ML20006D523
| ML20006D523 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 02/06/1990 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20006D522 | List: |
| References | |
| IEB-88-002, IEB-88-2, NUDOCS 9002140004 | |
| Download: ML20006D523 (2) | |
Text
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,.8 NUCLEAR RESULATORY COMMISSION e
,I wAsHawatoN, D, C. 20666 I
%,.....f EWCLOSURE-SAFETY. EVALUATION REPORT CLOSE0VT.0F. BULLETIN.88 02.!SSUES COMMONWEALTH EDISON COMPANY ZION. UNIT 1 DOCKET WO 50-296 1
1.0 INTRODUCTION
Commonwealth Edison Company (the licensee) submitted its response to NRC Bulletin I
88-02, " Rapidly Propagating Fatigue Cracks in Steam Generator Tubes
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cated March 24, April 8, and September 27, 1988.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 i
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.
i 2.0 DISCUSSION The. licensee reports that the Zion Unit I steam generators exhibit evidence of corrosion with magnitite at the uppermost support plate. Accordingly, items C.1 and C.2 of the bulletin are applicable to Zion Unit 1.
In accordance with item C.1 of the bulletin, the licensee has implemented an en.
i hanced primary-to-secondary leak rate monitoring program which is described in the licensee's March 24 and April 8,1988-submittals. -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.
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 Westinghouse report WCAP-11948 (Proprietary) which was submitted with the licensee's letter dated September 27, 1988. This report describes the analyses which were conducted to establish the susceptibility of the Zion steam generator tubes to rapidly propagating fatigue cracks and to identify any needed corrective actions.
The staff has reviewed the Westinghouse generic program and documented its evalua-L tion in Reference 1.
The staff concluded in Reference 1 that the Westinghouse L
program is an acceptable approach for resolving item C.2 of the Bulletin. The L
staff further concluded that the Westinghouse program, if properly implemented, will provide reasonable assurance against further failures of the kind which 9002140004 900206 9
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occurred at North Anna Unit 1.
The safety evaluation herein incorporates the staff's generic Reference 1 evaluation by reference, i
The analyses for the Zion Unit I steam generators conservatively assumed that i
all unsupported tubes are dented at the uppermost support plate.
In addition, 1
the stress ratio and fatigue estimates were based on the assumption of a full mean stress effect (i.e., yield stress), consistent with staff findings in i
Reference 1.
Stability ratios for the Zion Unit I steam generator tubes were determined from i
the FASTVIB computer cooe using thermal-hydraulic input from a 3-D ATH0S model f or assumed reference operating conditions (e.g., steam pressure and flow, cir-culation ratio) taken from current operating cycle paranaters.
Flow peaking factors were determined for the actual anti-vibration bar geometry at Zion Unit 1 on the basis of results f rom air model tests.
I The analyses documented in WCAP-11948 show that all presently unsupported tubes in the Zion Unit I steam generators satisfy the Westinghouse stress ratio criterion. The fatigue usage factor for the most liriiting tube is calculated to be 0.04 for a 40 year operating period with the current fuel cycle parameters (e.g., steam pressure and flow, circulation ratio).
Thus, the licensee has con-cluded that all tubes in the Zion Unit I steem generators are acceptable for continued service and that no haroware modifications, preventive tube plugging, or other measures are necessary to preclude fatigue crack initiation.
3.0 _ CONCLUSION The staff concluces that the 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 1, the above findings are subject to the development of administrative controls by the licensee to ensure that updated
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stress ratio ano fatigue usage calculations are performed in the event of any significant changes to the steam generator operating parameters (e.g., steam pressure and flow, circulation ratio) relative to the reference parameters assumed in the Zion Unit I analyses.
4.0 REFERENCE 1.-
Safety Evaluation Report, " Evaluation of Westinghouse Methodology to Address l-Item C.2 of NRC Bulletin 88-02" which was transmitted to Westinghouse by l
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