[Federal Register Volume 71, Number 189 (Friday, September 29, 2006)]
[Notices]
[Pages 57579-57584]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E6-16017]


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

NUCLEAR REGULATORY COMMISSION

[HLWRS-ISG-01]


Review Methodology for Seismically Initiated Event Sequences; 
Availability of Final Interim Staff Guidance Document

AGENCY: Nuclear Regulatory Commission.

ACTION: Notice of availability.

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

SUMMARY: The Nuclear Regulatory Commission (NRC) is announcing the 
availability of final interim staff guidance (ISG) document, ``HLWRS-
ISG-01, Review Methodology for Seismically Initiated Event Sequences,'' 
and NRC responses to the public comments received on that document. The 
ISG clarifies or refines the guidance provided in the Yucca Mountain 
Review Plan (YMRP) (NUREG-1804, Revision 2, July 2003). The YMRP 
provides

[[Page 57580]]

guidance to NRC staff for evaluating a potential license application to 
receive and possess high-level radioactive waste at a geologic 
repository constructed or operated at Yucca Mountain, Nevada.

ADDRESSES: The document HLWRS-ISG-01 is available electronically at 
NRC's Electronic Reading Room, at http://www.nrc.gov/reading-rm/adams.html. From this site, you can access NRC's Agencywide Documents 
Access and Management System (ADAMS), which provides text and image 
files of NRC's public documents. The ADAMS accession number for the ISG 
is ML062650140. 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 1-800-397-4209, or (301) 
415-4737, or (by e-mail), at [email protected].
    This document may also be viewed electronically on the public 
computers located at NRC's PDR, Mail Stop: O1F21, One White Flint 
North, 11555 Rockville Pike, Rockville, MD 20852. The PDR reproduction 
contractor will copy documents, for a fee.
    NRC RESPONSES TO PUBLIC COMMENTS ON HLWRS-ISG-1: In preparing final 
HLWRS-ISG-01, ``Review Methodology for Seismically Initiated Event 
Sequences,'' ADAMS ML062650140, the NRC staff reviewed and considered 
23 comments received from five different organizations during the 
public comment period. One commenter had 12 comments recommending 
specific clarifying changes to the ISG. One commenter questioned NRC 
using the ISG to clarify its regulatory intent, instead of addressing 
the issue of seismically initiated event sequences, more appropriately, 
in the YMRP. Two commenters questioned whether the ISG sets forth a 
more stringent standard for the seismic design of repository surface 
facilities than the existing criteria for reactors. One commenter was 
concerned that a specific methodology described in the ISG would bias 
the NRC staff's review against other methodologies that the U. S. 
Department of Energy (DOE) may propose that provide equal or better 
protection of public health and safety. One commenter was concerned 
that the specific methodology proposed in the ISG lacks both precedent 
and scientific support. Two commenters were concerned that the ISG 
methodology may not produce accurate results over the 100-year plus 
operating life of the Yucca Mountain repository preclosure operating 
period. Two commenters raised questions as to whether NRC has 
adequately considered the geometric consequence of closely spaced, 
recurring seismic events, in determining the seismic hazard and related 
failure probability of a structure, system, or component (SSC) 
important to safety (ITS). One commenter states that ``the ISG totally 
ignores the existence of Section 63.102(f) of the regulation.'' The 
following discussion indicates how the comments were addressed, and the 
changes, if any, made to the ISG as a result of the comments.
    Line numbers in the following comments refer to the draft HLWRS-
ISG-01, ADAMS ML061170532, which was made available for public comment 
on May 22, 2006 (71 FR 29369).
    1. Comment. The commenter recommends that the sentence starting at 
Line 38 be re-phrased as: ``The mean fragility curve for an SSC ITS may 
be estimated using: (1) Probability density functions for controlling 
parameters in a Monte Carlo analysis; (2) simplified methods outlined 
in Section 4 of Electric Power Research Institute, TR-103959 (Ref. 2); 
(3) a method that uses the Conservative Deterministic Failure Margin 
methodology to determine the 1percent probability of failure, and an 
estimate of the composite logarithmic standard deviation, as described 
by Kennedy (2001, pp. 44 to 45) and Ravindra (2006, p. 132); or (4) 
other methods that capture appropriate variability and uncertainty in 
parameters used to estimate the capacity of the SSCs ITS to seismic 
events.
    Response. NRC regulations grant DOE broad flexibility in choosing a 
method or methods for preclosure safety analysis of hazards at the 
geologic repository operations area (GROA). Although NRC staff has 
stated some example methods, in the ISG, for estimating the fragility 
curve, this does not imply that alternative methods would be 
unacceptable for demonstrating compliance with regulatory requirements. 
DOE may use an alternative method, if sufficient technical basis for 
the use of the method is provided.
    No changes were made to the ISG as a result of this comment.
    2. Comment. The commenter recommends that the following sentence be 
added at the end of the sentence on line 43:

    ``Where appropriate, assessment of fragility for an SSC may be 
based on fragility values for an identical or similar component as 
found in the literature.''

    Response. NRC agrees with the commenter that the fragility data for 
an SSC, developed and documented in databases and used at other 
facilities, may be used to estimate fragility for the SSCs at the 
repository, if the data are shown to be applicable to the repository 
SSCs.
    The ISG has been revised to add the following at the end of the 
sentence on Line 43:

    ``An estimate of fragility for an SSC may be based on fragility 
values for an identical or similar component as found in the 
literature, provided technical bases for the relevance of the data 
to the SSC under consideration are established.''

    3. Comment. The commenter recommends that an explanation be 
provided to address why the selection of the slope (Lines: 235 to 237, 
240 to 241: Page: 8) is appropriate. This explanation may include, for 
example, that this portion of the hazard curve was selected if it were 
the interval where the dominant contribution to risk arises. Text could 
be added at the end of the sentence on Line 241:

    ``The slope should be selected to focus on the portion of the 
curve where risk is expected to dominate the convolution.''

    Response. NRC agrees with the commenter that an explanation for the 
selection of the slope between probabilities of exceedance of 
10-6 and 10-5 should be added in the ISG.
    The ISG has been revised to add the following at the end of the 
sentence on Line 241:

    ``This slope was selected to represent the hazard accurately at 
probabilities of exceedance values close to the target annual 
threshold probability of 10-6 because this portion of the 
hazard curve may have a significant contribution to the risk.''

    4. Comment. The commenter suggests replacing the sentence starting 
on Line 263, with the sentence: ``For the purposes of illustration, a 
single response frequency of 10 hertz (Hz) is assumed for this 
evaluation.'' The commenter also suggests that an explanation of why a 
single frequency is appropriate should be added.
    Response. NRC believes that the essence of the comment, with the 
suggested change to the ISG, is adequately responded to by the 
sentences in lines 262 to 264 of the ISG. These sentences state that 
the evaluation typically would be performed at appropriate structural 
frequencies, based on the dynamic characteristics of the SSC, and that 
example evaluation is performed at a single frequency of 10 hertz. A 
single frequency was chosen in the example for illustration purposes 
only. As stated in the sentence in line 261, the evaluation typically 
would have to be performed for a number of structural frequencies of an 
SSC, based on its dynamic characteristics, to

[[Page 57581]]

appropriately assess the probability of failure of an SSC during a 
seismic event.
    No changes were made to the ISG as a result of this comment.
    5. Comment. The commenter suggests that text be added to include 
discussion of other non-seismic factors that may influence/mitigate the 
probability of occurrence of the event sequence. At line 262, a 
sentence should be inserted to read:

    ``Other non-seismic factors such as residency times, targeting 
factors, operational states, and design constraints, which may also 
influence the probability of occurrence of the complete event 
sequence, are not considered in this example.''

    Response. NRC agrees with the commenter that in the example, design 
constraints, such as the probability of failure of the canister during 
a potential drop event, are not considered. This is indicated in Lines 
276 to 277 of the ISG, and in the clarifying statement added in the ISG 
in response to comment 10. NRC believes that the clarifying statement 
recognizes that if the canister breach probability (given a drop) is 
demonstrated to be less than 1.0, the appropriate conditional 
probability of breach may be factored into the quantification of the 
event sequence. Therefore, NRC believes that a change to the ISG to 
clarify this factor in determining the probability of occurrence of the 
event sequences is not necessary. Other non-seismic factors mentioned 
in the comment appear to be related to the duration of operations at 
the proposed Yucca Mountain repository. NRC would need specific 
information on the Yucca Mountain repository operations and the 
technical bases for determining the values of these factors, to judge 
whether these factors are appropriate and can be used to calculate 
event sequence probability of occurrence in the preclosure safety 
analysis. NRC will review the use of these factors and their technical 
bases and make a determination of their acceptability during the 
potential future review of the DOE License Application for the proposed 
Yucca Mountain repository.
    No changes were made to the ISG as a result of this comment.
    6. Comment. The commenter recommends that the assumption made in 
the computation be clarified, and that each branch in the sequence be 
addressed in the description (Lines: 308 to 323: Page: 12). For 
instance, at the end of the sentence ending on Line 310, the text 
should be expanded to mention the other branches:

    ``Tracing Sequence 3 across the event tree shown in Figure B-1, 
this sequence also includes the STR-SHWL success branch and the 
assumed failure of the canister (CANIS-BRCH) * * *'' Additional text 
on Lines 310 to 323 should include: ``* * * the STR-SHWL success 
probability is the complement of the fragility of the failure branch 
* * *'' and ``* * * Therefore, the combined fragility of the three 
systems in the event sequence can be obtained by * * *.''

    Response. NRC agrees with the commenter that the Event Sequence 3, 
as shown in Figure B-1, implies that the concrete shear wall provides a 
confinement barrier to the release of radioactive materials before they 
pass through the Heating, Ventilation and Air-Conditioning (HVAC) 
system. However, for illustration purposes only, it was assumed in the 
example that, if the HVAC duct anchor system fails, all radioactive 
materials released because of the canister breach would be discharged 
through the HVAC system. To clarify this assumption, the ISG has been 
revised as follows:
    Add the following at the end of the sentence in Line 309:

    ``For simplicity, it is assumed, in this example, that if the 
HVAC duct anchor system were to fail, all radioactive materials 
released because of the potential canister breach would be 
discharged through the HVAC system, and that the concrete shear wall 
would be unable to provide a barrier to the release of radioactive 
materials.''

    7. Comment. The commenter suggests that Figure B-1 be revised for 
clarity, making the figure consistent with conventions for the 
construction of event trees in other NRC documents, such as NUREG-2300. 
The following changes are suggested to Figure B-1:

    (a) The figure be revised to indicate that the initiating event 
of the sequence is an earthquake;
    (b) The figure heading be revised to state the event in terms of 
success;
    (c) The missing branch be shown for the event that the crane 
does not drop the waste form.
    (d) The probability of canister breach, which has been assumed 
to be 1.0, be indicated.

    Response. NRC agrees with the suggested change in item (b), above, 
regarding revision of the figure headings and stating the event in 
terms of success, and has revised Figure B-1. Staff, however, does not 
agree with the other suggested changes because the title of the figure 
identifies the event sequence as initiated by a seismic event. This is 
also consistent with Section 11.2.6.2 of NUREG-2300. In addition, 
adding a success path for the crane not dropping the waste form would 
be superfluous to this example, and would not add any value to the 
illustration of the procedure for event sequence probability 
calculation. The probability of canister breach assumed as 1.0 is 
stated in section B of Appendix B.
    Figure B-1 has been revised as a result of this comment.
    8. Comment. Assuming that the text in lines 220 to 222 has broader 
applicability than just as part of the example, the commenter suggests 
that the sentence starting on Line 220 be deleted from Appendix A, 
moved to the Discussion section on page 1, and inserted into the text 
at Lines 54 to 63. The commenter also suggests changes to the text for 
insertion into the Discussion section on page 1, in comment 9.
    Response. NRC agrees with the comment. The ISG has been revised as 
follows:

    (a) The sentence starting on Line 220 and ending on Line 222, 
``The technical basis * * * staff review.'', has been deleted.

    (b) The following has been added at the end of the sentence on Line 
57:

    ``Technical bases for the development of the SSC ITS fragility 
curves should be available for staff review.''

    9. Comment. The commenter suggests that, the following sentence 
consistent with the Comment 8, should be inserted into the Discussion 
section on page 1 at Lines 54 to 63:

    ``It is necessary in developing seismic fragilities that the 
technical basis for the development of the applicable fragility 
parameters be available for staff review.''

    Response. NRC agrees with the essence of the comment. The ISG has 
been revised as shown in NRC staff response to comment 8, item (b).
    10. Comment. The commenter recommends adding the following phrase 
to the end of Line 277:

    ``* * * and it is assumed that probability of breach is 1.0 in 
all cases''. In addition, the commenter recommends adding, in Figure 
B-1, ``(Pf = 1.0),'' on the branch indicating potential for breach. 
The commenter also recommends adding text to state that when the 
probability of a breach (given a drop) is demonstrated to be less 
than 1.0, the appropriate conditional probability of breach may be 
factored into the quantification of the event sequence.

    Response. NRC agrees with the comment. The ISG has been revised to 
clarify that, for the example in Appendix B, it is assumed that the 
canister probability of failure (given a drop) is 1.0. The comment 
regarding the use of appropriate conditional probability of canister 
failure, in the event sequence probability calculation, has been 
addressed in response to comment 5.
    The ISG has been revised to add the following at the end of the 
sentence in Line 277:


[[Page 57582]]


    ``It is assumed that the canister probability of failure, given 
a drop, is 1.0.''

    11. Comment. The commenter suggests adding the following text in 
the sentence starting on Line 36:

    ``As a conservative assessment of probability, the probability 
of occurrence of an event sequence leading to an SSC ITS failure, or 
seismic performance, can be determined by * * *''

    Response. NRC agrees with the essence of the comment, and has added 
a new sentence to reflect the comment. The ISG has been revised to add 
the following sentence in Line 36:

    ``As a conservative assessment of the probability of occurrence 
of an event sequence, a single SSC ITS may be considered, instead of 
all SSCs ITS in the event sequence.''

    12. Comment. The commenter suggests that a brief statement be added 
at the end of line 232 and in Appendix B, as follows:

    ``Computations shown in the appendix can be performed either by 
hand computations or through the use of computer codes. A number of 
computer codes are available that can be used for probability 
computations.''

    Response. NRC agrees with the commenter that computations for the 
event sequence probabilities can be performed either by hand 
computations or through the use of computer codes. However, these 
options are available to the applicant for any calculations. Although 
the details of associated quality assurance requirements may be 
different for the computational method selected, the overall staff 
review strategy for the DOE analysis is not affected significantly by 
the computational method selected by DOE. Therefore, staff does not see 
the need to revise the ISG.
    No changes were made to the ISG as a result of this comment.
    13. Comment. The commenter refers to NRC Chairman Dale E. Klein's 
statement, on July 1, 2006, that regulatory stability is a crucial 
element in ensuring that NRC can complete its work in a timely manner, 
and states that HLWRS ISG-01 has the potential to create regulatory 
instability. Accordingly, the commenter encourages NRC to take 
advantage of the opportunity afforded by this comment period to 
reconsider issuing this ISG and to instead address the issue of 
seismically initiated event sequences, more appropriately, in the YMRP. 
The commenter is recommending this course of action for the following 
five reasons:
    (a) ISG is not the most effective means for NRC to clarify its 
regulatory intent and could lead to unforeseen consequences due to 
inadequate review (including not being reviewed by the Commission 
itself).
    (b) Use of an ``Interim Guidance,'' a vehicle that was meant to 
address emerging issues affecting multiple licensed activities, is 
unnecessary in a situation where there is only a single potential 
licensee that is not currently conducting any licensed activities.
    (c) Draft HLRWS ISG-01 lacks safety focus in that it sets forth a 
more stringent standard for the seismic design of repository surface 
facilities than currently exists for reactors, without recognizing the 
comparatively lower level of risk associated with the repository 
facilities. In doing this, HLRWS ISG-01 directly contradicts the very 
regulation (10 CFR Part 63) that it seeks to inform.
    (d) Providing guidance to staff that assumes a specific methodology 
for demonstrating compliance with 10 CFR 63.111 is likely to bias the 
staff's review against other methodologies, that DOE may propose, which 
provide equal or better protection of public health and safety. 
Furthermore, giving DOE the opportunity to first propose an acceptable 
method for meeting the regulation would allow for a more independent 
NRC review--avoiding a situation where NRC is both telling DOE how to 
demonstrate compliance and then determining if compliance was 
demonstrated as instructed.
    (e) The specific methodology proposed in this draft ISG lacks both 
precedent and scientific support.
    Response. Responses to each of the commenter's reasons are provided 
below:
    (a) In the commenter's view, the ISG is not an effective means for 
NRC to clarify its regulatory intent and could lead to unforeseen 
consequences because of inadequate review (including not being reviewed 
by the Commission itself).
    The ISG reflects a focused revision of the YMRP, with the scope of 
the revision limited to a specific technical issue. The ISG process 
allows for the rapid identification and resolution of specific 
technical issues that emerge as a result of staff interaction, with 
DOE, in preparation for the future License Application review. To 
increase regulatory efficiency and enhance clarity of communication 
with DOE and the public, NRC anticipates providing incremental updates 
to the YMRP in the form of ISGs. NRC believes it is unnecessary and 
inefficient to republish the YMRP, given the narrow scope of the 
technical issue addressed in the ISG. If re-publication of the YMRP is 
warranted (e.g., due to a major rule change or accumulation of a number 
of ISGs), staff will be able to insert the appropriate text directly 
from the ISG into the YMRP. The ISG remains available to provide 
background discussion and examples, to supplement text, in the YMRP, at 
a level of detail not normally found in a Standard Review Plan (SRP). 
Thus, staff sees the ISG process as an effective, efficient, and 
appropriate means for revising or supplementing the YMRP.
    An ISG provides guidance to NRC staff on approaches to use during 
the review of a potential license application. ISG guidance is for 
illustration purposes only, and does not imply a preferred method or an 
approach that an applicant must use. An ISG's review approach provides 
a framework for staff to conduct an efficient review, consistent with 
regulatory requirements. ISGs, that are revisions or supplements to the 
SRPs, are issued at the NRC Office Division level, because SRPs do not 
represent regulatory commitments, or staff interpretations. During the 
ISG development process, the technical and regulatory basis for the ISG 
is thoroughly reviewed by appropriate NRC technical, management, and 
legal staff. Also, the public and shareholders are informed of a 
proposed draft ISG and afforded the opportunity to comment. Comments 
from the public and stakeholders are considered in developing the final 
ISG.
    No changes were made to the ISG as a result of this comment.
    (b) In the commenter's view, ISGs are not necessary for the Yucca 
Mountain project because DOE is the only potential licensee for the 
proposed repository, and no licensing activities are being conducted 
currently. Although it is true that DOE is the only potential licensee 
and no licensing activities are currently underway, important technical 
issues continue to be identified in the complex, one-of-a-kind Yucca 
Mountain project during the prelicensing interaction with DOE. As these 
issues are being resolved, the ISG process provides an effective, 
efficient, and appropriate means for staff to revise or supplement the 
YMRP, as discussed in response to comment 13(a). The ISG process also 
allows staff to communicate with potential licensees on the scope of 
the staff reviews on specific technical issues, as NRC staff prepares 
to review the potential License Application in an effective and timely 
manner.
    No changes were made to the ISG as a result of this comment.
    (c) In the commenter's view, ISG-01 lacks safety focus and sets 
forth a more stringent standard for the seismic design

[[Page 57583]]

of repository surface facilities than for reactors even though the 
repository facility has a lower level of risk, which appears 
contradictory to the intent of 10 CFR Part 63. The commenter also 
questions the purpose of 10 CFR 63.102(f) and how it is accounted for 
in the draft HLWRS-ISG-01. Another commenter made a similar statement.
    NRC does not agree with the commenter that the ISG-01 proposed 
methodology for seismically initiated event sequences sets forth a more 
stringent standard for the seismic design of repository facilities than 
for reactors. NRC also does not agree that the ISG-01 contradicts the 
intent of Part 63. The methods discussed in the draft ISG do not 
mandate seismic design requirements, but present approaches that NRC 
staff could use to review the performance of SSCs ITS for seismically 
initiated event sequences, as required in Part 63.
    The preclosure compliance requirements in Part 63 are performance-
based, in that instead of specifying specific design loads and 
corresponding acceptance criteria (i.e., codes/standards) the 
regulations in 10 CFR 63.111, for the GROA, specify radiological dose 
limits to the public and workers. In the preclosure safety analysis 
(PCSA), DOE must demonstrate that the GROA design will meet these dose 
limits, taking into consideration credible event sequences.
    The ISG-01 provides a methodology to determine if a seismically 
initiated event sequence is a Category 2 event sequence, as defined in 
10 CFR 63.2, or if it is beyond Category 2 and can be screened out from 
further consideration. If the event sequence is determined to be a 
Category 2 event sequence, DOE has to demonstrate that the dose limit 
of 5 roentgen equivalent man (rem) at any point on the boundary of the 
site is met. These performance-based requirements in Part 63 
necessarily result in a different type of compliance demonstration than 
is traditionally used for reactor licensing.
    For reactors, a seismic event is directly related to the 
characteristics of a specified safe shutdown earthquake (10 CFR Part 
50, Appendix S), which is used as the design basis for each of the 
safety-related SSCs, and demonstration of compliance with regulations. 
In contrast, Part 63 does not specify seismic or other design bases or 
SSCs, but instead requires consideration of credible event sequences 
and their potential consequences. The guidance in the draft ISG shows 
how the fragilities of one or more SSCs in an event sequence can be 
combined with the seismic hazard curve to determine the likelihood of 
an entire event sequence, which is the metric used for compliance in 
Part 63. Section 63.102(f), which allows initiating events to be 
considered based on precedents adopted for nuclear facilities with 
comparable or higher risks, was not used in the ISG-01 because the 
compliance demonstration for Part 63 requires safe performance of SSCs 
in seismically initiated event sequences, instead of a single 
initiating seismic event (i.e., safe-shutdown earthquake) that is 
traditionally used as a design basis in reactor licensing.
    DOE will need to design to a level of performance sufficient to 
meet the requirements of Part 63, for seismically initiated event 
sequences. DOE is given broad flexibility in selecting a preferred 
design basis, and determining the degree of defense-in-depth contained 
within the GROA system. Although DOE must provide the basis for its 
proposed designs, compliance with Part 63 will be determined by the 
performance of the design during credible seismically initiated event 
sequences, not by adherence to a predetermined design basis for a 
seismic event.
    No changes were made to the ISG as a result of this comment.
    (d) In the commenter's view, the specific methodology in the ISG-01 
may bias the staff's review against other methodologies that DOE may 
propose, even if these alternatives provide equal or better protection 
of public health and safety. The commenter also raises the concern that 
NRC should not dictate to DOE how to demonstrate compliance with 
regulations because it does not allow for a more independent review of 
the future DOE License Application. NRC does not agree with the comment 
that providing a methodology for seismically initiated event sequences 
in ISG-01 may preclude DOE from proposing other methodologies for 
complying with Part 63. Similar to the YMRP, ISGs are prepared to 
provide guidance to the staff for review of any future License 
Application, from DOE, for the proposed Yucca Mountain repository, and 
are not mandatory. DOE has the option of proposing alternative 
methodologies to comply with the regulations, which the staff would 
evaluate during its review of the License Application. As discussed in 
response to Comment 1, presenting an example methodology in an ISG does 
not imply a preference for that method in licensing, and does not 
restrict the ability of an applicant to use an alternative method.
    No changes were made to the ISG as a result of this comment.
    (e) In the commenter's view, the specific methodology proposed in 
the draft ISG-01 lacks both precedent and scientific support. The 
commenter raises the concern that applying technical analysis to 
seismic events with probability of exceedance lower than one in 10,000 
per year to establish design bases is unprecedented, and that it would 
result in stringent design criteria. Staff disagrees with the 
commenter's concern because ISG-01 does not provide guidelines on the 
design bases or design criteria for the SSCs, of the GROA, at the 
repository, but provides one method for NRC staff to use in reviewing 
demonstration of compliance with the performance requirements for the 
SSCs in the PCSA. Additionally, the methodology proposed in the draft 
ISG has precedent in the mixed-oxide fuel fabrication facility at the 
Savannah River Site in South Carolina, where the applicant used a 
methodology similar to the one outlined in the draft ISG to demonstrate 
performance of the facility during seismic event sequences.
    NRC disagrees with the comment that the methodology proposed in 
ISG-01 lacks scientific support. The proposed ISG-01 methodology to 
evaluate seismic performance of an SSC ITS is consistent with the 
performance-based methodology in the consensus standard ASCE 43-05. The 
methodology has the scientific support of the experts in the industry, 
and is not beyond the state-of-the-art for performance evaluation of 
SSCs for seismic hazard.
    No changes were made to the ISG as a result of this comment.
    14. Comment. Two commenters stated that NRC's decision to approve 
the use of the methodology that is similar to the one outlined in ASCE 
43-05 appeared to be based on the method's recent use in licensing of 
the mixed-oxide fuel fabrication facility at the Savannah River Site. 
The MOX facility has a projected operating life of 20-40 years and it 
is assumed that the NRC operating license is for the same period of 
time. The commenters are concerned about the ability of ASCE 43-05 to 
appropriately account for uncertainty over the longer time-frame for 
Yucca Mountain, given that the preclosure operating period for the 
repository project could be 100 years or longer. The commenter adds 
that NRC should address this issue in the final staff guidance.
    Response. The commenters raise a concern that the ISG-01 
methodology, as suggested by ASCE 43-05, may not produce accurate 
results over a potential 100-year or longer operating life of the Yucca 
Mountain repository preclosure

[[Page 57584]]

operating period. The preclosure operating period of the Yucca Mountain 
repository may affect the ISG-01 methodology results in two ways: (i) 
In categorization of seismically initiated event sequences (e.g., one 
chance in 10,000 of occurrence during the preclosure period specified 
in Part 63 for category 2 event sequences); and (ii) in development of 
the SSCs ITS seismic fragility curves, with potential changes in 
material properties resulting from degradation during the preclosure 
period. Staff believes that the uncertainties, considered in the 
seismic hazard and SSCs ITS fragility curves development, would 
sufficiently account for potential materials degradation during the 
preclosure period.
    No changes were made to the ISG as a result of this comment.
    15. Comment. Two commenters stated that the example provided in 
Appendix A raises questions as to whether NRC has adequately considered 
the geometric consequence of closely spaced, recurring, seismic events 
in determining the mean seismic hazard and related failure probability 
of an SSC ITS. HLWRS-ISG-01 and/or the YMRP may need to be revised to 
ensure that such characteristics of seismic hazard and related failure 
probability are appropriately considered in computing SSC ITS 
probability of failure during a seismic event.
    Response. The example of Appendix A is based on a hypothetical 
seismic hazard curve selected only for illustrative purpose. However, 
for the development of the Yucca Mountain site-specific mean seismic 
hazard curves (Reference, Section 6.4), DOE's current approach 
evaluates the potential of closely spaced, recurring, seismic events by 
considering simultaneous multiple ruptures on parallel dipping faults, 
and increasing the ground motion parameters for a given probability of 
exceedance value. Since the effects of the closely spaced, recurring, 
seismic events are considered in the seismic hazard curve, staff 
believes that the ISG-01 methodology would result in an appropriate 
value of the failure probability of an SSC ITS, and that ISG-01 or the 
YMRP need not be revised.

    [Reference: Civilian Radioactive Waste Management System, 
Management and Operating Contractor (CRWMS, M&O), 1998, 
Probabilistic Seismic Hazard Analyses for Fault Displacement and 
Vibratory Ground Motion at Yucca Mountain, Nevada (I. G. Wong and J. 
C. Stepp, coordinators), report prepared for U. S. Geological 
Survey, 3 Volumes]

    No changes were made to the ISG as a result of this comment.
    16. Comment. It is unclear to the commenter whether the guidance 
directs NRC staff to use the suggested methodology or merely offers an 
alternative among possible methods. To reduce uncertainty, the 
commenter suggests that it would be helpful if NRC provided explicit 
guidance as to how the selection of an appropriate methodology would be 
made, and when, if at all, a given methodology might be unacceptable 
for use. The commenter believes that the discretion in choice of 
methods appears to introduce unwarranted ambiguity and uncertainty.
    Response. An ISG provides guidance to NRC staff on suggested 
methodologies to use during the review of a potential license 
application, and do not imply a preferred methodology that an applicant 
must use. The review approach in an ISG provides a framework for staff 
to conduct an efficient review, consistent with regulatory 
requirements. DOE has the option of proposing alternative methodologies 
to comply with the regulations, which the staff would evaluate during 
its review of the License Application. Methodologies that demonstrate 
compliance with the regulations, and have adequate technical bases, 
would be acceptable for staff review.
    No changes were made to the ISG as a result of this comment.

FOR FURTHER INFORMATION CONTACT: Jon Chen, Project Manager, Division of 
High-Level Waste Repository Safety, Office of Nuclear Material Safety 
and Safeguards, U.S. Nuclear Regulatory Commission, Washington, DC 
20005-0001 (Telephone: (301) 415-5526; fax number: (301) 415-5399; e-
mail: [email protected]); Mahendra Shah, Senior Level Advisor, Division of 
High-Level Waste Repository Safety, Office of Nuclear Material Safety 
and Safeguards, U.S. Nuclear Regulatory Commission, Washington, DC 
20005-0001 (Telephone: (301) 415-8537; fax number: (301) 415-5399; e-
mail: [email protected])

    Dated at Rockville, Maryland, this 22nd day of September 2006.

    For the Nuclear Regulatory Commission.
N. King Stablein,
Chief, Project Management Section B, Division of High-Level Waste 
Repository Safety, Office of Nuclear Material Safety and Safeguards.
 [FR Doc. E6-16017 Filed 9-28-06; 8:45 am]
BILLING CODE 7590-01-P