ML13085A190
| ML13085A190 | |
| Person / Time | |
|---|---|
| Site: | Cook  |
| Issue date: | 04/22/2013 |
| From: | NRC/NRR/DRA/APLA |
| To: | NRC/NRR/DRA/APLA |
| Snodderly M | |
| References | |
| TAC ME6629, TAC ME6630 | |
| Download: ML13085A190 (44) | |
Text
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 1 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
AS-01 The Large Loss of Coolant Accident (LOCA) event tree discussion of gross containment failure does not appear consistent with the stated assumption; in this instance the discrepancy probably does not change the conclusion that core damage occurs so late as to not qualify as part of Large Early Release Frequency (LERF).
(Significance Level C)
The numerical impact of this Fact and Observation (F&O) on the Internal Events Probabilistic Risk Assessment (PRA) results is judged very small. Since no fire consequentially causes a Large LOCA event, there is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution occurs in both the before and after results).
RAI Not Needed The licensee explains that Large LOCA is not a contributor to fire risk.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
AS-02 The medium LOCA event tree discussion on long term cooling states seems to indicate that more heat removal is required than the success criterion. (Significance Level C)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category (CC) II.
RAI submitted as RAI 21(a)1 In response to RAI 21(a) the licensee explains that the documentation related to success criteria was updated. The licensee also explains that consequential Medium LOCAs are very small contributors to fire risk.
Given the additional information the NRC staff finds this deficiency has been appropriately resolved by the licensee.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 2 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
AS-03 The Main Steam Line Break Inside Containment (SLBI)event tree discussion states that the Main Steam Line Break Outside Containment (SLBO )is covered by the SLBI tree, although the mitigation requirements of these events are different with respect to isolation and containment systems. (Significance Level C)
The numerical impact of this F&O on the Internal Events PRA results is judged very small. Since no fire consequentially causes a Main Steam Line Break (SLB) event, there is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution occurs in both the before and after results).
RAI Not Needed The licensee explains that Main Steam Line Break is a small contributor to fire risk.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
AS-04 In the event tree for Station Black-Out (SBO), existing logic for Auxiliary Feed Water (AFW) functions asked successively does not require correlation between success of the prior branch and the failure of the latter branch. (Significance Level C)
The numerical impact of this F&O on the Internal Events PRA results is judged to be zero. Either path through a failure branch requires three failed AFW flow paths.
Common Cause Failures (CCFs) are included in first possible failure path. Since there is no effect on the Internal Events PRA model, this F&O has no impact for NFPA-805.
RAI submitted as RAI 21(c)1 In response to RAI 21(c) the licensee explains that this minor logic error is only applicable to the internal events SBO event tree.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
AS-05 Although no inconsistencies in modeling of systems to meet the safety functions were identified, there is no discussion of functional failure criteria for the event trees. (Significance Level C)
This is a documentation issue that has no effect on the Internal Events PRA model, and, as a result, there is no effect of this F&O on NFPA-805.
RAI submitted as RAI 21(c)1 In response to RAI 21(c) the licensee explains that events tree modeling is based on the EOPs and system failures explicitly.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 3 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
AS-06 The success criterion for feed and bleed is conservative compared to the criterion used for some other 4-loop Westinghouse plants. (Significance Level C)
This F&O implies that the success criterion used for feed and bleed is also conservative in the Fire PRA. Given the abundant AFW availability at CNP, the numerical impact of this bleed and feed conservatism is judged very small for the Internal Events PRA model. The numerical effect of such conservatism would be to increase the Core Damage Frequency (CDF) for fires that affect the systems used to perform bleed and feed. This would cause the Fire PRA to be more conservative in feed and bleed scenarios since less fire-related damage can cause failure of the function. This additional conservatism would increase the fire risk estimated for NFPA-805 change evaluations.
RAI Not Needed The licensee explains that modeling used in the fire PRA is conservative.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 4 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
AS-07 Initiating event dependencies are not retained by event tree transfers. (Significance Level A)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a)1 In response to RAI 21(a) the licensee explains that 34 event tress were developed to explicitly consider transfer from one event tree to another given failure specific failures of support systems. The licensee listed the support systems and transfer modeling.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
AS-08 Reactor Coolant Pump (RCP) Seal LOCA modeling in the loss of service water event trees is different than in the SBO event trees and needs to be made consistent.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains the modeling and modeling assumptions specific to RCP seal LOCAs.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 5 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
AS-09 After recovery of Alternating Current (AC) power, after SBO, the PRA conservatively requires recovery for alignment of Safety Injection (SI), Charging and Volume Control System (CVCS),
Containment Spray (CTS), and AFW, however, the systems may not be needed or may be initiated automatically. (Significance Level C)
The numerical impact of this F&O on the Internal Events PRA results is judged very small. Pump recovery failure in the internal events PRA model is dominated by operator actions. The effect of this F&O would be an increase in predicted fire risk following fire-induced SBO, since op actions are more complicated. This would increase the fire risk estimated for NFPA-805 change evaluations.
RAI Not Needed The licensee explains that modeling used in the fire PRA is conservative.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
AS-10 The event trees do not include a heading for containment isolation failure, resulting in improper assignment of LERF.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that rather than changing all events trees to include CI, the LERF fault tree was modified to include CI.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
AS-11 Dependencies between the failures causing Interfacing Systems Loss of Coolant Accidents (ISLOCA) in the individual ISLOCA scenarios and the systems mitigating ISLOCA are not considered. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that the ISLOCA initiating event and event trees were revised to follow the guidance in NUREG/CR-5102 and that initiator and scenarios dependencies were explicitly considered..
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 6 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
AS-12 The ISLOCA analysis assumes low pressure piping failure probability based solely on engineering judgment. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that the ISLOCA initiating event and event tree have been revised to incorporate the guidance in NUREG/CR-5744 and NUREG/CR-5102.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
AS-A1, AS-B1, AS-B2, AS-B4, AS-B5, SY-C1 The split fraction dependencies and corresponding values (basis) are not well documented or described in the report. For example, top event "CSR" has 21 split fractions (each with a different value) identified in the CCW event trees. However, the only way to follow what the dependencies are and how the different fraction values are calculated, is to go to the WINNUPRA calculation files. Even using the WINNUPRA This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI submitted as RAI 21(c) 1 In response to RAI 21(c) the licensee explains that quantification was performed using linked fault trees not split fractions.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 7 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) files, not all boundary conditions could be determined.
AS-A1, QU-C3 The number of considered transfers is too large; even very unlikely transfer events are modeled (<1E-08/yr). For example, the Initiating Event frequency for Medium LOCA, transferred from a loss of DC is 8E-9/yr. Even at this low frequency, this combined event is still modeled and there is an event tree associated with This issue was addressed in a 2007 update that included simplifying the event tree transfers.
RAI Not Needed The PRA Model was updated in 2007 to address this concern.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
AS-A10, AS-B3, SC-A6 ISLOCA modeling should: (1) separate the human reliability (OIB) and hardware (valve) reliability when modeling potential isolation of the appropriate breaks, (2) address valve shutoff delta-P capability for valves credited for isolation, (3) valve failure rates indicative of functional degradation due to harsh environment for Residual Heat Removal (RHR) pump seal failure events, (4) operations procedures This issue relates specifically to the Internal Events PRA model. Since no fire consequentially causes an Interfacing Systems LOCA on one of the modeled pathways, there is no effect of this F&O on NFPA-805 change evaluations (i. the same contribution occurs in both the before and after results).
RAI Not Needed In the April 27 RAI response for this F&O, the licensee explains that no credit was given in the fire PRA mitigating fire induced ISLOCA.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 8 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) should address remote manual isolation of ISLOCA events.
AS-A10, SC-A12, SC-A18, SY-A3, SY-A5, SY-A6, SY-A7, SY-B1, SY-B16 Enhance AFW system modeling related to Condensate Storage Tank (CST) refill by adding a discussion about CST refill system boundaries, capacities and dependencies, and any potential CCF modes. Also, model the HEP separate from the hardware.
The numerical impact of this SR on the Internal Events PRA results is judged very small. The Gap assessment commented that the approach used was reasonable given the supporting information available, but that providing additional system detail and separating the HEP and hardware would be a benefit to the model. The Fire PRA has accounted for the possibility that one or more fire initiators could preclude refilling the CST, so the effect of this SR is implicit in NFPA-805 results.
RAI Not Needed In the April 27 RAI response for this F&O, the licensee explains that the internal events SBO event tree was not used in the fire PRA..
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
AS-A2-01 The accident sequence notebook does not identify the safety functions necessary to reach a safe and stable state to prevent core damage following the initiating events. (Finding)
The development of accident sequences did inherently consider safety functions required to reach a safe, stable state by virtue of the historical development of the Event Trees and by following the EOPs which are based on assuring that the safety functions are met. This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed In the April 27 RAI response for this F&O, the licensee explains that event tree modeling is based on EOP response and corresponding system function.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 9 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
AS-A5 Consider a review and update of MAAP analyses for success criteria pertaining to accident sequence development associated with SBO and RCP Seal LOCA.
The Internal Events PRA model was updated to account for the addition of SDGs to the plant in 2005. As part of this model update improved event tree modeling and success criteria were adopted for RCP seal LOCAs.
RAI Not Needed The SBO and RCP seal LOCA modeling was updated in 2005.
Based on this explanation, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
AS-B3, LE-D1, LE-D6 Address if there are any phenomenological interactions that should be considered as potential failure modes of containment isolation. In the containment isolation model, I&M should consider accounting for the possibility of up to 200 hrs per year (2.3E-02) containment purge operation when purge valves could be open and could randomly fail to close on demand.
The containment pressure relief operation the failure rate is so small that it has not been included in the fault tree model. The failure probability is the product of the time fraction and two isolation valves failing, which is more than two orders of magnitude lower than the than the highest failure cutset probability that results in the same effect on containment isolation. With respect to the remaining issue, the Gap assessment commented that there should not be any phenomenological interactions that are potential failure modes of containment isolation. This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed In the April 27 RAI response for this F&O, the licensee explains review of pressure relief operation was performed and model impact assessment performed that concluded low probability of occurrence.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 10 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
DA-01 The data collection notebook provides a description of the criteria, in terms of evidence, for performing a Bayesian update which is inconsistent with industry references and the guidelines for common cause failure data analysis seems to not be in accordance with the cited reference. (Significance Level C)
The Internal Events PRA model update in 2009 included revising failure data for Mitigating Systems Performance Indicator (MSPI) pumps and diesels, accounting for the latest plant-specific failure data. The 2009 calculation was performed in accordance with NURG/CR-6928, including Bayesian updating. In addition, a CCF update was also performed consistent with the latest industry and NRC guidance. The remaining issue from this F&O is related to updating the notebook to discuss/reference the methodologies employed. This remaining documentation issue does not affect NFPA-805.
RAI Not Needed Calculation and Bayesian updating has been performed. Documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
DA-02 The method used to perform Bayesian updates is often described as moment matching, which in some cases may yield misleading results. (Significance Level C)
The Internal Events PRA model update in 2009 included revising failure data for MSPI pumps and diesels, accounting for the latest plant-specific failure data. The 2009 calculation used generic data distributions from NURG/CR-6928, Bayesian updated per NUREG/CR-6928, and mapped the results to the natural conjugate. No moment matching was performed to obtain the prior distribution. The remaining issue is related to updating the documentation and does not affect NFPA-805.
RAI Not Needed Calculations have been performed per industry guidance. Documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 11 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
DA-03 General guidelines for test and maintenance data collection are provided, however, the boundaries of the components and subsystems to be included in maintenance basic event were not defined in the data collection sheets or in the data notebooks. (Significance Level C)
The Internal Events PRA model update in 2009 included revising failure data for MSPI pumps and diesels, accounting for the latest plant-specific failure data. The 2009 calculation identified the data to be collected and the sources for the data. The remaining issue is related to updating the documentation and does not affect NFPA-805.
RAI Not Needed Calculations have been performed per industry guidance. Documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
DA-04 Multiple Greek Letter (MGL) parameters from Table 5-11 of NUREG/CR-5485 were used to obtain CCFs for some components in the CNP PRA There are limitations on using such data, which I&M did not appear to observe At least one of the reviewers felt that the significance of this observation should be B, given the rationale above. AEP PRA personnel indicated that:
they understood the individual issues identified; that the issues regarding assumptions were primarily documentation rather than results impacts; and that the issue of which generic data source is appropriate is generic and, to some extent, a matter of preference, and that they are comfortable with the approach they have used. The significance C has been assigned on the The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains specific modeling changes to common cause modeling and the rationale for those changes related to three suggested resolutions.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 12 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) basis of this understanding, but AEP is encouraged to consider the suggested resolutions noted below. (Significance Level C)
DA-05 Plant specific screening of generic common cause failure data is insufficient and component boundaries defined for common cause modeling have not been verified.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that plant specificity was incorporated into the modeling for common cause terms having a Fussell Vessely importance measure greater than 1%. The licensee also confirmed that the data compilation was performed consistent with the component boundaries and explained common cause modeling related to
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 13 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) specific components.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
DA-06 The 4160 and 600VAC Electric System Notebook lists the common cause failure group for the Emergency Diesel Generators (EDGs) as being the Unit 1 EDGs, however, all four EDGs belong to a single common cause group. (Significance Level D)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that common cause failure groups were established per unit since EDGs are not shared and cannot be cross-tied.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
DA-07 The data notebook does not indicate that a systematic approach was used to identify plant specific common cause failure groups. (Significance Level C)
The Internal Events PRA model update in 2008 included revising CCF groups and data. This effort systematically redefined the CCF groups in a manner consistent with industry guidance. The remaining issue from this F&O is related to updating the notebook to discuss/reference the methodologies employed. This remaining documentation issue does not affect NFPA-805.
RAI Not Needed Calculations have been performed per industry guidance. Documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
DA-08 The degree of conservatism in the "unique unavailabilities" is not in all cases either best estimate known to be The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 14 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) conservative, but this is judged not to have a significant effect on the results at this time.
(Significance Level C)
Category II.
explains that the probability of PORV challenge following other plant transients is based on generic data updated with plant specific data and considers that the approach is reasonable given sparse data.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
DA-09 Data notebook was not signed by the reviewer indicated on its cover of notebook, so it is not clear the document has actually been reviewed by the reviewer, and, the document has not been approved.
(Significance Level C)
The data notebook has been updated twice (once completely, Rev. 3 and once partially, Rev. 3 CS-1) and additional supporting calculations have been completed since this F&O was written. All documents have been signed in accordance with applicable processes. No further disposition required for Internal Events PRA model. This F&O does not affect NFPA-805.
RAI Not Needed The documentation was updated.
Based on this explanation, the NRC staff finds this deficiency has been appropriately resolved by the licensee DA-C1 In many cases there is no description for the basic events or the description lacks sufficient detail to understand what the basic event is modeling.
This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI submitted as RAI 21(a) 1 In response to RAI 21(c) the licensee explains that cited information exists not in a single listing but several controlled documents.
Given the additional information, the NRC staff finds the disposition of the
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 15 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
F&O to be acceptable.
DA-C1 Provide a clear description of the boundaries that make up each basic event.
Such a description is provided only for the EDGs and is not provided consistently for other basic events.
The data collection did consider established boundaries but did not include references to the boundary definitions that were used.
This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed The licensee explains that data collection did consider established boundaries but documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
DA-C1 There is inconsistency in describing dominant basic events in the systems analyses. For example, the CCW system note book describes an important contributors to system unavailability as heat exchanger rupture but the basic event modeled is tube plugging This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI submitted as RAI 21(c) 1 In response to RAI 21(c) the licensee explains that the PRA system notebooks were revised subsequent to the F&O finding.
Given the additional information the NRC staff finds this deficiency has been appropriately resolved by the licensee.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 16 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
DA-D4-01 A Bayesian update process was used to combine industry-generic and plant-specific data for pumps and emergency diesel generators. No documentation or discussions with the utility demonstrated that the posterior distributions were checked for reasonableness. (Finding)
The Bayesian updated data have been reviewed and appear to be reasonable.
This issue impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed The licensee explains that Bayesian updated data have been reviewed for reasonableness.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
DA-D6-01 Review of the CCF Notebook shows that EDG CCFs for failure to start and failure to run were calculated s separately from the associated room cooling fans. These CCF values are based on NUREG/CR-5497 generic CCF parameters that include the EDG room fans within the boundary of the EDG. The explicit modeling of the EDG room cooling fans may lead to additional conservatism of EDG sub-components.
(Suggestion)
The numerical impact of this F&O on the Internal Events PRA results is judged very small. Since these failures are independent of fire initiators, there is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution occurs in both the before and after results).
RAI submitted as RAI 21(c) 1 In response to RAI 21(c) the licensee explains the decision to model EDH cooling fans separately from the EDG and that the modeling is conservative.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
DA-E2-01 Error factor (characterization of uncertainty) discrepancies were identified between supporting calculation reports.
(Finding)
This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI submitted as RAI 21(c) 1 In response to RAI 21(c) the licensee explains that discrepancy was a documentation error and further explains that the uncertainty analysis for the fire PRA is, in any event, performed separately.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 17 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
DA-E2-01 There is evidence to suggest that established component boundaries were considered in the data collection process, but the component boundary definitions that were used were not stated or referenced.
Component boundary definitions should be developed and included or cited as reference.
(Finding)
The data collection did consider established boundaries but did not include references to the boundary definitions that were used.
This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed The licensee explains that the data collection effort did consider established boundaries but did not reference the definitions used. The documentation remains to be improved.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
DA-E3-01 The sources of key uncertainty associated with the data analysis were not clearly documented. (Suggestion)
The numerical impact of this F&O on the Internal Events PRA results was judged very small. This issue will be separately addressed for the Fire PRA model developed in support of NFPA-805.
RAI Not Needed The licensee explains that documentation of key sources of uncertainty was performed for the fire PRA.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
DE-01 A procedure which is less systematic than NUREG/CR-4780 was used to develop the component groups for common cause, The disposition for Internal Events PRA model Significance Level B F&O DE-02 also addresses this F&O, so this Supporting RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that common cause failure
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 18 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) however, the groupings used are reasonable.
(Significance Level C)
Requirement was also brought up to at least Capability Category II.
(CCF) was modeled according to industry guidance and points out that CCF approach was reviewed in the 2009 focused scope peer review Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
DE-02 CCF groupings appear to be inconsistent with respect to whether running and standby components are grouped together. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that common cause failure (CCF) was modeled according to industry guidance and points out that CCF approach was reviewed in the 2009 focused scope peer review Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
HR-01 Cause-Based Decision Tree Method (CBDTM) was used for quantification of the post accident cognitive errors, but use of CBDTM alone may result in conservative and non plant specific Human Error Probability (HEP) values which may shadow actual important risk contributors. (Significance Level C)
This issue relates specifically to the Internal Events PRA model. Operator actions analyzed for Fire-response are performed in accordance with Fire PRA guidance.
Accordingly, there is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution from this issue occurs in both the before and after results).
RAI Not Needed Modeling used in the fire PRA is conservative.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 19 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
HR-02 Miscalibration of instruments was not addressed. This may result in underestimation of actual risk since miscalibration can affect multiple trains like a common cause failure. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that modeling pre-initiator human errors, including miscalibration errors, is included in the licensees Fault Tree Modeling Guidelines.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
HR-03 Use of bounding HEPs was identified.
Use of bounding HEPs will produce unrealistic results and may shadow actual dominant contributors to the risk.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that all operator actions were re-evaluated for use in the fire PRA and were peer reviewed for the fire PRA.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
HR-04 Table 1 in the Human Reliability Analysis (HRA) notebook is confusing and there should be some additional explanation provided about what the Table is presenting, to aid in understanding how the information is tied to the PRA model and results.
(Significance Level C)
This F&O was explicitly addressed by a subsequent HRA-related calculation. This is a documentation issue with no numerical effect on the Internal Events PRA model.
As a result, there is no effect of this F&O on NFPA-805.
RAI Not Needed This is a documentation issue.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 20 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
HR-C1 F&O #SY-9 stated that in part, In a couple of places in the SI and AFW system models, hardware failures are not explicitly modeled because they are dominated by operator action failures. The thrust of this F&O was on the AFW system as related to the modeling of CST refill. However, there was no apparent disposition of the SI model for combining a human action with hardware failures into a single basic event. These single HEP basic events can sometimes mask importance/success of the related hardware.
No examples of basic events that combined an operator action and hardware (or ignored hardware) in the Internal Events PRA SI system model were provided in the F&O, nor could any be identified. However, it is noted that in the PRA model prior to the 2003 F&O response update, RCP trip failure following loss of service water initiators only included HEPs. In this case, appropriate hardware failures were added into the fault tree as part of the 2003 F&O model update. Per the preceding SR discussion, the Fire PRA has addressed the CST portion of this SR, so the effect of this SR is implicit in NFPA-805 results.
RAI Not Needed The licensee explains that it could find no cases of basic events that combined operator actions and hardware but that in a previous version of the PRA that F&O had been made.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
HR-G6-01 There is no evidence that DC Cook checked the internal consistency of the HEPs either for the three Human Failure Events (HFEs) covered by this review or their complete set of HFEs, although there is no appearance of inconsistency. (Finding)
A complete review of the HEPs and a HEP dependency analysis have been performed for earlier versions of the Internal Events PRA model. These documents were not included in the list of references made available to the Peer Review team. This issue impacts the Internal Events PRA model documentation. This issue does not affect NFPA-change evaluations.
RAI Not Needed The licensee explains that a complete review of the HEPS and HEP dependency was performed for the internal events PRA. The response for RAI 211 provides further explanation that all HEPS were reevaluated for the fire PRA.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 21 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
IE-01 The sum of the mean values for the lognormal distributions that describe the generic single and dual unit loss of offsite power frequency derived are in the range of other generic estimates but their variance appears lower. (Significance Level C)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that the error factors were updated to consistent with generic estimates and the source of each error factor was added to initiating events notebook. The licensee also points out that initiating events error factors are not germane to the fire PRA.
Given the additional information the NRC staff finds this deficiency has been appropriately resolved by the licensee.
IE-02 There are several cases where an initiating event category with more severe impact is subsumed by another category with less severe impact. This may have a significant effect on the calculated CDF.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that in response to this Level B finding initiating event categories were reviewed and revised. The licensee also points out that internal event initiators were not used in the fire PRA.
Given the additional information the NRC staff finds this deficiency has been appropriately resolved by the licensee.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 22 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
IE-03 It appears that all initiator categories are accounted for, but a cross reference mapping categories to an acceptable source would ensure completeness. (Significance Level C)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that documentation was updated to include cross reference.
The licensee also points out that internal event initiators were not used in the fire PRA.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
IE-04 Although the initiators retained for system losses that lead to initiating events seem reasonable, the structured process used to obtain these results was not discussed in the model documentation.
(Significance Level C)
This issue relates specifically to the Internal Events PRA model. There is no effect of this F&O on NFPA-805 cha evaluations (i.e., the same contribution from this issue occurs in both the before and after results).
RAI Not Needed Documentation issue Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
IE-05 A single value is used for the probability of pressurizer Power Operated Relief valve (PORV) or safety valve challenge following all transients, without any other considerations for how the probability might depend on the transient. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that the probability of a PORV challenge were determined in accordance with methodology used in NUREG/CR for Sequoah and determined separate values following LOSP and loss of DC power.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 23 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
IE-07 The Loss of Direct Current (DC) initiating event was quantified by fault tree analysis and obtains results that are not comparable result to other plants.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that tin response to this Level B finding specific modeling changes were made.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
IE-C1 A good justification is lacking for why earlier years of operational experience are excluded (207 trips) in the plant trip initiation frequency. Note, however, that these trips are accounted for when evaluating the PORV challenge frequency.
This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI submitted as RAI 21(c) 1 The licensee explains that internal events PRA model initiators were not used in the fire PRA.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
IF-B3-01 Flood source flow rates and capacities were determined without considering source temperatures. It also appears that High Energy Line Break (HELB) was excluded from the Internal Flood Analysis because it was performed The numerical impact of this F&O on the Internal Events PRA results is judged very small. Since these failures are independent of fire initiators, there is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution occurs in both RAI Not Needed Fire induced flooding cannot have a significant impact on the fire PRA.
Based on this explanation, the NRC staff finds the disposition of the F&O to
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 24 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) separately. (Suggestion) the before and after results).
be acceptable.
IF-C3-01 The Systems, Structures and Components (SSCs) affected by any given flood are listed in an Attachment of a PRA calculation. However, the list does not identify the specific flood-induced failure mechanism for these SSCs. (Suggestion)
This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed Fire induced flooding cannot have a significant impact on the fire PRA.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
L2-01 Editorial Comment: Section 3.0 reference to DG-1061 should be changed to RG-1.174. (Significance Level D)
This documentation issue has no numerical implications for the Internal Events PRA model. Accordingly, there is no effect of this F&O on NFPA-805.
RAI Not Needed The editorial correction remains to be corrected.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
L2-02 The impact of containment failure prior to core mode has been evaluated with respect to LERF, assuming that offsite protective actions would have directed evacuation of the surrounding population prior to core damage, and this may not be This issue relates specifically to the Internal Events PRA model. There is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution from this issue occurs in both the before and after results).
RAI submitted as RAI 21(c) 1 In response to RAI 21(c) the licensee explains the assumption is related only Large LOCAs and Large LOCA is not a contributor to fire risk.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 25 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) reasonable for large LOCA initiated sequences. (Significance Level C)
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
L2-03 At CNP, actions to depressurize the Reactor Coolant System (RCS), e.g., open PORVs, at the onset of core damage have been moved from the Emergency Operating Procedures (EOPs) to the Severe Accident Mitigatino Guidelines (SAMGs), which may impact the generic containment failure probabilities in the simplified LERF model, since they implicitly include the likelihood of such action. (Significance Level C)
This issue identifies that the Internal Events PRA model conservatively determines LERF because no credit is taken for opening PORVs (whether directed by EOPs or SAMGs) to lower RCS pressure after the onset of core damage. Although this approach is consistent with the guidance provided for the simplified LERF model used, the CC for this sub-element is I. This over-estimate of LERF implies that NFPA-805 change evaluations will obtain more limiting delta-LERF values than a more realistic model achieving a CC II.
RAI Not Needed The model is conservative as no credit is taken for depressurizing RCS.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
L2-04 The LERF calculation omitted containment isolation failure from the LERF equation based on its low probability, but neglected the contribution due to "pre-existing failures." (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that in response to this Level B finding the containment isolation notebook was updated to address the cited omissions.
Given the additional information, the
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 26 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
NRC staff finds this deficiency has been appropriately resolved by the licensee.
L2-05 Generic split fractions from the NUREG/CR-6595 simplified LERF model are assumed to apply without any specific evaluation to support the assumption.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee justifies continued use of the split fractions from NUREG/CR 6595 based on similarity of design features between the Sequoyah and DC Cook plants.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
LE-A1, LE-B1, LE-C3, LE-C4, LE-D1, LE-E1 Use of NUREG/CR 6595 for containment modeling is considered conservative and is considered by RG-1.200 to be Capability Category I.
This SR implies that the LERF model is also conservative for the Fire PRA. The numerical effect of such conservatism would be to increase the predicted LERF for fire initiators. This conservatism would increase the fire risk estimated for NFPA-805 change evaluations based on LERF.
RAI Not Needed The modeling used in the fire PRA is conservative.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 27 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
MU-01 Draft PRA Maintenance and Update Procedure could be improved by focusing the list of criteria on key items with specific actions or directions for analysts.
(Significance Level C)
The PRA Maintenance and Update Procedure was finalized. Specific direction for calculation preparers and reviewers to use guidance in the applicable standards is included. This procedure revision was performed to satisfy an F&O from t Fire PRA Peer Review. Accordingly, there is no effect of this F&O on NFPA-805.
RAI Not Needed The PRA Procedure was updated in response to F&O.
Given this information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
MU-02 There is not currently a formal procedure or process for the control of the PRA models. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains their formal process and procedures involved for maintaining configuration control over the PRA per the recommendation in this Level B finding.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
QU-02 A house event structure for Loss of DC has been incorrectly developed in fault trees developed for Component Cooling Water (CCW) and possibly Essential Service Water (ESW). (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains the fault tree modeling has been reviewed and modified per the recommendation in this Level B finding.
Given the additional information, the
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 28 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
NRC staff finds this deficiency has been appropriately resolved by the licensee.
QU-03 The calculation of ISLOCA initiating event frequency does not consider correlated failures of valves in series, i.e., RHR discharge and suction (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that it adapted methodology from NUREG/CR 5744 to calculate ISLOCAs to consider correlated failures of valves in series to address this Level B finding.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
QU-04 Some loss of CCW and ESW sequences involving tripping the RCPs, depressurizing the RCS, and restoring CCW or ESW, appear to be overly optimistic.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee describes changes in models and documentation to explicitly address this Level B finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 29 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
QU-05 Multiple frequency truncation limits are used in the quantification for different events.
There is evidence that the model results tend to converge as the truncation is lowered, however, demonstration of results convergence could be enhanced.
(Significance Level C)
Adequate convergence is demonstrated by comparison of the latest Internal Events PRA model results for cases with truncation levels at least 5 and 7 orders of magnitude below event probability (e.g.,. CDF increase
< 5%). The remaining issue from this F&O is related to documentation and does not affect NFPA-805.
RAI Not Needed The licensee states that truncation was established using the convergence method but that documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
QU-06 A formal search performed for unique or unusual sources of uncertainty not present in the typical or generic plant analysis is not sufficiently documented. (Significance Level C)
The numerical impact of this F&O on the Internal Events PRA results was judged very small. This issue will be separately addressed for the Fire PRA model developed in support of NFPA-805.
RAI Not Needed Staff sees that sources of uncertainty were examined in the fire PRA Uncertainty and Sensitivity Analysis report (PRA-17663-014-LAR).
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
QU-07 There are inconsistencies in the HRA modeling of dependency between RCS cool-down and depressurization and transfer to recirculation. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that all operator actions were re-evaluated for use in the fire PRA.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 30 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
QU-F5 I&M should consider documenting the process used for independent review of computer code results as part of the PRA general guidance document.
This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed Formalization of computer code validation not likely to lead to PRA results impact.
Also, staff sees that PRA-RAI-002 Rev 0 Attachment 6C (PRA Update Study Report) shows QU-F6 met at CC II (Conditional).
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SC-A2 Explain the potential impact on the model and associated success criteria with regard to the conservative nature of MAAP analyses which assume a core damage temperature threshold of 1400F.
The impact of this core damage criterion is limited to a small number of scenarios, i.e.,
those for which the inventory makeup success criterion results in turning around the core temperature excursion after core temperature exceeds 140 F but before it reaches 2200 F. The numerical effect of such a conservatism may be a larger CDF than the assumption of the higher core damage threshold. Since thi conservatism depends on inventory make-up requirements, the estimated Fire CDF may also be larger. This would ten to increase the conservatism in NFPA-805 change RAI submitted as RAI 21(b) 1 In response to RAI 21(b) the licensee explains their MAAP run strategy and explains how it is conservative.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 31 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) evaluations since more degraded support states would benefit from the higher core damage threshold.
ST-02 Erroneous screening criteria used for Internal Flooding (IF) Analysis causes under-prediction of IF Risk. (Significance Level A)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that the approach to internal flooding is only related to internal events, and that the modeling does not affect fire PRA risk.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 32 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
SY-01 The calculation for Unfavorable Exposure Time (UET) for Anticipated Transient Without Scram (ATWS) pressure relief generally follows the approach defined in WCAP-11992, however, the calculated UETs are expressed as fraction of cycle without adjustment for use in annual CDF calculations, as specified in the WCAP.
(Significance Level C)
This issue relates specifically to the Internal Events PRA model. There is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution from this issue occurs in both the before and after results).
RAI submitted as RAI 21(c) 1 In response to RAI 21(c) the licensee explains that there are no fire-induced ATWS sequences in the fire PRA.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
SY-02 Assumptions about PORV block availability in the PORV logic models appear to be overly-conservative and not well supported, while the basis for the air bottle capacity appears to be inadequately documented. (Significance Level C)
The numerical impact of this F&O on the Internal Events PRA results is judged very small if PORV block valve availability is increased. The overly-conservative failure probability implies that fire scenarios that induce other PORV failure are more likely to proceed to core damage. This over-estimate of PORV failure implies that NFPA-805 change evaluations will be obtain more limiting delta-CDF values than a more realistic model.
RAI Not Needed The licensee explains how the modeling is conservative.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
SY-03 Editorial Comments for AFW Notebook Assumptions. (Significance Level D)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that the editorial correction cited in this Level D suggestion was incorporated.
Given the additional information, the
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 33 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-04 In the SI system notebook, the success criteria for the different combinations of pumps required for success are listed without specifying the scenarios to which they apply. (Significance Level C)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that the SI system notebook was enhanced to provide information requested by this Level C suggestion.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-05 Diversion flow paths that adversely affect success criteria or timing of events may have been eliminated without sufficient justification. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) April 27, 2012 In response to RAI 21(a) the licensee explains that modeling guidance was developed and implemented that include flow diversion path modeling based on this Level B finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 34 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
SY-06 Although the high pressure injection fault tree includes individual failures of two series valves whose closure prevents flow diversion, a small portion of the failure of this action is missed because the common cause factor is neglected. (Significance Level C)
The Internal Events PRA model update in 2008 included revising CCF groups and data. This effort added the identified CCF group and basic events to the model. This issue does not affect NFPA-805.
RAI Not Needed The PRA model was revised to model this common cause.
Based on this explanation, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-07 Although the PRA modeling approach for systems with a normally running pump on one train is reasonable, it is not explained in the SI Notebook where it is used for CVCS pumps. (Significance Level D)
This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed This is a documentation issue.
Documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
SY-08 Although the model correctly accounts for potential ESW and CTS fouling but the documentation could be improved.
(Significance Level D)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that description of how heat exchanger plugging was modeled was added.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 35 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
SY-09 In a couple of system models, component hardware failures are not modeled explicitly because they are dominated by operator action failures.
(Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that a firmer basis for the operator timing was developed and incorporated, and documentation improved to match for this Level B finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-10 There seems to be inconsistency between the success criteria and the implementation in the model for the pressure relief success criteria for ATWS. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that ATWS pressure relief modeling was redone to consider the appropriate success criteria for each unit to address for this Level B finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-11 Passive failure modeling in the loss of service water system initiating event logic is inadequate. (Significance Level A)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that modeling guidance was developed and implemented to passive
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 36 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) failures in both the initiator and system response modeling to address this Level A finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-12 Common cause failure modeling for 250V DC battery chargers was not included in the model. (Significance Level C)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that common cause failure of battery chargers was added to the DC power supply modeling to address for this Level C finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-13 The documentation provides a reasonable basis for performing the system analysis and, in general, maintains consistency with proven approaches, but does not include a discussion of the potential for initiating events due to system faults, a discussion of spatial dependencies, or a table of both support systems and the effect on the component of the loss of the support system.
This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed This is a documentation issue.
Documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 37 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
(Significance Level C)
SY-14 The only guidance available for systems analyses is the guidance from the Individual Plant Examination (IPE).
(Significance Level C)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that fault tree modeling guidance was developed and implemented to address this Level C finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-15 The system notebook for the EDGs states that the capacity of the EDG day tanks will supply the EDGs for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, but the notebook does not explicitly identify that fuel oil is included in the fail-to-run probability.
(Significance Level D)
This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed This is a documentation issue.
Documentation remains to be updated.
Given the information, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 38 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
SY-16 In the PORV fault tree for automatic operation, the hardware required for pressure sensing and signal generation is not modeled.
(Significance Level C)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that pressure sensors required to open the pressurizer PORVs have been added to all system models to address this Level C finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-17 Cross-tie for AFW from Unit 2 does not consider the need for AFW at Unit 2, so the fault trees presume both motor driven pumps at Unit 2 are available for supply to unit 1 in the event the three pumps at Unit 1 fail.
(Significance Level C)
The numerical impact of this F&O on the Internal Events PRA results is judged very small. Since no fire consequentially causes the other unit's AFW failure, there is [no]
effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution occurs in both the before and after results).
RAI Not Needed The licensee explains that the assumption cited in the F&O produces only a small impact to to fire risk.
Based on this the NRC staff finds the disposition to the F&O to be acceptable.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
SY-18 The fault trees have limited modeling of passive failures, functional failures and subtle interactions. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that modeling guidance was developed and implemented to that explicitly address the several issues
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 39 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) cited in this Level B finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-19 Recovery for ESW and CCW does not consider the cause of failure; NSAC-161 recovery factors are applied to all system failures evenly. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that the ESW and CCW modeling was modified to specifically address the differing recovery probabilities to address this Level B finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
SY-20 The model correctly includes sump strainer plugging, but the documentation is incorrect. (Significance Level D)
This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed This is a documentation issue.
Documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 40 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
SY-A15-01 The SDG model does not include pre-existing maintenance errors. Section 5.2 of the SDG notebook discusses pre-existing maintenance errors and the basis for excluding them. While the exclusion bases are appropriate, it is suggested that a tie to the screening criteria employed be included in the text. (Suggestion)
This issue only impacts the Internal Events PRA model documentation. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed This is a documentation issue.
Documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
SY-A19-01 The Supplemental Diesel Generator (SDG) notebook does not identify the environmental conditions such as excessive heat loads, excessive electrical loads or excessive humidity that might lead to SDG system failure. (Finding)
The SDG design did consider environmental factors since the SDGs are a backup for the EDGs. This is a documentation issue that has no numerical impact on the Internal Events PRA model.
This issue does not affect NFPA-805 change evaluations.
RAI Not Needed The licensee explains that it did consider environmental factors, but documentation remains to be updated.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
TH-01 The medium LOCA event tree analysis assumes that upon failure of AFW, feed and bleed is required, but success of AFW should not be required and feed and bleed should be unnecessary. (Significance Level C)
The numerical impact of this F&O on the Internal Events PRA results is judged very small. Since no fire consequentially causes a Medium LOCA event, there is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution occurs in both the before and after results).
RAI Not Needed The licensee explains that the assumption cited in the F&O produces only a small impact to on fire risk since no fire causes a consequential Medium LOCA.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 41 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
TH-02 Modular Accident Analysis Program (MAAP) runs evaluating containment overpressure timing for large LOCA obtained results that seemed overly conservative. The success criteria documentation would be improved by providing hard copies of some additional output (e.g., plots) of plant specific analyses (MAAP runs, etc.); this would also improve the ability to evaluate not only the correctness of the conclusion, but also the reasonableness of the overall results.
(Significance Level C)
This issue relates specifically to the Internal Events PRA model. Since no fire consequentially causes a Large LOCA event, there is no effect of this F&O on NFPA-805 change evaluations (i.e., the same contribution occurs in both the before and after results).
RAI Not Needed Large LOCA is not a contributor to fire risk.
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.
TH-03 There appears to be an inconsistency between the current event trees and the success criteria notebook text. (Significance Level D)
This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI submitted as RAI 21(c) 1 In response to RAI 21(c) the licensee explains that the events trees and fault trees do implement the correct success criteria but the notebook needs updating.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
TH-04 This is a suggestion for making the bases for Heating Ventilation Air Conditioning (HVAC) dependency modeling decisions clearer and more traceable. (Significance Level C)
The fire PRA has performed separate evaluations for room heat-up and need for HVAC. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed HVAC success and dependencies are separately evaluated for the fire PRA.
Based on the explanation, the NRC
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 42 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) staff finds the disposition of the F&O to be acceptable.
TH-05 The Containment Equalization (CEQ) fans are not included in the hydrogen control evaluation for LERF estimation. (Significance Level C)
The numerical impact of this F&O on the Internal Events PRA results is judged very small, since the overall failure probability of both trains of the CEQ fans is ~5E-4 and would increase the LERF conditional probabilities by less than 1% for non-SBO and non-bypass scenarios. For fire-induced scenarios, the possibility of consequential failure increases the CEQ fan failure probability. Assuming that at least one CEQ fan train is available following any fire initiator, the conditional LERF probability would increase by ~0.01, or which is roughly 10% to 20%. Since fire initiators do not consequentially cause SBO or containment bypass scenarios, this increase in fire-related LERF is small relative to the overall LERF value. As a result, the effect of this F&O should not significantly affect NFPA-805 change evaluations.
RAI Not Needed In the response to RAIs dated April 27, 2012, the licensee explains for this F&O that CEQ fans are not credited and that this represents conservative modeling.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
TH-06 The basic success criteria based on MAAP analyses were developed in the 1991/1992 time frame. (Significance Level B)
The disposition for the Internal Events PRA model brought the affected Supporting Requirement up to at least Capability Category II.
RAI submitted as RAI 21(a) 1 In response to RAI 21(a) the licensee explains that modeling guidance was developed and implemented to passive
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 43 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response) failures in both the initiator and system response modeling to address this Level A finding.
Given the additional information, the NRC staff finds this deficiency has been appropriately resolved by the licensee.
TH-08 Unable to establish basis for the time available to actuate bleed and feed for transients without steam conversion.
(Significance Level C)
The success criteria verification notebook explicitly states that the scenarios analyzed for feed and bleed timing represented a loss of main feedwater at the sequence initiation. The event timing provided in this notebook show that the feed and bleed HRA is based on a very conservative time estimate for transients without steam conversion. The success criteria verification notebook was prepared subsequent to the HRA to which the F&O refers. This discussion shows that the F&O has no effect on the Internal Events PRA model, although the referenced HRA modeling is conservative. This issue does not affect NFPA-805 change evaluations.
RAI submitted as RAI 21(b) 1 In response to RAI 21(b) the licensee explains that 92 new MAAP runs were made for the fire PRA to investigate timing given loss of AFW including time available for initiating for feed and bleed.
Given the additional information, the NRC staff finds the disposition of the F&O to be acceptable.
TH-09 There is a reference in the Event Tree notebook to NUREG-1335, and consideration should be given to changing this to a more current reference such as RG 1.174.
This is a documentation issue that has no numerical impact on the Internal Events PRA model. This issue does not affect NFPA-805 change evaluations.
RAI Not Needed This is a documentation issue.
Documentation remains to be updated.
Record of Review D.C. Cook LAR Attachment U - Table U-1 Internal Events PRA Peer Review - Facts and Observations (F&Os) 44 Facts and Observations (F&Os)
Licensee Disposition NRC Staff Findings (Note 1: from April 27, 2012 RAI Response)
(Significance Level C)
Based on this explanation, the NRC staff finds the disposition of the F&O to be acceptable.