Implementation of a Dose Standard After 10,000 Years, 10811-10830 [E9-5448]

Agencies

• NUCLEAR REGULATORY COMMISSION

[Federal Register Volume 74, Number 48 (Friday, March 13, 2009)]
[Rules and Regulations]
[Pages 10811-10830]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: E9-5448]



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Federal Register / Vol. 74, No. 48 / Friday, March 13, 2009 / Rules 
and Regulations

[[Page 10811]]



NUCLEAR REGULATORY COMMISSION

10 CFR Part 63

RIN 3150-AH68
[NRC-2005-0011]


Implementation of a Dose Standard After 10,000 Years

AGENCY: Nuclear Regulatory Commission.

ACTION: Final rule.

-----------------------------------------------------------------------

SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) is amending its 
regulations governing the disposal of high-level radioactive wastes in 
a proposed geologic repository at Yucca Mountain, Nevada. The final 
rule implements the U.S. Environmental Protection Agency's (EPA's) 
revised standards for doses that could occur after 10,000 years, but 
within the period of geologic stability. The final rule also specifies 
a range of values for the deep percolation rate to be used to represent 
climate change after 10,000 years, as called for by EPA, and specifies 
that calculations of radiation doses for workers use the same weighting 
factors that EPA is using for calculating individual doses to members 
of the public.

DATES: Effective Date: This final rule is effective on April 13, 2009.

ADDRESSES: Publicly available documents related to this rulemaking may 
be viewed electronically on the public computers located at the NRC's 
Public Document Room (PDR), Room O1F21, One White Flint North, 11555 
Rockville Pike, Rockville, Maryland. The PDR reproduction contractor 
will copy documents for a fee. Selected documents and information on 
this rulemaking can be accessed at the Federal rulemaking portal, 
https://regulations.gov by searching on rulemaking docket ID: NRC-2005-
0011.
    Publicly available documents created or received at the NRC are 
available electronically at the NRC's Electronic Reading Room at http:/
/www.nrc.gov/reading-rm/adams.html. From this site, the public can gain 
entry into the NRC's Agencywide Document Access and Management System 
(ADAMS), which provides text and image files of NRC's public documents. 
If you do not have access to ADAMS or if there are problems in 
accessing the documents located in ADAMS, contact the NRC Public 
Document Room (PDR) Reference staff at (800) 397-4209, (301) 415-4737, 
or by e-mail to pdr.resource@nrc.gov.

FOR FURTHER INFORMATION CONTACT: Timothy McCartin, Office of Nuclear 
Material Safety and Safeguards, U.S. Nuclear Regulatory Commission, 
Washington, DC 20555-0001, telephone (301) 492-3167, e-mail 
Timothy.McCartin@nrc.gov; Janet Kotra, Office of Nuclear Material 
Safety and Safeguards, U.S. Nuclear Regulatory Commission, Washington, 
DC 20555-0001, telephone (301) 492-3190, e-mail Janet.Kotra@nrc.gov; or 
Robert MacDougall, Office of Federal and State Materials and 
Environmental Management Programs, U.S. Nuclear Regulatory Commission, 
Washington, DC 20555-0001, telephone (301) 415-5175, e-mail 
Robert.MacDougall@nrc.gov.

SUPPLEMENTARY INFORMATION:

I. Background
II. Implementation of the Environmental Protection Agency's Final 
Standards for a Compliance Period Beyond 10,000 Years and Within the 
Period of Geologic Stability
III. Public Comments and Responses
    1. NRC Adoption of EPA Standards
    2. Clarifications on NRC's Implementation of Features, Events, 
and Processes for the Performance Assessment for the Period After 
10,000 Years
    3. Climate Change
    4. Use of Current Dosimetry
    5. Comments Beyond the Scope of This Rulemaking
IV. Summary of Final Revisions
V. Agreement State Compatibility
VI. Voluntary Consensus Standards
VII. Finding of No Significant Environmental Impact: Availability
VIII. Paperwork Reduction Act Statement
IX. Regulatory Analysis
X. Regulatory Flexibility Certification
XI. Backfit Analysis
XII. Congressional Review Act

I. Background

    On November 2, 2001 (66 FR 55732), NRC published its final rule, 10 
CFR Part 63, governing disposal of high-level radioactive wastes in a 
potential geologic repository at Yucca Mountain, Nevada. The U.S. 
Department of Energy (DOE) must comply with these regulations for NRC 
to authorize construction and license operation of a potential 
repository at Yucca Mountain. As mandated by the Energy Policy Act of 
1992 (EnPA), Public Law 102-486, NRC's final rule was consistent with 
the radiation protection standards issued by EPA at 40 CFR Part 197 (66 
FR 32074; June 13, 2001). EPA developed these standards pursuant to 
Congress' direction, in Section 801 of EnPA, to issue public health and 
safety standards for protection of the public from releases of 
radioactive materials stored or disposed of in a potential repository 
at the Yucca Mountain site. Such standards were to be ``based upon and 
consistent with'' the findings and recommendations of the National 
Academy of Sciences (NAS). The NAS issued its findings and 
recommendations, on August 1, 1995, in a report entitled Technical 
Bases for Yucca Mountain Standards.
    The State of Nevada and other petitioners challenged both the EPA 
standards and the NRC regulations in court. On July 9, 2004, the United 
States Court of Appeals for the District of Columbia Circuit upheld 
both EPA's standards and NRC's regulations on all but one of the issues 
raised by the petitioners. See Nuclear Energy Institute, Inc. v. 
Environmental Protection Agency, 373 F.3d 1251 (DC Cir. 2004) (NEI v. 
EPA). The court disagreed with EPA's decision to adopt a 10,000-year 
period for compliance with the standards and NRC's adoption of that 
10,000-year compliance period in NRC's implementing regulations. The 
court found that EPA's 10,000-year compliance period was not ``* * * 
based upon and consistent with'' NAS' findings, as required by Section 
801 of EnPA. See 373 F.3d at 1270. The NAS recommended EPA develop 
standards that provide protection when radiation doses reach their 
peak, within the limits imposed by long-term stability of the geologic 
environment. In addition, NAS found no scientific basis for limiting 
application of the individual-risk standard to 10,000 years. Thus, the

[[Page 10812]]

court vacated EPA's rule, at 40 CFR Part 197, to the extent that it 
specified a 10,000-year compliance period, and remanded the matter to 
EPA. The court also vacated NRC's rule, at 10 CFR Part 63, insofar as 
it incorporated EPA's 10,000-year compliance period.

EPA's Proposed Rule

    In response to the remand, EPA proposed revisions (70 FR 49014; 
August 22, 2005) to elements of its standards affected by the court's 
decision. EPA proposed to revise its individual-protection and human-
intrusion standards to incorporate the time of peak dose into the 
determination of compliance. EPA retained its 0.15 millisievert (mSv)/
year [15 millirem (mrem)/year] standards for 10,000 years after 
disposal, and added a 3.5 mSv (350 mrem) standard for the period after 
10,000 years, but within the period of geologic stability. EPA defined 
the period of geologic stability as ending at 1 million years after 
disposal. Further, EPA proposed that NRC base its determination of 
compliance with the post-10,000 year standards, based on the median of 
the projected doses from DOE's performance assessments, rather than on 
the arithmetic mean of the projected doses. The arithmetic mean was 
still retained as the compliance measure for the first 10,000 years 
after disposal.
    EPA also proposed to define how DOE should incorporate features, 
events, and processes (FEPs) in the performance assessment for the 
period after 10,000 years. EPA explained that the goal of the 
performance assessment ``is to design an assessment that is a 
reasonable test of the disposal system under a range of conditions that 
represents the expected case, as well as relatively less likely (but 
not wholly speculative) scenarios with potentially significant 
consequences. The challenge is to define the parameters of the 
assessment so that they demonstrate whether or not the disposal system 
is resilient and safe in response to meaningful disruptions, while 
avoiding extremely speculative (and in some cases, fantastical) 
events.'' (70 FR 49048; August 22, 2005). EPA proposed that DOE's 
performance assessments conducted to show compliance with the post-
10,000 year individual protection and human-intrusion standards shall 
project the continued effects of the FEPs included in the initial 
10,000 year analysis. EPA also proposed certain constraints on DOE's 
performance assessments for the post-10,000 year period. These are:
    (1) Seismic analysis may be limited to the effects caused by damage 
to the drifts in the repository and the failure of the waste packages;
    (2) Igneous analysis may be limited to the effects of a volcanic 
activity event directly intersecting the repository, and the igneous 
event may be limited to that causing damage to the waste packages 
directly, causing releases of radionuclides to the biosphere, 
atmosphere, or ground water;
    (3) Climate change analysis may be limited to the effects of 
increased water flow through the repository as a result of climate 
change, and that the nature and degree of climate change may be 
represented by sampling within a range of specified constant 
conditions; and
    (4) DOE must assess the effects of general corrosion on engineered 
barriers and may use a constant representative corrosion rate 
throughout the period of geologic stability, or a distribution of 
corrosion rates correlated to other repository parameters.
    With respect to climate change, EPA further proposed that NRC shall 
specify in regulation the values to be used to represent climate 
change, such as temperature, precipitation, or infiltration rate of 
water.
    Finally, in its definition of ``effective dose equivalent'' EPA 
proposed that DOE calculate annual committed effective dose equivalents 
using the weighting factors that would be incorporated in its 
regulations in a new Appendix A to 40 CFR Part 197. EPA believes this 
reflects the most recent application of current radiation science to 
the calculation of dose.

NRC's Proposed Rule

    Under the EnPA, NRC's regulations must be consistent with EPA's 
standards. On September 8, 2005, NRC proposed revisions to its 
regulations designed to achieve consistency with EPA's proposed revised 
standards (70 FR 53313; September 8, 2005). NRC proposed to incorporate 
the new post-10,000 year dose limit of 3.5 mSv/year (350 mrem/year) and 
statistical measure for compliance directly into its regulations for 
individual protection and human intrusion. Also, NRC proposed to adopt 
specific constraints EPA proposed for considering FEPs after 10,000 
years. NRC proposed to revise its requirements to be consistent with 
EPA's proposal that the performance assessment for the first 10,000 
years serve as the basis for projecting repository performance after 
10,000 years. NRC, supporting the use of current dosimetry, proposed to 
adopt the specific weighting factors provided in Appendix A of 40 CFR 
Part 197. Overall, NRC's proposed changes to Part 63 adopted the same 
or approximately the same wording as used by EPA in its proposed 
revisions to 40 CFR Part 197. Further, consistent with EPA's 
specification of dosimetry for calculating individual doses to members 
of the public, NRC proposed to revise its Part 63 regulations to allow 
DOE to use the same methods for calculating doses to workers during the 
operational period. Finally, in response to EPA's proposal, NRC 
proposed to specify, in its regulation, steady-state (constant-in-time) 
values that DOE should use to project the long-term impact of climate 
variation. NRC proposed that DOE represent future climate change in the 
performance assessment by sampling constant-in-time deep percolation 
rates from a log-uniform distribution, which varies between 13 and 64 
millimeters (mm)/year [0.5 and 2.5 inches (in.)/year].
    NRC's notice of proposed rulemaking invited comments on its 
proposal to implement EPA's proposed revisions to its standards, as 
well as on NRC's revisions for use of specific weighting factors for 
calculating worker doses, and on NRC's specification of values for 
climate change. NRC requested comments only on those provisions of Part 
63 that NRC proposed to change and noted that its existing regulations 
were not affected by this rulemaking except insofar as NRC's proposed 
rule adopts more up-to-date dosimetry for dose calculations. NRC 
notified potential commenters that comments on EPA's revised standards 
should be directed to EPA. In response to requests from the public, NRC 
extended the comment period, originally ending on November 7, 2005, to 
December 7, 2005 (70 FR 67098; November 4, 2005).

II. Implementation of the Environmental Protection Agency's Final 
Standards for a Compliance Period Beyond 10,000 Years and Within the 
Period of Geologic Stability

EPA's Final Rule

    EPA published final ``Public Health and Environmental Radiation 
Protection Standards for Yucca Mountain, Nevada,'' for the period after 
10,000 years at 40 CFR Part 197 on October 15, 2008 (73 FR 61256). EPA 
has finalized its proposals relating to: consideration of FEPs in the 
post-10,000 year period, and use of specific weighting factors that 
reflect current methods of dosimetry and updated models for calculating 
individual exposures from radiation. EPA's final rule differs from its 
proposal in two respects: the dose limit and the consideration of 
seismic activity.

[[Page 10813]]

    First, the EPA standards establish a 1.0 mSv/year (100 mrem/year) 
dose limit for the reasonably maximally exposed individual (RMEI) for 
the period after 10,000 years and within the period of geologic 
stability, rather than a 3.5 mSv/year (350 mrem/year) dose limit, as 
had been proposed. The EPA standards also provide that NRC base its 
determination of compliance with the post-10,000 year standards on the 
arithmetic mean of the projected doses, rather than on the median, as 
was proposed.
    Second, EPA's standards now require that analyses of seismic 
activity consider water table rise under Yucca Mountain caused by 
seismic activity. The final standards specify that NRC may determine 
the magnitude of the water table rise to be used in the performance 
assessment for the period after 10,000 years or, if this magnitude is 
found to be insignificant, not require its consideration in performance 
assessment. Alternatively, NRC may require DOE to demonstrate the 
magnitude of the water table rise and its significance in terms of 
repository performance in its license application.

NRC's Final Rule

    EnPA directs the Commission to modify its technical criteria to be 
consistent with EPA's standards for a geologic repository at the Yucca 
Mountain site. NRC's final rule achieves this consistency by 
incorporating the revised standards into its final revised 10 CFR Part 
63 regulations as transparently as possible. A brief description of the 
Commission's implementation of EPA's standards follows:
    (1) For the period after 10,000 years and within the period of 
geologic stability (up to 1 million years), NRC adopts EPA's 1.0 mSv/
year (100 mrem/year) dose limit for the RMEI in both the individual 
protection standard at 10 CFR 63.311 and the human intrusion standard 
at 10 CFR 63.321.
    (2) NRC adopts, in 10 CFR 63.303, EPA's specification of the 
arithmetic mean as the basis for determining compliance with the dose 
limit for the post-10,000-year period.
    (3) NRC adopts, in 10 CFR 63.305 and 63.342, EPA's specific 
requirements for the performance assessment DOE must use to evaluate 
the behavior of the repository for the period after 10,000 years. The 
FEPs selected for use in the performance assessment for the first 
10,000 years should also be used for projecting repository performance 
after 10,000 years. NRC adopts EPA's additional constraints for the 
inclusion of seismic activity, igneous activity, climate change, and 
general corrosion in the performance assessment for the period of time 
after 10,000 years. The seismic analysis must include the magnitude of 
the water table rise and its significance on the results of the 
performance assessment unless NRC, through rulemaking, decides to 
specify the magnitude of the water table rise to be used in the 
performance assessment after 10,000 years or to not require its 
consideration.
    (4) NRC adopts, in 10 CFR 63.102(o), EPA's specification of the 
weighting factors to be used for estimating potential radiation 
exposures for members of the public, which are provided in Appendix A 
of 40 CFR Part 197.
    In addition to the changes made for consistency with EPA's 
standards, NRC proposed to add a definition for ``weighting factor'' 
and to amend Sec.  63.111(a)(1) to allow DOE to use the weighting 
factors in Appendix A for calculating doses to workers. After 
consideration of the public comments, NRC chooses not to add the 
proposed definition for ``weighting factor'' to its regulations nor to 
amend Sec.  63.111(a)(1). Instead, NRC is providing a discussion 
regarding implementation of total effective dose equivalent (TEDE). NRC 
is adding text at Sec.  63.102(o) to clarify that the weighting factors 
specified in EPA's final standards should be used for dose calculations 
for workers and the public. Thus, TEDE calculations of potential 
radiation exposures to workers and the public are implemented 
consistently with a single set of weighting factors based on current 
dosimetry. The definition for TEDE is also revised to be consistent 
with NRC regulations at Part 20. This approach avoids the unnecessary 
complication and potential confusion that could result from the use of 
different definitions in Parts 20 and 63 and provides a single, clear 
statement on the proper implementation of TEDE in Part 63 thereby 
eliminating any need for further changes. (See response to comments 
under Use of Current Dosimetry, in this document.)
    EPA's rule requires DOE to assess the effects of climate change in 
the period after 10,000 years. This assessment is limited to the 
effects of increased water flow through the repository. The nature and 
degree of climate change may be represented by sampling within a range 
of constant climate conditions. EPA leaves it to NRC to specify, in 
regulation, the values to be used to represent climate change, such as 
temperature, precipitation, or infiltration of water. NRC's proposed 
rule sought public comment on its approach for representing the effect 
of future climate in performance assessments after 10,000 years. NRC 
proposed that the constant value to be used to represent climate change 
is to be sampled from a log-uniform distribution for deep percolation 
rates, which varies between 13 and 64 mm/year (0.5 and 2.5 in./year).
    After consideration of the public comments received on its 
proposal, NRC's final rule adopts its proposed approach with some 
modifications. NRC will require that DOE represent the effects of 
climate change by assuming constant-in-time climate conditions. The 
analysis may commence for the period beginning at 10,000 years after 
disposal and shall extend through the period of geologic stability. The 
constant-in-time value to be used to represent climate change is to be 
the spatial average of the deep percolation rate within the area 
bounded by the repository footprint. The constant-in-time deep 
percolation rates to be used now to represent climate change shall be 
sampled from a ``truncated'' lognormal distribution for deep 
percolation rates, which varies between 10 and 100 mm/year (0.39 and 
3.9 in./year). This ``truncated'' lognormal distribution has an 
arithmetic mean of 37 mm/year (1.5 in./year) for the deep percolation 
rate as compared to an arithmetic mean of 32 mm/year (1.3 in./year) 
based on the range and distribution in the proposed regulations. (See 
response to comments under Climate Change, in this document for further 
details on this approach and the consideration of public comments.)
    For a full description of changes NRC is incorporating into its 
Part 63 regulations, see Section IV of this document.

Water Table Rise From Seismic Activity

    NRC currently requires DOE to demonstrate the magnitude of the 
water table rise from seismic activity and its significance in its 
license application. The National Research Council (1992) conducted a 
comprehensive technical evaluation of mechanisms that could raise the 
water table at Yucca Mountain (National Research Council, Ground Water 
at Yucca Mountain: How High Can It Rise?, National Academy Press, 
Washington, DC, 1992). The Council considered both the dynamic response 
of the water table to propagation of seismic waves, as well as the 
long-term hydrologic response of the ground water system to permanent 
changes in rock stress after the seismic waves pass. The Council 
concluded that transient effects are not relevant to the performance of 
a repository. Of potential significance, however, are permanent changes 
to the

[[Page 10814]]

fluid pore pressure or rock permeability that may bring about long-term 
changes in the height of the water table. The report's authors 
evaluated historical accounts of relevant large earthquakes that have 
caused long-term changes to the regional hydrologic regime of ground 
water systems. The authors conducted site-specific quantitative 
analyses of the potential change in the level of the water table. They 
concluded that ``although the models are based on very limited data * * 
* [the] stress/strain changes resulting from an earthquake are 
inadequate to cause more than a few tens of meters rise in the water 
table based on the convergence of the results by a variety of models 
and assumptions, especially if the deep carbonate aquifer is as 
incompressible as the limited data suggest.'' Whatever approach DOE 
takes when determining the magnitude of the water table rise from 
seismic activity, NRC expects that DOE will consider the information 
provided by the National Research Council as referenced in the National 
Academy of Sciences report entitled, ``Technical Bases for Yucca 
Mountain Standards'' (1995) at page 94 (i.e., ``Results indicate a 
probable maximum transient rise on the order of 20 m or less'').
    Although EPA standards specify that NRC may determine the magnitude 
of water table rise and its significance, NRC is not planning such 
action. If, in the future, NRC decides to specify the magnitude of the 
water table rise and whether it is significant enough for consideration 
in DOE's performance assessment, NRC will do so in a future rulemaking.

III. Public Comments and Responses

    The NRC received 16 individual comment submittals, many of which 
contained numerous specific comments. In addition, NRC received more 
than 3000 submissions objecting, in nearly identical text, to NRC's 
adoption of EPA's standards because the commenters believed the 
proposed standards are inadequate and because NRC published its 
proposed revision to Part 63 before EPA issued final standards. NRC 
carefully reviewed and considered the range of comments received during 
the public comment period. The NRC staff grouped the comments into the 
following five major topic areas:
    (1) NRC Adoption of EPA Standards;
    (2) Clarifications on NRC's Implementation of FEPs for the 
Performance Assessment for the Period after 10,000 Years;
    (3) Climate Change;
    (4) Use of Current Dosimetry; and
    (5) Comments Beyond the Scope of this Rulemaking.

1. NRC Adoption of EPA Standards

    Issue 1: Must NRC supplement EPA's standards because they do not 
adequately protect public health and safety and the environment?
    Comment. Some commenters supported NRC's adoption of EPA's 
standards, while others opposed adoption because they believe EPA's 
proposed standards are inadequate to protect public health and safety 
and the environment. The State of Nevada recognized that EnPA requires 
NRC's regulations to be consistent with EPA's standards but claims this 
does not mean the two must be identical. Rather, the State asserts, NRC 
must recognize that compliance with EPA's standards is necessary but 
not sufficient to provide adequate protection of public health and 
safety and the environment. The State also asserts that NRC should 
promulgate supplemental standards, in its regulations, that will 
provide the additional protection the State believes is needed. With 
respect to EPA's proposed standards, the State and other commenters 
particularly objected to EPA's 3.5 mSv/year (350 mrem/year) post-10,000 
year standard and use of the median to assess compliance. The State and 
other commenters also objected to many other features of the EPA 
standards, including limitations on the FEPs, use of a two-tier 
standard, and defining the period of geologic stability as ending at 1 
million years. In support of its comments, the State attached a copy of 
the comments on the EPA proposed standards it had submitted to EPA.
    Response. While EnPA does not require NRC regulations to be 
identical to EPA's, EnPA does direct the Commission to modify its 
technical criteria to be consistent with EPA's standards for a geologic 
repository at the Yucca Mountain site. Thus, NRC is required to adopt 
EPA's post 10,000 year standard, and the NRC has done so. The NRC's 
notice of proposed rulemaking notified potential commenters that 
comments such as these on EPA's revised standards should be directed to 
EPA for EPA's response.
    Issue 2: Should NRC extend the compliance period beyond 1 million 
years if it is determined that the peak dose may occur beyond the 1 
million-year period?
    Comment. The State commented that EPA's requirement that the post-
10,000 year performance assessment should end at 1 million years is 
unnecessarily prescriptive. The State believes that if the trends in 
dose projection are not clear or heading upward and geologic stability 
is maintained, extending the assessment beyond 1 million years may be 
required to establish the performance of the entire repository system. 
The State believes that NRC has the authority to consider not only the 
magnitude of the peak, but also the timing and overall trends of dose 
projections as it evaluates the license application.
    Response. As explained in the response to the comment on Issue 1 
under NRC Adoption of EPA Standards of this document, EnPA requires the 
Commission to modify its technical criteria to be consistent with EPA's 
standards for a geologic repository at the Yucca Mountain site. The 
NRC's notice of proposed rulemaking notified potential commenters that 
comments such as these on EPA's revised standards should be directed to 
EPA for EPA's response.
    Issue 3: Has NRC illegitimately used rulemaking to resolve issues 
that must be resolved in an adjudicatory proceeding?
    Comment. The State of Nevada commented that the proposed rule 
violates fundamental principles of administrative law because it fails 
to conform to the usual distinctions in agency administrative processes 
between ``rulemaking'' and ``adjudication.'' This is because the rule 
includes what the commenter believes to be ``determinations of 
adjudicative fact'' that apply only to Yucca Mountain and that should 
be matters adjudicated in NRC's hearing on DOE's license application. 
According to the commenter, there are two critical distinctions between 
rulemaking and adjudication: ``First, a rule addresses the future while 
an order [the product of adjudication] addresses the past or the 
present. Second, a rule is based on general policy considerations or on 
what are sometimes called legislative facts, generalizations about 
people and things, while an order is based on specific facts about 
things and individuals, sometimes called adjudicative facts.'' The 
commenter believes that the proposed rule violates this distinction 
because ``[n]o agency may resolve a controversy over an adjudicative 
fact, relevant only to a single adjudication, by rulemaking.'' The 
State further asserts that NRC's alleged improper use of rulemaking to 
resolve adjudicatory factual issues constitutes an unlawful abrogation 
of Nevada's right, under section 189 of the Atomic Energy Act of 1954 
as amended (AEA), to an NRC licensing hearing on these factual issues.

[[Page 10815]]

    In the State's view, NRC cannot claim that it is permitted to 
resolve adjudicatory factual issues in its rulemaking simply because 
EPA did so and NRC must adopt EPA's standards. The commenter recognizes 
that the EnPA alters a straightforward demarcation between rulemaking 
and adjudication because ``EnPA does contemplate Yucca `rules' that by 
their nature depend on some facts relevant only to Yucca.'' However, 
the commenter contends that ``EnPA authorized only those EPA findings 
of adjudicatory fact that (1) are based on what the [National] Academy 
[of Sciences] considered necessary to support an EPA rule; and (2) are 
essential to promulgate limits on radiation exposures, concentrations, 
or quantities beyond the boundary of the Yucca Mountain site.'' This is 
because the grant of authority to EPA in EnPA to issue standards 
applicable only to Yucca Mountain is based on the previous delegation 
of rulemaking authority to EPA in section 121 of the Nuclear Waste 
Policy Act of 1982 (NWPA), which, in turn, relies upon the delegation 
of authorities to EPA in Reorganization Plan Number 3 of 1970 that 
identifies what standards EPA may issue. The commenter believes that 
the EPA standards that NRC is adopting are rife with ``adjudicative 
facts'' and go well beyond the narrow limits permitted by EnPA.
    The commenter cites eight ``determinations of adjudicative fact'' 
that appear in NRC's proposed rule, most of which NRC is adopting from 
EPA's standards:
    (1) The performance assessment for the period after 10,000 years 
must use a time-independent log-uniform probability distribution for 
deep percolation rates of from 13 to 64 mm/year;
    (2) Models and data used to develop FEPs (``features, events and 
processes'') for the assessment period before 10,000 years are 
sufficient for the post-10,000-year assessment period;
    (3) Seismic analyses for the post-10,000 year period may be based 
on seismic hazard curves developed for the pre-10,000-year period;
    (4) Seismic effects in the post-10,000-year period may be limited 
to effects on the repository's drifts and waste packages;
    (5) Igneous effects in the post-10,000 year period may be limited 
to effects on waste packages;
    (6) The effects of climate change in the post-10,000-year period 
may be limited to increased water flux through the repository;
    (7) Different types of corrosion of the waste packages must be 
considered in the pre-10,000-year period but only general corrosion at 
a constant rate may be considered in the post-10,000-year period; \1\ 
and
---------------------------------------------------------------------------

    \1\ The rule does not, in fact, restrict consideration of 
corrosion in the post-10,000 year period to general corrosion; other 
types of corrosion, if important, will be carried over from the pre-
10,000 year period and will also be considered.
---------------------------------------------------------------------------

    (8) Effects of climate change in the post-10,000-year period may be 
expressed by steady state (time independent) values.
    Response. The Commission disagrees with the comment. ``It is a 
well-settled principle of administrative law that the decision whether 
to proceed by rulemaking or adjudication lies within the broad 
discretion of the agency. See, SEC v. Chenery Corp., 332 U.S. 194, 202-
03 (1947)'' Wisconsin Gas Company v. Federal Energy Regulatory 
Commission, 770 F.2d 1144, 1166 (DC Cir. 1985). The Commission has 
properly exercised its discretion to resolve the issues referenced by 
the commenter through rulemaking rather than through adjudication.
    The commenter mischaracterizes as ``determinations of adjudicative 
fact'' what are in reality assumptions, derived from data, testing, and 
scientific analysis, that DOE is to use in its performance assessment 
to demonstrate compliance with regulatory standards. A performance 
assessment is used to take account of the considerable uncertainties 
inherent in projecting disposal system performance over times as long 
as 1 million years. The performance assessment is not intended to 
resolve issues arising in the past or present. Rather, it is intended 
to provide a reasonable test of the safety of the repository by 
modeling through computer simulations a large number of ``alternative 
futures,'' incorporating the features, events, and processes required 
by the rule to be included in the assessment to determine if there is a 
reasonable expectation that the disposal system will meet regulatory 
standards. The assumptions identified by the commenter impose certain 
limitations on the scope of the performance assessments. These 
limitations are based on the application of scientific reasoning to 
data, testing, and analysis at hand on these issues and are for the 
purpose of enabling a reasonable test of repository safety.
    NRC has made a policy judgment that rulemaking is the better 
procedural vehicle to use to determine how the performance assessments 
should be constructed and, in particular, what limitations are 
appropriate to avoid unbounded speculation and to provide a reasonable 
test of repository safety. How this testing should be conducted is 
preeminently a matter of scientific and technical analysis. To the 
extent that there may be disagreement in the scientific community as to 
the scientific soundness of the assumptions and any limitations on 
assumptions to be incorporated into the performance assessments, the 
notice and comment rulemaking process is of particular value because it 
allows equal access to all viewpoints and best assures achievement of 
the ultimate goal of making sure that the testing of the safety of the 
repository rests on the best science available. The determination of 
what assumptions and limitations on assumptions are best suited to form 
a reasonable test is not aimed at determining the rights or liabilities 
of particular individuals and thus, the adjudicatory process is not 
conducive to selecting the ingredients of the tests used to provide a 
reasonable expectation of repository safety.
    Because neither EPA nor NRC have made ``determinations of 
adjudicative fact,'' as explained above, the question of the extent of 
EPA's authority under EnPA to establish standards through rulemaking 
that the commenter believes would otherwise be determinations of 
adjudicative fact does not arise. EPA has adequately addressed its 
jurisdiction to issue the standards that NRC is adopting in this final 
rule.
    The commenter may also be asserting that all the issues in this 
rulemaking are adjudicatory issues simply because the rule applies to 
only one entity, DOE, and the licensing of a repository at one site. A 
``rule,'' as defined in the Administrative Procedure Act, ``means the 
whole or part of an agency statement of general or particular 
applicability and future effect designed to implement, interpret, or 
prescribe law or policy'' 5 U.S.C. 551(4) (emphasis added). Thus, the 
fact that NRC's rule applies only to DOE and only to DOE's activities 
at one site does not, per se, turn the issues considered in this 
rulemaking into adjudicative issues determining adjudicative facts (See 
Attorney General's Manual on the Administrative Procedure Act, 1947, p. 
13 (``[R]ule'' includes agency statements not only of general 
applicability but also those of particular applicability applying 
either to a class or to a single person''); Anaconda Company v. 
Ruckelshaus, 482 F.2d 1301, 1306 (10th Cir. 1973)).
    The cases cited by the commenter, Heckler v. Campbell, 461 U.S. 458 
(1983), Broz v. Heckler, 711 F.2d 957 (11th Cir. 1983) (Broz II), and 
Opinion Modified on Denial of Rehearing by Broz

[[Page 10816]]

v. Heckler, 721 F.2d 1297 (11th Cir. 1983) (Broz III), in support of 
its view that NRC may not make ``determinations of adjudicatory fact'' 
in a rulemaking are similarly not relevant because no such 
determinations are being made in the final rule. These cases do not 
establish the broad principle stated by the commenter; i.e., that 
``[n]o agency may resolve a controversy over an adjudicative fact, 
relevant only to a single adjudication, by rulemaking.'' In Heckler v. 
Campbell, the Supreme Court upheld the Secretary of Health and Human 
Service's (HHS) reliance on rulemaking to establish guidance for the 
determination that jobs existed in the national economy within the 
capability of the disabled claimant against a claim that such a 
determination must be made in an individual adjudication. Broz 
considered the same guidance with respect to its application to the 
effect of age on disability determinations. Ultimately, in Broz III, 
the Eleventh Circuit of the U.S. Court of Appeals based its decision 
that this must be an individualized determination reached in an 
adjudication on its interpretation of Congress' intent in amending the 
Social Security Act (SSA) rather than on more sweeping statements about 
an agency's choice to use rulemaking or adjudication to achieve its 
mission.\2\ Finally, the Commission does not agree that resolving the 
issues the commenter has labeled ``determinations of adjudicative 
fact'' deprives the State of its right to a hearing under section 189a. 
of the AEA on these issues. As the Supreme Court has stated, ``the 
statutory requirement for a hearing * * * does not preclude the 
Commission from particularizing statutory standards through the 
rulemaking process and barring at the threshold those who neither 
measure up to them nor show reasons why in the public interest the rule 
should be waived'' (Federal Power Commission v. Texaco, Inc., 377 U.S. 
33, 39 (1964)).\3\
---------------------------------------------------------------------------

    \2\ The Eleventh Circuit initially construed the provisions of 
the SSA in terms of the distinction between adjudicative facts and 
legislative facts and concluded that the effect of age on disability 
was an adjudicative fact that could not be determined in a 
rulemaking. Broz v. Schweiker, 677 F.2d 1351 (11th Cir. 1982) (Broz 
I) Certiorari Granted, Judgment Vacated by Heckler v. Broz, 461 U.S. 
952 (1983). Upon remand for reconsideration in light of Campbell, 
the Eleventh Circuit, in Broz II, reaffirmed its original decision 
upon finding that the Supreme Court had left open the validity of 
the guidance with respect to its use in determining the effect of 
age on disability.
    \3\ The commenter believes that the rules which resolve these 
issues will be incapable of actually being applied as written 
because they will turn out to be based on outdated scientific 
evidence. If this should happen, any person can petition to amend 
the rules. In addition, NRC's procedural rules enable a party to an 
adjudicatory proceeding to petition that application of a rule be 
waived in circumstances when the rule would not serve the purposes 
for which it was adopted. See, 10 CFR 2.335(b).
---------------------------------------------------------------------------

    The commenter also believes that, as explained in its comments to 
EPA, EPA's ``findings of adjudicative fact,'' in its final rule, now 
being adopted in NRC's final rule, are without any technical basis and 
are contrary to sound science, and for that reason violate both EnPA 
and the AEA. The NRC's notice of proposed rulemaking notified potential 
commenters that comments such as these on EPA's revised standards 
should be directed to EPA for EPA's response.
    Issue 4: Should NRC have waited to propose its regulations until 
after EPA had finalized its standards?
    Comment. A number of commenters objected to the process NRC used to 
conduct this rulemaking, namely issuing a proposed rule adopting EPA's 
proposed standards before EPA issued its final standards. Commenters 
expressed the view that NRC conveyed the impression that EPA's proposed 
standards would be adopted in NRC's final rule, such that public 
comment on EPA's proposal would have no effect; that if NRC cared what 
potential commenters thought about EPA's proposal, it should have 
waited, considered the comments received by EPA, and developed NRC's 
rule based on EPA's final rule; that having the public comment period 
for both rules at the same time is confusing for concerned citizens and 
makes it difficult for them to comment on the NRC rule; and that NRC 
should provide an additional comment period on its rule if EPA's final 
rule departs substantially from its proposed rule.
    Response. NRC's process for conducting this rulemaking was intended 
to put in place revised regulations, consistent with EPA's final 
revised standards, because the court had vacated NRC's rule insofar as 
it incorporated EPA's 10,000 year compliance period. NRC also sought to 
inform potential commenters on both rules, of how NRC envisioned 
implementing the EPA's proposed standards. It was hoped that such 
information would be of value in developing comments on both proposals. 
NRC's intention has always been, consistent with its statutory 
obligations, to conform its final regulations to the final standards 
EPA would issue after EPA duly considers the comments it received.
    NRC emphasized in its notice of proposed rulemaking that comments 
on EPA's revised standards were to be addressed to EPA and that the 
scope of NRC's revised rule was limited to its adoption of EPA's 
revised standards, its proposal to allow DOE to use the same methods 
for calculating doses to workers during the operational period as those 
required for calculating public doses and its proposal to specify use 
of a deep percolation rate to represent the effect of future climate in 
performance assessments after 10,000 years. Thus, the narrow focus of 
NRC's rulemaking only required potential commenters to focus on two 
technical issues beyond the issues involved in EPA's proposal (i.e., 
setting a value for the deep percolation rate and use of modern 
dosimetry for estimating worker exposures). NRC extended the comment 
period by one month in response to public comments. For these reasons, 
we believe the public was given a fair opportunity to comment on NRC's 
proposal. NRC regrets any misimpression that NRC was assuming that 
EPA's proposed rule would become final as proposed without modification 
and that comments provided to EPA would have no effect. NRC made no 
such assumption and EPA has in fact made changes to its proposed rule 
in light of the comments it received.
    Finally, with respect to the request for an additional comment 
period if EPA's final rule is substantially different from its proposed 
rule, as stated above (see Background section of this document), EPA's 
final rule differs from its proposed rule in only two respects: the 
dose limit is set to 1.0 mSv/year (100 mrem/year) with the arithmetic 
mean as the statistical metric to be used to assess compliance; and its 
requirement that NRC either establish the magnitude of the water table 
rise and its significance as part of the seismic assessment, or require 
DOE to do this assessment. The first change responds favorably to the 
numerous public comments urging use of a dose limit lower than 3.5 mSv/
year (350 mrem/year) and use of the arithmetic mean as the measure of 
compliance. Similarly, in its final regulations, NRC requires DOE to 
include the magnitude of the water table rise and its significance in 
its seismic assessment submitted with the license application. As a 
result, this information would also be subject to litigation, absent 
any future NRC rulemaking on this subject. Because of these changes, 
the Commission believes there is no need for an additional comment 
period.

[[Page 10817]]

2. Clarification of NRC's Implementation of FEPs for the Performance 
Assessment for the Period After 10,000 Years

    Issue 1: Are the FEPs considered for the first 10,000 years after 
repository closure the only FEPs that need be considered for the entire 
post-closure period?
    Comment. The Nuclear Energy Institute (NEI) agreed with NRC's 
adoption of EPA's requirement that the same FEPs identified and 
screened for inclusion in performance assessments to show compliance 
with the standards for the initial 10,000 years after closure be used 
in performance assessments to show compliance with the post-10,000 year 
standards. However, NEI believes NRC should clarify that FEPs that are 
screened-in for the first 10,000 years after repository closure are the 
only FEPs that need be considered for the entire post-closure period. 
NEI provided the example that if DOE provides an adequate basis to 
screen-out post-closure criticality or microbially-influenced corrosion 
(MIC) effects during the first 10,000 years after repository closure, 
the Yucca Mountain Review Plan (YMRP) should specify that no additional 
consideration of criticality or MIC in the post-10,000 year period is 
necessary.
    Response. The requirements for FEPs to be included in the 
performance assessment for the period after 10,000 years are specified 
at Sec.  63.342. DOE is required to include those FEPs that are 
screened into the performance assessments for the first 10,000 years 
after repository closure and the four FEPs specifically identified for 
inclusion, i.e., seismicity, igneous activity, climate change, and 
general corrosion. Based on the requirements at Sec.  63.342, the 
specific FEPs (criticality or MIC) identified by the commenter would 
only be included in the performance assessment after 10,000 years if 
they were also included in the performance assessment for the first 
10,000 years (i.e., could not be screened out of the performance 
assessment for the first 10,000 years). The Commission does not believe 
further clarification to the regulation is necessary.
    Issue 2: Do the proposed changes to Sec.  63.114 ``Requirements for 
performance assessment'' impose additional limits on the performance 
assessment for the period after 10,000 years?
    Comment. The State of Nevada believes that Sec.  63.114(b) appears 
to include another limit beyond the limits in Sec.  63.342(c) on the 
post-10,000 year performance assessment and asks for clarification. NEI 
believes that NRC should more clearly assert that performance 
assessment methods meeting existing Part 63 requirements are also 
adequate for the post-10,000 year period.
    Response. The changes to Sec.  63.114 impose no additional limits 
on the performance assessment for the period after 10,000 years. The 
changes ensure consistency between NRC's regulations and EPA's final 
standards. In particular, EPA's final standards specify that FEPs used 
for the first 10,000 years should be used for estimating performance 
after 10,000 years. Thus, Sec.  63.114(b) specifies that the same 
performance assessment methods used for the first 10,000 years are to 
be used for the period after 10,000 years. For example, parameter 
ranges used in the performance assessment for the first 10,000 years 
would be used in the performance assessment for the period after 10,000 
years. Additional technical basis for selection of FEPs, beyond that 
developed for the performance assessment for the first 10,000 years, is 
not required. Thus, the changes at Sec.  63.114 ensure the performance 
assessment methods, such as the support and treatment of FEPs will be 
the same for the periods before and after 10,000 years, subject to the 
limits on performance assessments at Sec.  63.342. Some minor revisions 
have been made to Sec.  63.114(b) to further clarify the Commission's 
intent.
    Issue 3: Does the proposed treatment of a potential igneous event 
during the period after 10,000 years limit consideration of the effects 
of magma on spent fuel?
    Comment. The State of Nevada commented that the proposed regulation 
at Sec.  63.342(c)(1)(ii) specifies that the effects of an igneous 
event are limited to the effects of damage directly to the waste 
package. The State is concerned that NRC will not consider the effects 
of magma on the radioactive waste inside the waste package. The State 
asserts that effects on the contents of the waste package could be 
important for igneous events that occur at times after waste packages 
are already breached because of other processes (such as corrosion) and 
the radioactive waste may be more vulnerable to igneous events.
    Response. The regulations do not exclude consideration of the spent 
fuel in the treatment of a potential igneous event during the period 
after 10,000 years. The rule, at Sec.  63.342(c)(1)(ii), requires the 
igneous analysis to include damage to the waste package directly. Waste 
package is defined in Sec.  63.2 to mean ``the waste form and any 
containers, shielding, packing, and other absorbent materials 
immediately surrounding an individual waste container'' and waste form 
is defined in the same section to mean ``the radioactive waste 
materials and any encapsulating or stabilizing matrix.'' Thus, 
consideration of damage to the waste package would include 
consideration of damage to the radioactive waste materials inside the 
waste package.
    Issue 4: Should the seismic analysis exclude seismic activity from 
magma movement?
    Comment. NEI agreed with NRC's proposal to limit analysis of long-
term effects of seismicity to effects on the drifts in the repository 
and the waste package but requested that NRC clarify that seismic 
activity from magma movement need not be considered in the analysis. 
NEI suggests such a limitation is appropriate based on an Electric 
Power Research Institute (EPRI) analysis that demonstrates that seismic 
activity induced from magma movement is very minor, compared to seismic 
activity caused by tectonism.
    Response. Seismic activity includes activity from both tectonism 
and magma movement. Current methods to develop and quantify seismic 
ground motions, such as DOE's current Probabilistic Seismic Hazard 
Assessment, include consideration of seismic activity from volcanism or 
magma movement. Volcanic sources of seismic activity are often included 
as part of the background seismic source term. Therefore, the 
commenter's request for clarification, which would exclude seismic 
activity caused by magma movement from the seismic analysis, is not 
appropriate.
    Issue 5: Should NRC's rule set a requirement for assuring the 
statistical significance of DOE's modeling results in its performance 
assessments?
    Comment. The State of Nevada stated that NRC's rule should 
establish a requirement for DOE to prove mathematically that its 
modeling results are statistically significant (i.e., a sufficient 
number of ``runs'' or the set of probabilistic simulations used to 
simulate the wide range of possible future behaviors of the repository 
system have been performed).
    Response. The current regulations provide specific requirements at 
Sec.  63.114 for the performance assessment. Among these, for example, 
are proper consideration of uncertainty and variability in parameter 
values. The Commission believes it is neither necessary nor appropriate 
to further specify measures of statistical significance. Fundamental to 
any approach for representing uncertainty and variability is 
demonstrating how the results accurately represent the uncertainty and 
variability, for example,

[[Page 10818]]

by performing a sufficient number of probabilistic simulations. 
Determining what number of ``runs is sufficient'' is best left for DOE 
to present and defend, based on the approach used in the performance 
assessment and an understanding of the results. NRC is confident that 
its regulations for performance assessment require DOE to provide 
sufficient information for NRC to judge if DOE has performed enough 
probabilistic simulations.
    Issue 6: Will FEPs associated with atmospheric releases of 
radioactivity and exposure of residents downwind of Yucca Mountain be 
considered in the performance assessment for the period after 10,000 
years?
    Comment. Two commenters expressed concern over how FEPs associated 
with atmospheric releases of radioactivity and exposure of residents 
downwind of Yucca Mountain will be considered in the performance 
assessment for the period after 10,000 years, including FEPs associated 
with seismic and igneous FEPs.
    Response. The performance assessment for the period after 10,000 
years must include consideration of potential atmospheric releases of 
radioactivity. The NAS report, Technical Bases for Yucca Mountain 
Standards (1995), pp. 6-7, recommended that the exposure scenario be 
specified in the standards because of the difficulties in projecting 
where people may reside and how exposures might occur in the distant 
future (e.g., thousands to hundreds of thousands of years in the future 
and longer). Accordingly, EPA specified characteristics of the RMEI 
(66-FR 32134; June 13, 2001).
    Issue 7: Does the fact that the limitations on FEPs in the 
performance assessments are being established through rulemaking rather 
than adjudication, based on data available in 2005, mean that there 
will be no flexibility to take into account data and models used in 
DOE's license application or that DOE will have no incentive to further 
reduce uncertainties?
    Comment. The State of Nevada believes that the assumptions being 
used to account for uncertainty in the post-10,000 year period, and 
which are incorporated through this rulemaking into the limitations on 
the FEPs to be considered in DOE's performance assessments, are 
premature and render the rule inflexible because they are based on data 
available in 2005. NRC's rules must be sufficiently flexible to take 
into account data and models used in DOE's license application. The 
State fears that because the rules are premised on uncertainties as 
perceived through 2005 data and models, DOE will have a disincentive to 
reduce these uncertainties and add realism to its post-10,000 year 
performance assessment because it will wish to preserve the 
uncertainties and conservatisms that form the basis for the rules.
    Response. NRC's regulations afford DOE the flexibility to account 
for uncertainty in data and models. Such flexibility provides neither 
incentive nor disincentive to reduce uncertainties. The regulations, at 
Sec.  63.114, require DOE to account for the uncertainties in data and 
models in the performance assessment over the initial 10,000 years, and 
these same uncertainties are to be included in the performance 
assessment beyond 10,000 years. On June 3, 2008, DOE submitted a 
license application to NRC for authorization to construct a repository 
at Yucca Mountain. The NRC will review DOE's treatment of the 
uncertainties. DOE has the flexibility to decide where to reduce 
uncertainties; however, it must demonstrate there is a reasonable 
expectation that the performance objectives will be met. NRC 
regulations afford DOE appropriate flexibility for selecting and 
supporting its performance assessment, including the consideration of 
uncertainties, given the unique and difficult task of estimating 
performance of a geologic repository over thousands of years.
    The regulations do provide certain limitations, as specified in 
EPA's final standards, with respect to certain FEPs (i.e., seismicity, 
igneous activity, climate change, and general corrosion). Uncertainties 
in data and models for these FEPs are limited to those aspects of the 
FEPs considered most important to performance and the treatment of the 
uncertainties used in the performance assessment for the initial 10,000 
years (see also the response to Issue 2 under this topic). For example, 
the consideration of seismic events in the performance assessment for 
the period after 10,000 years would be based on the same seismic hazard 
curve, including its uncertainties, that was used in the performance 
assessment for the initial 10,000 years. However, the analysis for the 
period after 10,000 years would only consider the aspects of the 
seismic events that might be the most important to repository 
performance (i.e., damage to the drifts in the repository, failure of 
the waste package, and magnitude of the water table rise under Yucca 
Mountain).
    Finally, the commenter believes that the rules which resolve these 
issues will be incapable of actually being applied as written because 
they will turn out to be based on outdated scientific evidence. If this 
should happen, any person can petition to amend the rules. In addition, 
NRC's procedural rules enable a party to an adjudicatory proceeding to 
petition that application of a rule be waived in circumstances when the 
rule would not serve the purposes for which it was adopted (See, 10 CFR 
2.335(b)).

3. Climate Change

    Issue 1: Can the future climatic regime be bounded by the observed 
range of conditions over past glacial-interglacial cycles?
    Comment. One commenter indicated it is incorrect to presume that 
future climate conditions at Yucca Mountain can be bounded by the 
observed range of conditions over past glacial-interglacial cycles. To 
the extent this comment may refer to human-induced influences on 
climate, those influences are considered under a separate issue.
    Response. The Commission believes the future climatic regime can be 
bounded by the observed range of conditions over past glacial-
interglacial cycles. All climate predictions are based on and 
calibrated to evidence of past climates contained in the geologic 
record. The values specified for deep percolation rates adopted in the 
final regulation capture the range of temporal variability, 
uncertainty, and magnitude of deep percolation expected as a 
consequence of future climate change.
    The NAS committee (1995) was familiar with the science behind 
predicting future climate changes and stated, in its recommendations on 
Yucca Mountain standards, that a future ice age in the next few hundred 
years is ``unlikely but not impossible,'' in the next 10,000 years is 
``probable but not assured.'' However, over a 1-million-year time 
frame, the climate is much more likely to pass through several glacial-
interglacial cycles (i.e., ice ages). The NAS indicated there is a 
reasonable data base from which to infer past changes and noted that 
``(a)lthough the range of climatic conditions has been wide, 
paleoclimate research shows that the bounding conditions, the envelope 
encompassing the total climatic range have been fairly stable'' and 
that ``(b)ased on this record, it seems plausible that the climate will 
fluctuate between glacial and interglacial stages during the period 
suggested for the performance assessment calculations.'' Further, in 
its 1995 findings, the NAS stated that ``enough of the important 
aspects [of climate change] can be known within reasonable limits of 
uncertainty, and these properties and processes are sufficiently 
understood and stable over the long time scales of interest to make 
calculations possible

[[Page 10819]]

and meaningful, we believe that there is a substantial scientific basis 
for making such calculations, taking uncertainty and natural 
variability into account.''
    Issue 2: Should human-induced influences on climate be considered 
when bounding the future climatic regime?
    Comment. One commenter noted that human-induced (i.e., 
anthropogenic) influences on climate from fossil fuel combustion and 
the resulting persistence of greenhouse gases in the atmosphere are the 
main issues to consider in predicting future climatic conditions. These 
anthropogenic effects might cause substantial reorganization of 
atmospheric systems, both before and after 10,000 years, that increase 
the number and intensity of extreme storm events at Yucca Mountain. The 
commenter believed that the highly non-linear hydrologic response of an 
arid system like Yucca Mountain to such extreme events would affect the 
performance of the repository and invalidate the use of the long-term 
average climate proposed in the Part 63 revisions. The same commenter 
also noted that the predictive challenges of simulating these 
postulated extreme events could be met through use of existing and 
soon-to-be-available global circulation models (GCMs) that explicitly 
incorporate atmospheric composition and evolution in predicting future 
climate conditions. In presuming use of these models, this commenter 
noted that uncertainties in climate prediction do not change in the 
period beyond 10,000 years, at least in terms of the range of climate 
conditions that could occur, but rather that their detailed timing may 
change. Another commenter speculated that the same anthropogenic 
climate effects might delay the onset and reduce the magnitude of full 
glacial cycles, resulting in longer interglacial periods that would be 
warmer and drier than present-day conditions. Accordingly, this second 
commenter felt that the use of long-term average climate conditions 
represented by the values specified for deep percolation rates in the 
proposed Part 63 revisions was overly conservative and that less water 
would reach the repository horizon.
    Response. NRC considered the effects of anthropogenic influences on 
climate change. Based on that evaluation, the NRC believes the range of 
values specified for deep percolation rates adopted in the final rule 
captures the range of temporal variability, uncertainty, and magnitude 
of deep percolation expected as a consequence of future climate change.
    The magnitude and timing of the anthropogenic effects suggested by 
the commenter are likely to be more pronounced during the first 10,000 
years. The final regulation addresses only the 10,000 to 1 million year 
time period, during which any anthropogenic effects are anticipated to 
diminish. Anthropogenic effects, as represented in the GCMs cited by 
the commenter, might persist for 100,000 year time periods, but they do 
not fluctuate periodically and they decrease with time after an initial 
peak. Therefore, NRC believes that these effects can be captured by the 
long-term average infiltration values adopted in the final regulation 
because the range of values for the sampled population bounds these 
effects in an appropriately conservative manner.
    Atmospheric reorganization and increased frequency and magnitude of 
extreme events might result from natural or anthropogenic climate 
change. However, extreme 10-to 20-year events effectively become long-
term averages that are incorporated into the range specified for deep 
percolation in the final regulation, when simulating a time period of 1 
million years.
    The Paintbrush non-welded tuff unit (PTn unit) overlying the 
potential repository dampens the effects of transient phenomena 
associated with shorter time frames (Manepally, C., et al., ``The 
Nature of Flow in the Faulted and Fractured Paintbrush Nonwelded 
Hydrogeologic Unit,'' San Antonio, TX: Center for Nuclear Waste 
Regulatory Analyses, April 2007) in the system's response to external 
hydrologic events. The NAS also recognized that long-term net 
infiltration averages can bound and describe Yucca Mountain hydrology 
adequately, stating that ``the subsurface location of the repository 
would provide a temporal filter for climate change effects on 
hydrologic processes'' The commenter also acknowledged this, quoting 
Cohen, ``no evidence shows that high-frequency fluctuations (a few 
years or shorter) penetrate to the depth of the potential repository'' 
(Cohen, S., ``Assumptions, Conservatisms, and Uncertainties in Yucca 
Mountain Performance Assessm