[Federal Register Volume 67, Number 195 (Tuesday, October 8, 2002)]
[Rules and Regulations]
[Pages 62628-62634]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 02-25521]


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NUCLEAR REGULATORY COMMISSION

10 CFR Part 63

RIN 3150-AG91


Specification of a Probability for Unlikely Features, Events and 
Processes

AGENCY: Nuclear Regulatory Commission.

ACTION: Final rule.

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SUMMARY: The U. S. Nuclear Regulatory Commission (NRC) is amending its 
regulations governing the disposal of high-level radioactive wastes in 
a

[[Page 62629]]

potential geologic repository at Yucca Mountain, Nevada, to define the 
term ``unlikely'' in quantitative terms. NRC regulations now specify a 
range of numerical values for use in determining whether a feature, 
event or process, or a sequence of events and processes, should be 
excluded from certain required assessments. NRC is taking this action 
to clarify how it plans to implement two of the environmental standards 
for Yucca Mountain issued by the U.S. Environmental Protection Agency 
(EPA). Specifically, EPA's standards require the exclusion of 
``unlikely'' features, events or processes, or sequences of events and 
processes, from the required assessments for the human-intrusion and 
ground-water protection standards. In accordance with the Energy Policy 
Act of 1992, NRC has adopted EPA's standards in its recently published 
technical requirements for a potential geologic repository at Yucca 
Mountain.

EFFECTIVE DATE: November 7, 2002.

ADDRESSES: The final rule and any related documents are available on 
NRC's rulemaking Web site at http://ruleforum.llnl.gov. For information 
about the interactive rulemaking Web site, contact Carol Gallagher 
(301) 415-5905; e-mail nrc.gov">cag@nrc.gov.
    The documents may also be examined at the NRC Public Document Room 
(PDR), Room O-1F23, 11555 Rockville Pike, Rockville, MD.
    NRC maintains an Agencywide Document Access and Management System 
(ADAMS), which provides text and image files of NRC's public documents. 
These documents may be accessed through NRC's Public Electronic Reading 
Room on the Internet at http://www.nrc.gov/reading-rm/adams.html. If 
you do not have access to ADAMS, or if there are problems in accessing 
the documents located in ADAMS, contact the NRC PDR Reference staff at 
1-800-397-4209, or 301-415-4737; or by e-mail to: nrc.gov">pdr@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) 415-7285, e-mail: 
nrc.gov">tjm3@nrc.gov; or Clark Prichard, Office of Nuclear Material Safety and 
Safeguards, U.S. Nuclear Regulatory Commission, Washington, DC 20555-
0001, telephone (301) 415-6203, e-mail: nrc.gov">cwp@nrc.gov.

SUPPLEMENTARY INFORMATION:

I. Background

    On November 2, 2001 (66 FR 55732), NRC published a final rule, 10 
CFR part 63, governing disposal of high-level radioactive wastes in a 
potential geologic repository at Yucca Mountain, Nevada. We are now 
finalizing one particular matter that specifies a probability for 
unlikely features, events, and processes (FEPs). These are the 
regulations that the U.S. Department of Energy (DOE) must meet in any 
license application for construction and operation of a potential 
repository. As mandated by the Energy Policy Act of 1992, Pub. L. 102-
486, NRC's rule adopts the radiation protection standards established 
by EPA in 40 CFR Part 197 (66 FR 32074; June 13, 2001). EPA's standards 
for disposal include an individual-protection standard (40 CFR 197.20); 
a human-intrusion standard (40 CFR 197.25); and ground-water protection 
standards (40 CFR 197.30). These EPA standards have been incorporated 
into NRC's regulations at 10 CFR 63.311, 63.321, and 63.331, 
respectively.
    DOE's performance assessments are required to consider the 
naturally occurring features, events, and processes that could affect 
the performance of a geologic repository (i.e., specific conditions or 
attributes of the geologic setting; degradation, deterioration, or 
alteration processes of engineered barriers; and interactions between 
natural and engineered barriers). EPA's standards include limits on 
what DOE must consider in performance assessments undertaken to 
determine whether the repository will perform in compliance with the 
standards (40 CFR 197.36). EPA's standards state that DOE's performance 
assessments shall not include consideration of ``very unlikely'' FEPs, 
which EPA defines to be those FEPs that are estimated to have less than 
one chance in 10,000 of occurring within 10,000 years of disposal. In 
addition, EPA's standards require NRC to exclude ``unlikely'' FEPs, or 
sequences of events and processes, from the required assessments for 
demonstrating compliance with the human-intrusion and ground-water 
protection standards. EPA did not define unlikely FEPs in its 
standards, but, rather, left the specific probability of the unlikely 
FEPs for NRC to define. The Commission explained in its rulemaking 
establishing part 63 that it `` * * * fully supports excluding unlikely 
FEPs from analyses for estimating compliance with the standards for 
human intrusion and ground-water protection * * *,'' and that it ``* * 
* plan[ned] to conduct an expedited rulemaking to quantitatively define 
the term `unlikely''' (66 FR 55734; November 2, 2001).
    NRC published a proposed rule, ``10 CFR Part 63: Specification of a 
Probability for Unlikely Features, Events, and Processes,'' on January 
25, 2002 (67 FR 3628), and requested public comments. That proposed 
rule defined the term ``unlikely'' in quantitative terms. This action 
was taken to allow NRC to implement EPA's final standards for a 
potential repository at Yucca Mountain, Nevada. The Commission was 
careful to point out that its specification for unlikely events was in 
the context of very specific assessments (i.e., those made to assess 
compliance with ground-water protection and human-intrusion standards) 
over a long time frame, and this specification was not intended to 
suggest or imply precedent for other significantly different 
applications that used the term ``unlikely'.
    Unlike the broader purposes served by the performance assessment 
for the all-pathway individual-protection standard, the performance 
assessments used to determine compliance with the human-intrusion 
standard and the ground-water protection standards serve narrow, 
focused objectives. In the case of the performance assessment for human 
intrusion, the purpose is to evaluate the robustness of the repository 
system, assuming the occurrence of a prescribed human-intrusion 
scenario. In the case of the performance assessment for ground-water 
protection, the purpose is to evaluate potential degradation of the 
ground-water resource. Although EPA's final standards did not specify a 
numerical value to define unlikely FEPs in quantitative terms, the 
preamble to the standards stated that the exclusion of unlikely FEPs is 
intended to focus these assessments on the ``expected'' or ``likely'' 
performance of the repository.\1\
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    \1\ For example, the preamble states: (1) ``[T]he assessment of 
resource pollution potential is based upon the engineering design of 
the repository being sufficiently robust under expected conditions 
to prevent unacceptable degradation of the ground-water resource 
over time'' (66 FR 32114; June 13, 2001); and (2) the term 
``undisturbed,'' which is used in connection with demonstrating 
compliance with the ground-water protection standards, means the ``* 
* * * disposal system is not disturbed by human intrusion but that 
other processes or events that are likely to occur could disturb the 
system'' (66 FR 32104; June 13, 2001).
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    From a probabilistic perspective, any FEP with an annual 
probability of 10-4 or higher would have a high probability 
of occurring within the 10,000 year compliance period.\2\ As the 
Commission

[[Page 62630]]

described in the proposed rule, likely FEPs should include not only 
FEPs very likely to occur, but also those reasonably likely to occur. 
Given uncertainties in estimating the occurrence of FEPs over a 10,000 
year time period, the Commission believed a prudent decision was to 
consider FEPs with 10 percent or greater chance of occurring within the 
10,000 year compliance period as likely FEPs. Thus, the Commission 
sought public comment on its proposal that unlikely FEPs be defined as 
those FEPs with less than a 10 percent chance, but greater than or 
equal to a 0.01 percent chance, of occurring within the 10,000 year 
compliance period (i.e., annual probability less than 10-5, 
but greater than or equal to 10-8, which is the upper 
boundary for very unlikely events). As mentioned previously, the focus 
of the performance assessments for human intrusion and ground-water 
protection is to be on expected conditions. The Commission believes an 
upper bound for unlikely FEPs of a 10 percent chance of occurring 
within the compliance period will focus the assessments for ground-
water protection and human intrusion on the likely performance of the 
repository.
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    \2\ Estimating a high probability of occurrence for an FEP 
creates an expectation that an FEP will occur; however, it does not 
guarantee such an occurrence. There is a chance that even high-
probability FEPs will not occur. Likewise, in a probabilistic sense, 
having a low probability of occurrence does not mean that an FEP 
will not occur.
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II. Public Comments and Responses

    The 75-day comment period for the proposed rule closed on April 10, 
2002. Comments were received from the following five organizations: 
EPA; State of Nevada and the Nevada Agency for Nuclear Projects; DOE; 
Nuclear Energy Institute (NEI); and Exelon Generation. Commenters 
differed on the quantitative values NRC should use for defining 
unlikely FEPs. Although some commenters supported the proposed values, 
others provided different numbers and associated rationales. In 
preparing the final rule, the NRC staff carefully reviewed and 
considered these comments. The commenters that suggested alternative 
values did not provide a convincing basis for rejecting NRC's proposed 
range and adopting a different range; therefore, the Commission has 
decided to finalize the rule as originally proposed. The NRC's 
consideration of each of the comments is provided below.

1. EPA Comments

    Comment 1.1: The upper value for the probability range for unlikely 
FEPs should be an annual probability of 10-6. An annual 
probability of 10-6 as a demarcation separating likely FEPs 
from unlikely FEPs is reasonable because it is the middle of the range 
between FEPs that are nearly certain to occur (i.e., annual probability 
of 10-4), and FEPs that are very unlikely to occur (i.e., 
annual probability of 10-8). Placing the demarcation closer 
to either end of the range could be perceived as biased, either too 
liberal or too conservative, whereas the middle of the range avoids 
those implications. The NRC proposal, which is a factor of 10 reduction 
(from the 10-4 annual probability level), could be perceived 
as an arbitrary selection, whereas an annual probability of 
10-6 is a factor of 100 reduction and is likely to be more 
widely accepted.
    Response 1.1: The Commission stated in the proposed rulemaking (67 
FR 3629; January 25, 2002) that the specification of a value to 
quantitatively define the probability for unlikely FEPs is complicated 
because of the subjective nature of the term ``unlikely.'' The 
Commission did consider the merits of using an annual probability of 
10-6 rather than 10-5 for the demarcation between 
likely and unlikely FEPs. These two probability values represent 
approximately a 1 percent and 10 percent chance of occurring over the 
10,000 year regulatory period. The Commission considered a 1 percent 
chance of occurring (i.e., annual probability of 10-6 over 
10,000 years) neither expected nor likely and, therefore, an 
inappropriate value for the demarcation between likely and unlikely 
FEPs (67 FR 3630; January 25, 2002). The Commission continues to 
believe an annual probability of 1 x 10-5 (i.e., 10 percent 
chance of occurring within the 10,000 year compliance period) is a 
protective and prudent value for defining the upper limit of unlikely 
FEPs and is retaining the proposed range for defining unlikely FEPs.
    EPA has suggested that a probability value which represents the 
middle of a particular range (only when displayed on a logarithmic 
scale) contains some inherent justification for its selection. EPA also 
suggests that the NRC proposal, which is a factor of 10 less than an 
annual probability of 10-4, may be considered too high by 
some, whereas the EPA recommended value of 10-6, which is 
100 times lower than 10-4, is likely to be more acceptable. 
The issue is not whether a particular value lies within the middle of a 
range (when plotted in a particular manner), or that the value is 10 
rather than 100 times less than another value. The issue for NRC is to 
determine an appropriate value that is protective of public health and 
safety and the environment, and consistent with EPA's standards. EPA's 
standards exclude unlikely FEPs from the required assessments for 
ground-water protection and human intrusion so that these assessments 
may focus on the likely performance of the repository. This is the 
context in which the definition of a specific probability value should 
be viewed. The Commission and other commenters consider the NRC 
proposal (i.e., 10 percent chance of occurring over 10,000 years 
defines demarcation between likely and unlikely FEPs) consistent with 
the intended focus of the assessments for ground-water protection and 
human intrusion, and protective of public health and safety and the 
environment (see Comments 3-5).
    Comment 1.2: Given the significant uncertainty in estimating the 
probability for rare events (e.g., events with an annual probability of 
10-5), specification of an annual probability value of 
10-6 for the demarcation between likely and unlikely FEPs 
will provide greater confidence that all likely FEPs are considered in 
the assessments for ground-water protection and human intrusion. There 
is no need to be restrictive about the probability limits because both 
standards and regulations allow for excluding FEPs that have no 
significant impact on performance results. Use of an annual probability 
of 10-6 assures a reasonably conservative approach is taken 
for screening FEPs.
    Response 1.2: EPA has suggested that the Commission adopt a more 
conservative approach for selecting the demarcation between likely and 
unlikely FEPs. The Commission disagrees with this approach advocated by 
EPA for the following reasons: (1) The proposed value of 
10-5 (i.e., 10 percent chance of occurrence over 10,000 
years) already represents a reasonably conservative value for the 
demarcation between likely and unlikely FEPs; (2) introducing 
additional conservatism for screening of FEPs, by selecting an annual 
probability of 10-6, will detract from the intended purpose 
of the assessments to focus on likely performance; and (3) 
understanding and addressing uncertainties in the quantitative 
estimates for the probabilities of FEPs is preferred over selection of 
more conservative screening values.
    The Commission acknowledges that selection of a more conservative 
value (i.e., annual probability of 10-6) for the demarcation 
between likely and unlikely FEPs could provide additional assurance by 
considering a broader range of FEPs. Such an approach, however, would 
not be consistent with the intent that the required assessments focus 
on likely behavior. EPA, in

[[Page 62631]]

describing what level of expectation will meet the standards, has 
pointed out negative aspects of an overly conservative approach (e.g., 
conservatism can bias analyses and deflect attention from questions 
critical to developing an adequate understanding of the FEPs) (66 FR 
32102; June 13, 2001). The Commission understands that EPA believes its 
recommendation (i.e., annual probability of 10-6) is 
``reasonably'' conservative. However, the Commission views EPA's 
recommendation, which would identify FEPs with as little as a one-in-a-
million chance of occurring in a year (i.e., one percent chance of 
occurring over 10,000 years) as likely FEPs, as overly conservative and 
thus not appropriate. The Commission, as well as other commenters (see 
Comments 4 and 5), support the annual probability of 10-5 
(i.e., 10 percent chance of occurrence over 10,000 years) as a 
reasonably conservative value for the demarcation between likely and 
unlikely FEPs. The Commission continues to believe the specification of 
an annual probability of 10-5 is consistent with the focus 
on likely performance for the assessments of ground-water protection 
and human intrusion.
    There will be uncertainty in estimating performance of any geologic 
repository, including the uncertainty in estimating the probabilities 
of FEPs. NRC's regulation for Yucca Mountain contains specific 
requirements for addressing uncertainty in estimating performance, 
which includes uncertainty for estimating probabilities for FEPs. The 
Commission believes it is prudent to understand and evaluate the 
uncertainty in the probability estimates rather than set a more 
conservative screening value as a means to address uncertainty in 
estimating probabilities of FEPs. Reasonable expectation, as specified 
in EPA standards (40 CFR 197.14) and NRC regulations (10 CFR 63.304), 
in compliance with the postclosure standards of the repository, 
dictates that uncertainties be understood and evaluated even when they 
may be difficult to precisely quantify (e.g., accounting for the 
inherently greater uncertainties, in making long-term projections of 
the performance of the Yucca Mountain disposal system, does not exclude 
important parameters from assessments and analyses simply because they 
are difficult to precisely quantify to a high degree of confidence). In 
the preamble to the final standards, EPA asserted that ``[T]he 
reasonable expectation approach is aimed simply at focusing attention 
on understanding the uncertainties in projecting disposal system 
performance so that regulatory decision making will be done with a full 
understanding of the uncertainties involved'' (66 FR 32102; June 13, 
2001). The Commission believes its requirements for the performance 
assessments provide for a thorough evaluation and understanding of 
uncertainties in estimating repository performance. Thus, selection of 
a more conservative probability value for the demarcation between 
likely and unlikely FEPs is unnecessary. As discussed previously, the 
Commission continues to believe the proposed value (i.e., 10 percent 
chance of occurring within 10,000 years) ensures the assessments for 
ground-water protection and human intrusion focus, as intended, on 
likely performance, whereas the use of more conservative values to 
define unlikely FEPs would inappropriately distort the estimation of 
likely performance.
    Comment 1.3: Variation in dose assessments for Yucca Mountain is 
sufficiently broad (e.g., two orders of magnitude--a factor of one-
hundred) that it is reasonable to adopt an annual probability value of 
10-6 as the demarcation between likely and unlikely FEPs 
because this value represents a numerically similar difference (i.e., 
two orders of magnitude) between it and the probability for events 
nearly certain to occur within the 10,000 year period (i.e., an annual 
probability value of 10-4). Whereas NRC's proposed value 
(i.e., an annual probability value of 10-5) is only a factor 
of 10 (i.e., one order of magnitude) different from the probability for 
events nearly certain to occur.
    Response 1.3: The performance assessments for evaluating individual 
protection for the proposed repository at Yucca Mountain evaluate 
performance probabilistically; therefore, the estimates of repository 
performance are represented by a range of values. The variation in 
repository performance results from including uncertainty and 
variability in the models and parameters of the performance assessment 
used to represent FEPs associated with the site conditions and the 
natural and engineered barriers of the repository. EPA's observation 
that the variation in estimates of repository performance and the 
difference between the EPA recommendation of an annual probability 
value of 10-6 and the probability of FEPs nearly certain to 
occur within the 10,000 year period (i.e., an annual probability value 
of 10-4) are both two orders of magnitude does not justify 
EPA's recommendation, nor does it imply that NRC's proposed value of 
10-5 is inappropriate. EPA has not provided information to 
support the relevance of this observation to the specification of a 
value for the demarcation of likely and unlikely FEPs. The performance 
assessments for Yucca Mountain involve complex models, for FEPs, that 
consider the uncertainty and variability in natural processes and the 
degradation of engineered materials. Performance assessments are 
expected to continue to evolve over time as new information is 
collected and evaluated and the variation in performance assessment 
results is also expected to change. A logical conclusion of the EPA 
comment is that the demarcation between likely and unlikely FEPs should 
change if future assessments of Yucca Mountain cause the variation of 
results to deviate from the current two orders of magnitude range. The 
Commission believes the determination of an annual probability for the 
demarcation between likely and unlikely FEPs should not be tied to the 
performance assessment results nor any other particular assessment of 
site conditions (see also response to Comment 1.4).
    Comment 1.4: The selection of the probability for the demarcation 
between likely and unlikely FEPs should be divorced from the site 
conditions.
    Response 1.4: The Commission agrees that site conditions should not 
be used to determine the probability for the demarcation between likely 
and unlikely FEPs. NRC's proposed rulemaking did not use any site 
conditions to determine an appropriate probability value. In the 
proposed rule, the Commission did identify a few selected FEPs, as a 
matter of reference, to inform the public of the kinds of FEPs that 
might be included and excluded by the proposed probability range for 
unlikely FEPs (67 FR 3630; January 25, 2002).

2. State of Nevada and the Nevada Agency for Nuclear Projects Comments

    Comment 2.1: Unlikely FEPs should be defined by the same 
quantitative value used to define very unlikely FEPs (i.e., annual 
probability less than 10-8). The EPA standard requires the 
Commission to set the quantitative level for unlikely FEPs, but it does 
not require that it be higher than the value used to define very 
unlikely FEPs.
    Response 2.1: The EPA standards provide that a numerical value to 
define unlikely FEPs is to be specified by NRC, and the preamble to the 
standards clearly indicates that any such value would be higher than 
the value used to define very unlikely events. More

[[Page 62632]]

specifically, the preamble to the final standards states: ``[W]e 
intended to establish another demarcation for excluding unlikely 
features, events, and processes with a higher probability. * * *'' (66 
FR 32100; June 13, 2002). The Commission does not consider the State's 
proposal (i.e., unlikely FEPs be specified with the same numerical 
value used to define very unlikely FEPs) consistent with EPA's intent 
for the standards or common understanding of the two terms ``unlikely'' 
and ``very unlikely,'' which imply a difference in likelihood. The 
Commission believes its proposal, which specified a numerical range for 
unlikely FEPS above the range for very unlikely FEPs, is consistent 
with the EPA standards, as required by statute, and is fully protective 
of public health and safety and the environment.
    Comment 2.2: Preservation of ground-water quality must not be 
compromised. Therefore, the assessment for protection of ground water 
should be no less rigorous than the assessment used to evaluate 
individual protection, which is required to consider unlikely events.
    Response 2.2: The State is correct in pointing out that the 
individual protection assessment is the only assessment that includes 
unlikely FEPs; however, the EPA standards are clear that ``unlikely'' 
FEPs are to be excluded from the performance assessments for ground-
water protection and human intrusion (40 CFR 197.36). The State of 
Nevada's recommendation is not consistent with EPA's standards that 
specify different assessments for determining compliance with the 
ground-water protection and individual-protection standards. EPA's 
intent for the assessments for ground-water protection and human 
intrusion is to focus on the likely performance of the repository; 
thus, unlikely events are to be excluded from these two assessments 
(see Response 1.2). Unlikely FEPs should not be included in the 
assessments for ground-water protection and human intrusion, because 
inclusion would inappropriately emphasize the contribution of these 
less likely FEPs when determining the likely behavior of the 
repository. Exclusion of low-probability FEPs ensures that the 
assessments for ground-water protection and human intrusion are as 
intended (i.e., on likely repository performance).
    Ground water is an important resource, and potential contamination 
of ground water is evaluated in all three assessments (i.e., ground-
water protection, human intrusion, and individual protection) required 
by regulations and standards. More specifically, the assessment for 
ground-water protection must demonstrate compliance with stringent 
safety standards [e.g., 0.04 millisievert/year (mSv/yr) (4 millirem/
year (mrem/yr))] for the potential contamination of drinking water. The 
assessment for individual protection must demonstrate compliance with a 
0.15 mSv/yr (15 mrem/yr) exposure limit from all potential exposure 
pathways (e.g., drinking contaminated water, consuming crops that are 
assumed to be irrigated with contaminated water, consuming animal 
products that are assumed to be raised with contaminated water and 
feed) and include unlikely FEPs. The assessment for human intrusion 
must demonstrate compliance with a 0.15 mSv/yr (15 mrem/yr) exposure 
limit from all potential exposure pathways, and assume that a human 
intrusion results in a borehole that provides a direct pathway for 
water to transport waste to the water table (i.e., the ground-water 
resource). The Commission considers the multiple and overlapping 
assessments for ground-water protection, individual protection, and 
human intrusion, and the associated standards, to provide a 
comprehensive evaluation of potential ground-water contamination that 
is protective of the ground-water resource. Requiring the assessments 
for ground-water protection and human intrusion to include ``unlikely'' 
FEPs is not necessary for protection of the ground-water resource nor 
consistent with the EPA standards.
    Comment 2.3: NRC's proposed value for unlikely events would, but 
should not, allow the exclusion of igneous activity from consideration 
in the performance assessments for ground-water protection and human 
intrusion because it could be the largest contributor to dose. The 
proposed definition for unlikely events is subjective to the extreme 
because the largest risk contributor is excluded.
    Response 2.3: The State's recommendation that igneous activity be 
included because, as currently assessed, igneous activity is the 
largest contributor to risk, is not consistent with EPA's standards. 
EPA's standards specify that NRC is to determine FEPs are either 
``unlikely'' or ``very unlikely,'' based on the likelihood of 
occurrence of the FEPs and not on other considerations, such as risk. 
The Commission explained, in its proposed rule (67 FR 3629; January 25, 
2002), that EPA's intent for the assessments for ground-water 
protection and human intrusion was to focus on the likely performance 
of the repository; thus, unlikely events are to be excluded from these 
two assessments. Unlikely FEPs should not be included in the 
assessments for ground-water protection and human intrusion because 
inclusion would inappropriately emphasize the contribution of these 
less likely FEPs when determining the likely behavior of the 
repository. Exclusion of such low-probability FEPs ensures that the 
assessments for ground-water protection and human intrusion are as 
intended (i.e., on likely repository performance), and are not 
considered ``subjective to the extreme,'' because of this exclusion.
    Exclusion of igneous activity in the assessments for ground-water 
protection and human intrusion is not expected to have a significant 
effect on either assessment. The assessment for ground-water protection 
is not affected because the dose from an igneous event is predominately 
through the air pathway and not the ground-water pathway. The 
assessment for human intrusion is not affected because the assumed 
intrusion (i.e., single borehole to the water table) scenario leads to 
a ground-water pathway, whereas the igneous event primarily involves 
the air pathway. As the State has indicated, the air pathway is 
considered in the assessment for individual protection.
    Comment 2.4: The performance assessments for human intrusion and 
individual protection should consider similar FEPs, to provide a 
meaningful comparison of repository resilience.
    Response 2.4: As discussed in the previous responses (under 
Comments 2.2 and 2.3), each of the three performance assessments (i.e., 
those conducted to demonstrate compliance with the standards for 
individual protection, ground-water protection, and resiliency to an 
assumed human intrusion) has its own specific purpose, assumptions, and 
standards. The EPA standards and NRC's regulations do not require that 
direct comparisons be made between any of these assessments. The 
performance assessment for human intrusion demonstrates the resilience 
of the repository by assuming a specified intrusion occurs and by 
requiring potential exposures to comply with the same overall exposure 
limit [i.e., 0.15 mSv/yr (15 mrem/yr) from all pathways] used for 
individual protection. Although the EPA standards clearly state 
``unlikely'' FEPs are not to be included in the assessment for human 
intrusion and ground-water protection (40 CFR 197.36), the performance 
assessments for individual protection, ground-water protection, and 
human intrusion provide a comprehensive evaluation of FEPs to inform 
the licensing decision. Regardless of which aspect of repository 
performance is the largest risk contributor, the regulatory

[[Page 62633]]

requirements for all assessments must be met.
    Comment 2.5: The possibility of multiple intrusions into the 
repository should be considered as a likely event and included in the 
evaluation of human intrusion rather than the ``single'' intrusion 
prescribed in the EPA standards and adopted in NRC's regulations.
    Response 2.5: The State raised a similar concern (i.e., 
consideration for multiple intrusions) during the public comment period 
for part 63. The Commission addressed this issue when it finalized part 
63, stating:

    Another related issue is whether the stylized calculation should 
consider multiple intrusions. The final EPA standards resolve this 
issue in favor of a single intrusion. Moreover, in its findings and 
recommendations, NAS [National Academy of Sciences] argued against 
analyses of whether and how often exploratory drilling would occur 
at Yucca Mountain because of the complexities associated in such 
assessments. Simply stated, the NAS felt that no one can accurately 
predict the characteristics of future human society and their 
technology. In the context of human intrusion, estimating the 
probability of exploratory drilling for a given resource relies on 
an ability to predict certain economic and technical factors that 
influence supply of, and demand for, that resource. In fact, NAS 
noted that the continued advances in noninvasive geophysical 
techniques may, in fact, reduce the number and frequency of 
exploratory boreholes * * *
    Consequently, any consideration for the drilling of multiple 
exploratory boreholes or later drilling of more boreholes further 
increases the speculative nature of the intrusion scenario with 
potentially little increase in understanding repository resilience.

    The EPA standards provide for consideration of a single borehole at 
the earliest time that human intrusion into the waste package can occur 
without recognition by the drillers. The Commission believes this is an 
appropriate test for evaluating repository resilience. Moreover, the 
suggested alternative to evaluate multiple intrusions for the human 
intrusion calculation fails to reflect the purpose of the human 
intrusion calculation, that is to test the resilience of the 
repository, not to evaluate the speculative issue of frequency of the 
intrusion (66 FR 55761; November 2, 2001).

3. DOE Comments

    DOE supports NRC's proposed probability range for defining unlikely 
FEPs as a reasonable and conservative choice.
    Comment 3.1: For assessing operational safety of the repository, 
NRC's regulations specify that operational events that occur one or 
more times during the operational period are considered reasonably 
likely to occur. Applying this definition (i.e., one or more times) to 
the specification of a value to define unlikely FEPs results in an 
upper bound of one chance of occurrence within 10,000 years (i.e., 
approximately 10-4 annual probability). Thus, NRC's proposal 
of an upper bound of one chance in ten of occurring within 10,000 years 
(i.e., 10-5 annual probability) for unlikely FEPs is a 
reasonable and conservative approach.
    Response 3.1: During the development of the proposed rulemaking, 
NRC considered an annual probability of 10-4 for the 
demarcation between likely and unlikely FEPs, but ultimately decided on 
a probability of one chance in ten of occurring within 10,000 years 
(i.e., annual probability of 10-5) as a prudent value, given 
the uncertainties in estimating the occurrence of FEPs over the very 
long compliance period. The Commission was careful to point out that 
its specification for unlikely events was in the context of very 
specific assessments (i.e., those made to assess compliance with 
ground-water protection and human-intrusion standards) over a long time 
frame, and this specification was not intended to suggest or imply 
precedent for other significantly different applications that used the 
term ``unlikely'' (67 FR 3630; January 25, 2002). Similarly, 
significantly different applications such as requirements for the 
safety assessment of the operational period (e.g., significantly 
shorter time period, inclusion of worker activities) should not imply a 
precedent for specifying a value for unlikely FEPs.

4. NEI Comments

    NEI supports NRC's proposed probability range for defining unlikely 
FEPs. NEI stated that the proposed definition of unlikely FEPs will 
facilitate a reasonable and prudently conservative analysis of these 
aspects of repository performance (i.e., ground-water protection and 
human intrusion).

5. Exelon Generation Comments

    Exelon Generation supports NRC's proposed probability range for 
defining unlikely FEPs.

III. Changes From the Proposed Rule

Section 63.342 Limits on Performance Assessments

    The word ``should'' has been replaced with the word ``shall'' to be 
consistent with the final EPA standard (40 CFR 197.36).

IV. Section-by-Section Analysis

Section 63.342 Limits on Performance Assessments

    This section specifies how DOE will determine which features, 
events, and processes will be considered in the performance assessments 
described in subpart L of part 63.

V. Voluntary Consensus Standards

    The National Technology Transfer and Advancement Act of 1995, Pub. 
L. 104-113, requires that Federal agencies use technical standards that 
are developed or adopted by voluntary consensus standards bodies unless 
using such a standard is inconsistent with applicable law or is 
otherwise impractical. In this rule, NRC is establishing probability 
limits for unlikely FEPs at a potential geologic repository for high-
level radioactive waste at Yucca Mountain, Nevada. This action does not 
constitute the establishment of a standard that contains generally 
applicable requirements.
    VI. Finding of No Significant Environmental Impact: Availability
    Pursuant to section 121(c) of the Nuclear Waste Policy Act, this 
rule does not require the preparation of an environmental impact 
statement under section 102(2)(c) of the National Environmental Policy 
Act of 1969 or any environmental review under subparagraph (E) or (F) 
of section 102(2) of such act.

VII. Paperwork Reduction Act Statement

    This rule does not contain new or amended information collection 
requirements subject to the Paperwork Reduction Act of 1995. (44 U.S.C. 
3501 et seq.). Existing requirements were approved by the Office of 
Management and Budget (OMB), approval number 3150-0199.

Public Protection Notification

    If a means used to impose an information collection requirement 
does not display a currently valid OMB control number, NRC may not 
conduct nor sponsor, and a person is not required to respond to, the 
information collection.

VIII. Regulatory Analysis

    The Commission has prepared a regulatory analysis on this 
regulation. The analysis examines the costs and benefits of the 
alternatives considered by the Commission. It is available for 
inspection in the NRC Public Document Room, One White Flint North, 
11555 Rockville Pike, Rockville, MD 20852.

[[Page 62634]]

Single copies of the analysis may be obtained from Clark Prichard, 
Office of Nuclear Material Safety and Safeguards, U.S. Nuclear 
Regulatory Commission, Washington, DC 20555-0001, telephone (301) 415-
6203, e-mail: nrc.gov">cwp@nrc.gov.

IX. Regulatory Flexibility Certification

    In accordance with the Regulatory Flexibility Act [5 U.S.C. 
605(b)], the Commission certifies that this rule will not, if 
promulgated, have a significant economic impact on a substantial number 
of small entities. This rule relates to the licensing of only one 
entity, DOE, which does not fall within the scope of the definition of 
``small entities'' set forth in the Regulatory Flexibility Act or the 
size standards established by the NRC (10 CFR 2.810).

X. Backfit Analysis

    NRC has determined that the backfit rule does not apply to this 
rule and, therefore, that a backfit analysis is not required, because 
this rule does not involve any provisions that would impose backfits as 
defined in 10 CFR chapter 1.

XI. Small Business Regulatory Enforcement Fairness Act

    In accordance with the Small Business Regulatory Enforcement Act of 
1996, the NRC has determined that this action is not a major rule and 
has verified this determination with the Office of Information and 
Regulatory Affairs of OMB.

List of Subjects in 10 CFR Part 63

    Criminal penalties, High-level waste, Nuclear power plants and 
reactors, Nuclear materials, Reporting and recordkeeping requirements, 
Waste treatment and disposal.

    For the reasons set out in the preamble and under the authority of 
the Atomic Energy Act of 1954, as amended; the Energy Reorganization 
Act of 1974, as amended; the Nuclear Waste Policy Act of 1982, as 
amended; and 5 U.S.C. 552 and 553, the NRC is adopting the following 
amendments to 10 CFR part 63.

PART 63--DISPOSAL OF HIGH-LEVEL RADIOACTIVE WASTES IN A GEOLOGIC 
REPOSITORY AT YUCCA MOUNTAIN, NEVADA

    1. The authority citation for part 63 continues to read as follows:

    Authority: Secs. 51, 53, 62, 63, 65, 81, 161, 182, 183, 68 Stat. 
929, 930, 932, 933, 935, 948, 953, 954, as amended (42 U.S.C. 2071, 
2073, 2092, 2093, 2095, 2111, 2201, 2232, 2233); secs. 202, 206, 88 
Stat.1244, 1246 (42 U.S.C. 5842, 5846); secs. 10 and 14, Pub. L. 95-
601, 92 Stat. 2951 (42 U.S.C. 2021a and 5851); sec. 102, Pub. L. 91-
190, 83 Stat. 853 (42 U.S.C. 4332); secs. 114, 121, Pub. L. 97-425, 
96 Stat. 2213g, 2238, as amended (42 U.S.C. 10134, 10141); and Pub. 
L. 102-486, sec. 2902, 106 Stat. 3123 (42 U.S.C. 5851).


    2. Section 63.342 is revised to read as follows:


Sec.  63.342  Limits on performance assessments.

    DOE's performance assessments shall not include consideration of 
very unlikely features, events, or processes, i.e., those that are 
estimated to have less than one chance in 10,000 of occurring within 
10,000 years of disposal. DOE's assessments for the human-intrusion and 
ground-water protection standards shall not include consideration of 
unlikely features, events, and processes, or sequences of events and 
processes, i.e., those that are estimated to have less than one chance 
in 10 and at least one chance in 10,000 of occurring within 10,000 
years of disposal. In addition, DOE's performance assessments need not 
evaluate the impacts resulting from any features, events, and processes 
or sequences of events and processes with a higher chance of occurrence 
if the results of the performance assessments would not be changed 
significantly.

    Dated at Rockville, Maryland, this 2nd day of October, 2002.

    For the Nuclear Regulatory Commission.
Annette Vietti-Cook,
Secretary of the Commission.
[FR Doc. 02-25521 Filed 10-7-02; 8:45 am]
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