ML20209G371

Provides Final Root Cause Evaluation Re GL 94-03, Intergranular Stress Corrosion Cracking of Core Shrouds in Bwrs, for Unit 1

ML20209G371
Person / Time
Site:	Nine Mile Point
Issue date:	07/12/1999
From:	Abbott R NIAGARA MOHAWK POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-94-03, GL-94-3, NMP1L-1451, NUDOCS 9907190139
Download: ML20209G371 (2)

Text

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Niagara @ Mohawk' Richard B. Abbott Phone: 315.349.1812 Vicz President Fax: 315.349.4417 NuclearEngineering July 12, 1999 NMPIL 1451 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 RE: Nine Mile Point Unit 1 Docket No. 50-220 DPR-63

Subject:

Generic Letter 94-03, "Intergmnular Stress Corrosion Cmcking of Core Shmuds in Boiling Water Reactors, "Mnal Root Cause Evaluation of a Cap Screw Failurefor a Stabilizer Assembly (Tie Rod)for Nine Mile Point Unit 1 Gentlemen:

By letter dated May 21,1999, Niagara Mohawk Power Corporation (NMPC) requested Staff approval of a proposed modification to each of the four tie rods pursuant to 10CFR50.55a(a)(3)(i). As stated in that letter, the modification replaces the design function of the failed cap screw and other cap screws that have the potential for future failure in the upper spring. NMPC also stated that the root cause evaluation was preliminary and that it would provide a final root cause evaluation within 30 days following restart of the unit. The purpose of this letter is to provide the final root cause evaluation.

The cause, as stated in our letter dated May 21,1999, was confirmed to be intergranular stress corrosion cracking in the alloy X-750 cap screw material due to large sustained stresses. J Specifically, these stresses were from differential thermal expansion of dissimilar materials  !

fastened by the cap screw.

As also stated in that letter, a potential contributing cause was the sustained stresses that were attributed to the torquing of the cap screw associated with the original assembly of the tie rods.

NMPC has determined that the torquing process was properly controlled during initial assembly of the tie rods to provide reasonable assurance that the cap screws were not over torqued. Therefore, this potential contributing cause has been eliminated.

Furthermore, as stated in the same letter, a second potential contributing cause was the stresses associated with friction between the reactor pressure vessel wall and the upper spring contact points. Based on a re-examination of the wear marks on the reactor pressure vessel wall and \

destructive testing of two cap screws from the 166 degree tie rod upper spring upper contact, \O NMPC has eliminated friction as a potential contributing cause.

9907190139 990712 [1v p-l PDR ADOCK 05000220 - 1 P PDR . m vomumw stauon ru. Box 63, Lycoming. New York 13093-0J63

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i Page 2 The modification is unaffected by these conclusions in that it was designed to address the combined effect of these three sources of stress on the subject caps crews. Accordingly, NMPC continues to conclude that the proposed modification provides an acceptable level of quality and safety.

Very truly yours, I db Richard B. Abbott Vice President - Nuclear Engineering RBA/KWK/ kap xc: Mr. H. J. Miller, Regional Adntinistrator, Region I Mr. S. S. Bajwa, Section Chief PD-1, Section 1, NRR Mr. G. K. Hunegs, NRC Senior Resident Inspector Mr. D. S. Hood, Senior Project Manager, NRR Records Management l

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