ML16341F712
| ML16341F712 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 05/25/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML16341F713 | List: |
| References | |
| IEB-88-002, IEB-88-2, NUDOCS 9005310446 | |
| Download: ML16341F712 (6) | |
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ENCLOSURE i t
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFETY EVALUATION REPORT PACIFIC GAS AND ELECTRIC COMPANY DIABLO CANYON POWER PLANT UNIT NOS ~
1 AND 2 DOCKET NOS ~ 50-275 AND 50-323 1 ~ 0 INTRODUCTION Pacific Gas and Electric Company
( the licensee) submitted its response to NRC Bu 1 letin 88-02, "Rapidly Propagating Fatigue Cracks in Steam Generator Tubes by letters dated March 24 and December 12, 1988, and February 6, 1990 Bulletin 88-02 requested that licensees for plants with Westinghouse steam enerators employ ing carbon steel support plates take certa in actions specified in the bulletin) to minimize the potential for a steam generator tube rupture event caused by a rapidly propagating fatigue crack such as the one that occurred at North Anna Unit 1 on July 15, 1987
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2. 0 D IS CUSS ION The licensee reports that the Diablo Canyon Unit 1 steam generators exhibit indications of corrosion with magnetite deposits and denting at the uppermost support plate.
The Diablo Canyon Unit 2 Steam generators were a iso obser ved to have magnetite deposits at a number of the support p 1 ate intersections and were conservatively assumed to be dented.
Accordingly, Items C. 1 and C. 2 of the bulletin are applicable to both Units 1 and 2 ~
In accordance with Item C ~ 1 of the bulletin, the licensee has implemented an enhanced primary-to-secondary leak rate monitoring program which is described in the 1 icensee' March 24, 1988 submittal
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This enhanced leak rate monitoring program was an interim compensatory measure pending compl etion of the actions requested in Item C ~ 2 of the bul letin and NRC staff review and approval of these actions The 1 icensee subsequently implemented the generi c program deve 1 oped by Westinghouse to resolve Item C.2 of the bul 1etin.
The 1 icensee' imp 1 ementati on of this program for D iab 1 o Canyon Units 1 and 2 is described in Westinghouse reports WCAP-1 2064
( Proprietary Version) and WCAP-12065
( Non-Pr oprietary Version) which were submitted with the 1 icensee '
letter dated December 12, 1988.
These reports descr ibe the analyses which were conducted to establish the susceptibility of the D iab1 o Canyon steam generator tubes to rapidly propagating fatigue cracks and to identify any needed corrective actions.
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The staff has reviewed the Westinghouse generic program and documented its evaluation in Reference 1.
The staff concluded in Reference 1 that the Westinghouse program is an acceptable approach for resolving Item C.2 of the Bulletin.
The staff further concluded that the Westinghouse program, if properly implemented, wi 11 provide reasonable assurance against further fai lures of the kind which occurred at North Anna Unit 1.
The safety evaluation herein incorporates the staff's generic Reference I evaluation by reference.
Stability ratios for the Diablo Canyon steam generator tubes were determined from the FASTVIB computer code using thermal-hydraulic input from a 3-D ATHOS model for assumed reference operating conditions (e.g.,
steam pressure and flow, circulation ratio) which are conservative for current operating cycle parameters.
Flow peaking factors were determined for the anti-vibration bar (AYB) geometry at Diablo Units I and 2 on the basis of results from air model tests.
Staff questions regarding the conservatism of the assumed flow peaking factors were addressed in the licensee's letter dated February 6, 1990 (see additional discussion in next paragraph) and in subsequent phone conversations with Westinghouse.
As part of it's response to the staff's questions, the licensee performed a complete review of the AVB locations reported previously in WCAP-12064 for both Diablo Canyon Units I and 2.
This review resulted in a number of corrections to the AVB maps and a reevaluation of flow peaking and tube fatigue susceptibility at a few tube locations.
The analyses documented in WCAP-12064 show that three unsupported tubes in Unit 1 and five unsupported tubes in Unit 2 failed to satisfy the Westinghouse stress ratio criterion.
These tubes have either been plugged with a sentinel plug or stabilized with a cable damper and subsequently plugged with a solid plug.
In its letter dated February 6, 1990, the licensee reported that one additional tube at Diablo Canyon Unit 1 was now calculated not to satisfy the stress ratio criterion, and the licensee stabilized and plugged this tube.
This finding was the result of a Westinghouse assessment to resolve a staff comment concerning a potential non-conservatism in the assumed flow peaking factor for this tube.
All other unsupported tubes in the Diablo Canyon steam generators satisfy the Westinghouse stress ratio criterion and are acceptable for continued service.
3.0 CONCLUSION
The staff concludes that actions taken by the licensee resolve the issues identified in Bulletin 88-02 and are, therefore, acceptable.
Consistent with staff finding No.
11 in reference I, the above conclusion is subject to the development of administrative controls by the licensee to ensure that if any significant changes occur in the steam generator operating parameters (e.g.,
steam pressure and flow, circulation ratio) relative to the reference parameters assumed in the analyses for Diablo Canyon Units I and 2, updated stress ratio and fatigue usage calculations will be performed.
Implementation of the additional administrative controls is subject to verification by an NRC staff inspection at some future time.
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4.0 REFERENCE 1.
Safety Evaluation Report, "Evaluation of Westinghouse Methodology to Address Item C.2 of NRC Bulletin 88-02," transmitted to Westinghouse by letter dated October 2, 1989.
Principal contributors:
E. Murphy H.
Rood
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