ML070670425
| ML070670425 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 03/15/2007 |
| From: | Dyer J Office of Nuclear Reactor Regulation |
| To: | Keenan J Pacific Gas & Electric Co |
| Wang, A B, NRR/DORL/LPLIV, 415-1445 | |
| References | |
| TAC MD4150, NRR-07-003 | |
| Download: ML070670425 (5) | |
Text
March 15, 2007 CAL No. NRR-07-003 Mr. John S. Keenan Senior Vice President and Chief Nuclear Office Pacific Gas and Electric Company Diablo Canyon Power Plant P.O. Box 770000, Mail Code B32 San Francisco, CA 94177-0001
SUBJECT:
CONFIRMATORY ACTION LETTER, DIABLO CANYON POWER PLANT, UNIT NO. 2 (TAC NO. MD4150)
Dear Mr. Keenan:
This letter confirms commitments by Pacific Gas & Electric Company (PG&E) in regard to Alloy 82/182 butt welds in the pressurizer at Diablo Canyon Power Plant (DCPP), Unit No. 2.
The discovery, in October 2006, of five circumferential indications in three dissimilar metal (DM) welds on the pressurizer at the Wolf Creek Generating Station (Wolf Creek) raised safety concerns based on the size and location of the indications. At Wolf Creek, 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 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 Nuclear Regulatory Commission (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. The size of the relief nozzle-to-safe end flaw is also of concern, as this flaw has a much larger aspect ratio than those assumed in the estimates used to establish the basis for completing the baseline inspections required by the industry-sponsored MRP. Larger aspect ratios could result in achieving a critical flaw size and rupture before the onset of detectable leakage.
The long-term resolution of this issue is expected to involve changes to the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code), and will involve changes to the NRC regulations in Title 10 of the Code of Federal Regulations (10 CFR),
Part 50, Section 50.55a, Codes and standards. The development of the NRC regulations,
J. S. Keenan whether the rule adopts the ASME Code standards or defines separate requirements, will likely benefit from additional operating experience, continuing assessments, and analysis being conducted by the NRC and the MRP.
Until NRC regulations are revised, it is necessary to establish a minimum set of enhanced reactor coolant system (RCS) DM butt weld inspection expectations for nickel-based Alloy 82/182 pressurizer surge, spray, safety, and relief nozzle butt welds, including safe end welds, to supplement existing inspection and other requirements of the ASME Code and NRC regulations. In addition, enhanced monitoring of RCS leakage is needed to promptly identify any through-wall flaws in the pressurizer surge, spray, safety, or relief nozzle DM butt welds or safe end DM butt welds to prevent additional degradation from occurring. The above actions provide reasonable assurance that there is no undue risk to the health and safety of the public while the NRC regulations are revised.
The NRC communicated the need for near-term enhancements to the industry through public meetings held on November 30, 2006, December 20, 2006, and February 2, 2007. Licensees submitted letters voluntarily committing to the enhanced inspection and leakage monitoring requirements. After teleconferences with specific licensees held between February 12 through February 23, 2007, the licensees submitted supplemental commitment letters addressing the NRC staffs concerns regarding inspection, compensatory actions, and reporting.
In your letter dated February 20, 2007 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML070580128), you described actions you will take at DCPP, Unit No. 2 for the pressurizer dissimilar metal butt welds containing Alloy 82/182 material.
These commitments address: 1) completion schedules for inspection/mitigation of the welds;
- 2) RCS leak monitoring frequency, action levels, and actions; 3) reporting requirements, and
- 4) reinspection frequencies.
The NRC staff has reviewed these actions and commitments and agrees the actions and commitments are appropriate to address the potential of PWSCC of the applicable pressurizer dissimilar metal butt welds containing Alloy 82/182 material with clarifications on two of the commitments as indicated in bold below regarding RCS leak monitoring action levels and notification of changes to commitments.
- If a quantity of leakage can be assigned to a source other than the pressurizer, and that quantity decreases the unidentified leakage below both thresholds (0.1 gpm [gallon per minute] step increase or 0.25 gpm above baseline), this will negate the requirement for shutdown and/or pressurizer Alloy 600/82/182 butt weld bare metal examination.
- PG&E will inform the NRC in writing prior to any revision of the pressurizer butt weld inspection schedules or commitments described in this letter.
These clarifications were discussed with and agreed upon by your staff during a telephone discussion on March 8, 2007, between Donna Jacobs, Vice President - Nuclear Services and Michele Evans, Director - Division of Component Integrity.
J. S. Keenan Pursuant to Section 182 of the Atomic Energy Act, 42 U.S.C. 2232, you are required to:
- 1) Notify me immediately if your understanding differs from that set forth above;
- 2) Notify me if for any reason you cannot complete the actions and commitments within the specified schedule and advise me in writing of your modified schedule in advance of the change; and
- 3) Notify me in writing when you have completed the actions and commitments addressed in this Confirmatory Action Letter.
Issuance of this Confirmatory Action Letter does not preclude issuance of an order formalizing the above commitments or requiring other actions on the part of the licensee; nor does it preclude the NRC from taking enforcement action for violations of NRC requirements that may have prompted the issuance of this letter. In addition, failure to take the actions addressed in this Confirmatory Action Letter may result in enforcement action.
In accordance with 10 CFR 2.390 of the NRC's "Rules of Practice," a copy of this letter and your response will be made available electronically for public inspection in the NRC Public Document Room or from the NRCs ADAMS, accessible from the NRC Web site at http://www.nrc.gov/reading-rm/adams.html. To the extent possible, your response should not include any personal privacy, proprietary, or safeguards information so that it can be made available to the Public without redaction. If personal privacy or proprietary information is necessary to provide an acceptable response, then please provide a bracketed copy of your response that identifies the information that should be protected and a redacted copy of your response that deletes such information. If you request withholding of such material, you must specifically identify the portions of your response that you seek to have withheld and provide in detail the bases for your claim of withholding (e.g., explain why the disclosure of information will create an unwarranted invasion of personal privacy or provide the information required by 10 CFR 2.390(b) to support a request for withholding confidential commercial or financial information). If safeguards information is necessary to provide an acceptable response, please provide the level of protection described in 10 CFR 73.21.
Sincerely,
/RA/
J. E. Dyer, Director Office of Nuclear Reactor Regulation Docket No. 50-323 License No. DPR-82 cc: See next page
ML070670425 OFFICE CPNB DCI LPL4/PM LPL4/LA LPL4/BC TECH ED DIRS/TA NAME TLupold ESullivan AWang LFeizollahi DTerao HChang RPascarelli DATE 3/13/07 3/13/07 3/12/07 3/9/07 3/12/07 2/28/07 3/12/07 OFFICE CPNB/BC DRP/D RGN4 DORL/D DCI/D ADES/AD NRR/D NAME TChan AHowell, III CHaney MEvans JGrobe JDyer DATE 3/14/07 3/14/07 3/19/07 3/14/07 3/14/07 3/15/07 Diablo Canyon Power Plant, Units 1 and 2 cc:
NRC Resident Inspector Antonio Fernández, Esq.
Diablo Canyon Power Plant Pacific Gas & Electric Company c/o U.S. Nuclear Regulatory Commission P.O. Box 7442 P.O. Box 369 San Francisco, CA 94120 Avila Beach, CA 93424 City Editor Sierra Club San Lucia Chapter The Tribune ATTN: Andrew Christie 3825 South Higuera Street P.O. Box 15755 P.O. Box 112 San Luis Obispo, CA 93406 San Luis Obispo, CA 93406-0112 Ms. Nancy Culver Director, Radiologic Health Branch San Luis Obispo State Department of Health Services Mothers for Peace P.O. Box 997414, MS 7610 P.O. Box 164 Sacramento, CA 95899-7414 Pismo Beach, CA 93448 Mr. James D. Boyd, Commissioner Chairman California Energy Commission San Luis Obispo County 1516 Ninth Street (MS 31)
Board of Supervisors Sacramento, CA 95814 1055 Monterey Street, Suite D430 San Luis Obispo, CA 93408 Mr. James R. Becker, Vice President Diablo Canyon Operations Mr. Truman Burns and Station Director Mr. Robert Kinosian Diablo Canyon Power Plant California Public Utilities Commission P.O. Box 56 505 Van Ness, Room 4102 Avila Beach, CA 93424 San Francisco, CA 94102 Jennifer Tang Diablo Canyon Independent Safety Field Representative Committee United States Senator Barbara Boxer ATTN: Robert R. Wellington, Esq. 1700 Montgomery Street, Suite 240 Legal Counsel San Francisco, CA 94111 857 Cass Street, Suite D Monterey, CA 93940 Regional Administrator, Region IV U.S. Nuclear Regulatory Commission Harris Tower & Pavillion 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011-8064 January 2007