ML20236W427
| ML20236W427 | |
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|---|---|
| Issue date: | 07/31/1998 |
| From: | NRC (Affiliation Not Assigned) |
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| NUDOCS 9808050269 | |
| Download: ML20236W427 (7) | |
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p g"g*, UNITED STATES s* j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 2006dk 0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION APPROVAL OF WCAP-14333P (PROPRIETARY) AND WCAP-14334NP (NON-PROPRIETARY).
"PROBABILISTIC RISK ANALYSIS OF THE RPS AND ESFAS TEST TIMES AND COMPLETION TIMES"
1.0 INTRODUCTION
By letter dated June 20,1995, the Rochester Gas and Electric (RG&E) Company, as the lead plant, submitted proposed Technical Spec?ication (TS) changes for the Ginna Nuclear Power Plant, based on the generic Westinghouse Owners Group (WOG) topical report WCAP-14333P (Reference 1). In the submittal, the WOG proposed the following changes to the TS: I
- 1. I Increase the test bypass times and the a,1 owed outage times (AOTs) for both the solid j state protection system and reactor protection system (RPS) and engineered safety featured actuation system (ESFAS) designs:
For analog channels, increare the AOTs from 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, and the test bypass time from 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> to 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. For logic cabinets, master relays and slave relays, increase the AOTs from 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
- 2. Revise action statement for an inoperable slave relay:
Current TS with the Technical Specification Optimization Program (TOP) changes implemented require a plant shutdown if the slave relay is not returned to service after a 6-hour AOT for maintenance plus 6 additional hours for the plant operating mode change.
WCAP-14333P proposes two changes:
(i) The AOT is increased to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> for maintenance plus 6 additional hours for the mode chcnge, l
(ii) After the slave relay AOT has expired, the component affected by the inoperable slave relay should be declared inoperable, and the TS Action for this component should be followed. This change applies to (a) slave relays wMse failure affects the actuation of a single component only, and (b) multiple slave relays or slave relays that actuate multiple componen's, providing that the affected components are all in ,
the same train of a single spiem. To implement this, the high head, intermediate i head, and low head subsystems of the emergency core cooling system (ECCG) must each be considered a single system.
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- 3. For the case where logic cabinets and trip breakers both cause their train to be inoperable when in test or maintenance, reactor trip breakers could be bypassed for a ,
( period of time equivalent to the bypass time for the logic cabinets provided both are i tested at the same time.
P 9800050269 980731 Enclosure PDR TOPRP EMVWEST C PDR l
2 WCAP-14333P states that these improvements will allow additional time to perform tests and maintenance, enhance safety, provide additional operational flexibility, and reduce the potential for forced outages related to complying with the TS for the RPS and ESFAS instrumentation.
Following its initial review of the topical report WCAP-14333P, the staff, by letter dated June 28, 1996 (Reference 2), requested additionalinformation. TP,e WOG responded by letter dated December 20,1996 (Reference 3). In addition, by letter dated September 27,1995, the WOG submitted Appendix A and B to the WCAP-14333P and WCAP-14334NP (Reference 4). Based
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on discussions with the staff on the proposed TS changes, the WOG withdrew the changes identified by item 2(ii) above and by letter dated February 24,1990, submitted the revised Appendix A to the WCAP-14333P and WCAP-14334NP (Reference 5). By letter dated March 30, 1998, the WOG submitted an errata page, A28, to Appendix A, Revision 1, to WCAP-14333P (Reference 6).
2.0 BACKGROUND
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l The NRC staff formed a Task Group in August 1983 to investigate problems and recommend improvements concoming surveillance testing required by TS. The results of the Task Group study were published in November 1983 (Reference 7) in NUREG-1024, " Technical Specifications - Enhancing the Safety impact." NUREG-1024 recommended that the staff (1) review the bases for TS test frequencies, (2) ensure that the TS required tests promote safety and do not degrade equipment, and (3) review surveillance tests to ensure that they do not l unnecessarily burden personnel.
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The Technical Specifications Improvement Program (TSIP) was established in December 1984 to provide the framework for addressing the NUREG-1024 recommendations, and for rewriting I and improving the Standard TS. As an element of the TSIP, TS surveillance requirements were comprehensively examined as recommended in NUREG-1024. The results of the TSIP effort are presented in NUREG-1366, " improvements to Technical Specifications Surveillance Requirements"(Reference 8). The TSIP study concluded that, while some testing at power is essential, safety can be improved, equipment degradation decreased, and unnecessary personnel burden prevented by reducing the amount of testing performed at power. The TSIP {
study formed the basis for the WOG requested TS relaxation in WCAP-14333P.
l 3.0 EVALUATION The staff used a three-tiered aparoach in its evaluation of the risk associated with the proposed TS changes in RPS and ESFAS test bypass times and AOTs. The review approach is consistent with the guidance in Draft Regulatory Guide DG-1065, "An Approach for Plant-Specific, Risk-Informed Decision Making: Technical Specifications" (Reference 9). The first tier evaluates the probabilistic risk analysis (PRA) model and the impact of the change on plant operational risk.
The second tier addresses the need to preclude potentially high risk configurations, should additional equipment outages occur during the proposed AOT period. ' The third tier evaluates the licensee's configuration risk managemeret program to ensure that equipment outage due to maintenance, testing, or random failure immediately pnor to or during the proposed AOT will be appropriately sSsessed from a risk perspective.
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3.1 Tier 1: PRA Capability and insiahts Westinghouse used traditional PRA methodology to evaluate the requested TS changes. The Tier-one staff review of the PRA involved two aspects: (1) evaluation of the PRA model and application to the proposed changes, and (2) evalustion of PRA results and insights stemming from the application.
(1) Evaluation of PRA Model and its Application to the AOT Extension The staff's review focused on the approach taken to develop a model and its capability to analyze the risk stemming from the proposed changes. The approach taken by Westinghouse included a survey of plant test and maintenance data io justify the proposed changes, an analysis of the reliability of the RPS and ESFAS actustion signals, and a risk analysis addressing the impact of the proposed changes. The plant survey showed that the typical times for test and maintenance j would remain less than those requested while the number of reactor trips is expected to be '
reduced as a result of the increased flexibility obtained through the proposed relaxation in AOT and test-bypass times. To evaluate the signal unavailability, Westinghouse selected representative signals which included safety injection < etuation, Auxiliary Feedwater (AFW) pump stari and reactor trip initiation. After selecting the representative signals, a fault tree analysis was performed to assess the impact of the proposed changes on the unavailability of these signals. Most of the fault trees used in this analysis came directly from the WCAP-10271 (Reference 10) analysb that had been previously reviewed and approved by the staff. Several additional fault trees were also developed for this analysis. The fault trees were quantified using a computer code, called WesSAGE, whose output provided the main probability of failure and the associated cutsets. The fault tree analysis indicated that the unavailabilties of the signals with 2/4 logic are lower than those with 2/3 logic, and common cause failure contributes significantly to the unavailabilities. However, the proposed TS changes have only a smallimpact on the overall system unavailabilities. Westinghouse used the Vogtle PRA as a representative model to quantify the overall plant risk. The signal unavailabilities developed from the fault tree analysis {
replaced the signal unavailabilities in the Vogtle PRA. The Vogtle PRA was then requantified with a computer code called WESQT to determine the risk associated with the proposed TS changes.
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' Westinghouse provided the impact of the proposed changes on the Level 1 and Level 2 risk.
This assessment also provided single AOT risks of the proposed changes in the form of incremental conditional core damage probabilities (ICCDPs). The staff performed a cross- ;
comparison of the impact on risk using the Surry NUREG-1150 model(Reference 11) and confirmed that the results are sirdlar. Major aspects of the PRA modeling used in the WCAP-14333P were also evaluated, including data, screening and truncation limits, level of detail, sensitivity and uncertainty. The staff found that these aspects of the PRA modeling to be reasonable. Westinghouse also included a reasonable common cauce failure analysis (Multiple Greek Letter method) to account for dependent failures which is one of the dominant contributors to the risk from instrumentation and control systems. Independent evaluations of the potential impact on the Level 2 and Level 3 risk were performed by the staff and ext 0 mal event risk was ;
also considered in the staff's evaluation.
4 (2) Evaluation of PRA Results and insights The results of the staff's review indicated that the increase in core damage frequency (CDF) from intemal events due to the proposed TS changes is small. The average CDFs are summarized as fallows:
Pre-TOP
- Proposed 2/3 Logic: 5.72x10-5/yr 5.83x10-5/yr 5.89x10-5/yr 2/4 Logic: 5.71x10-5/yr 5.80x104/yr 5.84x105/yr
(* TOP indicates that the plant has previously implemented the changes identified in WCAP-10271. Some licensees have not yet implemented the TOP changes.)
The above values correspond to a smallincrease in CDF of approximately 3.1% (from Pre-TOP) and 1.0% (from TOP) for 2/3 RPS and ESFAS logic and of 2.3% (from Pre-TOP) and 0.6% (from TOP) for 2/4 RPS and ESFAS logic. The increase in large early release frequency (LERF) is also small for both 2/3 logic and 2/4 logic. The average LERFs are summarized as follows:
Pre-TOP TOP Proposed 2.24x104/yr 2.31x104/yr 2.33x104/yr Compared with the pre-TOP and TOP cases, the LERF would increase only by 4% and 1%,
respectively, with the proposed changes in place.
The ICCDPs for each AOT have also been calculated and are considered small. For analog channels (test or maintenance), master relays (maintenance), and slave relays (maintenance),
the proposed relaxation in AOT results in a very smallICCDP (<1x104 ) for a single AOT. For logic cabinets (maintenance), the proposed AOT results in an ICCDP of about 4x104 for a single AOT.
In response to the staff request, Westinghouse performed sensitivity studies on several important parameters assumed in the risk analysis, i.e., human error probability, maintenance frequency, common cause failure probability, and anticipated transient without scram (ATWS) mitigating system actuation circuitry (AMSAC) availability, in order to examine the impact of the assumptions made for the risk analysis. The studies indicated that the impact of the variation in those parameters on plant risk is insignificant. The staff finds this determination to be reasonable.
The staff also considered the impact of the proposed TS changes on the risk from extemal events such as fire and earthquake events qualitatively, insights from the Suny NUREG-1150 study were included in this consideration. From its review, the staff finds that the proposed TS changes will hai a only a very small impact on the risk from extemal events, and are acceptable to the staff.
WCAP-14333P discusses the risk benefits of the proposed changes, mainly the averted risk from avoiding potential reactor trips. A number of reactor trips are known to have occuned during
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startup and shutdown and during instrumentation testing and maintenance. Westinghouse calculated that the averted risk associated with avoiding one reactor trip is comparable to the risk increase from the proposed changes. Westinghouse stated that the transition and shutdown risk can also be averted by avoiding potential reactor trips.
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The staff finds that the applicability of the Vogtle PRA analysis for this proposed TS change to other Westinghouse plants is not clear. This conclusion is based on the design variation in i actuated systems among Westinghouse plants and the variation of the contribution to the total I plant risk from the accident classes that are affected from the proposed TS changes.
Westinghouse indicated in its response to the staff RAI that using one plant as representative of all Westinghouse plants is appropriate due to the level of similarity in RPS and ESFAS design, function, and initic.ing event frequency across plants. While the staff acknowledges these similarities, it nevertheless believes that individual licensees referencing WCAP-14333P in license amendments need to confirm applicability of the PRA analysis to their plants. The staff expects each licensee to address any difference between their plant and the representative plant, Vogtle, that could potentially increase the AOT risk significance. Westinghouse is developing implementation guidelines to assist licensees in this effort. These guidelines are not part of the staff approval of WCAP-14333P discussed in this SER.
Based on the Tiwr 1 evaluation, the staff finds that the PRA model used for the proposed TS changes in WCAP-14333P is reasonable, and the risk ~ impact of the changes is small and supports the TS changes. Huwever, referencing licensees must confirm that the VogtW PRA analysis is applicable to their plant when they apply for a plant cpecific license amendment.
3.2 Tier 2: Avoidance of Risk-Significant Plant Confiouraticu Westinghouse conducted an importance analysis for 25 top events in the event irees for each of the test or maintenance configurations associated with the proposed TS changes. The results were compared with that of a base cece. For the cases of analog channels, master relays, and slave relays, the rank of importance arnong the systems involved did not change. In the case of logic cabinet in maintenance, however, several systems had a relatively significant increase in rank of importance, including auxiliary feedwater, reactor trip, high pressure injection, containment cooling, and low pressure injection. This provided system-levelinsights, but did not identify any specific components of importance whose simultaneous outages should be avoided.
Based on the staff's evaluation using the Surry NUREG-1150 model, however, the staff believes that concurrent removal from service of some equipment during the proposed relaxed AOTs may result in a risk significant configuration on a plant specific basis. For example, the configurational risks were evaluated for the following two cases: a train of RPS is unavailable and a train of ESFAS is unavailable. The results show for both cases that simultaneous outage of some equipment, i.e., AC circuit breakers, increased risk significantly (a conditional core damage 4
frequency greater than ir.10 /yr was calculated). Each licensee referencing WCAP-14333P must, therefore, examine the need for and place necessary restrictions on concurrent equipment outages when entering the proposed AOTs in order to avoid risk significant configurations.
3.3 Tier 3: Risk-Informed Plant Configuration Control and Manaoement The staff generally requests that licensees develop a reasonable configurational risk management program as part of the approval of risk-informed license amendment requests. The configuration risk management program is expected to be proceduralized and to include the
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capability to assess the risk associated with both planned :nd unplanned work activities. The staff expects future individual plant-specific submittals referencing WCAP-14333P to address the Tier 3 issue noted above.
4.0 CONCLUSION
The staff review of the proposed TS changes identified in WCAP-14333P for relaxation in RPS and ESFAS test bypass times and AOTs used a three-tiered approach, consistent with the guidance of DG-1065 in order to evaluate the risk associated with the proposed TS changes.
The staff review included severalindependent assessments using a different Westinghouse plant PRA model to verify the results in WCAP-14333P and to address areas in the PRA that were not completely considered in WCAP-14333P, i.e., extemal events and Levr,l 2 and 3 risks.
For Tier 1, the staff finds that the PRA model used in WCAP-14333P is generally valid and the risk impact of the proposed TS changes is small. The use of the Vogtle PRA as a representative model is generally reasonable, but a degree of uncertainty 6 introduced when the representative model and the associated results are applied to a spec?ic plant due to modeling, design, and operational differences. While the staff recognizes the t/ fort by Westinghouse to address the generic applicability of the representative PRA modl, thera are issues that rernain for resolution on a plant-specific basis. Therefore, each licensee referenceg WCAP-14333P in a plant specific license amendment request must confirm that the WCAP-14333P analysis and its results are applicable to their plant.
For Tiers 2 and 3, the staff finds that plant-specific information is needed because when entering the proposed AOTs potentially risk significant configurations should be avoided. Therefore, licensees are expected to confirm that the necessary restrictions will be placed on concurrent equipment outages in order to avoid risk significant configuration. Further, licensees are expected to describe the provision of their configuration risk management programs consistent with the guidance of DG-1065 for assessing risk associated with various planned and unplanned work activities.
In summary, the staff concludes that the risk analysis in WCAP-14333P supports the proposed TS changes and is acceptable, subject to the following conditions which must be addressed in referencing licensee's plant-specific license amendment requests:
(1) Confirm the applicability of the WCAP-14333P analyses for their plant.
(2) . Address the Tier 2 and Tier 3 analyses including the CRMP insights, by confirming that these insights are incorporated into the referencing licensee's % sion-making process before taking equipment out of service.
Principal Contributor: H. Garg Date: July 31, 1998 1
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5.0 REFERENCES
- 1. Westinghouse Topical Reports WCAP-14333P and WCAP-14334NP, "Probabilistic Risk Analysis of the RPS and ESFAS Test Times and Completion Times," dated May 1995, transmitted to NRC by Robert C. Meeredy (Rochester Gas & Electric Company for Ginna) letter dated June 20,1995.
- 2. Guy S. Vissing (NRC) letter to R. C. Mccredy (RG&E) dated June 28,19S6, " Westinghouse Owners' Group- Request for Additional Information, Probabilistic Risk Analysis of the RPS and ESFAS Test Times and Completion Times."
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- 3. N. J. Stringfellow (WOG) letter to USNRC, dated December 20,1996, " Response to Request for Additional Information (RAl) regarding WCAP-14333P, Entitled, 'Probabilistic Risk Analysia of the RPS and ESFAS Test Times and Completion Times.'"
& B to WCAP-14333P (Proprietary) and WCAP-14334NP (Non-Proprietary), Entitled,
'Probcbilistic Risk Analysis of the RPS and ESFAS Test Times and Completion Times.'" I S. Louis F. Liberatori Jr. (WOG) letter to USNRC, dated February 24,1998, " Transmittal of Appendix A , Revision 1, to WCAP-14333P and WCAP-14334NP, Entitled, 'Probabilistic Risk Analysis of the RPS and ESFAS Test Times and Completion Times.'"
- 6. Jack Stringfellow (WOG) letter to USNRC, dated March 30,1998, " Errata Page to Appendix A, Revision 1, to WCAP-14333P (Proprietary) and WCAP-1433NP (Non-Proprietary),
entitled, 'Probabilistic Risk Analysis of the RPS and ESFAS Test Times and Completion Times.'"
- 7. NUREG-1024, " Technical Specification - Enhancing the Safety impact," dated November 1983.
- 8. NUREG-1366, " Improvement to Technical Specification Surveillance Requirements," dated December 1992.
- 9. NRC Draft Regulatory Guide DG-1065, Revision 6, dated March 13,1997,"An Approach for Plant Specific, Risk-Informed Decision Making: Technical Specifications."
- 10. " Evaluation of Surveillance Frequencies had Out-of-Service Times for the Reactor Protection Instrumentation System," WCAP-10271-PA, May 1986.
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" Evaluation of Surveillance Frequencies and Out-of-Service Times for the Reactor l Protection Instrumentation System, Supplement 1," WCAP-10271, Supplement 1-PA, May 1986.
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" Evaluation of Surveillance Frequencies and Out-of-Service Times for the Engineered Safety l
Features Actuation System," WCAP-10271-P-A, Supplement 2, Revision 1. 1
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- 11. NUREG-1150,
- Severe Accident Risks: An Assessment for Five US Nuclear Power Plants,"
October 1990.
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