[Federal Register Volume 72, Number 55 (Thursday, March 22, 2007)]
[Notices]
[Pages 13534-13537]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 07-1404]


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

NUCLEAR REGULATORY COMMISSION

[HLWRS-ISG-02]


Preclosure Safety Analysis--Level of Information and Reliability 
Estimation; Availability of Final Interim Staff Guidance Document

AGENCY: Nuclear Regulatory Commission.

ACTION: Notice of availability.

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

SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) is announcing the 
availability of the final interim staff guidance (ISG) document HLWRS-
ISG-02, ``Preclosure Safety Analysis--Level of Information and 
Reliability Estimation,'' 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 guidance to NRC staff to evaluate a 
potential license application for a high-level radioactive waste at a 
geologic repository constructed or operated at Yucca Mountain (YM), 
Nevada.

ADDRESSES: The document HLWRS-ISG-02 is available electronically at 
NRC's Electronic Reading Room, at http://www.nrc.gov/reading-rm.html. 
From this site, a member of the public 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 
ISG-02 is ML070260204. If an individual does 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: O-1F21, 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-02: In preparing 
final HLWRS-ISG-02, ``Preclosure Safety Analysis--Level of Information 
and Reliability Estimation,'' ADAMS ML070260204, the NRC staff reviewed 
and considered 23 comments, including two editorial comments, received 
from two organizations during the public comment period. Two of the 
comments were identical; three comments were related to the ISG 
process; one comment endorsed NRC's recognition of the use of the 
published reliability values for structures, systems, and components 
(SSCs); and the remaining comments included recommendations on specific 
clarifying changes to the ISG. Three comments on the ISG process were 
consistent with the comments made earlier on HLWRS-ISG-01, and were 
addressed in responses to public comment on HLWRS-ISG-01 [see 71 FR 
57582, Comments 13(a) and (b)].

[[Page 13535]]

    The following discussion indicates how the comments were addressed, 
and the changes, if any, made to ISG-02 as a result of the comments. 
Line numbers in the following comments refer to the draft HLWRS-ISG-02, 
ADAMS ML062360241, which was made available for public comment on 
September 29, 2006 (71 FR 57584).
    Comment 1. One commenter was concerned that the changes in the 
YMRP, recommended in ISG lines 59-66, 222-224, and 271-273, appear to 
suggest that information regarding ``design bases and design criteria'' 
for non-important to safety (non-ITS) SSCs be similar to those for ITS 
SSCs. Since non-ITS SSCs have been determined not to be necessary to 
assure compliance with 10 CFR Part 63 preclosure performance 
objectives, the commenter states that subsection 63.21(c)(3) does not 
appear to support inclusion of information related to design bases and 
design criteria for non-ITS SSCs. The commenter recommends specific 
changes to ISG lines 62, 222, 239, 254, 258, 263, 266, and 272, to 
clarify its position.
    Response. NRC agrees that information required for non-ITS SSCs 
would be less than for ITS SSCs. Subsection 63.21(c)(3) requires a 
description and discussion of the design of the YM geologic repository 
operations area, that is sufficient to permit an evaluation of the 
preclosure safety analysis (PCSA). DOE will have to provide sufficient 
information to discuss how the proposed design would function. This 
also includes the general arrangements of SSCs, capacities of SSCs, and 
levels at which the SSCs are operated. Staff agrees with the commenter 
that 10 CRF Part 63 requires the design bases and design criteria for 
ITS SSCs, and not for non-ITS SSCs.
    ISG lines 62, 222, 254, and 272 have been revised to state that 
design bases and design criteria refer to SSCs that have been 
designated as ITS. ISG lines 239, 258, 263, and 266 have not been 
revised, because these lines refer to estimating the reliability of 
SSCs sufficient for performing the PCSA and identifying ITS SSCs, as 
per 63.112.
    Comment 2. The commenter stated that, in lines 57-259, it would be 
more appropriate to use ``accept,'' instead of ``recognize,'' because 
it is unclear. The same commenter also noted that lines 276-284 do not 
include an acceptance criterion element related to ``acceptability of 
codes and standards,'' as proposed in lines 258-259, and supplemented 
in lines 121-124.
    Response. NRC disagrees that the word ``recognize'' is unclear in 
the context of the sentence in lines 257-259. Staff believes that the 
use of the word ``accept'' would be inappropriate here, because the 
codes and standards do not provide explicit reliability values 
requiring acceptance. Staff also disagrees with the commenter's 
recommendation on the addition of a new acceptance criterion item (7), 
regarding the use of codes and standards to obtain a probability of 
unacceptable performance. Staff believes that, as stated in ISG lines 
121-124, the application of the codes and standards, to the design and 
operation of an ITS SSC, is an accepted engineering practice, and is 
addressed as new item (2), of ``Acceptance Criterion 2,'' in ISG lines 
276-277.
    No changes to the ISG were made as a result of this comment.
    Comment 3. The commenter states that the phrases ``risk-
significant'' or ``risk-significance'' have a multiplicity of meanings. 
For example, in nuclear power plant probabilistic risk assessment 
applications, the terms refer to a metric of risk that is a function of 
both probability (or frequency) of occurrence, and consequences. 
However, in the context of Part 63, event sequence categorization is 
performed on the basis of probability, only. The consequences of 
interest (public and worker doses) are deterministic in nature. The 
commenter recommended that the terms ``risk-significant'' or ``risk-
significance'' be avoided or defined specifically in the context of 
this ISG.
    Response. NRC agrees that use of the terms ``risk-significant'' or 
``risk-significance'' in the ISG requires clarification where reference 
is to the consequences only and not to the ``risk,'' which includes 
both the probability and the consequences. Changes to lines 41 and 162 
were made to either clarify or remove redundancy of the ``risk'' term. 
Specific changes to the ISG, suggested by the commenter on lines 210, 
268, 289, 382, and 574, are not made, because these lines refer to the 
``risk'' consistent with the traditional definition (U.S. Nuclear 
Regulatory Commission, White Paper on Risk-informed and Performance-
based Regulation, SECY-98-144, June 22, 1998, as revised by the Staff 
Requirements Memorandum, March 1, 1998).
    The ISG has been revised as follows:
    Line 41: Change ``risk-significant'' to ``significant.''
    Line 162: Delete ``risk-significance or * * *''
    Comment 4. The commenter suggested that the lines 86 and 240 of the 
ISG be revised to state that ``Explicit quantitative reliability 
estimates of software failure modes during event sequences are beyond 
the state-of-the-art and are not expected for the PCSA. It is 
acceptable to use reliability estimates of digital control units, which 
would implicitly include hardware and software effects.''
    Response. NRC disagrees that revisions to lines 86 and 240 are 
needed. For SSCs where the reliability estimates include hardware and 
software effects, it is acceptable to use the reliability estimates, 
without explicit consideration of software failures. However, for SSCs 
where such data are not available, an estimate for reliability needs to 
include consideration of hardware and software failures. NRC believes 
that ISG lines 86 and 240 do not need to be revised because these 
statements allow the U.S. Department of Energy (DOE) the flexibility to 
consider hardware and software failures with appropriate technical 
bases.
    No changes to the ISG were made as a result of this comment.
    Comment 5. The commenter states that the sentence starting at line 
89 be revised by replacing ``event'' with ``event sequences.''
    Response. NRC agrees with the suggested change.
    ISG line 89 has been revised to change ``events'' to ``event 
sequences.''
    Comment 6. The commenter recommends that a definition of the mean 
value of a probability distribution be included after line 90 of the 
ISG.
    Response. NRC disagrees that the mean value of a probability 
distribution needs to be defined in the ISG. The mean of a distribution 
is a clear and unambiguous statistical term.
    No changes to the ISG were made as a result of this comment.
    Comment 7. The commenter states that items 2 and 3, in lines 129-
132 of the ISG, ``* * * appear to contradict the indication that a 
quantitative reliability estimate is needed,'' and recommends revising 
the ISG to clarify that quantitative reliability estimates are needed.
    Response. NRC disagrees that the changes recommended by the 
commenter are necessary. As stated in the ISG, items 1, 2, and 3 are 
given as examples of methods that may be used, in combination with a 
code and standard, to obtain quantitative reliability estimates, and do 
not contradict the need for the quantitative reliability estimates.
    No changes to the ISG were made as a result of this comment.
    Comment 8. The commenter states that the use of the term 
``procedure,'' in ISG line 229, does not recognize that

[[Page 13536]]

many of the actions associated with repository operations, such as 
crane and trolley operations, will also be skill-based. The commenter 
recommends that the ISG line 229 be revised to clarify that the review 
will be of ``procedures and activities,'' related to the controls and 
the human interactions associated with each SSC.
    Response. NRC agrees with the commenter.
    ISG line 229 has been revised to add ``and activities'' after 
``procedures.''
    Comment 9. The commenter states that, in Appendix A of the ISG, the 
probability of dropping a heavy load is estimated with empirical data, 
then multiplied by the number of times that heavy loads are lifted, to 
arrive at a number that is characterized as the ``expected number of 
drops.'' The use of the word ``expected'' is misleading, because it 
implies expected value, which is often used as a synonym for the mean 
value. The product of these two point estimates cannot be construed as 
a mean or expected value of the number of drops, because the underlying 
probability distributions were not developed for them. The commenter 
recommends that the phrase ``expected number of drops'' in ISG line 451 
should be changed to ``point estimate number of drops.''
    Response. NRC disagrees with the change recommended by the 
commenter. However, the ISG has been revised to clarify the staff's 
approach. Whereas the staff agrees that the use of the phrase 
``expected number of drops'' may be misleading, the staff disagrees 
with the reason given in the comment. The ISG calculation uses a 
classical statistical approach. With this approach, the number of drops 
in L lifts has a binomial distribution which is typically approximated 
by a Poisson distribution. The expected value of the Poisson 
distribution is the product of the drop probability and the presumed 
number of lifts that may occur in the preclosure facility. Since the 
drop probability is estimated in this case, the expected number of 
drops is also estimated.
    The ISG has been revised to add the above approach after line 449. 
ISG line 451 has been revised to change ``expected'' to ``estimated.'' 
Also, ISG lines 432 and 489 have been similarly changed.
    Comment 10. Two commenters stated that scientific and technical 
precedent point to the use of the mean value of a frequency 
distribution as the appropriate metric for event sequence 
categorization. One commenter adds that, contrary to this, ISG lines 
465-472 appear to point to the use of a fraction of a confidence 
interval, on which to base a conclusion about categorization of an 
event sequence. The commenter recommends deleting the sentence, 
beginning on line 467, and changing lines 470-472 to read as, ``The 
number of expected drops, in this example, would be the mean value of a 
joint probability distribution of both the conditional drop probability 
and the number of lifts.''
    Response. In Appendix A of ISG-02, empirical data were used to 
derive a point estimate for the probability of dropping a cask. To 
address uncertainty in this point estimate, staff chose a standard 
statistical approach of the confidence interval method, to determine 
the confidence level in categorization of the event sequence for the 
example.
    NRC does not agree that the sentence beginning on line 467 should 
be deleted, because it provides an example of a method to illustrate 
consideration of uncertainty. The 48-percent level of confidence is 
analogous to reporting the descriptive level of significance, which is 
often used in reporting the results of a test of a hypothesis.
    According to the ``Statement of Considerations'' for Part 63, 
November 2, 2001 (66 FR 55742), the approach in the rule is to provide 
DOE with the flexibility to select the type of analysis it believes 
most appropriate for the license application. Whatever approach DOE 
uses will need to be supported, taking into account uncertainties. 
Therefore, analyses relying on point values (e.g., best-estimate 
values) will need to discuss how uncertainties are taken into account.
    NRC agrees that DOE can use the mean value of an event sequence 
frequency distribution to categorize an event sequence. However, DOE 
should to consider the uncertainty in any mean value used to categorize 
event sequences. In particular, DOE should to provide the technical 
bases for developing the event sequence frequency distribution, 
including consideration of uncertainties in performance of individual 
SSCs, the choice of distribution type, and the values of the 
parameters.
    ISG lines 470-472 have been deleted, because these lines refer to 
the estimated conditional drop probability for a specific confidence 
level, which is not discussed in the ISG.
    Comment 11. The commenter states that ISG line 592 be revised to 
clarify that the design bases are associated with SSCs and not with an 
event sequence category, as stated in the ISG.
    Response. NRC agrees with the comment.
    ISG line 592 has been revised to read as follows: ``Design bases 
(e.g., loadings on SSCs associated with Category 1 and Category 2 event 
sequences, such as a canister drop event); and * * *.''
    Comment 12. The commenter states that the definition of ``S = C/
D,'' in line 617, appears to be inconsistent with the definition in 
Figure B-2 of the ISG. The commenter recommends that either the 
definition of ``S,'' in line 617, be revised, or that Figure B-2 be 
revised.
    Response. NRC disagrees with the commenter that definition of ``S'' 
in ISG line 617, and Figure B-2 are inconsistent. Figure B-2 is 
consistent with the commonly used definition of the limit state 
function in the form of S = C/D, as shown in line 617, where C and D 
are the capacity and demand, respectively. Staff, however, recognizes 
that Y-axis labeling in Figure B-2, and description of the ISG lines 
680-681, may have resulted in an appearance of inconsistency. As stated 
in ISG line 676, Figure B-2 shows the cumulative distribution function 
of S, with the probability of failure defined as the probability that S 
is less than or equal to 1. The curve, shown in Figure B-2, is for the 
constant demand D = 497 mega pascals (MPa) [72 kips per square inch 
(ksi)]. Similar curves are derived for two other values of demand 
values, listed in Appendix B, using a log-normal distribution of the 
capacity, C, divided by a constant demand, D (see Ref. B.3), and are 
included in the revised Figure B-2 in the ISG. Probability of failure 
values for three different demand values, along with their 
corresponding ratios of American Society of Mechanical Engineers (ASME) 
code allowable stress to demand, are shown in Table B-3. The results 
show, as expected, that the probability of failure decreases as the 
demand decreases. The ISG has been revised as follows:
     Figure B-2 has been revised to include plots for all three 
demand values shown in Table B-3, and the caption has been revised to 
include ``for three demand values'';
     Label for the ordinate axis has been changed from 
``Probability of Failure (x 10-5)'' to ``Cumulative 
Probability,'' and is replotted in the log-scale;
     Line 622: The phrase, `` * * * which is traditionally 
defined as the limit state function'' is added at the end of the 
sentence.
     Line 680: A new sentence, ``Failure probabilities for 
various values of demand are shown in Figure B-2.'' has been added;
     Lines 680-681: sentence has been revised to ``Failure 
probabilities for various values of ratios of ASME

[[Page 13537]]

allowable stress to these corresponding demand values are given in 
Table B-3.''
    Comment 13. The commenter stated that the paragraph beginning with 
line 156 of the ISG specifies that the NRC staff will verify that 
uncertainty is addressed in the PCSA. The commenter is concerned that 
this may be interpreted as requiring excessive conservatism in the 
analysis, and that such an approach would be the opposite of the intent 
of risk-informed regulation. The commenter recommends that text of the 
discussion on uncertainty be revised to explicitly recognize this 
intent.
    Response. NRC agrees that excessive conservatism should be avoided 
in considering uncertainty. DOE has the flexibility to choose the 
method to demonstrate that the performance objectives are met. For 
example, DOE could perform a bounding calculation. As stated in the 
``Statement of Considerations,'' for Part 63, ``* * * whatever approach 
DOE uses will need to be supported, taking into account 
uncertainties.'' For example, if DOE is to portray its PCSA results as 
best estimates, this term will need to be defined because it has no 
statistical meaning (see ``A Review of Staff Uses of Probabilistic Risk 
Assessment,'' NUREG-1489, March 1994). Staff believes that the 
paragraph on uncertainty, beginning with ISG line 156, is sufficiently 
clear, and that no changes are required.
    No changes to the ISG were made as a result of this comment.
    Comment 14. The commenter stated that the screening criteria in ISG 
lines 127-128 presume a preclosure period of 100 years by specifying 
that the lower bound of Category 2 event sequence frequency is 
10-6 failures/yr. Instead, the staff should be consistent 
with Part 63 in referring to the lower bound of Category 2 event 
sequence frequency as the one chance in 10,000 during the period of 
operation.
    Response. NRC agrees with the comment. Unless there is a reason to 
state otherwise, the staff will refer to the terminology, used in Part 
63, for Category 2 event sequence frequency as having at least one 
chance in 10,000 of occurring during the preclosure period. The 
quantitative frequency limit of a Category 2 event sequence is 
determined by the duration of the preclosure period.
    ISG line 127 has been revised to change ``(e.g., <=10-6 
failures/year)'' to ``(e.g., <= one chance in 10,000 of occurring 
during the preclosure period).''
    ISG line 128 has been revised to delete ``(e.g., <=10-6 
failures/year).''
    Comment 15. The commenter stated that, in ISG line 136, the NRC 
staff recognizes various sources of reasonable input to the PCSA. It is 
important that such information does not have to be created under an 
NRC-licensed quality assurance program. The sources cited in the ISG 
[e.g., ``Generic Data Base, developed by Savannah River Site,'' and the 
Equipment Performance and Information Exchange (EPIX) System], for 
reliability input, are reasonable, based on actual operating data, and 
not skewed by conservatism. Even though applying conservatism is 
acceptable for safety analysis purposes (e.g., for analytical 
simplification or bounding uncertainties), doing so distorts the 
foundation of risk-informed regulation by implying higher risks than 
actually exist.
    Response. NRC agrees that DOE can use reliability information from 
published references. However, DOE must provide the technical basis to 
demonstrate that any reliability information is applicable to the 
proposed design of the GROA.
    No changes to the ISG were made as a result of this comment.
    Comment 16. The commenter stated that, in ISG lines 157-168, the 
staff should apply additional scrutiny or focus in its review, in cases 
where a reliability estimate is close to a Category 1 or 2 limit. The 
ISG should not be taken to imply that DOE is required to submit any 
additional analysis with its license application. The guidance should 
be clarified to explicitly recognize that it is incumbent on DOE to 
determine both if and when a reliability estimate is sufficiently close 
to a Category 1 or 2 limit to warrant additional consideration, in the 
license application, as well as the specific nature and extent of any 
such consideration in the application.
    Response. NRC has not specified criteria for determining when a 
sequence frequency is close enough to a category limit to warrant 
additional scrutiny. DOE is expected to provide NRC with enough 
information to demonstrate that sequences have been correctly 
categorized.
    No changes to the ISG were made as a result of this comment.
    Comment 17. The commenter stated that the demand in ISG lines 636-
638 is a function of several parameters (e.g., modulus of elasticity, 
dimension, thermal expansion). The commenter adds that these parameters 
would affect the material capacity, not the demands placed on the 
material, and recommends that this sentence be revised by deleting the 
words ``modulus of elasticity, dimensions, thermal expansion.''
    Response. NRC agrees with the comment. Demand on an SSC because of 
an event, such as a drop or a natural event, would not depend on the 
modulus of elasticity, dimension, and thermal expansion.
    ISG lines 636-637 have been revised to delete ``modulus of 
elasticity, dimensions, thermal expansion.''
    Comment 18. One commenter suggested the following editorial 
changes: Lines 587-588: Revise ``* * * including major components of 
canister structure, internals'' to read ``* * * including major 
components of canister structure, and its internals''; Line 622: Revise 
``function can developed'' to read ``function can be developed.''
    Response. NRC agrees with the comment. The ISG has been revised to 
reflect the suggested changes.
    In addition to the changes described above, the ISG has also been 
revised, as follows, for clarification:
    Line 91: The sentence ``DOE should identify the key SSCs in an 
event sequence.'' was deleted because ``key'' SSCs is not formally 
defined; a new sentence to replace the deleted sentence has been added;
    Line 446: The definition of [lgr] (now p was reworded for clarity;
    Lines 445: Though 453: [lgr] was changed to, to distinguish this 
quantity from [lgr], which often is used to indicate a rate in the 
Poisson distribution, and that the quantity is an estimate;
    Line 622: Clarifying words were added.

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 
20555-0001 [Telephone: (301) 415-5526; fax number: (301) 415-5399; e-
mail: [email protected]];
    Robert Johnson, Senior Project Manager, Division of High-Level 
Waste Repository Safety, Office of Nuclear Material Safety and 
Safeguards, U.S. Nuclear Regulatory Commission, Washington, DC 20555-
0001 [Telephone: (301) 415-6900; fax number: (301) 415-5399; e-mail: 
[email protected]].

    Dated at Rockville, Maryland this 8th day of March, 2007.

    For the Nuclear Regulatory Commission.,
N. King Stablein,
Chief, Project Management Branch B, Division of High-Level Waste 
Repository Safety, Office of Nuclear Material Safety and Safeguards.
[FR Doc. 07-1404 Filed 3-21-07; 8:45 am]
BILLING CODE 7590-01-P