ML13331A295
| ML13331A295 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 01/06/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML13331A294 | List: |
| References | |
| IEB-88-002, IEB-88-2, NUDOCS 8901110383 | |
| Download: ML13331A295 (2) | |
Text
SAFETY EVALUATION REPORT SOUTHERN CALIFORNIA EDISON COMPANY SAN ONOFRE UNIT 1 CLOSEOUT OF BULLETIN 88-02 ISSUES INTRODUCTION By letters dated April 6, August 30, and November 22, 1988, Southern California Edison Company (the licensee) submitted its response to NRC Bulletin 88-02, "Rapidly Propagating Fatigue Cracks in Steam Generator Tubes".Bulletin 88-02 requested that licensees for plants with Westinghouse steam generators employing carbon steel support plates take certain 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 occurred at North Anna Unit 1 on July 15, 1987.
DISCUSSION The licensee reports that the San Onofre Unit 1 steam generators exhibit evidence of denting at the uppermost support plate. Accordingly, items C.1 and C.2 of the bulletin are applicable to San Onofre Unit 1.
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 licensee's April 6, 1988, submittal. This enhanced leak rate monitoring program is an interim compensatory measure pending completion of the actions requested in item C.2 of the bulletin and NRC staff review and approval of these actions. Although the licensee plans to discontinue the enhanced leak rate monitoring program upon closeout of item C.2 in the bulletin, the licensee does plan to implement modifications to the air ejector monitor to enhance its reliability and operability and to provide the operators.with real-time steam generator leakage information.
The licensee has implemented the generic program developed by Westinghouse to resolve item C.2 of the bulletin. The licensee's implementation of this program is described in its November 22, 1988, submittal which included Westinghouse reports WCAP-11974 (Proprietary version) and WCAP-11975 (Non-Proprietary Version),
"San Onofre Unit 1, Evaluation for Tube Vibration Induced Fatigue, September 1988.
These reports provide a detailed description of the analyses which were conducted to establish the susceptibility of the San Onofre Unit I steam generator tubes to rapidly propagating fatigue cracks and to identify any needed corrective actions.
The staff has previously reviewed the Westinghouse generic program. The staff's Safety Evaluation of the Westinghouse generic program is incorporated as part of this Safety Evaluation Report as Attachment 1 (proprietary version) and (non-proprietary version). As stated in the attachments, the staff has concluded that the Westinghouse program is an acceptable approach for resolv
.ing item C.2 of the bulletin. The staff has further concluded that the Westing 8901110383 890106 PDIR ADOCK 05000206 P
-2 house program, if properly implemented, will provide reasonable assurance against future failures of the kind which occurred at North Anna Unit 1. The staff's review of WCAP-11974 indicates that the Westinghouse generic program has been fully implemented for San Onofre Unit 1. Key elements of the Westinghouse pro gram are summarized in Section 3.3.6 of this SER.
The analyses documented in WCAP-11974 show that all unsupported tubes in the San Onofre Unit 1 steam generators satisfy the Westinghouse stress ratio criterion. The fatigue usage factor for the most limiting tube is calculated to be 0.088 for a 30 year operating period with the current fuel cycle (i.e., cycle 8 and 9) parameters (e.g., steam pressure and flow, circulation ratio). Thus, the licensee has concluded that all tubes in the San Onofre Unit 1 steam genera tors are acceptable for continued service and that no hardware modifications, preventive tube plugging, or other measures are necessary to preclude fatigue crack initiation.
The San Onofre Unit 1 analyses conservatively assumed that all unsupported tubes are dented at the uppermost support plate. In addition, the stress ratio and fatigue estimates were based on the assumption of a full mean stress effect (i.e., yield stress), consistent with staff finding No. 4 in Section 4 of the Westinghouse SER.
The San Onofre Unit 1 stability ratio analysis was performed with the FASTVIB computer code using thermal-hydraulic input from the 3-D ATHOS Code for a reference operating cycle (i.e., cycles 8 and 9 for San Onofre Unit 1). -Flow peaking factors were determined from air model tests which considered the actual San Onofre anti-vibration bar geometry.
CONCLUSION The licensee's analyses documented in WCAP-11974 fully resolve the issues identified in Bulletin 88-02 and are acceptable. These analyses indicate that there is reasonable assurance that rapidly propagating fatigue cracks of the type which occurred at North Anna Unit I should not occur at San Onofre Unit
- 1. These findings are subject to the development of administrative controls by the licensee to ensure that updated stress ratio and fatigue usage calcula tions are performed in the event of any significant changes to the steam generator operating parameters (e.g., steam flow and pressure, circulation ratio) relative to the reference parameters assumed in the WCAP-11974 analyses.