ML20198S141
| ML20198S141 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 12/29/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20198S136 | List: |
| References | |
| NUDOCS 9901110175 | |
| Download: ML20198S141 (8) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS. 62 AND 48 TO FACILITY OPERATING LICENSE NOS. NPF-87 AND NPF-89 TEXAS UTILITIES ELECTRIC COMPANY COMANCHE PEAK STEAM ELECTRIC STATION. UNITS 1 AND 2 DOCKET NOS. 50-445 AND 50-446
1.0 INTRODUCTION
By application dated August 2,1996 (TXX-96434), as supplemented by letters dated October 2, l
1998 (TXX 98215), and November 13,1998 (TXX-98241 and TXX-n '44), Texas Utilities Electric Company (TU Electric /the licensee) requested changes to the *: 3chnical Specifications (Appendix A to Facility Operating License Nos. NPF-87 and NPF-89) for the Comanche Peak Steam Electric Station (CPSES), Units 1 and 2. The proposed changes would increase the allowed outage time (AOT) for one centrifugal charging pump (CCP) declared inoperable while in MODES 1,2,3, or 4 from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 7 days. The affected TSs are 3/4.1.2.4," Charging Pumps - Operating," and 3/4.5.2, "ECCS Subsystems - T,9 > 350*F." The licensee is also adding a Configuration Risk Management Program to Administrative Controls section of TS 6.8.3 f. The licensee is requesting this change to accommodate potential replacement of the rotating element of an inoperable CCP. __ _ _
The October 2 and November 13,1998, letters provided additional information and changed the no significant hazards consideration material to include a Configuration Risk Management Program in TS 6.8.3 f and was beyond the scope of the original scope of the August 2,1996, application. The markup to the improved Technical Specifications (ITS) (Attachment 4 to TXX-l-
98241) is not addressed in the following safety evaluation because it was provided for information purposes, it will be addressed in connection with the CPSES ITS conversion dated May 15,1997 (TXX-97105).
2.0 EVALUATION The Comanche Peak charging system consists of two redundant centrifugal charging pumps and one reciprocating charging pump that operate as part of the Chemical and Volume Control
- System during ncrmal operation and as part of the Emergency Core Cooling System (ECCS) following a LOCA.
F During normal operation the charging system maintains the proper water inventory in the RCS, provides seal water flow to the RCP seals, and maintains water purity and boron concentration in the reactor coolant. In the event of a loss of coolant accident the CCPs act as components i
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2-of the Emergency Core Cooling System and deliver high pressure water to the reactor coolant system cold legs. Tne current Final Safety Analysis Report indicates that the Chapter 15 transients are analyzed assuming one pump in operation. These functions, both normal and emergency operation, can be met with the use of one CCP.
4 Currently the TS allows for one CCP to be inoperable in MODES 1,2,3,4 for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. The licensee is requesting to increase this AOT to seven days. This proposed AOT of 7 days is an extension of the current ACTION statement and has no impact on the current safety analysis.
Therefore, the safety analysis remains valid, and it is concluded that there is no difference in the deterministic safety sign lficance of a 72-hour AOT for the Comanche Peak CCP and a 7-day AOT. The difference in the current TS versus the proposed extension lies in the added risk due to the extension of the AOT which is reviewed in the following section of this evaluation.
2.1 Evaluation of the Probabilistic Risk Assessment (PRA) Used to Sucoort the Prooossd Comanche Peak Steam Electric Station (CPSES) Amendment The licensee requested an allowed outage time (AOT) extension from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 7 days for an inoperable centrifugal charging pump (CCP). The extended AOT would provide a sufficient time for performing most repairs on an inoperable CCP and could help avoid an unnecessary plant shutdown. Use of PRA insights constitutes the primary justification for the proposed change. The licensee's risk analysis indicates that the risk impact of the extended AOT for a l
CCP is small.
To gain risk insights, the staff used a three-tiered approach to evaluate the risk associated with the proposed amendment. The first tier evaluated the PRA model and the impact of the change on plant operational risk. The second tier addressed the need to preclude potentially high risk configurations if additional equipment will be taken out of service simultaneously or other risk significant operational factors such as concurrent system or equipment testing are involved.
The third tier evaluated the licensee's configuration risk management program, to ensure that equipment removed from service prior to or during the proposed AOT will be appropriately assessed from a risk perspective. Each tier and associated findings are discussed below.
' Tier 1: PRA Evaluation of AOT Extensions The Tier 1 staff review of the licensee's PRA involved two aspects: (1) evaluation of the PRA model and its application to the proposed AOT extension, and (2) evaluation of PRA results and insights stemming from the application.
(1)
Evaluation of PRA Model and Application to the AOT Extension.
The staff's review focused on the capability of the licensee's PRA model to analyze the risk stemming from the modified AOTs for CCPs. This activity, however, did not involve an in-depth review of the CPSES PRA to the extent necessary to validate the licensee's overall quantitative estimates. This was based on the staff's initial screening process which considered the information contained in the licensee's submittal, the Comanche Peak Individual Plant
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. Exarnination (IPE), the IPE for External Events (IPEEE) and the Staff EvaluMion Report (SER)'
on risk-informed inservice testing (RI-IST). Recent component reliability and availability experience and plant-specific features such as emergency core cooling system configurations were also considered. The staff subsequently determined that an in-depth review of the CPSES PRA would not be needed for this application.
The licensee's PRA used to support the proposed change is the same as the original IPE and IPEEE. The IPE has been reviewed by the Nuclear Regulatory Commission (NRC) Office of Research (RES) and RES staff has concluded that the CPSES IPE met the intent of Generic Letter 88-20. CPSES indicated that the IPE is adequate for this application due to the following considerations: (1) The IPE shows a relatively flat profile, meaning that the CDF is uniformly distributed by soquence type and initiating events. A significant change in plant design or operation would be necessary to significantly change this profile; (2) Plant changes and modifications have generally made the affected systems more reliable, and in some cases more redundant or diverse. For example, the new high temperature reactor coolant pump seals have been installed. CPSES indicates that the IPE was essentially revalidated to represent the as-built and as-operated plant prior to submitting the IPEEE in 1995. The licensee determined that there were no significant impacts on the models that would make the results non-conservative; (3) The updated plant-specific data, which has not been incorporated into the IPE model, shows that, in most cases, failure rates have decreased. Both internal and externalindependent reviews were conducted for the IPE and IPEEE process. A final independent review was performed after the IPE study was completed. The licensee intends to update the PRA periodically about every two refueling outages.
The recently completed NRC review of the licensee's RI-IST submittalincludec' a review of the CPSES PRA, which include analyses stemming from internal and external initiating events for both at-power and shutdown modes, used in support of the submittal. Though the focus of the PRA review was to examine whether the PRA was of sufficient quality, scope and level of detail for use in support of that particular application, much of the same focus was determined to remain applicable for this charging pump application. The RI IST SER concluded that the CPSES PRA was of sufficient quality, scope and level of detail to support the conclusions made in the IST submittal. Specifically, the IST PRA review found no significant deficiencies or shortcomings in areas such as initiating event types and frequencies, event trees and success criteria, data, common cause failures, and human reliability analysis.
The staff evaluated several areas in detail that are closely associated with the proposed charging pump AOT extension. For example, the success criteria and the reliability and unavailability data for CCPs were examined. The staff finds that the average unavailability of the Chemical and Volume Control (CVCS) trains for both units during last three years has been consistently lower than those used in the IPE Likewise, the CCP reliability has been high and exceeds the iPE data. For this period, the run failure rate was estimated to be 2E-6/hr (3.4E-6/hr in the IPE) and the start failure rate 0.0/ demand (3.3E 3/ demand in the IPE). The data used in the CPSES IPE was based on the PLG 500 Database.
' Letter.
- Approval of Risk-Informed Inservics Testing Program for CPsEs. units 1 and 2,* August 14,1998
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! A plant-specific data update was performed for the four-year period between 1991 to 1994.
The update showed that standby pump failure data used for IPE had been conservative. The CPSES PRA models the CVCS functions and their corresponding success criteria in the both accident sequence analysis and the system fault tree models. Review of both the system level success criteria and accident sequence level success critoria found that the CCPs, in general, were adequately modeled. The following summarizes the accident sequence level success criteria:
Feed and Bleed; One of two CCPs or one of two Safety injection Pumps (SIPS) and one of two Power Operated Relief Valves - Transient and small LOCA initiating events Safety injection; One of two CCPs or one of two SIPS - LOCA and Steam Generated Tube Rupture (SGTR) initiating events Recirculation; One of two CCPs or one of two SIPS and one of two residual heat i
remcval pumps - Transient, LOCA and SGTR initiating events Seal LOCA; Thermal barrier cooling by component cooling water or seat injection from CVCS - Non-LOCA Transient initiating event Long term Shutdown; Boration of the RCS using CVCS - Anticipated Transient Without Scram initiating event l
The supporting bases for these success criteria were based either on the design bases documents or on supporting calculations performed as part of the IPE study. The staff found that the success criteria and their bases are generally consistent with other IPEs. In addition, it is noted that the Safety injection system essentially works as a redundant system for the accident sequences that are associated with feed and bleed, safety injection and high pressure recirculation.
The licensee performed sensitivity studies to examine the robustness of the importance measures for the CVCS system for the various failure modes. The studies showed that the CDF at CPSES is relatively insensitive to changes in CCP unavailability or reliability. The CDF changed less than 0.1% when the test and maintenance unavailability was doubled and less than 0.5% for a doubled demand failure rate.
l The staff finds that the CPSES PRA used in support of the proposed change in CCP AOT l
extension is of sufficient quality, scope and level of detail for the application.
(2) Evaluation of PRA Results and Insights Associated with the Proposed Change The IPE and IPEEE estimated the CDF contributions to be about 5.7E-5/yr and 2.5E-5/yr, respectively from internal and external initiating events. Fire and tornado initiators were the main contributors to the IPEEE CDF.
Risk measures on which the staff evaluation is based include CDF change, incremental conditional core damage probability (ICCDP) for a single outage, large early release frequency (LERF) change, and incremental conditional large early release probability (ICLERP) for a l
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,... i single outage. To calculate the risk impact, CPSES increased the expected maintenance unavailability by the ratio of 2.59, based on the PLG Database correlation, which relates corrective maintenance duration to TS AOT. The test unavailability was increased by a factor of 2.33. Summarized below are the results of the calculations for those risk measures; Risk Measures Results ACDF 3E-8/yr ICCDP for a single outage 1E-6
-ALERF 9E-9/yr ICLERP for a single outage 2E-8 L
These results are based on both internal and external initiating events (fire and tomado), and L
both test and maintenance unavailabilities are included. Review of the risk measures I
considered shows that the risk impact of the proposed change is small. In particular, the changes in average CDF and LERF are estimated to be much below the staff criteria in RG 8
1.174. The single AOT risk for CPSES is about 1E-6 and is slightly in excess of the guideline 8
- value of SE-7 in RG 1.177 ; however, the result from extemal initiators, which constitute 50% of the total, has been included. This relatively large ICCDP, calculated with some conservatism, indicates that the instantaneous risk given a CCP out of service at CPSES is also relatively significant. The licensee should thus try to minimize the time in this configuration. The staff -
examined the licensee's response to the Tier 2 and Tier 3 guidance in this regard, and found that the licenses's Tier 2 and Tier 3 process could provide a reasonable means to accomplish
. the intent of the guidance. The licensee also provided a detailed discussion on the potential safety benefit stemming from being able to perform maintenance at power instead of having to -
shut the plant down to perform the maintenance, in several studies quoted in the licensee's RAI response, the averted risk from transition to shutdown was relatively significant as compared to the increased risk from performing the maintenance at power. The ICLERP result was still less than 5E 8. Therefore, the staff concludes that the risk impact of the proposed L
change is small and below the staff criteria and guidelines in RG 1.174 and 1.177.
4 Tier 2: Avoidance of Risk Significant Plant Configurations CPSES identified a list of components and systems whose simultaneous unavailability might place the plant in a high risk configuration, based partially on their Risk Achievement Worth value. These components and systems include the following:
Electric Power - Opposite train motive and control power Refueling Water Storage Tank - Tank and its associated discharge valves Service Water System - Opposite train Component Cooling Water System - Opposite train Emergency Diesel Generator - Opposite train L
ECCS Injection / Recirculation flow path valves 2RG 1.174, "An Approach for using Probabilistic Risk Assessment in Risk informed Decisions on Plant-specific Changes to the Ucensing Basis," July 1998 8RG 1.177. "An Approach for P'*,t-specific. Risk Informed Decisionmaking: Technscal specifications,' september 1998
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Electrical power - Same train motive and control power Service Water - Same train CVCS - Opposite train l
This list was identified when CPSES developed their risk matrix. The risk matrix and work l
control procedure control simultaneous maintenance activities on these systems and l
components. To avoid or reduce the potential for risk significant configurations from either emergent or planned work, CPSES has put in place a set of administrative guidelines which are l
consistent with the CRMP in Tier 3. The licensee has proposed to revise the Administrative Controls in the CPSES TS to include a CRMP in TS 6.8.3 f that is consistent with RG 1.177.
The licensee stated that these guidelines control configurational risk by assessing the risk impact of equipment outage during all modes of operation to assure that the plant is being operated within acceptable risk. The weekly online maintenance schedules are train based and prohibit the scheduling of opposite train activities without additional review, approvals, and/or compensatory measures. In addition, restrictions are placed on the number and combination of systems / trains allowed to be simultaneously unavailable for scheduled work. Therefore, the staff finds that the licensee adequately addresses the intent of the Tier 2 guidance.
Tier 3: Risk-Informed Plant Configuration Management Various procedures at CPSES embody programs that provide reasonable assurance that the risk impact of equipment outages is approprit.tely evaluated prior to and while performing any maintenance activity. The upper level procedure, Procedure No. STA-604," Work Scheduling,"
prescribes the methods and assigns responsibilities for scheduling of test and maintenance acCvPs during at-power operations and shutdown. The risk impact of on-line maintenance is required to be evaluated as prescribed by Instruction No. WCl-203, " Weekly Surveillance /
Work Scheduling." For risk assessment of online maintenance activities, risk categories were developed using a methodology consistent with the overall public safety goal and the available industry guidelines, such as the draft EPRI PSA Applications Guide. The risk categories were defined based on internal events CDF as the following:
Risk Category 1: High Risk Risk Category 2: High-Medium Risk Risk Category 3: Medium Risk Risk Category 4: Low Risk Scheduling of online maintenance activities that fallin Categories 1 and 2 are generally prohibited. A risk matrix is provided for the work schedulers with the means of determining the category into which a work activity would fall. CPSES plans to acquire and use a safety monitor, a computerized online risk monitoring software. In the future.
An instantaneous risk measure for a specific plant configuration is used to control the instantaneous risk level and the duration. This was accomplished by keeping the configuration-specific core damage probability, i.e., the instantaneous CDF times the duration of a specific configuration, less than 1E-6.
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, CPSES assesses risk when scheduling planned outage activities. Procedure No. STA-627, "
Control of Planned Outages," requires that an independent risk assessment of the outage
- schedule be performed. The Outage Risk Assessment and Management (ORAM) softwara tool is used to provide a means to evaluate the risk associated with planned outage activities.
The licensee has proposed to revise the Administrative Controls in the CPSES TS to include a
'CRMP that is consistent with RG 1.177. The proposed changes also revise appropriate BASES sections to reference the CRMP in TS 6.8.3 f.
1 The staff finds that the licensee has implemented a risk-informed CRMP to assess the risk associated with the removal of equipment from service prior to, or during, the proposed AOT.
Therefore, the staff concludes that CPSES has met the Tier 3 guidance in RG.1.177.
2.2 Imolementation and Monitorina The staff expects the licensee to implement these TS changes in accordance with the three-tiered approach described above. The licensee has also indicated that the maintenance scheduling practice and the tools used to implement a means of evaluating the impact of maintenance activities on plant configurations are consistent with the Maintenance Rule
_ (10 CFR 50.65). The AOT extension will allow efficient scheduling and performance of on-line maintenance within the boundaries established by implementing the Maintenance Rule. The licensee will monitor CCP/CVCS performance in relation to the Maintenance Rule performance criteria. Therefore, application of these implementation and monitoring strategies will help to ensure that an extension of TS CCP AOT does not degrade operational safety over time and that the risk expected when a CCP is taken out of service is minimized.
3.0 EVALUATION CONCLUSION The staff has reviewed the licensee submittal proposing to increase the AOT for one CCP.
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The staff concludes that because (1) there is no change to the Chapter 15 Safety Analysis, and (2) this is an extension of a condition for which the plant has already been analyzed, the
- deterministic aspect of this change is acceptable.
LThe CPSES PRA used in support of the proposed change in charging pump AOT extension is believed to be of sufficient quality, scope and level of detail for the proposed application. The staff did not identify any significant weaknesses or deficiencies associated with the licensee's risk analysis used to support the proposed change that could impact the overall quantitative conclusion. The results of the risk analysis indicate that the risk impact of the proposed change
. would be small. The licensee has put in place tools and guidance that provide a reasonable assurance to avoid potential risk significant configurations. The licensee has also implemented - --
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a risk-informed CRMP to assess the risk associated with the removal of equipment from service prior to or during the AOT, and to take appropriate measures in response to the outcome of the risk assessment. The staff concludes that the results and insights of the PRA analysis support the proposed charging pump AOT extension from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 7 days. Therefore, the staff concludes that the proposed TS changes to TS 3.1.2.4, TS 3.5.2, and TS 6.8.3 f are acceptable, and the Bases have been changed to reflect these TS changes.
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4.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Texas State official was notified of the proposed issuance of the amendments. The State official had no comments.
5.0 ENVIRONMENTAL CONSIDERATION
The amendments change a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20. The NPC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cum'ulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (63 FR 65617). The amendment also changes recordkeeping, reporting, or administrative procedures or requirements. Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9) and (10). Pursuant to 10 CFR 51.22(b) no environmentalimpact statement or environmental assessment need be prepared in connection with the issuance of the amendments.
6.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by
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operation in the proposed manner, (2) such actisities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
I Principal Contributors: 1. Jung, SPSB -
S. Brewer, SRXB Date: December 29, 1998 l
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