Preclosure Safety Analysis-Level of Information and Reliability Estimation; Availability of Final Interim Staff Guidance Document, 13534-13537 [07-1404]
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of March, 2007.
For the Nuclear Regulatory Commission.
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New Reactor Licensing, Office of New
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[FR Doc. E7–5247 Filed 3–21–07; 8:45 am]
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[FR Doc. E7–5249 Filed 3–21–07; 8:45 am]
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[HLWRS–ISG–02]
Florida Power Corporation; Notice of
Consideration of Issuance of
Amendment to Facility Operating
License, Proposed No Significant
Hazards Consideration Determination,
and Opportunity for a Hearing;
Correction
Nuclear Regulatory
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ACTION: Notice of Issuance; Correction.
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AGENCY:
SUMMARY: This document corrects a
notice published in the Federal Register
on March 13, 2007 (72 FR 11381), which
informs the public that the NRC is
considering issuance of an amendment
to Renewed Facility Operating License
No. DPR–72. This action is necessary to
correct the name of the licensee.
FOR FURTHER INFORMATION CONTACT:
Beverly A. Clayton, Office of Nuclear
Reactor Regulation, Nuclear Regulatory
Commission, Washington, DC 20555–
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NRC’s Agencywide Document Access
and Management System (ADAMS),
which provides text and image files of
NRC’s public documents. The ADAMS
accession numbers for the documents
related to this notice are:
Jkt 211001
Preclosure Safety Analysis—Level of
Information and Reliability Estimation;
Availability of Final Interim Staff
Guidance Document
Nuclear Regulatory
Commission.
ACTION: Notice of availability.
AGENCY:
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
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constructed or operated at Yucca
Mountain (YM), Nevada.
The document HLWRS–
ISG–02 is available electronically at
NRC’s Electronic Reading Room, at
https://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 pdr@nrc.gov.
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)].
ADDRESSES:
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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–
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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 ‘‘risksignificant’’ 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 ‘‘risksignificance’’ 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
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13535
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
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many of the actions associated with
repository operations, such as crane and
trolley operations, will also be skillbased. 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
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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.
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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 lognormal 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
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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).’’
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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
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13537
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;
ˆ
Line 446: The definition of λ (now p
was reworded for clarity;
Lines 445: Though 453: λ was
changed to, to distinguish this quantity
from λ, 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:
jcc2@nrc.gov];
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:
rkj@nrc.gov].
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]
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[Federal Register Volume 72, Number 55 (Thursday, March 22, 2007)]
[Notices]
[Pages 13534-13537]
From the Federal Register Online via the Government Printing 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 https://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 pdr@nrc.gov.
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: jcc2@nrc.gov];
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:
rkj@nrc.gov].
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
