ML053470284

Initiative 5b LAR RAI 1 Response

ML053470284
Person / Time
Site:	Limerick
Issue date:	12/12/2005
From:	Cowan P Exelon Generation Co, Exelon Nuclear
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML053470284 (12)

Text

10 CFR 50.90 December 12.2005 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Limerick Generating Station, Units 1 and 2 Facility Operating License Nos. NPF-39 and NPF-85 NRC Docket Nos. 50-352 and 50-353

Subject:

Response to Request for Additional Information License Amendment Request - Proposed Technical Specifications Change to Relocate Surveillance Test Intervals to a Licensee-Controlled Program (Risk-Informed Initiative 5b)

References:

(1) Letter from M. P. Gallagher, Exelon Generation Company, LLC, to U. S.

Nuclear Regulatory Commission, dated June 11, 2004 (2) Letter from T. R. Tjader, U. S. Nuclear Regulatory Commission, to B. Bradley, Nuclear Energy Institute, and M. P. Gallagher, Exelon Nuclear, dated April 12, 2005.

In Reference 1, Exelon Generation Company, LLC (Exelon), requested a change to the Technical Specifications (TS), Appendix A, of Facility Operating License Nos. NPF-39 and NPF-85 for Limerick Generating Station (LGS), Units 1 and 2, respectively. The proposed change relocates the surveillance test intervals (STls) of various TS surveillance requirements from the TS to a new licensee program, the Surveillance Frequency Control Program, which is being added to the Administrative Controls section of TS. This license amendment request (LAR) was submitted as a pilot in support of the Boiling Water Reactor Owners' Group (BWROG) Risk-Informed Initiative 5b, "Relocate Surveillance Test Intervals to Licensee Control."

In Reference 2, the NRC requested additional information concerning the LGS LAR (Enclosure 1 to the Reference 2 letter) and the Nuclear Energy Institute (NEI) proposed process methodology document NEI 04-10 (Enclosure 2 to the Reference 2 letter). Exelon has been working with various organizations, including the NRC, BWROG and NEI, through meetings and conference calls over the past several months to further refine the Risk-Informed Initiative 5b methodology and develop the responses to the NRC questions. The attachment to this letter restates the NRC questions concerning the LGS LAR and provides Exelon's response to each question. The response to the methodology document questions will be submitted by NEI under a separate cover letter.

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR December 12,2005 Page 2 Exeion has concluded that the information provided in this response does not impact the conclusions of the: (1) Technical Analysis, (2) No Significant Hazards Consideration under the standards set forth in 10 CFR 50.92(c), or (3) Environmental Consideration as provided in the original submittal (Reference 1).

There are no regulatory commitments contained within this letter; however, as indicated in the response to Question 10 in the Attachment to this letter, a summary of the results of a quantitative assessment for an additional STI example will be provided in a supplemental response once the assessment is completed. The supplement will be provided by February 28, 2006.

If you have any questions or require additional information, please contact Glenn Stewart at 61 0-765-5529.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 12'h day of December 2005.

Respectfully, Pamela B. Cbwan Director - Licensing & Regulatory Affairs Exelon Generation Company, LLC

Attachment:

Response to Request for Additional Information cc:

Regional Administrator - NRC Region I NRC Senior Resident Inspector - Limerick Generating Station NRC Project Manager, NRR - Limerick Generating Station NRC Project Manager - BWROG NRC Project Manager - RlTS Task Force Director, Bureau of Radiation Protection - Pennsylvania Department of Environmental Protection wlattac h m en t s I1 11 I1 I1

Page 1 of 10 ATTACHMENT Limerick Generating Station, Units 1 and 2 Docket Nos. 50-352 and 50-353 Proposed Technical Specifications Change to Relocate Surveillance Test Intervals to a Licensee-Controlled Program (Risk-Informed Initiative 5b)

Response to Request for Additional Information In Reference 1, Exelon Generation Company, LLC (Exelon), requested a change to the Technical Specifications (TS), Appendix A, of Facility Operating License Nos. NPF-39 and NPF-85 for Limerick Generating Station (LGS), Units 1 and 2, respectively. The proposed change relocates the surveillance test intervals (STls) of various TS surveillance requirements from the TS to a new licensee program, the Surveillance Frequency Control Program, which is being added to the Administrative Controls section of TS. This license amendment request (LAR) was submitted as a pilot in support of the Boiling Water Reactor Owners' Group (BWROG) Risk-Informed Initiative 5b. "Relocate Surveillance Test Intervals to Licensee Control."

In Reference 2, the NRC requested additional information concerning the LGS LAR (Enclosure 1 to the Reference 2 letter). Each NRC question is restated below followed by our response.

Question 1.

An important element of the proposed risk-informed Surveillance Test Interval (STI) revision process is the categorization of structures, systems and components (SSCs) into high safety significance (HSS) and low safety significance (LSS). It is not clear whether Limerick proposes to use its Maintenance Rule SSC categorization or the categorization guidance developed for 10 CFR 50.69 (NEI-00-04) or a combination of both. For example, on page 4 of Attachment 4 it is stated: "Since the LGS Maintenance Rule Expert Panel has already classified this system as risk significant, compliance with NEI 00-04 is achieved. The MRule HSS classification is retained as permitted by the Initiative 5b methodology, 'I From the review of the Initiative 5b methodology, the staff understands that the Maintenance Rule classification will be retained o for SSCs classified as HSS.

Please clarify.

Resoonse In response to the RAls, the methodology is being modified to not differentiate between high safety significance components (HSSCs) and low safety significance components (LSSCs) any more. The revised methodology thus addresses the NRC concern stated in the RAI. Limerick will follow this revised process when issued.

Question 2.

It is stated (page 1 of Attachment 1) "...changes will be evaluated in accordance with the licensee-controlled program, and the STls may be revised as appropriate, based on the evaluation results without prior NRC approval. "This statement may be interpreted to imply that the NRC approved methodology for changing surveillance testing intervals may be changed by the licensees, which is not the intent of Initiative 5b. Therefore, this statement needs to be reworded to clarify that STI changes will be evaluated in accordance with the NRC approved process and methodology.

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 2 of 10 ResDonse The full statement from page 1 of Attachment 1 referred to above clearly states that future changes to STls will be evaluated in accordance with the licensee-controlled program, and that STIs may be revised without prior NRC approval. There is no wording in this sentence, or in, to suggest that licensees can make changes to the NRC-approved methodology without prior NRC approval. In fact, previously, on page 1 of Attachment 1, we make the statement that revisions to the STls will be made in accordance with the Surveillance Frequency Control Program which is being added to the Administrative Section of TS and is predicated on use of the NRC-approved methodology. Specifically, as indicated on the TS markup pages 6-14d provided in Attachments 2 and 3 of Reference 1, the Surveillance Frequency Control Program being added to the Administrative Section of TS requires: "Changes to the Frequencies listed in the Surveillance Frequency Control Program shall be made in accordance with NEI 04-1 0, "Risk Informed Method for Control of Surveillance Frequencies." As a result, there is no need to provide revised wording for page 1 of Attachment 1 of Reference 1.

Question 3.

It is stated (page 1 of Attachment 1): "various TS surveillance requirements, including in some cases their associated STls, were established based on commitments to Regulatory Guides, or based on implementation of NRC-approved Licensing Topical Reports. Within the licensee-controlled program, the surveillance requirements themselves will not be changed........; however, associated STls may be modified in accordance with the licensee-controlled program. "The staff agrees that licensees should consider the intent of commitments and evaluate the impact of any changes in commitments on overall plant safety in accordance with the NRC approved process. However, no guidance is included into the current version of the proposed "Methodology for Implementing a Surveillance Frequency Control Program" to ensure that the impact of changing commitments will be fully captured. Such guidance is needed because licensee commitments are made to address a variety of deterministic and probabilistic issues, such as defense-in-depth, safety margins, uncertainties in PRA assumptions and lack of detailed modeling or analysis. Please discuss.

Response

Section 4.1, Steps 1 through 4 of the Initiative 5b methodology document, NEI 04-10, provide the steps for identifying commitments and evaluating the effect of changing commitments on overall plant safety. In particular, Step 3, "Change the Commitment," states:

"In Step 3, change the commitment using NRC-approved process such that the Frequency can be revised using the SFCP process. Changing the NRC commitment is a separate activity. Return to the SFCP process after the commitment has been changed. If the NRC does not permit the commitment change, go to Step 4,...'I Step 3 directs the use of a separate NRC-approved process for evaluating the commitment changes. In Regulatory Issue Summary 2000-1 7, "Managing Regulatory Commitments Made by Power Reactor Licensees to the NRC Staff," the NRC endorsed the industry guideline NEI 99-04, "Guideline for Managing NRC Commitment Changes," as an acceptable method for managing and changing regulatory commitments. Rather than spell out specific detailed steps for reviewing and evaluating commitment changes within the methodology document, Step 3 will continue to direct the use of an NRC accepted process for making changes to commitments.

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 3 of 10 However, this step will be revised to make specific reference to the industry guidelines for making commitment changes.

In addition, since Step 2 is the step where the determination of whether or not making the commitment change is acceptable to the NRC is performed, Step 2 will be revised to reference NEI 99-04 as well. Both Steps 2 and 3 will be revised as indicated below.

Step 2: Can Commitments be Changed?

In Step 2, a check is made to determine if the NRC commitments can be changed.

Evaluating changes to NRC commitments is a separate activity based on a method acceptable to the NRC for managing and changing regulatory commitments, e.g., NEI 99-

04. If the commitments can be changed without prior NRC approval, go to Step 3 for changing the commitments. If the commitments cannot be changed without prior NRC approval, go to Step 4.

Step 3: Change the Commitments In Step 3, change the commitments using a method acceptable to the NRC, e.g., NEI 99-04, such that the STI can be revised using the SFCP process. Return to the SFCP process after the commitments have been changed and continue the SFCP process with Steps 5 and 6.

Question 4.

It is stated (page 2 of Attachment 4):

' I...the cumulative impact of all risk-informed STI revisions on all PRAs (i.e., internal events, external events and shutdown) is compared to the risk acceptance criteria in Regulatory Guide 7.774. 'I According to this statement CDF and LERF increases will be assessed (from both internal and external events at power and during shutdown operation) for all STI changes (i.e., STI changes associated with both HSS and LSS SSCs). Please verify. The staff believes this statement must be included in the proposed "Methodology for Implementing a Surveillance Frequency Control Program."

Response

The statement 'I...the cumulative impact of all risk-informed STI revisions on all PRAs (i.e.,

internal events, external events and shutdown) is compared to the risk acceptance criteria in Regulatory Guide 1.174." is consistent with Step 19 of the Draft Rev. 1 version of NEI 04-10, dated December 2004. All quantitative results of the sample STI changes were summed up appropriately and compared to the limits of Regulatory Guide (RG) 1.I 74.

NEI 04-1 0 is being revised to clarify the comparison of the realistic PRA results and the bounding risk analysis results to their respective acceptance criteria as well as to clarify the consideration of the qualitative results by the IDP. Limerick will follow the revised guidance when issued.

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 4 of 10 Question 5.

It is stated (page 2 of Attachment 4): For those cases where the STl cannot be modeled in the PRA, a bounding analysis is performed to provide some indication of the impact on the PRA results. Bounding analyses are either quantitative carried out with available PRA models or qualitative using deterministic considerations. Please explain how the results of bounding analyses indicating a significant impact on the PRA results will be incorporated in the decision-making process. Will the results of bounding analyses be combined with the results of best estimate analyses? Also, please explain how qualitative analyses using deterministic considerations will be considered in the decision-making process.

ResDonse NEI 04-10 is being revised to clarify the comparison of the realistic PRA results and the bounding risk analysis results to their respective acceptance criteria, as well as to clarify the consideration of the qualitative results by the IDP. In the updated NEI 04-10 methodology, the results of the quantitative bounding analysis will be combined with best-estimate analysis unless the results of the bounding analysis are less than 1 E-7 CDF and 1 E-8 LERF. Results of the qualitative analysis would be reviewed and sent to the IDP for its consideration. Limerick will follow the revised guidance when issued.

Question 6.

The criteria used for selecting examples of candidate STI extensions are listed on page 3 of Attachment 4. It is stated: The selection of STl candidates in these categories was intended to provide the opportunity to exercise all possible legs of the risk-informed methodology. The staff agrees with the stated goal of exercising the risk-informed methodology. However, a review of the selected examples indicates that such a goal was not achieved. None of these examples go through the important steps of quantifying the risk impact associated with the proposed STI extension, such as Step 18 (Evaluate Cumulative Effect on CDF and LERF), Step 16 (Perform Bounding Risk Analysis) and Step 21 (Perform Sensitivity Studies). Please provide one or more examples that exercise these important steps. Carefully selected examples could provide helpful insights that would help improve the proposed Methodology for Implementing a Surveillance Frequency Control Program. Since these steps focus on the quantitative aspects of the methodology, IDP participation may not be necessary. Please discuss.

Response

Step 18 of the Draft Rev. 1 version of NEI 04-10, dated December 2004, was exercised by example STls 3 through 6, which analyzed for internal events CDF and LERF and tracked the cumulative total. Step 16 of the Draft Rev. 1 version of NEI 04-10 was exercised in the control rod notch STI example (STI #l). The individual control rods are not modeled in the Limerick PRA and could not easily or practically be modeled. The bounding analysis performed used a global basic event representing mechanical failure of the control rods to insert into the reactor core. This is the failure being looked for in the surveillance requirement. Since this event represented failure to insert sufficient rods to bring the core subcritical, modifying this event bounded the surveillance requirement being evaluated.

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 5 of 10 The cumulative effect on CDF and LERF and sensitivity study evaluations were performed for STls 3 through 6 which were analyzed quantitatively. Results of the sensitivity studies were presented to the IDP for consideration of the STI assessment.

An additional candidate STI change will be developed that more fully exercises the determination of the cumulative effects on CDF and LERF and considers the impact on the results from additional sensitivity studies. See the response to Question 10 below.

Question 7.

Example STIM (Control Rod Drive Exercise Test) is categorized as HSS but no additional performance monitoring is proposed above what is required by the Maintenance Rule (i.e.,

to monitor the number of unplanned inoperable rods occurring in 24-month periods).

Also, it is concluded that no phased implementation is necessary. Please explain how the basis for such a conclusion supports the assumption made in the risk analysis that no new common cause failure mechanisms, in addition to the failure of the mechanical portion of the reactor protection system, are likely due to the STI extension.

Response

No phased implementation was deemed necessary because the performance history showed that the equipment was unlikely to have issues at the new frequency. Given that there is no change in the normal operation, maintenance or environmental conditions for the SSCs and the interval extension (one week to one month) is not large, it was considered unlikely that a new common cause failure mechanism would occur.

Given that the Maintenance Rule performance criteria specifically monitors the attribute that would indicate that the increased surveillance interval was allowing additional failures, the IDP determined that the Maintenance Rule performance criteria was sufficient since it directly addressed the identification of a change in failures for the components impacted by the STI change. If an increase in the number of unplanned inoperable rods did occur after the implementation of the revised STI that warranted an (a)(l) characterization of the system under the Maintenance Rule, the most likely corrective action would be to revert to the original STl.

This could be handled under the Maintenance Rule, but would also require re-evaluation as part of the periodic reassessments performed by the IDP (Refer to the discussion in Steps 19 and 20 of the revised NEI 04-1 0 methodology).

Question 8.

A major objective of the pilot application of Initiative 5b at Limerick is to provide feedback input to the methodology document (NEI-04-10). Please list important insights or lessons learned from this pilot application that have or should have been included in the methodology document.

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 6 of 10

Response

The important insights or lessons learned from the pilot application at Limerick are provided below. The items involving a revision to the methodology document are also annotated.

1.

2.

3.

4.

5.

A step was added in the methodology to review each proposed STl for impact against existing station commitments and pursue changes where appropriate, including evaluation of NRC related commitments per NEI 99-04. Limerick identified the need for this step during evaluation of the control rod exercise test (STI #1) that referenced a vendor recommendation. This insight has been incorporated into the methodology document.

The methodology is risk-informed rather than risk-based. The PRA (quantitative) input is only part of the process. In fact, the insights gained during site Integrated Decision-making Panel (IDP) meetings associated with the pilot clearly yielded that qualitative aspects were far more significant for each STI. Qualitative factors included equipment performance history, surveillance test history, vendor recommendations, 1 OCFR50.65 Maintenance Rule program reliability and unavailability data and performance indicator trends, plant-specific and industry operating experience, and the existence of alternate testing. The cumulative quantitative risk impact of all six pilot STls was minimal (CDF increase of 2E-9 and LERF increase of 1 E-1 1). This insight is characterized as an observation. Therefore, a revision to the methodology document was not necessary.

Risk categorization of each STI using the NEI 00-04 process is only one aspect of the Risk-Informed Initiative 5b pilot but would entail a significant resource effort by itself. The risk categorization process is more rigorous and resource intensive than the approved site Maintenance Rule process. Limerick and industry personnel noted that few U.S. nuclear plants have incorporated the 1 OCFR50.69 risk categorization process. Therefore, Limerick retained all site Maintenance Rule risk categorizations and evaluated the lone STI characterized as containing low safety significance components by using PRA quantitative analysis which was reserved for HSSC systems per the methodology guideline. In effect, all systems in the pilot were treated and analyzed using the HSSC protocol. Based on Limerick and industry feedback, and as noted in the response to Question 1 above, the methodology guideline will be revised to eliminate the risk categorization aspect.

Limerick used screening criteria for the selection of the six test case STls to evaluate in the pilot process. Each STI considered for optimization required a tangible benefit or savings.

The benefits considered included nuclear safety aspects such as minimization of reactivity management events and plant transients. The benefits considered also included radiation dose savings, burden reduction (i.e., resources), outage duration savings, simplification of work management coordination for on-line work, reduction of production risk affecting both load drops and plant trips, and improvement of component reliability by reducing number of demands. This insight is being incorporated into the methodology document. A sample STI evaluation form containing a line entry for benefits will be added to the methodology.

Limerick confirmed that, just as with the Maintenance Rule Expert Panel, the IDP complemented the PRA results associated with each STI. PRA does not model all plant SSCs that are included in surveillance requirements. Also, as the Maintenance Rule process shows, PRA is not the only input considered. The IDP composition and expertise ensured that equipment performance issues, maintenance practices, operational experience and impacts, surveillance test history and insights, and component-specific insights were factored into the STI review orocess. The IDP moved to be a verv valuable comoonent of

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 7 of 10 the STI pilot process. This was not unexpected based on the similar panel utilized in the station Maintenance Rule program. This insight is characterized as an observation.

Therefore, a revision to the methodology document was not necessary.

6. Compliance to RG 1.200 is a component of the STI evaluation process and is becoming a pre-condition for any future risk-informed license submittal relying on PRA. Limerick is one of five licensees piloting RG 1.200. The Regulatory Guide requires comparison of the PRA to ASME PRA Standard RA-S-2002 (and addenda). This addition requires significant PRA resources. Limerick was well positioned for RG 1.200 usage since Limerick personnel had already performed a gap analysis against the ASME PRA Standard. A subsequent NRC RG 1.200 assessment team visited Limerick primarily to assess the Regulatory Guide effectiveness but also assessed Limerick's conformance to the Regulatory Guide. This insight is characterized as an observation. Therefore, a revision to the methodology document was not necessary.

7. PRA models will require a revision to facilitate STI evaluation if a standby failure rate term does not already exist in the PRA. Prior to this pilot, the Limerick PRA did not contain any standby failure rate terms. The Limerick PRA was revised to include standby failure rate terms, as appropriate, for the test case STls. Many other licensees will have a similar situation with their station PRA. This task also requires risk management resources for the PRA model revision. Limerick plans to revise the PRA as needed as each STI is evaluated.

This insight is characterized as an observation. Therefore, a revision to the methodology document was not necessary.

8. The Limerick IDP identified one STI test case in which equipment reliability history did not support a proposed STI change at this time. This was Limerick STI test case #4, the Loss of Coolant Accident (LOCA) - Loss of Offsite Power (LOOP) surveillance test. The PRA analysis supported the proposed STI change from two years to four years. However, the surveillance test actuates certain relays that are only cycled in this surveillance test. These specific relays have a potential contact resistance issue due to build up of an oxidation layer that is mitigated by cycling the contacts every two years. This issue is addressed in the station preventive maintenance program. Thus, extending the interval beyond two years will challenge the reliability of the relays and success rate of the surveillance test. The IDP evaluated this issue and disapproved the STI change pending further reliability analysis or alternate testing. This insight confirms our observations from Item Nos. 2 and 5 above that conclude the STI methodology relies primarily on qualitative aspects and IDP review, and that the PRA input, although necessary to risk-inform the process, is only one contributing component to the process. This insight is characterized as an observation. Therefore, a revision to the methodology document was not necessary.

9. Limerick recognized that, although no single STI nor the cumulative impact of all six test case STls challenged the RG 1.I 74 limits specified in the methodology document, the potential exists that one single high risk impact STI could consume a majority of the margin allowed in RG 1.1.74. Limerick did not pre-screen STls for this effect. However, the benefit of consuming a significant portion of this regulatory margin would have to be balanced against the forfeiture of multiple other proposed STI optimizations. From our experience, the likelihood of a licensee implementing an STI change that consumes the bulk of the RG 1.1 74 margin is very low. More benefit is achieved by having the capability to optimize multiple STls. This insight is characterized as an observation. Therefore, a revision to the methodology document was not necessary.

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 8 of 10

10. Limerick pilot personnel, including IDP members, noted that alternate testing of SSCs affected by an STI adjustment is an important element in the STI evaluation process. If SSCs affected are tested as often or more frequently than in the current STI through other methods or tests, then supporting the proposed interval change from a qualitative and reliability perspective is more easily justified. However, when the only test that exercises the affected SSCs evaluated for extension is the candidate STI itself, then the evaluation process requires more rigor and technical bases to justify the STI adjustment. This insight is characterized as an observation and is already included in the current methodology document. Therefore, a revision to the methodology document was not necessary.

11. Limerick pilot personnel contributed to various other revisions of the methodology document during its evolution up to and including NEI 04-10, Revised Draft 1, issued in December 2004. These changes included a wholesale redesign of the SFCP process flow chart, the placement of key steps in the process and supporting text for each step. These changes simplified the process and clarified the purpose and requirements for each step. Limerick continues to work with industry personnel on the pending revision of the methodology document as cited in this response as well as the parallel response by NEI for RAls associated with the NEI 04-1 0 methodology. The methodology document has been revised to incorporate Limerick feedback associated with these insights.

Question 9.

The staff expects the final methodology document (NEI-04-10) to include guidance on acceptable ways of integrating the impact of external events (e.g., internal fires, internal floods and seismic) and events occurring during plant shutdown, in the decision-making process. The staff also expects the final methodology document to include minimum quality expectations of risk assessments for external events and shutdown operation.

Attributes for available information, approaches and tools (e.g., screening analyses, approximate event trees and risk insights) should be identified and discussed. Please discuss Limerick's capability to integrate safely the impact of external events and shutdown operation in the decision-making process. Since this issue has not been finalized yet in the methodology document, Limerick can provide useful input to this document. The staff expects Limerick to demonstrate its capability to integrate safely the impact of external events and shutdown operation in the decision-making process in accordance with the guidance that will be included in an NRC approved final methodology document.

Resfsonse As demonstrated in the response to Question 8 above, Limerick has actively participated in the improvement of the NEI 04-1 0 methodology. The revised methodology has been significantly enhanced to provide explicit guidance on integration of the impact of external and shutdown events. Limerick will follow the final revised guidance, relating to the integration of the impact of external events and shutdown events in the decision-making process, when issued.

Any PRA used quantitatively for the purpose of changing surveillance frequencies would have to meet appropriate available quality requirements just as was done for the internal events PRA in of our submittal.

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 9of 10 Question 10.

A precondition for implementing Initiative 5b by a licensee is to demonstrate the technical adequacy of the base PRA model that will be used to assess risk changes associated with STI extensions. Limerick states (page 3 of Attachment 5) that the technical adequacy of their base PRA model is, in general, based on Capability Category II of the ASME PRA Standard (endorsed by RG 1.200) for all ASME Supporting Requirements (SRs). On page 3 of Attachment 5 it is also stated that in cases where SRs are identified as less than Capability Category II, the PRA technical adequacy for the application is shown to have little or no impact on the calculated results and the decision making. It is stated that the impact assessment is addressed by one or more of the following techniques:

(1) model change, (2) sensitivity calculations, or (3) bounding risk-informed arguments.

The staff agrees that this process is consistent with RG 1.200. However, this process was implemented only for the six example STI assessments, performed so far, which do not go through the important steps of quantifying the risk impact associated with the proposed STI extension. Thus, the objective of demonstrating PRA technical adequacy for Initiative 5b in cases where SRs are identified as less than Capability Category II was not completely achieved. Please discuss actions that would provide additional confidence in Limericks PRA technical adequacy to support Initiative 5b. Actions that the staff believe would provide such confidence are listed below.

- Perform additional risk assessments for carefully selected STI extension examples (as discussed also in RAI #6) to provide helpful insights regarding SRs that are identified as less than Capability Category II. In these examples, a broader scope of PRA gap analysis findings and key sources of uncertainty than those associated with the examples performed so far should be addressed.

- Implement planned PRA upgrades related to documentation, identification of key model uncertainties, data and human reliability analysis, treatment of repair and completeness of initiating events.

- Perform an independent review of the PRA upgrades upon completion.

Response

As previously indicated in response to Question 6, certain of the example STI changes were quantitatively assessed for risk impact.

The evaluation against RG 1.200 and the ASME PRA standard looked at the entirety of the Level 1 and Level 2 internal events PRA against the Capability Category II supporting requirements. Where the PRA model did not fully meet the intent of Capability Category II for a SR, a gap was identified. These gaps were evaluated for this application in two fashions. First, identifying whether any of the gaps were applicable to all surveillance frequency changes. This would include items such as broad use of generic rather than plant-specific component failure rates. These were evaluated for impact on the results, focusing on sensitivity evaluations such as varying the failure rates upward and downward and reviewing the impact on the numeric results. Second, identifying any of the gaps that were applicable to specific surveillance requirements, such as lack of modeling of the diesel generator initiation logic affecting the surveillance requirement to verify that the diesel generator starts in response to the LOCA or LOOP initiation signal. These were generally addressed by modifying the logic of the model for

Response to Request for Additional Information Limerick Risk-Informed Initiative 5b Pilot LAR Attachment Page 10 of 10 that surveillance interval change. Any change like this was carried forward in analysis of additional surveillance interval changes.

Key areas of uncertainty were also identified and evaluated for impact on either a specific interval change or globally on the complete process in Attachment 5 of our original submittal (Ref. 1). It is intended that this process continue in future interval evaluations.

Limerick will select and evaluate one additional STl example for interval adjustment on a high risk system that should challenge the limits of RG 1.174. Preliminarily, we have selected the High Pressure Coolant Injection (HPCI) system due to the overall high risk importance of this system based on its Risk Achievement Worth (RAW) in the Limerick PRA. The specific STI to be evaluated is the HPCl Pump Valve and Flow test that implements a quarterly surveillance requirement. This evaluation is a hypothetical example since it would require relief from the American Society of Mechanical Engineers (ASME)Section XI requirements for quarterly pump and valve testing. Nonetheless, this STI will be evaluated but the assessment will be limited to a PRA quantitative perspective only. No qualitative aspects, such as those cited in the response to Question 8, insight number 2, will be assessed. Also, per our discussions with the NRC during a public meeting addressing these RAls on June 3, 2005, no IDP review will be performed as this example is limited to a quantitative assessment only. The analysis will include the information requested, including PRA gap analysis findings relative to ASME Capability Category II and also key sources of uncertainty. The results of this quantitative assessment will follow the revised methodology to consider total and cumulative impacts from all risk contributors for the HPCl STI adjustment, and will be summarized in a supplement to this response once the assessment is completed. The supplement will be provided by February 28, 2006.

Though not relevant to this submittal which fully addressed PRA quality, the status of items identified in Question 10 are described below. An update of the PRA model has been completed since the submittal of the original license amendment request (Ref. 1). The update included adoption of plant-specific failure rates for significant components, update of common cause failure to the most current data set, update of HRA events to current methodologies, re-evaluation of use of repair terms in the model and further expansion of initiating events in the scope of the model. Due to the broad scope of changes to the model, Exelon had a peer review performed on the revised model during the week of October 10-14, 2005. Although no findings are expected to require an immediate update to the model, the final report is pending. The results of the review (i.e., 'B' findings and instances where Capability Category II of the ASME Standard are not met) will be used as input to the SFCP process during the performance of sensitivity studies as delineated in the revised NEI 04-10 methodology.

References:

1.

2.

Letter from M. P. Gallagher, Exelon Generation Company, LLC, to U. S. Nuclear Regulatory Commission, dated June 11, 2004 Letter from T. R. Tjader, U. S. Nuclear Regulatory Commission, to B. Bradley, Nuclear Energy Institute, and M. P. Gallagher, Exelon Nuclear, dated April 12, 2005.