failures occurring
relatively late in the period. We believe that setting a 1 million year
limit is a cautious but reasonable approach consistent with the NAS
position on bounding performance assessments for uncertain elements
affecting disposal system performance. Finally, explicitly defining the
period during which our standards apply will focus attention on times
for which the geologic setting and associated processes are more
quantifiable and boundable, rather than entering debate on disposal
system performance in time periods where the fundamental geologic
regime may have sufficiently changed so that the ``scientific basis for
performance assessment is substantially eroded and little useful
information can be developed.'' (NAS Report p. 72)
9. How Will NRC Judge Compliance?
    Today's final rule directs NRC to use the arithmetic mean of the
distribution of projected doses to determine compliance with both the
150 [mu]Sv/yr (15 mrem/yr) dose standard applicable for the first
10,000 years after closure and the 1 mSv/yr (100 mrem/yr) peak dose
standard applicable between 10,000 and 1 million years after closure.
In reaching this decision, we considered comments raising legal,
technical, and policy points. Foremost among these were comments
focusing on a statement by the NAS committee: ``We

recommend that the mean values of calculations be the basis for
comparison with our recommended standards.'' (NAS Report p. 123)
    After considering public comments, the NAS Report, and the DC
Circuit decision, we conclude that the use of the arithmetic mean to
determine compliance at all times, without conditions or restrictions,
is straightforward and clearly consistent with the NAS recommendation,
pursuant to the EnPA. Consistent with our proposal, we are specifying
that the ``mean'' to be used is the arithmetic mean, as this is
consistent with the intent of 40 CFR part 191 and its implementation at
WIPP. See section 7 of the Response to Comments document for more
discussion of the points raised in public comments.
10. How Will DOE Calculate the Dose?
    Today's final rule requires DOE to calculate the annual committed
effective dose equivalent (CEDE) for comparison to the storage,
individual-protection, and human-intrusion standards using the
radiation- and organ-weighting factors in ICRP Publication 60 (``1990
Recommendations of the ICRP''), rather than those in ICRP Publication
26 (``1977 Recommendations of the ICRP''). As we described in our
proposal, this action will incorporate updated scientific factors
necessary for the calculation, but will not change the underlying
methodology. We explained in some detail the use of the terms
``effective dose equivalent'' and ``effective dose'' in the EnPA, the
DC Circuit decision, the ICRP publications, and our previous actions to
support our position that use of the weighting factors in ICRP 60 (and
its follow-on implementing Publication 72) is consistent with
calculation of effective dose equivalent, as required by the EnPA. (70
FR 49046-49047)
    We received some comment disagreeing with our conclusion that use
of the term ``effective dose equivalent'' is consistent with the use of
the ICRP 60 weighting factors. As we discussed in our proposal, we
believe a close reading of ICRP 60 supports our interpretation that
effective dose equivalent and effective dose are synonymous concepts.
ICRP defined two weighting factors in ICRP 26, the radiation quality
factor, Q, and the tissue weighting factor, WT. In ICRP 60,
the quality factor was replaced by the radiation weighting factor,
WR, with the same values assigned to alpha, beta, and gamma
radiation. In ICRP 26, the tissue weighting factor was presented as a
rigid construct with defined values for specific organs. In ICRP 60,
the tissue weighting factor was redefined as a set of recommended
values for an expanded set of organs (which could be modified in cases
where scientific information was available to support using alternative
factors), and it was explained that the attributes of the tissue
weighting factor include the components of detriment cited by the
comments (fatal and non-fatal cancers, length of life lost, and
hereditary effects). However, ICRP made a clear distinction between its
renaming of the doubly weighted dose quantity from ``effective dose
equivalent'' (ede) to ``effective dose'' (E) and its redefining of
WT. The association of effective dose equivalent with the
ICRP 26 tissue weighting factors is thus coincidental but not required.
We cited ICRP to that effect in our proposal:

    The weighted equivalent dose (a doubly weighted absorbed dose)
has previously been called the effective dose equivalent but this
name is unnecessarily cumbersome, especially in more complex
combinations such as collective committed effective dose equivalent.
The Commission has now decided to use the simpler name effective
dose, E. The introduction of the name effective dose is associated
with the change to equivalent dose, but has no connection with
changes in the number or magnitude of the tissue weighting factors *
* *

ICRP Publication 60, p. 7, paragraph 27, Docket No. EPA-HQ-OAR-2005-
0083-0421, emphasis added.

    Similarly, ICRP also states:

    The values of both the radiation and tissue weighting factors
depend on our current knowledge of radiobiology and may change from
time to time. Indeed, new values are adopted in these
recommendations * * *. It is appropriate to treat as additive the
weighted quantities used by the Commission but assessed at different
times, despite the use of different values of weighting factors. The
Commission does not recommend that any attempt be made to correct
earlier values. It is also appropriate to add values of dose
equivalent to equivalent dose and values of effective dose
equivalent to effective dose without any adjustments.

ICRP Publication 60, p. 9, paragraph 31, Docket No. EPA-HQ-OAR-2005-
0083-0421, emphases added.

    In summary, we believe the intent of Congress in specifying
effective dose equivalent is that the Yucca Mountain standards be based
on a doubly weighted dose quantity, not that the assessment of that
quantity be tied to factors developed at a particular time, when newer
science indicates those factors should be updated. We use effective
dose equivalent for consistency with the terminology used in the EnPA,
but are adopting in today's final rule the current recommended values
for WT. Our approach is thus fully consistent with both the
current ICRP recommendations and the EnPA.
    Today's final rule does incorporate a change to the proposed
definition of ``effective dose equivalent'' in Sec.  197.2 to make it
consistent with language in Appendix A regarding the potential use of
future ICRP recommendations. We received some comments suggesting that
the appendix should not include specific weighting factors, but state
only that doses are to be calculated in accordance with the methods of
ICRP 60/72. The commenter believes this is appropriate because NRC's
proposed licensing requirements included the tissue weighting factors,
but not the radiation weighting factors. Further, the commenter points
out that dose coefficients in ICRP 72 (and Federal Guidance Report 13)
consider a somewhat different set of organs than do the tissue
weighting factors. We prefer not to adopt the commenter's suggestion,
which we believe could lead to questions regarding the appropriate
factors to use. We note that ICRP 60, unlike ICRP 26, is not tied to a
specific set of weighting factors, and allows for the possibility that
users will substitute their own preferred set of factors. Stating only
that the methods of ICRP 60/72 be used to calculate dose, without the
additional stipulations in the appendix, would not provide sufficient
clarity on this point. Therefore, we are adding language to the
definition in Sec.  197.2 to the effect that NRC can direct that other
weighting factors be used to calculate dose, consistent with the
conditions presented in Appendix A. We believe this will effectively
address the commenter's concern.

B. How Will This Final Rule Affect DOE's Performance Assessments?

    Today's final rule requires DOE to demonstrate compliance with the
individual-protection standard through use of performance assessment. A
performance assessment is developed by first compiling lists of
features (characteristics of the disposal system, including both
natural and engineered barriers), events (discrete and episodic
occurrences at the site), and processes (continuing activity, gradual
or more rapid, and which may occur over intervals of time) anticipated
to be active during the compliance period of the disposal system. These
items are collectively referred to as ``FEPs'' (features, events, and
processes). Once FEPs are identified, they are evaluated for their
probability of occurrence (i.e., how likely they are to occur during
the compliance period) and their effect on the results of the
performance assessment (i.e., do they significantly

affect projected doses from the disposal system during the first 10,000
years after disposal). Addressing these aspects of performance
assessment for a compliance period of 1 million years was a central
aspect of our proposal and is the focus of this section.
    After considering public comments, we are retaining Sec.  197.36 as
proposed, with two modifications. First, the probability threshold for
FEPs to be considered for inclusion in performance assessments
conducted to show compliance with Sec.  197.20(a)(1) is now stated as
an annual probability of 1 in 100 million (10-8 per
year).\33\ Because the same FEPs included in these performance
assessments will also be included in performance assessments conducted
to show compliance with Sec.  197.20(a)(2), the same probability
threshold applies in all cases. Second, we are adding a provision to
address a potential effect of seismicity on hydrology that was
identified by NAS. The final rule now requires the potential effects of
a rise in the ground-water table as a result of seismicity to be
considered. If NRC determines such effects to be significant to the
results of the performance assessment, it shall specify the extent of
the rise for DOE to assess.
---------------------------------------------------------------------------

    \33\ Only FEPs with an annual probability greater than or equal
to 10-5 need to be considered in performance assessments
to show compliance with Sec. Sec.  197.25(b) and 197.30. FEPs below
this probability threshold, but still above 10-8 per
year, are defined by NRC as ``unlikely''.
---------------------------------------------------------------------------

    Our 2001 rule set forth three basic criteria for evaluating FEPs
for their potential effects on site performance and their incorporation
into the scenarios used for compliance performance assessments (Sec.
197.36). These criteria retained the same limitations originally
established in 40 CFR part 191, which were developed to apply to any
potential repository for spent nuclear fuel, high-level waste, or
transuranic radioactive waste. We believe that approach remains
reasonable for the site-specific Yucca Mountain standards, and we
believe it is desirable to maintain consistency between the two
regulations for geologic repositories in the basic criteria for
evaluating FEPs. The criteria for evaluating FEPs are:
     A probability threshold below which FEPs are considered
``very unlikely'' and need not be included in performance assessments;
     A provision allowing FEPs above the probability threshold
to be excluded from the analyses if they would not significantly change
the results of performance assessments; and
     An additional stipulation that in addition to ``very
unlikely'' FEPs, ``unlikely'' FEPs need not be considered in
performance assessments conducted to show compliance with the human-
intrusion and ground-water protection standards.
    As an initial step, a wide-ranging set of FEPs that potentially
could affect disposal system performance is identified. The term
``potentially'' is key here, because at this early stage, the list is
deliberately broad, focusing more on ``what could happen'' rather than
``what is likely to happen at Yucca Mountain.'' Under the 2001 rule,
each of these FEPs is then examined to determine whether it should be
included in an assessment of disposal system performance over a 10,000-
year period by evaluating the probability of occurrence at Yucca
Mountain and, as appropriate, the effects of the FEP on the results of
the performance assessment. Based on these evaluations, a FEP may be
excluded from the assessment of disposal system performance on the
basis of probability, or if the results of the performance assessments
would not be changed significantly by its exclusion.
    We included in our proposal provisions describing how FEPs should
be incorporated into assessments of disposal system performance during
the period of geologic stability, defined as ending at 1 million years
after closure. Our purpose was to build upon the provisions applicable
to the 10,000-year compliance period in our 2001 rule to address the
complexities introduced by extending the compliance period to 1 million
years. In general, the database of FEPs applicable to Yucca Mountain
should be the same, regardless of the period covered by the
assessments. In developing our proposal, however, we considered how
these general provisions might change when the compliance period
extends to 1 million years. We also proposed specific provisions to
address climate change, seismicity, and igneous events, which were
identified by NAS as potential ``modifiers'' whose effects could be
bounded within the period of geologic stability.
    Some commenters questioned whether our authority to establish
public health protection standards for Yucca Mountain extended to
specifying how FEPs must be considered, contending that this function
properly lies with the implementing authority (NRC). We disagree. While
NRC clearly has authority to specify such provisions, it is also within
our purview to stipulate such conditions as are necessary to place our
regulations in context and ensure they are implemented as we intended.
For analyses covering 1 million years, it is important to focus on
those factors most affecting performance, if necessary by excluding
other aspects that are more likely to have little or no significance.
We believe this approach is consistent with the direction from NAS. NAS
was charged with providing advice to EPA on ``reasonable standards for
protection of public health and safety'' (EnPA section 801(a)(2)). NAS
provided its findings and recommendations in the context of standards
to be developed by EPA, including discussion of FEPs, for example:
``the radiological health risk from volcanism can and should be subject
to the overall health risk standard to be required for a repository at
Yucca Mountain.'' (NAS Report p. 95) Further, NAS discussed the
question of uncertainty in quantifying physical and chemical processes
and their operation over long time periods and the inevitability of
``residual, unquantifiable uncertainty,'' stating ``[t]he only defense
against it is to rely on informed judgment.'' (NAS Report p. 80)
Therefore, we believe it appropriate to specify, where necessary,
additional provisions for the treatment of FEPs in disposal system
assessments to avoid boundless speculation. We have explained our
understanding of the proper use of bounding performance scenarios, and
we believe we are consistent with the NAS on this point. Bounding
assessments addressing uncertainty in understanding the long-term
behavior of the site should be constructed using informed judgment, not
speculative assumptions without credible supporting evidence.
    Two of the criteria for evaluating FEPs, probability and
significance of the impacts on performance assessments, are of primary
importance in considering how the provisions applicable to the 10,000-
year period might change when the compliance period is extended to 1
million years. In the proposed rule, we concluded that the 10,000-year
FEPs screening could serve as an adequate basis for longer-term
assessments because it is sufficiently inclusive to be appropriate for
the entire 1 million-year compliance period, while at the same time
reasonably bounding the scenarios that must be considered over the
longer time frame. We thought our statements in the preamble on this
point were sufficiently clear, but we understand that the way we
structured Sec.  197.36 of the proposal, essentially separating the two
time periods, may have caused some confusion. For example, we did not
intend to indicate or imply that the

post-closure performance assessments would consist of two separate and
dramatically different calculations, with each having distinctly
different scenario construction, parameter value distributions, or
other attributes. Regardless of the standard against which compliance
is being judged, the probability of occurrence and the significance of
the impacts on performance assessment are the two primary criteria for
including a FEP in the compliance analysis. The screening for FEPs is
done for the 10,000-year performance assessment and then used with
certain additions set forth in the rule for the 1 million-year peak
dose performance assessment. The initial screening provides a database
of FEPs, which is then used for both the 10,000-year and post-10,000-
year peak dose analyses, with some additional stipulations for the
period beyond 10,000 years. The discussion that follows addresses each
of these screening criteria in turn.

Probability

    In the proposed standards, we defined the probability threshold for
``very unlikely'' FEPs as a 1 in 10,000 chance of occurrence within
10,000 years, or roughly a 1 in 100 million (10-8) chance
per year of occurring. In today's final rule, the probability threshold
is now stated only as an annual probability of 1 in 100 million
(10-8). We believe it is appropriate to clarify that FEPs
have associated probabilities of occurrence that generally do not
change over time. That is, the database of FEPs deemed sufficiently
probable would serve equally well as the basis for assessments covering
1,000, 10,000, 100,000, or 1 million years. These probabilities of
occurrence are established by examining the geologic record and
considering potential mechanisms for components of the repository and
its natural setting to undergo changes. FEPs with a probability of
occurrence greater than 1 chance in 100 million per year should be
considered for inclusion in the performance assessments to show
compliance with the 10,000-year individual-protection standard, and the
same FEPs included in those assessments should be used to develop the
performance assessment scenarios to be analyzed for the peak dose
performance assessments between 10,000 and 1 million years. We believe
that this is an inclusive threshold level that fully considers a range
of low-probability FEPs, while at the same time limiting speculation
over highly improbable FEPs. We believe the probability screening
threshold provides the foundation for a reasonable test of the disposal
system, as discussed further below.
    Although we discussed the meaning of the probability threshold in
some detail in our proposal, we emphasize it again as the foundation
for constructing the performance assessment. A 1 in 100 million annual
probability of occurrence, when considered over a 10,000-year period,
includes FEPs with a cumulative chance of occurring of one one-
hundredth of one percent (0.01%). Similarly, over 1 million years, the
cumulative probability increases to only a one percent (1%) chance of
occurrence within that time frame. We believe that the database of
information necessary to assess FEPs at this low probability is the
same as that necessary for examining their importance over the entire 1
million-year compliance period. We believe this probability criterion
leads to an inclusive set of potential FEPs for both the 10,000-year
and peak dose assessments, and in our view would support a reasonable
test of the disposal system that encompasses the climate change,
seismic, igneous, and corrosion scenarios specified in our proposal.
    In our proposed rule, we concluded that the 10,000-year FEPs
screening could serve as an adequate basis for longer-term assessments
because it is sufficiently inclusive to be appropriate for use in
developing performance scenarios applicable to the entire 1 million-
year compliance period. That is, we did not propose to require DOE to
consider FEPs with an annual probability lower than 10-8 to
accommodate the lengthened compliance period. We believe excluding FEPs
with less than a 1% chance of occurrence in 1 million years is
consistent with the principles of reasonable expectation. We believe
that lowering the annual probability level below 10-8 would
allow for speculative scenarios to be considered in the peak dose
performance assessment, which would be neither reasonable nor
justifiable, as explained below.
    Some commenters disagreed, stating that, because we are extending
the compliance period by a factor of 100, the probability threshold for
excluding FEPs should also be extended by a factor of 100, resulting in
a threshold of 1 chance in 10 billion of occurrence per year.
Similarly, we received some comments questioning altogether the need
for or validity of a probability threshold. The comments suggest that,
because the effects are weighted by the probability of occurrence, any
potential FEP, no matter how unlikely, should be characterized and
assessed because its influence will be mitigated by its low
probability. They cite NAS to the effect that ``all these scenarios
need to be quantified'' with respect to probability and consequence.
(NAS Report p. 72) Therefore, the commenters conclude that our concerns
about introducing excessive speculation are unfounded. We disagree. We
addressed this topic in our proposal, in the expectation that we would
be encouraged to adjust the probability threshold by two orders of
magnitude (i.e., widening the probability range by a factor of 100) to
account for the similarly lengthened compliance period. We believe that
simply extending the approach of using a one in 10,000 probability over
a 1 million-year period to give 1 in 10 billion chance per year of
occurring (10-10) would result in the inclusion of FEPs that
are so speculative as to be unreasonable (70 FR 49052). Nor do we
believe it would be consistent with NAS's view that the overall goal
was ``to define a standard that specifies a high level of protection
but that does not rule out an adequately sited and well-designed
repository because of highly improbable events.'' (NAS Report p. 28)
Further, NAS itself suggested situations in which scenarios need not be
quantified. NAS discusses, in the context of volcanism, a
10-8 annual probability of occurrence as a level that
``might be sufficiently low to constitute a negligible risk'' below
which ``it might not be necessary to consider'' how the event might
contribute to releases from the disposal system. (NAS Report p. 95) We
believe this example is instructive, given that volcanism is the single
scenario resulting in direct release of radioactive material from the
repository into the biosphere, resulting in relatively immediate
exposures. We believe it is reasonable to extend the concept expressed
by NAS as ``negligible risk'' to FEPs whose influences are seen in the
gradual release and transport of radionuclides over long periods of
time. Therefore, we believe that lowering the probability threshold, or
eliminating it altogether, would be inconsistent with the important NAS
cautions to focus assessment efforts on FEPs that can be bounded within
the limits of geologic stability.
    In our view, were we to lower or eliminate the probability
threshold, it would be necessary to consider and describe FEPs that
might have been present or occurred only the initial years of the
planet's existence. Similarly, FEPs with an annual probability of
10-10 may be only hypothetical, since the age of the Earth
is generally considered to be ``only'' 4.6

x 10\9\ years, suggesting that these FEPs may have less than a 50%
chance of occurring within the entire history of the Earth. Indeed, the
volcanic rocks comprising Yucca Mountain and its surroundings are only
on the order of 10-12 million years old (~10\7\ years). In determining
the probability of particular FEPs, the geologic record at the site is
the source of information to identify what FEPs have occurred at the
site in the past and may occur in the future (through the period of
geologic stability). Since the host rock formations at the site are
only about 10 million years old, an annual probability cut-off of
10-10 would mean that probability estimates for some FEPs
would have to be made in spite of the fact that there is no evidence
for their occurrence at the site in the past. As it is, the
10-8 probability threshold presents a significant challenge
to characterize FEPs with some degree of confidence, given the limits
of today's science and technology. ICRP makes a similar point in its
2007 recommendations: ``The use of probability assessment is limited by
the extent that unlikely events can be forecast. In circumstances where
accidents can occur as a result of a wide spectrum of initiating
events, caution should be exercised over any estimate of overall
probabilities because of the serious uncertainty of predicting the
existence of all the unlikely initiating events.'' (Publication 103,
Docket No. EPA-HQ-OAR-2005-0083-0423, paragraph 269) (Note that this
discussion is in the context of ``potential'' exposures, which include
releases that may occur in the far future from disposal facilities.
Therefore, the term ``accidents'' should not be taken as limited to
operational activities.) Overall, we believe events with a lower annual
probability than 10-8 would introduce speculation beyond
what is appropriate to define a reasonable test of disposal system
performance.
    We also received comments stating that maintaining the probability
screening criteria for the extended compliance period undermines our
arguments for increasing uncertainty. To the contrary, we believe the
physical meaning of the probability threshold (0.01% chance of
occurrence within 10,000 years, but a 1% chance within 1 million years)
appropriately incorporates the concept of uncertainty increasing with
time, while still applying a substantially conservative screening
criterion.
    We believe that the guidance we have provided for executing a FEPs
evaluation and screening process assures that it is executed in a
thorough manner. For example, we have stated that the geologic record
through the Quaternary Period (a period extending back approximately 2
million years from today) at and around the site should be examined to
identify relevant FEPs. While we believe that the Quaternary Period
offers the most reliable data for identifying and characterizing site
geologic FEPs, we do not believe that evidence preserved in older
portions of the geologic record should be ignored in the FEPs
identification process. We did not mean to imply that DOE need only
consider the previous 10,000 years when developing evidence for the
probability of occurrence of future events. Rather, our statements
regarding the Quaternary Period as an appropriate geologic record were
intended to confirm that, where available, reliable geologic records
for earlier time periods should be consulted. For example, determining
the probability of seismic and igneous events would make use of the
geologic record at the site for as far back in time as reliable
estimates of past events can be made so that defensible probability
estimates can be made. We believe the Quaternary Period offers the best
information to quantify the probabilities and consequences of geologic
FEPs relevant to site performance. However, we did not intend that
significant information about FEPs be ignored simply because that
information appears in the geologic record at the site prior to the
Quaternary Period.
    In fact, a longer portion of the geologic record has been examined
by DOE and NRC in developing FEP probabilities. For example, to
determine the nature and frequency of volcanic activity around Yucca
Mountain, volcanic activity around the site through the Quaternary
Period was extensively examined, as well as volcanic activity prior to
that time (ACNW Workshop on Volcanism at Yucca Mountain, September 22,
2004--Docket No. EPA-HQ-OAR-2005-0083-0373 and 0378). We believe that
the information necessary to evaluate FEPs against the probability
threshold we established (10-8 annual probability) will be
extensive, and that increasing the compliance period from 10,000 to 1
million years does not require that additional studies be performed
beyond those necessary to derive the FEPs probabilities under the
screening process done for the 10,000-year time frame assessments. As
we have noted previously, the probabilities for individual FEPs are
determined once, and the same probabilities are used in both the
10,000-year and 1 million-year assessments.
    On this last point, we stress that the revised Sec.  197.36(a)
issued today should not be interpreted as compelling DOE to extend the
databases for its technical justifications. We are restating the
probability screening criterion, not recasting the entire framework for
the analysis. We recognize that in any licensing process the burden of
proof is on the applicant to demonstrate that the necessary factors and
influences have been evaluated. It must also be recognized that there
will always be limits to the ability of science and technology to
characterize FEPs and their effects on the disposal system. However,
NAS has stated that many of these processes and their uncertainties are
boundable. In our judgment, given the capabilities of today's science
and technology, it would be contrary to the principle of reasonable
expectation to require DOE to demonstrate the same level of confidence
in assessments covering 1 million years as it would for a much shorter
10,000-year analysis.
    Similarly, we believe that this clarification does not create the
prospect of speculative scenarios of very low probability (from
combinations of FEPs) being proposed, thereby opening the performance
assessments to unbounded speculation. For example, if two low
probability independent FEPs were proposed to occur simultaneously
because of the longer time horizon under consideration, the probability
of that combination would be the product of their respective
probabilities. In other words, the probability of the combined FEPs
occurring during the same year will be much lower, by possibly orders
of magnitude, than the probability of either FEP occurring
individually. Therefore, since the contributions of various FEPs (or
scenarios) to the dose assessments is the product of their respective
probabilities and consequences, the consequence of the combined FEPs
would need to be inversely proportionally higher, typically by orders
of magnitude, than the combined consequences of the individual FEPs
considered separately, in order to make a significant change in the
overall dose assessment.
    We did receive some comment suggesting that we had inappropriately
excluded the type of volcanic events that created the Yucca Mountain
tuff some 12 to 14 million years ago, instead focusing on the past
several million years. However, as we stated in our proposal, the
geologic record of the past several million years in the area around
the site indicates that basaltic volcanism is the type of volcanism
that has occurred recently and has the potential to recur in the
future. The earlier events

were of a much different, cataclysmic nature, producing rock units more
than 6000 ft (1800 m) thick. The type of volcanic activity that created
Yucca Mountain and the surrounding area has not recurred over the
approximately 10 million years since the deposits were originally laid
down and is extremely unlikely to occur within the next 1 million years
(Docket No. EPA-HQ-OAR-2005-0083-0050, pp. 7-42 through 7-49). Further,
we question whether such cataclysmic events could be reasonably
considered to fall within the bounds of geologic stability as
envisioned by NAS. Inclusion of such events in the peak dose assessment
up to 1 million years would be inconsistent with the intent of the NAS
when it noted that long-term performance can be assessed (because
physical and geologic processes are sufficiently quantifiable, and the
related uncertainties sufficiently boundable) when the geologic system
is relatively stable or varies in a boundable manner. (NAS Report p. 9)
However, NAS noted that ``[a]fter the geologic environment has changed,
of course, the scientific basis for performance assessment is
substantially eroded and little useful information can be developed.''
(NAS Report p. 72) We believe that volcanism of that magnitude would
result in fundamental change of the geologic environment and would not
represent a reasonable test of the disposal system. Therefore, we
continue to see no basis for requiring this type of event be included
in the performance assessment.
    Some may view our approach using a single probability threshold for
determining which FEPs should be considered for inclusion in the
performance assessments as inconsistent with the application of
different dose standards for the initial 10,000 years and the period up
to 1 million years. We do not see an inconsistency primarily because
the nature and effects of uncertainty on event probability and dose
projections are dissimilar. The overall uncertainty in projecting doses
using a model simulating the complex interplay of the disposal system
components over long times, each of which has inherent uncertainties in
their characteristics, and the associated difficulty in relying on such
projections for regulatory decisions, should not be confused with the
uncertainty implied in assigning a probability of occurrence to a
particular FEP, which in many cases derives from an examination of the
geologic record at the site. We have noted the difficulty in
extrapolating performance to very long times, and believe it is
appropriate to address this difficulty by establishing a somewhat
higher, but still protective, dose limit for the period beyond 10,000
years. FEP probabilities are assigned based on observations that may
cover long periods of time, such as for geologic processes, or from
laboratory testing and the extrapolation of such results to conditions
that may exist in the distant future, such as for corrosion processes.
In today's final rule, the FEP probability threshold that must be
considered in developing performance assessments represents a policy
judgment about how such events should be addressed in order to meet the
regulatory challenge recognized by NAS, supported by technical
reasoning about the nature of the site database for identifying and
characterizing FEPs.

Significance

    The second criterion for evaluating FEPs, the evaluation of the
significance of the impacts on performance assessment, allows FEPs
above the probability threshold to be excluded from the analyses if
they would not significantly change the results of performance
assessments. In other words, this evaluation is intended to identify
those FEPs whose projected probability would otherwise make them
candidates for inclusion in the performance assessment, but whose
effect on repository performance (however probable) can be demonstrated
not to be significant. We are retaining the provisions presented in the
proposed rule related to screening FEPs for their effects on the
performance assessment results, and, for the reasons discussed below,
are adding an additional provision regarding the analysis of seismic
FEPs in Sec.  197.36(c).
    Today's final rule continues to focus on seismic and igneous events
that cause direct damage to the engineered barrier system (e.g.,
repository drifts and waste packages). Regardless of other effects of
these events on the disposal system, the timing and degree of waste
package degradation has a significant effect on peak dose. The
longevity of waste packages, when considering periods of hundreds of
thousands of years, is uncertain and dependent on a number of factors.
Therefore, the aspect of primary interest in evaluating seismic and
igneous FEPs is their potential to breach waste packages and make
radioactive material available for transport by infiltrating water (or,
in the case of volcanic events, for direct release into the biosphere).
    We believe that the use of the significance criterion of Sec.
197.36(a) would assure a reasonable test of disposal system performance
through the period of geologic stability. We recognize that setting
forth the significance screening criterion in Sec.  197.36(a) of our
proposal as pertaining to the 10,000-year period could be construed as
creating a situation in which important long-term processes could be
excluded altogether from the analysis if they were not significant in
the earlier period. However, we do not believe it is reasonable to
interpret the significance criterion in this way. We have taken
specific steps to ensure that significant long-term FEPs will be
considered in the assessments. Consistent with NAS, we have addressed
the long-term effects of seismic, igneous, and climatic FEPs. In
addition, as described below, we have directed that the effects of
general corrosion on the engineered barrier system be evaluated.
Further, contrary to some comments, we explicitly required that FEPs
included in the 10,000-year analysis must continue to be included for
the longer-term (10,000 years to 1 million years) assessment. That is,
FEPs included in the initial 10,000-year assessments will continue to
operate throughout the period of geologic stability. These FEPs are
already identified as appropriate for inclusion, and include
fundamental physical and geologic processes that play roles in the
release and transport of radionuclides, regardless of the time period
covered by the assessment.
    As noted above, to further bolster the significance screening
criterion, in our proposal we considered whether it might be possible
that FEPs eliminated from consideration during the first 10,000 years
should be included in the longer-term assessment if they would have a
significant bearing on performance at later times, even if they could
legitimately be dismissed for the initial 10,000-year period. We
focused our attention on FEPs affecting the engineered barriers since,
as noted above, waste package failure is the dominant factor in the
timing and magnitude of the peak dose, and is the primary reason for
considering time frames up to 1 million years. To illustrate one
consideration, thermal conditions in the repository change dramatically
within the initial 10,000-year period, affecting the relative
importance of some FEPs during and after the thermal pulse. However,
FEPs involved in release and transport of radionuclides would generally
be the same, regardless of when the waste package fails. Further, while
FEPs associated with the natural characteristics of the site are active
today or can be observed in the geologic

record, FEPs related to engineered barrier longevity involve
extrapolation of shorter-term testing data. The degree to which natural
FEPs can contribute to the breaching of waste packages is dependent to
a large extent on the condition of those packages over time, making
FEPs specific to the engineered barriers of particular importance. We
took this approach for two reasons. First, we needed to clearly outline
the reasons why a FEP that could be excluded on the basis of
significance from the performance assessments for the initial 10,000-
year period might potentially need to be re-considered for the
lengthened compliance period. Second, we wanted to further our goal of
issuing an implementable standard by limiting potentially unconstrained
speculation over the longer compliance period. By discussing the
considerations involved in evaluating FEPs that could be previously
excluded, we hoped to lay out clearly the reasoning that could be used
to justify inclusion of additional FEPs beyond those identified by the
NAS committee.
    We explicitly addressed general corrosion of the waste packages and
other engineered barriers in our proposal because it is likely to be a
significant degradation process at later times. We identified this FEP
as being significant at times greater than 10,000 years because we
believe it is the principal process FEP that could lead to ``gross
breaching'' of the waste package over those extended time frames.
Processes and events that could lead to ``gross breaching'' are of
greatest significance to long term performance because, as noted by the
NAS, ``canisters are likely to fail initially at small local openings
through which water might enter, but out of which the diffusion of
dissolved wastes will be slow until the canister is grossly breached.''
(NAS Report p. 86) It is the time of ``gross breaching'' that
determines the time of more rapid release of dissolved wastes from the
repository and hence may have a significant effect on the time and
magnitude of the peak dose within 1 million years. Although the general
corrosion process is slow, tends to decrease with decreasing
temperature, and may not lead to significant releases for the first
10,000 years (depending on DOE's design of the waste package), we
believe this FEP could be significant enough over the long term to
require inclusion in the assessment of performance during the time of
geologic stability, regardless of the screening decision in the first
10,000 years. Further, consideration of the uncertainties involved in
extrapolating general corrosion data for the proposed waste package
materials supports the inclusion of this potentially highly significant
process (``Assumptions, Conservatisms, and Uncertainties in Yucca
Mountain Performance Assessments,'' Docket No. EPA-HQ-OAR-2005-0083-
0085, section 5.4.1). Therefore, we believe that general corrosion, in
addition to those FEPs related to seismicity, igneous activity and
climate change identified by NAS, requires explicit inclusion in the
assessments during the time of geologic stability.
    We did, as one commenter pointed out, consider providing NRC more
latitude to identify FEPs if they would significantly affect the peak
dose. After further consideration, we decided against this approach,
believing the provisions outlined above and the specification of
general corrosion would adequately address this situation, provide a
reasonable test of disposal system performance, and give DOE the
necessary assurance that the important factors have been explicitly
identified in the rule. As we noted above, we identified general
corrosion of engineered barriers as a FEP potentially significant to
the peak dose, and specified its inclusion because it is likely to be a
significant degradation process at later times. Similarly, consistent
with the NAS recommendations, we have specified the inclusion of
climate change, seismicity, and igneous scenarios. We view the
requirement to include general corrosion, as well as the climate,
seismic, and igneous scenarios identified by NAS, as leading to an
effective and extensive assessment, which can fairly be represented as
a reasonable test of the disposal system. As we discussed in our
proposal, the search for additional FEPs that might be significant at
some point beyond 10,000 years can rapidly become highly speculative
and limited in benefit. Therefore, we continue to believe that our
approach represents ``informed judgment'' and a reasonable test of
repository performance over time frames as long as 1 million years for
the Yucca Mountain disposal system.
    We also note that DOE submitted, as part of its comments on the
proposed rule, the results of analyses based on a simplified peak dose
model (Docket No. EPA-HQ-OAR-2005-0083-0352, Appendix 1). DOE states
that it had compiled a database of FEPs, independent of compliance
period, and evaluated them for inclusion in a 10,000-year analysis. DOE
``subsequently re-evaluated the FEPs over the period beyond 10,000
years'' and concluded that those FEPs excluded on the basis of
significance within 10,000 years would also not have significant
effects on performance projections beyond 10,000 years. DOE reached its
conclusion both for FEPs excluded ``on a low consequence basis that is
not affected by time'' and for ``gradual and continuing processes''
that ``are time dependent.''
    Also as part of its comments, DOE submitted an analysis that
identified three reasons why gradual and/or infrequent FEPs excluded on
the basis of significance within 10,000 years would also not have
significant effects on performance projections beyond 10,000 years: (1)
An excluded FEP was determined to be of secondary importance to the
primary significant degradation FEP, which was included in the
analysis; (2) the inclusion of the FEP would tend to lower the peak
dose during the time of geologic stability because it resulted in
earlier and more diffuse releases (hence the exclusion of the FEP would
be conservative from a peak dose perspective); or (3) the FEP is
correlated in some way with temperature (e.g., in the rate with which
it operates), so it would be less significant at later times due to the
lower temperature in the repository over time. (Docket No. EPA-HQ-OAR-
2005-0083-0352, Appendix 1, section 6.1 and Table 24) DOE considered
FEPs of this nature associated with both the engineered and natural
barrier systems. DOE concluded, for example, that some longer-term
processes, such as general corrosion, may contribute to waste package
failure, and disruptive seismic events may contribute to rockfall and
other physical mechanisms leading to release.
    We also considered public comments on this topic. Most commenters
who disagreed with our proposal cited the limited data available on
various corrosion mechanisms that could affect the waste packages. Many
of these commenters seem to believe that we have excluded all corrosion
mechanisms except general corrosion. This is not the case. We have
explicitly directed that general corrosion be considered because it is
likely to be the most significant such process at longer times;
however, other corrosion mechanisms (such as localized corrosion) are
more likely in the early period after disposal when temperatures inside
the repository are high. If DOE determines these processes to be
insignificant within 10,000 years, they are not likely to be more
significant than general corrosion at later times. If they are included
in the 10,000-year analysis,

they must be included in the longer-term assessments. One commenter
highlighted our discussion of criticality as excluding one of the
``most worrisome threats to the repository'' over the long term. We
cited an NRC technical study to support our conclusion that such an
event is unlikely to be significant to the results of the assessments.
Further, the DOE reference cited above concludes that all criticality
scenarios fall below the probability screening threshold. An
alternative view on the FEPs screening process was expressed in a
report by the Electric Power Research Institute (EPRI): ``Thus, the
current EPA screening limit is very conservative compared to the
[Negligible Incremental Dose] level suggested by [NAS]. It is likely
that there are many FEPs that DOE has already included in their
analysis using the EPA approach that would not have been included if
the [NAS]-recommended approach had been followed. Given that many
additional FEPs are already included, it should be unnecessary to
include any additional FEPs if the regulatory compliance period is
extended beyond 10,000 years.'' (``Yucca Mountain Licensing Standard
Options for Very Long Time Frames,'' April 2005, pp. 3-5 and 3-6,
Docket No. EPA-HQ-OAR-2005-0083-0087) Taking all of this information
into account, we continue to believe it is reasonable that, with the
exception of the specific FEPs identified in 197.36(c), a FEP
determined to be insignificant in the first 10,000 years may continue
to be excluded in the post-10,000-year analyses.
    As we noted above, we are modifying the proposed rule regarding the
provisions related to seismic events in Sec.  197.36(c). We noted in
our proposal the NAS statement that ``[w]ith respect to the effects of
seismicity on the hydrologic regime, the possibility of adverse effects
due to displacements along existing fractures cannot be overlooked''
but that ``such displacements have an equal probability of favorably
changing the hydrologic regime.'' (NAS Report p. 93). We argued that
these effects would likely be minimal given the many small-scale
changes that would be possible in the connectivity of the fracture
networks, and that these effects would likely be small compared to the
effects of climate change on the hydrologic behavior of the disposal
system. We did not mean to imply that the seismic and climate events
would involve the same hydrologic characteristics and processes or
produce the same effects on the ground-water flow regime, but that the
effects of one were likely to outweigh the effects of the other. While
we still believe that is likely, we have concluded, after further
consideration, that the issue of hydrologic effects resulting from
seismic events needs to be examined in sufficient detail to address the
point made by NAS. We believe the effects of fault displacement on the
hydrologic regime will be adequately addressed by the variation in
parameters such as hydraulic conductivity (i.e., evaluating reasonable
variation in ground-water flow parameter values, whether seismically-
induced or not, will illustrate the range of effects that might result
from seismicity). However, NAS also identified another seismic effect
on hydrology, namely the potential for transient rise in the ground-
water table. In this instance, NAS did not simply state that such
potential could be bounded, but noted site-specific studies suggesting
``a probable maximum transient rise on the order of 20 m or less.''
(NAS Report p. 94) Therefore, we now require that the effects of a rise
in the ground-water table as a result of seismicity be considered. We
are not specifying the extent of the rise to be considered, but leave
that conclusion to be determined by NRC. NRC may choose to estimate the
magnitude of ground-water table rise itself, or require DOE to include
such estimates in its license application. In this case, however, we
are also allowing NRC to make a judgment as to whether such a rise in
ground water would be significant to the results of the performance
assessment. If NRC determines that such a reasonably bounded scenario
would not be significant, DOE would not be required to evaluate its
effects.
    We believe deferring to NRC on this point is the appropriate
approach. The above quote from page 93 of the NAS Report makes it clear
that changes to the hydrologic regime from seismic events would be
equally likely to enhance or reduce transport of radionuclides.
However, it would seem unlikely for changes to occur that would all
combine to enhance transport to the saturated zone and then through the
controlled area, such that concentrations of radionuclides at the RMEI
location would be significantly increased. It seems more likely that
localized changes would occur, which in sum would not significantly
increase overall transport of radionuclides. Further, as noted above,
we believe these seismically-induced changes are likely to be
approximated by the normal variation in flow parameters. Changes in the
hydrologic system from climate change (e.g., increases in infiltration)
are expected to be quantitatively more significant than such changes
resulting from seismic activity. We believe NRC is better positioned to
make judgments regarding the significance and extent of such changes.
We note that a dozen years of site characterization, scientific study,
and performance assessments have been conducted since the NAS Report in
1995. NRC has conducted its own analyses as well as participated in
ongoing technical exchanges with DOE over this period. We view
deferring to NRC's judgment in this case as comparable to the approach
we have taken with climate change. In that instance, we outlined the
primary issues and overall approach, but specified that NRC would
establish the details required to implement our standard.
    Finally, we are retaining the provision related to climate change
as it was proposed. We believe this is a reasonable approach, which
allows NRC to characterize climate change beyond 10,000 years using
constant conditions. This approach has the advantage of avoiding
speculation regarding the timing and magnitude of climatic cycles,
while addressing the important aspects of climate change. We received
some comments that appear to have misinterpreted our proposal. Some
comments suggested that our citation of the NAS statement to the effect
that ``climate changes on the time scale of hundreds of years would
probably have little if any effect on repository performance'' (NAS
Report p. 92) as implying that we are ``ignoring longer-term changes''
such as ``glacial periods covering thousands of years.'' This
represents a fundamental misunderstanding of our proposal, which would
allow the future climate to be represented by what is essentially a
glacial transition period lasting 990,000 years, but in any event
placed no limits on the duration of periods of increased precipitation.
Similarly, some commenters expressed the view that we ``required'' the
future climate to be represented by constant conditions, or that we
were suggesting that a single climate be used in all realizations. On
the contrary, we cited the NAS conclusion that ``a doubling of the
effective wetness'' might be significant as one justification for
stating that it would be reasonable to represent far-future climate by
constant conditions. Today's final rule, consistent with our proposal,
leaves it to NRC to determine the parameter values that would define
the future climate, including influential parameters other than
precipitation,

such as temperature. Our specification of the outcome of ``increased
water flow through the repository'' provides NRC with the flexibility
to specify basic parameters, such as precipitation and temperature,
that must be assumed by DOE, or to derive estimates of water flow
directly. This is consistent with our current belief that the dominant
mechanisms and flow paths for water to move from the surface through
the repository and beyond should be determined by NRC rather than EPA.
Further, we anticipated that ``constant climate conditions'' would be
used as another parameter in the probabilistic assessment. That is,
each realization would select its constant conditions from among a
distribution of such conditions developed to reflect estimates of
different future climate states. This is exactly the approach that NRC
has taken in its proposal, i.e., that a range of deep percolation
values be used (70 FR 53313-53320, September 8, 2005).
    Some commenters disagreed with the approach of specifying constant
climate conditions leading to a higher rate of water flow through the
repository, stating that the ``non-linear'' nature of the disposal
system would be more sensitive to a dynamic, cyclical representation of
climate. This is not necessarily true, as the effects on the disposal
system would be highly affected by the timing of waste package failures
(e.g., whether they fail during a wetter or drier cycle). Some comments
cite recent climate research suggesting that anthropogenic climate
influences will postpone the next glacial cycle by roughly 500,000
years, or that today's climate at Yucca Mountain will actually be more
representative of future climates than would the wetter conditions
known to have occurred in the past. We believe that our final rule's
approach to climate change provides a reasonable approach to address a
point of fundamental uncertainty regarding long-term climate change and
its role in the performance assessments, an uncertainty that cannot be
removed by additional research into past climate cycles or modeling of
present or future climate behavior. We refer to NAS on this point:
``Although the typical nature of past climate changes is well known, it
is obviously impossible to predict in detail either the nature or the
timing of future climate change.'' (NAS Report p. 77, emphasis added)
Although continuing research will provide better understanding of past
climate fluctuations, we believe that predicting with high confidence
the timing and extent of climate fluctuations into the far future will
remain an unrealistic goal. We believe that the understanding of past
climate fluctuations and their potential effects on the Yucca Mountain
hydrologic system is valuable information and should be applied to
define the climate-related parameter values. As noted above, NRC has
used such information to propose climate-related parameter values,
which DOE will use to project the behavior of hydrologic processes at
the site. We believe that this approach to treatment of a ``residual,
unquantifiable uncertainty'' by the application of ``informed
judgment'' is consistent with NAS guidance. (NAS Report p. 80)

IV. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under Executive Order 12866 (58 FR 51735, October 4, 1993), this
action is a ``significant regulatory action'' because it raises novel
legal or policy issues arising out of the specific legal mandate of
section 801 of the Energy Policy Act of 1992. Accordingly, EPA
submitted this action to the Office of Management and Budget for review
under Executive Order 12866 and any changes made in response to OMB
recommendations have been documented in the docket for this action.

B. Paperwork Reduction Act

    This action does not impose an information collection burden under
the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.
Burden is defined at 5 CFR 1320.3(b). We have determined that this rule
contains no information collection requirements within the scope of the
Paperwork Reduction Act. This final rule establishes requirements that
apply only to DOE.

C. Regulatory Flexibility Act

    The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
    For purposes of assessing the impacts of today's rule on small
entities, small entity is defined as: (1) A small business as defined
by the Small Business Administration's (SBA) regulations at 13 CFR
121.201; (2) a small governmental jurisdiction that is a government of
a city, county, town, school district or special district with a
population of less than 50,000; and (3) a small organization that is
any not-for-profit enterprise which is independently owned and operated
and is not dominant in its field.
    After considering the economic impacts of today's final rule on
small entities, I certify that this action will not have a significant
economic impact upon a substantial number of small entities. This final
rule will not impose any requirements on small entities. This final
rule establishes requirements that apply only to DOE.

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public
Law 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with ``Federal mandates'' that
may result in expenditures to State, local, and tribal governments, in
the aggregate, or to the private sector, of $100 million or more in any
one year. Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and adopt
the least costly, most cost-effective or least burdensome alternative
that achieves the objectives of the rule. The provisions of section 205
do not apply when they are inconsistent with applicable law. Moreover,
section 205 allows EPA to adopt an alternative other than the least
costly, most cost-effective or least burdensome alternative if the
Administrator publishes with the final rule an explanation why that
alternative was not adopted. Before EPA establishes any regulatory
requirements that may significantly or uniquely affect small
governments, including tribal governments, it must have developed under
section 203 of the UMRA a small government agency plan. The plan must
provide for notifying potentially affected small governments, enabling
officials of affected small governments to have meaningful and timely
input in the development of EPA regulatory proposals with significant
Federal intergovernmental mandates, and informing, educating, and
advising small governments on compliance with the regulatory
requirements.

    Today's final rule contains no Federal mandates (under the
regulatory provisions of Title II of UMRA) for State, local, or tribal
governments or the private sector. This final rule implements
requirements specifically set forth by the Congress in section 801 of
the EnPA and establishes radiological protection standards applicable
solely and exclusively to the Department of Energy's potential storage
and disposal facility at Yucca Mountain. The rule imposes no
enforceable duty on any State, local or tribal governments or the
private sector. Thus, today's rule is not subject to the requirements
of sections 202 and 205 of UMRA.
    EPA has determined that this rule contains no regulatory
requirements that might significantly or uniquely affect small
governments. This final rule implements requirements specifically set
forth by the Congress in section 801 of the EnPA and establishes
radiological protection standards applicable solely and exclusively to
the Department of Energy's potential storage and disposal facility at
Yucca Mountain. The rule imposes no enforceable duty on any small
governments. Thus, today's rule is not subject to the requirements of
section 203 of UMRA.

E. Executive Order 13132: Federalism

    Executive Order 13132, entitled ``Federalism'' (64 FR 43255, August
10, 1999), requires EPA to develop an accountable process to ensure
``meaningful and timely input by State and local officials in the
development of regulatory policies that have federalism implications.''
``Policies that have federalism implications'' is defined in the
Executive Order to include regulations that have ``substantial direct
effects on the States, on the relationship between the national
government and the States, or on the distribution of power and
responsibilities among the various levels of government.''
    This final rule does not have federalism implications. It will not
have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. This final rule implements
requirements specifically set forth by the Congress in section 801 of
the EnPA and establishes radiological protection standards applicable
solely and exclusively to the Department of Energy's potential storage
and disposal facility at Yucca Mountain. Thus, Executive Order 13132
does not apply to this rule. In the spirit of Executive Order 13132,
and consistent with EPA policy to promote communications between EPA
and State and local governments, EPA specifically solicited comment on
the proposed rule from State and local officials.

F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments

    Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000),
requires EPA to develop an accountable process to ensure ``meaningful
and timely input by tribal officials in the development of regulatory
policies that have tribal implications.'' This final rule does not have
tribal implications, as specified in Executive Order 13175. This final
rule will regulate only DOE on land owned by the Federal government.
The rule does not have substantial direct effects on one or more Indian
tribes, on the relationship between the Federal Government and Indian
tribes, or on the distribution of power and responsibilities between
the Federal Government and Indian tribes. Thus, Executive Order 13175
does not apply to this rule.
    Although Executive Order 13175 does not apply to this rule, EPA
specifically solicited additional comment on this proposed rule from
tribal officials and consulted with tribal officials in developing this
rule. EPA directly contacted more than 20 tribal governments and
conducted three conference calls with members of tribal governments. In
recognition of the importance of government-to-government consultation
with tribes and the significance of tribal governments as sovereign
nations, EPA extended the public comment period for tribal governments
to December 31, 2005. Comments related to tribal issues, and our
responses to them, may be found in Section 13 of the Response to
Comments document associated with this final rule (docket ref).

G. Executive Order 13045: Protection of Children From Environmental
Health & Safety Risks

    This final rule is not subject to Executive Order 13045 because it
is not economically significant as defined in Executive Order 12866,
and because the Agency does not have reason to believe the
environmental health risks or safety risks addressed by this action
present a disproportionate risk to children.

H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution, or Use

    This action is not a ``significant energy action'' as defined in
Executive Order 13211 (66 FR 28355 (May 22, 2001)), because it is not
likely to have a significant adverse effect on the supply,
distribution, or use of energy. This final rule will apply only to DOE.
Construction, operation, and closure of the repository at Yucca
Mountain would fulfill the Federal government's commitment to manage
the final disposition of spent nuclear fuel from commercial power
reactors. However, there is no direct link between operation of the
repository and an increased use of nuclear power. Other economic,
technical, and policy factors will influence the extent to which
nuclear energy is utilized.

I. National Technology Transfer and Advancement Act

    Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (``NTTAA''), Public Law No. 104-113, 12(d) (15 U.S.C. 272
note) directs EPA to use voluntary consensus standards in its
regulatory activities unless to do so would be inconsistent with
applicable law or otherwise impractical. Voluntary consensus standards
are technical standards (e.g., materials specifications, test methods,
sampling procedures, and business practices) that are developed or
adopted by voluntary consensus standards bodies. NTTAA directs EPA to
provide Congress, through OMB, explanations when the Agency decides not
to use available and applicable voluntary consensus standards.
    This rulemaking involves technical standards. Therefore, the Agency
conducted a search to identify potentially applicable voluntary
consensus standards. In our original 1999 proposal (64 FR 46976), we
requested public comment on potentially applicable voluntary consensus
standards that would be appropriate for inclusion in the Yucca Mountain
rule. However, we identified no such standards, and none were brought
to our attention in comments. Therefore, the standards promulgated in
2001 and today's final revisions are site-specific and developed solely
for application to the Yucca Mountain disposal facility.

J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-income Populations

    Executive Order (EO) 12898 (59 FR 7629 (Feb. 16, 1994)) establishes
federal executive policy on environmental justice. Its main provision
directs federal agencies, to the greatest extent practicable and
permitted by law, to make environmental justice part of their mission
by identifying and addressing, as appropriate, disproportionately high
and adverse human health or environmental effects of their programs,
policies, and activities on minority populations and low-income
populations in the United States.
    EPA lacks the discretionary authority to address environmental
justice in this final rulemaking. This final rule implements
requirements specifically set forth by the Congress in section 801 of
the EnPA and establishes radiological protection standards applicable
solely and exclusively to the Department of Energy's potential storage
and disposal facility at Yucca Mountain. Section 801(a)(1) of the EnPA
directs EPA to ``promulgate, by rule, public health and safety
standards'' that `` prescribe the maximum annual effective dose
equivalent to individual members of the public'' from releases of
radioactive material from the Yucca Mountain repository. This final
rule fulfills this statutory direction.

K. Congressional Review Act

    The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the
Small Business Regulatory Enforcement Fairness Act of 1996, generally
provides that before a rule may take effect, the agency promulgating
the rule must submit a rule report, which includes a copy of the rule,
to each House of the Congress and to the Comptroller General of the
United States. Section 804 exempts from section 801 the following types
of rules: (1) Rules of particular applicability; (2) rules relating to
agency management or personnel; and (3) rules of agency organization,
procedure, or practice that do not substantially affect the rights or
obligations of non-agency parties. 5 U.S.C. 804(3). EPA is not required
to submit a rule report regarding today's action under section 801
because this is a rule of particular applicability. This final rule
will apply only to DOE, and is issued by EPA in response to direction
from Congress in the EnPA.

List of Subjects in 40 CFR Part 197

    Environmental protection, Nuclear energy, Radiation protection,
Radionuclides, Uranium, Waste treatment and disposal, Spent nuclear
fuel, High-level radioactive waste.

    Dated: September 30, 2008.
Stephen L. Johnson,
Administrator.

0
40 CFR part 197 is amended as follows:

PART 197--PUBLIC HEALTH AND ENVIRONMENTAL RADIATION PROTECTION
STANDARDS FOR YUCCA MOUNTAIN, NEVADA

0
1. The authority citation for part 197 continues to read as follows:

    Authority: Sec. 801, Pub. L. 102-486, 106 Stat. 2921, 42 U.S.C.
10141n.

Subpart A--Public Health and Environmental Standards for Storage

0
2. Section 197.2 is amended by revising the definition of ``Effective
dose equivalent'' to read as follows:

Sec.  197.2  What definitions apply in Subpart A?

* * * * *
    Effective dose equivalent means the sum of the products of the dose
equivalent received by specified tissues following an exposure of, or
an intake of radionuclides into, specified tissues of the body,
multiplied by appropriate weighting factors. Annual committed effective
dose equivalents shall be calculated using weighting factors in
appendix A of this part, unless otherwise directed by NRC in accordance
with the introduction to appendix A of this part.
* * * * *

Subpart B--Public Health and Environmental Standards for Disposal

0
3. Section 197.12 is amended by revising paragraph (1) of the
definition of ``Performance assessment'' and the definition of ``Period
of geologic stability'' to read as follows:

Sec.  197.12  What definitions apply in Subpart B?

* * * * *
    Performance assessment means an analysis that:
    (1) Identifies the features, events, processes, (except human
intrusion), and sequences of events and processes (except human
intrusion) that might affect the Yucca Mountain disposal system and
their probabilities of occurring;
* * * * *
    Period of geologic stability means the time during which the
variability of geologic characteristics and their future behavior in
and around the Yucca Mountain site can be bounded, that is, they can be
projected within a reasonable range of possibilities. This period is
defined to end at 1 million years after disposal.
* * * * *

0
4. Section 197.13 is revised to read as follows:

Sec.  197.13  How is Subpart B implemented?

    The NRC implements this subpart B. The DOE must demonstrate to NRC
that there is a reasonable expectation of compliance with this subpart
before NRC may issue a license.
    (a) The NRC will determine compliance, based upon the arithmetic
mean of the projected doses from DOE's performance assessments for the
period within 1 million years after disposal, with:
    (1) Sections 197.20(a)(1) and 197.20(a)(2) of this subpart; and
    (2) Sections 197.25(b)(1), 197.25(b)(2), and 197.30 of this
subpart, if performance assessment is used to demonstrate compliance
with either or both of these sections.
    (b) [Reserved]

0
5. Section 197.15 is revised to read as follows:

Sec.  197.15  How must DOE take into account the changes that will
occur during the period of geologic stability?

    The DOE should not project changes in society, the biosphere (other
than climate), human biology, or increases or decreases of human
knowledge or technology. In all analyses done to demonstrate compliance
with this part, DOE must assume that all of those factors remain
constant as they are at the time of license application submission to
NRC. However, DOE must vary factors related to the geology, hydrology,
and climate based upon cautious, but reasonable assumptions of the
changes in these factors that could affect the Yucca Mountain disposal
system during the period of geologic stability, consistent with the
requirements for performance assessments specified at Sec.  197.36.

0
6. Section 197.20 is revised to read as follows:

Sec.  197.20  What standard must DOE meet?

    (a) The DOE must demonstrate, using performance assessment, that
there is a reasonable expectation that the reasonably maximally exposed
individual receives no more than the following annual committed
effective dose equivalent from releases from the undisturbed Yucca
Mountain disposal system:

    (1) 150 microsieverts (15 millirems) for 10,000 years following
disposal; and
    (2) 1 millisievert (100 millirems) after 10,000 years, but within
the period of geologic stability.
    (b) The DOE's performance assessment must include all potential
pathways of radionuclide transport and exposure.

0
7. Section 197.25 is revised to read as follows:

Sec.  197.25  What standard must DOE meet?

    (a) The DOE must determine the earliest time after disposal that
the waste package would degrade sufficiently that a human intrusion
(see Sec.  197.26) could occur without recognition by the drillers.
    (b) The DOE must demonstrate that there is a reasonable expectation
that the reasonably maximally exposed individual will receive an annual
committed effective dose equivalent, as a result of the human
intrusion, of no more than:
    (1) 150 microsieverts (15 millirems) for 10,000 years following
disposal; and
    (2) 1 millisievert (100 millirems) after 10,000 years, but within
the period of geologic stability.
    (c) The analysis must include all potential environmental pathways
of radionuclide transport and exposure.

0
8. Section 197.35 is removed and reserved.

Sec.  197.35  [Removed and Reserved]

0
9. Section 197.36 is revised to read as follows:

Sec.  197.36  Are there limits on what DOE must consider in the
performance assessments?

    (a) Yes, there are limits on what DOE must consider in the
performance assessments.
    (1) The DOE's performance assessments conducted to show compliance
with Sec. Sec.  197.20(a)(1), 197.25(b)(1), and 197.30 shall not
include consideration of very unlikely features, events, or processes,