ML20247G672

Forwards TR WCAP-15054, Metallurgical Investigation & Root Cause Assessment of Part Length CRDM Housing Motor Tube Cracking at Prairie Island Nuclear Generating Plant,Unit 2, in Response to NRC 980218 RAI Re Leak Found on CRDM

ML20247G672
Person / Time
Site:	Prairie Island
Issue date:	05/15/1998
From:	Sorensen J NORTHERN STATES POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML20247G683	List: ML20247G672 ML20247G701
References
NUDOCS 9805200317
Download: ML20247G672 (3)

Text

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' *" ) Prairie Island Nuclear Generating Plant 1717 Wakonade Dr. East Welch, Minnesota 55089 May 15,1998 I

l U S , Nuclear Regulatory Commisston i Attn: Document Control Desk l Washington, DC 20555 l

PRAIRIE ISLAND NUCLEAR GENERATING PLANT I Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Transmittal of Westinghouse WCAP - 15054:

MetallurgicalInvestigation and Root Cause Assessment of Part Length CRDM i Housing Motor Tube Cracking at Prairie Island Nuclear Generating Plant Unit 2 By letter dated February 18,1998, the NRC Staff requested additional information l concerning the leak found on a Prairie Island Unit 2 part length control rod drive mechanism (CRDM). The requested information was provided in response to that Request for Additional Information (RAI) by letter dated February 23,1998. In the [

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February 23,1998 Response to Request for Additional Information, NSP committed j j that the formal root cause analysis report for the flaw in part length CRDM G-9 will be completed and submitted to the NRC by April 6,1998 (in a related submittal dated l March 6,1998, NSP provided a CRDM housing design description and A-ferrite requirements for the weld and buttering metal used in fabrication.) ,

A preliminary summary root cause evaluation was provided by letter dated April 6, f ))T

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1998. This letter noted that the formal Westinghouse WCAP was delayed due to the l

volume of inquiries being directed at the Westinghouse laboratory by parties outside of NSP.

The formal root cause analysis has been completed and documented in the attached Westinghouse WCAP - 15054. The report summarizes the results of metallurgical investigations conducted by Westinghouse to establish the mechanism and root cause of cracking and leakage in the Unit 2 part length CRDM motor tube weld at location G9.

The investigation included NDE and surface examinations, metallographic and

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USNRC NORTHERN STATES POWER COMPANY May 15,1998 Page 2 l

fractographic examinations, chemistry and mechanical property assessments and mechanistic and root cause considerations. The results of the investigation indicate the motor tube weld cracking was caused by " solidification cracking" during weld fabrication followed by crack extension and additional cracking during post weld heat treatment. Examinations uncovered no evidence of service related growth. The root cause was identified as contamination by sulfur, copper, boron, and zinc during weld fabrication. Approximately 60 other similar welds in the industry, including five other welds from Prairie Island part length CRDMs fabricated during the same time period as the G9 CRDM, showed no evidence of cracking.

In this letter we have made no new Nuclear Regulatory Commission commitments.

Please contact Richard Pearson (612-388-1121) if you have any questions related to this follow-up response.

Joel P Sorensen  !

Plant Manager i Prairie Island Nuclear Generating Plant i

c: Regional Mministrator - Region lil, NRC Senior Rv Alt insptfor, NRC NRR Project Manager, NRC i J E Silberg Attachments:

1. Westinghouse WCAP - 15054, Metallurgical Investigation and Root Cause Assessment of Part Length CRDM Housing Motor Tube Cracking at Prairie Island Nuclear Generating Plant Unit 2," dated April 1998.

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L USNRC NORTHERN STATES POWER COMPANY May 15,1998 Attachment 1 Westinghouse WCAP - 15054 MetallurgicalInvestigation and Root Cause Assessment of Part length CRDM Housing Motor Tube Cracking at Prairie Island Nuclear Generating Plant Unit 2 1

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