ML20235Q034

Safety Evaluation Supporting Util 880324,0425,0610,0815 & 1006 Responses to NRC Bulletin 88-002, Rapidly Propagating Fatigue Cracks in Steam Generator Tubes

ML20235Q034
Person / Time
Site:	Mcguire
Issue date:	02/27/1989
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20235Q030	List: ML20235Q028 ML20235Q034 ML20235Q041
References
IEB-88-002, IEB-88-2, NUDOCS 8903020472
Download: ML20235Q034 (3)

Text

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION j RESOLUTION OF BULLETIN 88-02 ISSUES -

l DUKE POWER COMPANY- -

DOCKET N0.-50-370 MCGUIRE NUCLEAR STATION, UNIT'2

-1.0- INTRODUCTION By letters' dated March 24, April'25, June 10, August 15,'and October 6, 1988, Duke Power Company (the licensee) submitted its response to NRC Bulletin 88-02,

" Rapidly Propagating Fatigue Cracks in Steam Generator. Tubes" for McGuire Unit

2. .Bulletin 88-02 requested 'that licensees for plants with Westinghouse steam generators employing carbon steel support plates take certain actions, specified .in the bulletins 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.

2.0 DISCUSSION The -licensee reports that the McGuire Unit 2 steam generators exhibit evidence

. of support plate " corrosion with magnetite" at the uppermost support plates.

L . Accordingly, items C.1 and C.2 of the bulletin are applicable to McGuire Unit 1 '2.

L In accordance with item C.1 of the bulletin, the' licensee has implemented an enhanced primary-to-secondary leak rate monitoring program. 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. This program is intended to ensure that leakage from a rapidly propagating fatigue crack will be detected and monitored in a timely manner to ensure that the plant is shutdown before rupture occurs.

l 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 was first described in its August 15, 1988, submittal which included Westinghouse preliminary report SG-88-08-0006 (Westinghouse proprietary),

"McGuire Unit 2, Evaluation for Tube Vibration Induced Fatigue." By letter dated October 6,1988, the licensee transmitted this report in final form as WCAP-11935 (proprietary) and WCAP-11936 (non-proprietary). This report

provides a detailed description of the analyses which were conducted to 1

establish the susceptibility of the McGuire Unit 2 steam generator tubes to  !

rapidly propagating fatigue cracks and to identify any needed corrective actions.

8903020472 890227

• PDR ADOCK 05000370 Q PDC L

1 i .i l 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 Attachment 2 (non-proprietary version). As stated in the attachments, the staff has concluded that the Westinghouse program is, an acceptable approach for resolving item C.2 of the bulletin. The staff has further concluded that the Westinghouse 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 Westinghouse report SG-88-08-006 (or WCAPs 11935 and 11936) indicates that the Westinghouse generic program has been fully implemented for McGuire Unit 2. Key elements of the Westinghouse program are summarized in Section 3.3.6 of Attachments 1 and 2 of this SER. l The analyses documented in the Westinghouse report show that all unsupported tubes in the McGuire Unit 2 steam generators satisfy the Westinghouse stress ratio criterion. The fatigue usage factor for the most limiting tube is calculated to be less than 0.01 over the projected lifetime of the plant.

Thus, the licensee has concluded that all tubes in the McGuire Unit 2 steam generators are acceptable for continued service and that no hardware modifications, preventive tube plugging, or other measures are necessary to preclude fatigue crack initiation.

The McGuire Unit 2 analyses conservatively assumed that all unsupported tubes were 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 1 of Attachments 1 and 2. The McGuire Unit 2 peaking factor assessment was based on air model tests which considered the McGuire antivibration bar (AVB) geometry and AVB depth measurement uncertainties. The McGuire Unit 2 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., cycle 4). The 3-D ATH0S analysis for McGuire Unit 2 employed a more refined model (i.e., a finer grid or mesh) than has been used previously for plants such as North Anna and Beaver Valley, thus adding additional precision to the analysis.

CONCLUSION The licensee's analyses documented in Westinghouse report SG-88-08-006 (or WCAPs 11935 and 11936) 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 McGuire Unit 2.

These 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 paranieters (e.g., steam flow and pressure, circulation ratio) relative to the reference parameters assumed in the Westinghouse analyses.

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i l Based on the satisfactory resolution of item C.2 of Bulletin 88-02, the licensee may, .at its option, terminate its comitment to the enhanced leak rdte lionitoring program described in the licensee's June.10,19P28 submittal )

l (for McGuire Unit 2 only). Nevertheless, the staff encourages the licensee to review its leak rate monitoring procedures for McGuire Unit 2 to ensure the ,

continued effectiveness of these procedures for the timely detection, monitoring, and trending of rapidly increasing leak rates.

1 Attachments:

1. Evaluation of W Program (Nonproprietary)

2. Evaluation of H Program (Proprietary) j

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