ML20055H915

Safety Evaluation Accepting Actions Taken to Resolve NRC Bulletin 88-002, Rapidly Propagating Fatigue Cracks in Steam Generators Tubes

ML20055H915
Person / Time
Site:	Catawba
Issue date:	07/27/1990
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20055H914	List: ML20055H913 ML20055H915
References
IEB-88-002, IEB-88-2, NUDOCS 9007310126
Download: ML20055H915 (2)

Text

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l' ENCLOSURE

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,. 3 NOCLEAR REGULATORY COMMISSUN n 1 wAssiwovow. o. c. rosss s,,.....  :

i SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION CLOSE0VT OF BULLET W 88-02 ISSUES .

DUKE POWER COMPANY CATAWBA NUCLEAR STATION. UNIT 1 DOCKET N0. F +413

1.0 INTRODUCTION

Duke Power Company (the licensee) submitted its response to NRC Bulletin 88-02,  !

" Rapidly Propagating Fatigue Cracks in Steam Generator Tubes by letters dated March 24, April 25, June 10, August 15, October 6,1988, January 16 and February 20, 1989. Bulletin 88-02 requested that licensees for plants with Westinghouse steam generators employing carbon steel support plates take certain L

actions (specified in the bulletin) to minimize the potential for a steam

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generator tube rupture event caused by a rapidly propagating fatigue crack l

such as occurred at North Anna Unit 1 on July 15, 1987.

2.0 DISCUSSION The licensee reports that the Catawba Unit 1 steam generators exhibit evidence ,

of magnitude in t k tube-to-support crevices at the uppermost support plate. .

Accordingly, iteam C.1 and C.2 of the bulletin are applicable to Catawba 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 June 10, 1988 letter. This enhanced leak rate monitoring program is an interim compensatory measure pending completion of the actions c requested in item C.2 of the bulletin ano NRC stadf review and approval of c I

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-12125 (Proprietary) which wa:. submitted with the licensee's letter dated February 20, 1989. This report described the analyses which were conoucted to establish the susceptibility of the Catawba steam generator tubes to rapidly propagating fatigue cracks and to identify any needed corrective actions.

The hRC staff has reviewed the Westinghouse generic progran and docu m ted its .

evaluction in Reference 1. The stoff concluded in Reference 1 that the  !

l Westinghouse program is an acceptable approach for resolving Item C.2 of the bulletin. The staff further concluded that the Westinghouse program, if .

I properly implemented, will provide reasonable assurance against further failures of the kind which occurred at North Anna Unit 1. The Safety Eysluction herein incorporates the staff's generic Reference 1 evaluation by reference.

9007310126 900727

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The analyses for the Catawba Unit I steam generators conservatWely assumed i o 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 asan stress ef fect (i.e., yield stress), consistent with staff ,

fincings in Reference 1.

Stability ratios for the Catawba Unit 1 steam generator tubes were determined f rom dttailed analyses perforced for McGuire Unit 2 whose thernnl-hydraulic conditions are slightly more limiting than those for Unit 1. Theu analyses  ;

incluoed tube instebility analyses performed with the FASTVIB computer coce using thermal-hyoraulic input from a 3-0 ATHOS model. Flow peaking factors

• were determined for the actual anti-vibration bar geometry at Catawba Unit 1 on the basis of results from air model tests. '

The analyses documented in WCAP-12125 show that only one tube failed to satisfy the Westinghouse stress ratio criterion. A multiple cable tube damper was installed in this tube and the tube was subsequently plugged. The purpose of the damper is to reduce the vibration response of the tube to an acceptable level to ensure that the tube remains intact after it has been plugged. Dampers and plugh were also installed in two other tubes as a conservative measure, even though these tubes satisfied the stress ratio criteria and exhibited acceptable fatigue usage.

3.0- CONCLUSION The NRC staff concludes that the actions taken by the licensee resolve the issues identified in Bulletin 88-0? and are, therefore, acceptable. Consistent with staff fincing No. 11 in Reference 1, the above findings are subject to the development of administrative controls by the licensee to ensure that updated stress ratio and 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 t'9 reference parameters assumed in the Catawba Unit I analyses.

4.0 REFERENCE

1. Safety Evaluation Report, " Evaluation of Westinghouse Methodology to -

Address Item C.2 of HRC Bulletin 88-02" which was transnitted to Westinghouse by letter dated October 2, 1987.

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