ML20195J006

Safety Evaluation Supporting Licensee Responses to NRC Bulletin 88-002, Rapidly Propagating Fatigue Cracks in Steam Generator Tubes, Subj to Util Adopting Addl Administrative Controls

ML20195J006
Person / Time
Site:	North Anna
Issue date:	11/29/1988
From:	Office of Nuclear Reactor Regulation
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Shared Package
ML20195J003	List: ML20195H998 ML20195J006
References
IEB-88-002, IEB-88-2, NUDOCS 8812010166
Download: ML20195J006 (4)

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! o NUCLEAR REGULATORY COMMISSION D E WASH WGTON, D. C. 20666 ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION VIRGINIA ELECTRIC AND POWER COMPANY CLOSE0VT OF BULLETIN 88-02 ISSUES

1. INTRODUCTION By letter dated March 24, 1988 (Reference 1), Virginia Electric and Power 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 tha bulletin) to minimize the potential for a steam generator tube rupture (SGTR) event caused by a rapidly propagating fatigue crack which occurred at North Anna Unit 1 on July 15,1987.

2. NORTH ANNA UNIT 1 2.1 Discussion Actions taken by the licensee and their Westinghouse consultant to investigate and correct the conditions which led to the July 15, 1987 SGTR event were provided in (1) the licensee's report, "North Anna Unit 1 July 15,1987, Steam Generator Tube Rupture Report," September 15, 1987, Revision 1 (Reference 2),

and (2) Westinghouse reports WCAP-11601 (proprietary version) and WCAP-11602 (non-proprietary version) (Reference 3).

The failure mechanism was established by Westinghouse as fatigue due to excessive flow induced vibration associated with a fluid-elastic instability. To prevent future fatigue crack initiation, the licensee installed a downcomer flow resis-tance plate in each steam generator to reduce the steam generator crossflow velocities and, thus, the stability ratios for all tubes. In addition, extensive preventive plugging was performed on tubes with potentially high stability ratios which were unsupported by anti-vibratiun bars (AVBs). This preventive plugging program included all unsupported tubes beyond row 8 and unsupported row 8 tubes which apprared to be subject to lucalized flow peaking effects. These corrective actions were estimated by Westinghouse to reduce the maximum stability ratio for unsupported tubes remaining in service by at least 22% compared to the stability ratio for the tube which ruptured. This reduction in stability ratio satisfies the 10% criterion developed by Westinghouse to ensure that alternating stress levels are sufficiently small to preclude fatigue crack initiation for the re-maining lifetire of the plant.

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, In addition to the above corrective actions, the iicensee comitted to and implemented an enhanced primary-to-secondary leak rate monitoring program which provides added assurance that the plant will be shut down in a timely fashion before a leaking through-wall crack can propagate to failure. In addition, by letter dated December 4, 1987 the licensee submitted c license amendment request to incorporate this enhanced monitoring program as part of the Technical Specifications.

By letter dated December 11, 1987 (Reference 4), the staff issued its Safety Evaluation authorizing North Anna Unit 1 to return to 100% of licensed power.

The staff concluded in the Safety Evaluation that the licensee had implemented acceptable diagnostic and corrective actions to prevent a similar fatigue f ailure in the future. The licensee's actions and the staff's December 11, 1987 Safety Evaluation pre-dated issuance of Bulletin 88-02 on February 5, 1988.

Although the December 11, 1987 Safety Evaluation endorsed the licensee's enhanced leak rate monitoring program, it did not address the licensee's  !

proposal to include this enhanced program as part of the Technical Specifications. The staff expects to complete action on the requested change to the Technical Specifications within the next few weeks.

As part of its March 24, 1988 response to Bulletin 88-02, the licensee enclosed Revision 2 of its "Steam Generator Tube Rupture Report" (Reference 5). The licensee concludes that the actions documented in the Revision 2 report (Reference 5) and WCAP-11601 (Reference 3) resolve the issues identified in Bulletin 88-02.

l 2.2 Evaluation The staff has reviewed the above-mentioned Revision 2 report (Reference 5) and finds that the updated inforhtation contained in this report does not affect the essential facts upon which the staff relied in its December 11, 1987 Safety Evaluation (SE) authorizing North Anna Unit 1 to return to 100% of licensed power operation.

A discussed earlier, the staff's conclusions in its December 11, 1987 SE were based in part on information contained in Westinghouse reports WCAP-11601 and WCAP-11602 (Reference 3). Information contained in more recent Westinghouse reports issued on behalf of other plants (e.g., Reference 6) indicate that local flow peaking effects associated with certain "as built" AVB insertion depth configurations play a more important role in causing high stability ratios than was recognized in Reference 3. The staff's generic evaluation of the more recent Westinghouse studies of local flow peaking effects is documented in Reference 7. ,

However, the staff's review of Reference 3 indicates that regions of the most  ;

significant flow peaking effects were included in the licensee's preventive '

plugging program at North Anna Unit 1. Therefore, the staff finds that direct consideration of the higher flow peaking effects now being estimated by Westinghouse would further increase the estimated stability ratio reductions  !

which have been achieved i.t North Anna Unit 1 through preventive plq ging. Thus, the more recent information which has become available concerning flow peaking effects does not impact the staff's conclusions in the Decerber 11, 1987 Safety Evaluation.

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3. NORTH ANNA UNIT 2 3.1 Discussion The program implemented at North Anna Unit 2 to minimize the likelihood of rapidly propagating fatigue cracks is described in Westinghouse report SG-88-03-016 (Reference 8) which was enclosed with the licensee's March 24, 1988 submittal.

This program is very similar to that implemented at Unit 1. This included installation of downcomer flow resistance plates and preventive plugging of all tubes beyond row 8 which are not effectively supported by AVBs. These actions are estimated to reduce the stability ratios for the most limiting tubes remaining in service (i.e., tubes in row 8) by 24% compared to the tube which ruptured at North Anna Unit 1. This reduction satisfies the 10% reduction criteria developed by Westinghouse for North Anna Unit 1 and which was reviewed by the staff in its December 11, 1987 SE. The estimated reduction assumes flow peaking factors for tubes remaining in service equal to the 1.47 value exhibited by the tube which ruptured at North Anna Unit 1. Westinghouse states that row 8 tubes remaining in service have negligible potential for flow peaking and thus can be expected to exhibit stability ratios significantly smaller than 76% of that associated with the tube rupture at Unit 1.

The licensee's program for Unit 2 also included implementation of the enhanced primary-to-secondary leak rate monitoring program implemented at North Anna Unit 1. The licensee's request to incorporate this enhanced monitoring program as part of the Technical Specificatinns, dated December 4, 1987, applies to Unit 2 as well as Unit 1.

3.2 Evaluation The licensee's program for Unit 2 is essentially the same as that implemented for North Anna Unit I which was reviewed and approved in the staff's SE dated December 11, 1987. The staff's review of the Westinghouse report for Unit 2, however, indicates that several tubes in row 8 may in fact exhibit a degree of flow peaking, contrary to the finding reached by Westinghouse. Although Westing-house did not specifically address the flow peaking factors for these tubes, it is the staff's judgement, based on its review of flow peaking factor test data published in more recent Westinghouse reports (e.g., Reference 6) that the finw peaking factor for these row 8 tubes would be well within the 1.64 upper bound obtained from Westinghouse test data for the most limiting AVB configurations observed at any pWR to date. Naking the extrerely conservative assumption that certain row 8 tubes exhibit a flow peaking factor of 1.64, the staff estimates that the maximum stability ratios for row 8 tubes would still be 15% smaller than the stability ratio for the tube which ruptured at North Anna Unit 1.

This still satisties the 10% acceptance criteria developed by Westinghouse.

Thus, the staff finds that the licensee's program for North Anna Unit 2 is acceptable.

4. CONCLUSION Corrective actions at North Anna Units 1 and 2 in the form of installation of downcomer flow resistance plates and preventive plugging effectively minimize the likelihood of rapidly propagating fatigue cracks of the type which led to

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O the SGTR event in July 1987. The enhanced primary-to-secondary leak rate monitoring program implemented at both units provides added assurance that rapidly increasing leaks will be detected, ensuring tinely plant shutdown before rupture occurs. The staff concludes that the licensee has satisfactorily resolved the issues identified in Bulletin 88-02. Consistent with conclusions reached by the staff for other plants (see Reference 7), these findings are subject to the development of administrative controls by the licensee to ensure that updated stability ratio and fatigue usage calculations are performed in the event of any significant changes to the steam (e.g., steam flow and pressure, circulation ratio) generator relativeoperating parameters to the reference parameters assumed in References 3 and 8 for Units 1 and 2, respectively.

REFERENCES

1. Licensee's letter dated March 24, 1988, "North Anna Power Station Units 1 and 2, Response to Bulletin 88-02." NRC Accession No. 8803300328.

2. Licensee's letter dated September 15, 1987, enclosing licensee's report "North Anna Unit 1 July 15, 1987, Steam Generator Tube Rupture Report,"

Revision 1. NRC Accession No. 8709160239,

3. Westinghouse report WCAP-11601 (Proprietary Version) and WCAP-11602 (Non-Properietary Version), "North Anna Unit 1 Steam Generator Tube Rupture and Remedial Actions Technical Evaluation," September 1987. NRC Accession Nos.

8710050087 and 8710050084.

4. NRC letter dated Decerrber 11, 1987 to tir. W. L. Stewart, Virginia Electric and Power Company, enclosing proprietary and non-proprietary versions of staff's Safety Evaluation authorizing 100% power operation of North Anna Unit 1 following steam generatur tube rupture event on July 15, 1987.

5. Licensee's report, "North Anna Unit 1 July 15,1987, Steam Generator Tube Rupture Report," Revision 2. (This report is docketed as part of Reference 1above).

6. Westinghouse Report WCAP-11799 (Proprietary Version) and WCAP 11800 (Non-Proprietary Version), "Beaver Valley Units 1 and 2 - Evaluation for Tube Vibration Induced Fatigue," April 1988. NRC Accession t;u. 88051600'3.

7. NRC staff memorandum dated August 19, 1988, C. Cheng to D. Neighbors, "Evaluation of Westinphouse Pethodology to Address item C.2 of NRC Bulletin 88-02."

8. Westinghouse report STD-7.2.2.1-8079, SG 88-03-016. "North Anna 2 Steam i Generator Tube fatigue Evaluation and Retredial Actions Report," dated j March 16, 1988. NRC Accession No. 8803300330.

Dated: November 29, 1988 Principal Contributor:

I E. Ilurphy  ;

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