ML20195B960

Safety Evaluation Accepting Proposed long-term C/As for Assuring Operability of MSSVs

ML20195B960
Person / Time
Site:	South Texas
Issue date:	11/06/1998
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20195B692	List: ML20195B689 ML20195B960
References
NUDOCS 9811160331
Download: ML20195B960 (3)

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pa arg g t UNITED STATES s* j NUCLEAR REGULATORY COMMISSION WASHINGTON, FLC,906M#M

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION MAIN STEAM SAFETY VALVE CORRECTIVE ACTIONS l

STP NUCLEAR OPERATING COMPANY l l

SOUTH TEXAS PROJECT. UNITS 1 AND 2 (STP)

DOCKET NOS. 50-498 AND 50-499

1.0 BACKGROUND

The main steam safety valve (MSSV) setpoints have been found to be outside the required technical specification (TS) tolerance of +/-3% at STP and have been reported in Licensee Event Reports97-005 and 97-009 for Unit 1 and 97-001 for Unit 2. In a letter dated May 4, 1998, STP Nuclear Operating Company (the licensee) submitted information regarding the operability of the plant MSSVs and committed to submit a letter at least 90 days prior to the Unit 2 Fall 1998 refueling outage to detail the planned long-term corrective actions to prevent recurrence of MSSV inoperability. The May 4,1998, submittal contained commitments to inform the Nuclear Regulatory Commission (NRC) when an MSSV fails a test until permanent corrective actions are completed and to seek an interpretation of the American Society of Mechanical Engineers (ASME) Code regarding additional testing required in the event an MSSV fails to function properly under the augmented testing program being conducted. The May 4,1998, submittal also provided a safety analysis of the consequences of the inoperability of the MSSVs which had failed setpoint testing. In a letter dated July 2,1998, the licensee submitted the proposed long-term corrective actions for assuring operability of the MSSVs.

2.0 PROPOSED ACTIONS The licensee evaluated the root cause of the MSSVs found outside the required TS tolerance and determined the cause of the inoperability to be a bonding of the valve nozzle seat and mating disk surfaces. The licensee further determined the bonding phenomena to be caused by past maintenance activities which have involved use of lapping compounds that result in too fine a surface finish of the valve disks and seats. As a result, the licensee has determined that maintenance procedures should be revised to result in a lapping technique that will leave a less fine gray finish. The licensee also stated that test results have demonstrated that the total valve populdon susceptible to the valve bonding are limited to MSSVs where seat repair maintenance was performed. These valves are identified as 7430A and 7430B in Unit 1 and 7410A,7430,7430A, and 7440 in Unit 2. The licensee's proposed corrective actions for these MSSVs consist of setpoint testing the six susceptible MSSVs approximately every 60 days until the next refueling outage for each unit and revising the MSSV maintenance procedure to leave a gray finish when lapping the disks and seats. During the next refueling outages (Unit 1 Spring 1999 and Unit 2 Fall 1998), these six valves will be opened, inspected, and refurbished as required using the revised maintenance procedures (i.e., lapping to a gray finish).

9811160331 981106 PDR ADOCK 05000498 P PDR

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, 3.0 EVALUATION The staff has reviewed the licensee's proposed corrective actions for the MSSVs and finds that it adequately addresses the concerns regarding MSSV operability. The licensee has used

previous operating experience and reviewed past practices to determine an improved

maintenance procedure and has focused on the suoset of valves which have exhibited the disk i to seat bonding. Further, the staff agrees that setpoint testing of the susceptible MSSVs at an I

increased frequency of approximately every 60 days until the next refueling outages is an

! acceptable approach, it is noted that the licensee has performed these tests on the six valves during the period from June 14 through July 25,1998, as outlined in NRC Inspection Report 50-498/98-07; 50-499/98-07 dated August 11,1998, and the valves performed acceptably The 4

staff's finding is further supported by the licensee's safety analysis which shows that the consequences of the inoperability of the MSSVs which has been experienced, are bounded by j licensing basis events.

l The staff also finds that the licensee has satisfied the commitments to inform the NRC of MSSV

! test failures and to seek an ASME Interpretation regarding additional testing necessary if any of the six MSSVs should fail the 60-day tests. By letter dated June 2,1998, the licensee

! submitted a request to the ASME, and by letter dated July 15,1998, in a response to the

licensee's request, the ASME provided the following interpretation

j "lNQUIRY: Is it a requirement of ASME Section XI paragraph IWV-3513 that i additional tests are to be performed if a safety valve or relief valve fails i during testing other than that performed under paragraph IWV-35107 RESPONSE: No."

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it should be noted the. tne ASME response is more general than the licensee's specific i circumstances. Although it may not be required by the Code, it is the staff's belief that circumstances could exist in which it would be prudent to expand relief valve testing regardless of when or how often testing is performed. However, in the circumstances at STP described above, the staff finds that the licensee has met ASME Section XI requirements and has an adequate basis for determining the scope of the inoperability due to the MSSV disk and seat bonding which has been experienced, and that the proposed 60-day testing of the six susceptible MSSVs is acceptable.

4.0 CONCLUSION

Based on the review of the informatiun submitted by the licensee, the staff cor.cludes that the licensee's corrective actions regarding the STP MSSVs are adequate.

Principal Contributor: G. Hammer Date: November 6, 1998

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