ML071160436
| ML071160436 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 05/10/2007 |
| From: | Catherine Haney Plant Licensing Branch III-2 |
| To: | Muench R Wolf Creek |
| Donohew J N, NRR/DORL/LP4, 415-1307 | |
| References | |
| TAC MD4198 | |
| Download: ML071160436 (4) | |
Text
May 10, 2007 Mr. Rick A. Muench President and Chief Executive Officer Wolf Creek Nuclear Operating Corporation Post Office Box 411 Burlington, KS 66839
SUBJECT:
WOLF CREEK GENERATING STATION - PRESSURIZER SURGE, SPRAY, SAFETY, AND RELIEF NOZZLE WELD SUSCEPTIBILITY TO PRIMARY WATER STRESS-CORROSION CRACKING (TAC NO. MD4198)
Dear Mr. Muench:
This letter acknowledges the receipt of information from Wolf Creek Nuclear Operating Corporation (the licensee) pertaining to the pressurizer dissimilar metal (DM) surge, spray, safety, and relief nozzle and safe-end welds at Wolf Creek Generating Station (WCGS).
In Refueling Outage 15 (October 2006), the licensee performed full-structural weld overlays on the five pressurizer steam-space piping connections after having inspected the piping connections. This is the mitigative or repair action required by the American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME) Code to repair crack indications in these piping connections. By letter dated May 19, 2006 (ET 06-0021, Agencywide Documents Access and Management System (ADAMS) Accession No. ML061450319), the licensee requested relief from certain ASME Code,Section XI, requirements at WCGS. In its application, the licensee submitted Relief Request (RR) I3R-05 and proposed alternatives to the ASME Code requirements in (1) Code Cases N-504-3 and N-638-1, and (2) Appendix VIII, Supplement 11, for the purpose of performing pre-emptive or repair full-structural weld overlays on the pressurizer spray, relief, safety, and surge nozzle safe-ends at WCGS. RR I3R-05 was verbally authorized by the Nuclear Regulatory Commission (NRC) staff in a conference call on October 5, 2006, and the structural weld overlays were performed on the pressurizer spray, relief, safety, and surge nozzle safe-ends during the Refueling Outage 15. The NRC staff issued its letter authorizing RR I3R-05 in its letter dated April 3, 2007 (ADAMS Accession No. ML070670514).
The licensee also provided (1) the results of the ultrasonic examinations of the full-structural weld overlays on the pressurizer nozzle welds and (2) its response to a request for additional information related to the pre-weld overlay examination of the pressurizer nozzle to safe-ends DM welds. These were provided in its letters dated (1) November 17, 2006 (ET 06-055, ADAMS Accession No. ML063330060) and January 31, 2007 (ET 07-0002, ADAMS Accession No. ML070430455), and (2) November 29, 2006 (ET 06-0049, ADAMS Accession No. ML063380456), respectively.
The discovery, in October 2006, by the licensee of five circumferential indications in three Alloy 82/182 DM welds on the pressurizer at WCGS raised safety concerns based on the size and location of the indications. The licensee inspected these welds in Refueling Outage 15 before performing full-structural weld overlays in the outage, and requested a conference call
R. A. Muench with the NRC staff to discuss these indications in the outage. Three indications were in the pressurizer surge nozzle-to-safe end weld, and two separate indications were in the safety and relief nozzle-to-safe end welds. These findings also indicated to the NRC staff that significant concerns might exist with the inspection schedules for addressing the pressurizer weld concerns issued by the industry-sponsored Materials Reliability Program (MRP), in Primary System Piping Butt Weld Inspection and Evaluation Guideline (MRP-139).
The NRC is concerned about the pressurizer surge nozzle-to-safe end weld indications, as this is the first time that multiple circumferential primary water stress-corrosion cracking (PWSCC) indications have been identified in a weld. This condition calls into question the degree of safety margin present in past structural integrity evaluations for flawed DM welds susceptible to PWSCC, since multiple stress-corrosion cracking flaws may grow independently and ultimately grow together, significantly reducing the time from flaw initiation to leakage or rupture.
Based on discussions with the NRC staff, licensees of plants susceptible to this condition have committed to an enhanced inspection frequency and reactor coolant system (RCS) leakage monitoring until actions to repair/mitigate the potential of PWSCC in the affected welds have been completed. You were not requested to commit to an enhanced inspection frequency and RCS leakage monitoring because the full-structural weld overlays were performed on the DM butt welds in Refueling Outage 15.
In the licensees letter dated January 31, 2007, the licensee stated that the pressurizer surge, spray, safety, and relief nozzle-to-safe end welds containing Alloy 82/182 material have been inspected and repaired/mitigated through application of full-structural weld overlays. As a result, the NRC staff concludes that the licensee has adequately addressed the concerns that the NRC staff has at this time regarding PWSCC susceptibility of these DM welds.
Sincerely,
/RA/
Catherine Haney, Director Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-482 cc: See next page
R. A. Muench with the NRC staff to discuss these indications in the outage. Three indications were in the pressurizer surge nozzle-to-safe end weld, and two separate indications were in the safety and relief nozzle-to-safe end welds. These findings also indicated to the NRC staff that significant concerns might exist with the inspection schedules for addressing the pressurizer weld concerns issued by the industry-sponsored Materials Reliability Program (MRP), in Primary System Piping Butt Weld Inspection and Evaluation Guideline (MRP-139).
The NRC is concerned about the pressurizer surge nozzle-to-safe end weld indications, as this is the first time that multiple circumferential primary water stress-corrosion cracking (PWSCC) indications have been identified in a weld. This condition calls into question the degree of safety margin present in past structural integrity evaluations for flawed DM welds susceptible to PWSCC, since multiple stress-corrosion cracking flaws may grow independently and ultimately grow together, significantly reducing the time from flaw initiation to leakage or rupture.
Based on discussions with the NRC staff, licensees of plants susceptible to this condition have committed to an enhanced inspection frequency and reactor coolant system (RCS) leakage monitoring until actions to repair/mitigate the potential of PWSCC in the affected welds have been completed. You were not requested to commit to an enhanced inspection frequency and RCS leakage monitoring because the full-structural weld overlays were performed on the DM butt welds in Refueling Outage 15.
In the licensees letter dated January 31, 2007, the licensee stated that the pressurizer surge, spray, safety, and relief nozzle-to-safe end welds containing Alloy 82/182 material have been inspected and repaired/mitigated through application of full-structural weld overlays. As a result, the NRC staff concludes that the licensee has adequately addressed the concerns that the NRC staff has at this time regarding PWSCC susceptibility of these DM welds.
Sincerely,
/RA/
Catherine Haney, Director Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-482 DISTRIBUTION PUBLIC LPLIV r/f cc: See next page RidsNrrDorl RidsNrrDorlLpl4 RidsNrrPMJDonohew RidsNrrLALFeizollahi RidsOgcRp RidsAcrsAcnwMailCenter RidsRegion4MailCenter BMoroney, NRR RidsNrrDciCpnb (TChan) TLupold, NRR ACCESSION NO.: ML071160436 OFFICE NRR/LPL4/PM NRR/LPL4/LA NRR/CPNB/BC NRR/LPL2-2/PM NRR/LPL4/BC DORL/D NAME JDonohew LFeizollahi TChan BMoroney THiltz CHaney DATE 5/10/07 5/10/07 5/8/07 5/10/07 5/10/07 5/10/07 OFFICIAL RECORD COPY
Wolf Creek Generating Station cc:
Jay Silberg, Esq. Chief, Radiation and Asbestos Control Pillsbury Winthrop Shaw Pittman LLP Section 2300 N Street, NW Kansas Department of Health Washington, D.C. 20037 and Environment Bureau of Air and Radiation Regional Administrator, Region IV 1000 SW Jackson, Suite 310 U.S. Nuclear Regulatory Commission Topeka, KS 66612-1366 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011 Vice President Operations/Plant Manager Wolf Creek Nuclear Operating Corporation Senior Resident Inspector P.O. Box 411 U.S. Nuclear Regulatory Commission Burlington, KS 66839 P.O. Box 311 Burlington, KS 66839 Supervisor Licensing Wolf Creek Nuclear Operating Corporation Chief Engineer, Utilities Division P.O. Box 411 Kansas Corporation Commission Burlington, KS 66839 1500 SW Arrowhead Road Topeka, KS 66604-4027 U.S. Nuclear Regulatory Commission Resident Inspectors Office/Callaway Plant Office of the Governor 8201 NRC Road State of Kansas Steedman, MO 65077-1032 Topeka, KS 66612 Attorney General 120 S.W. 10th Avenue, 2nd Floor Topeka, KS 66612-1597 County Clerk Coffey County Courthouse 110 South 6th Street Burlington, KS 66839 February 2006