ML20236V550
| ML20236V550 | |
| Person / Time | |
|---|---|
| Issue date: | 07/28/1998 |
| From: | Lainas G NRC (Affiliation Not Assigned) |
| To: | Terry C NIAGARA MOHAWK POWER CORP. |
| Shared Package | |
| ML20236V551 | List: |
| References | |
| TAC-M93925, NUDOCS 9808040037 | |
| Download: ML20236V550 (6) | |
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.o NUCLEAR REGULATORY COMMIS810N WASHINGTON. D.C. 20000 4001 July 28,1998 CarlTerry, BWRVIP Chairman Niagara Mohawk Power Company Post Office Box 63 Lycoming, NY 13093 ,
SUBJECT:
FINAL SAFETY EVALUATION OF THE BWR VESSEL AND INTERNALS PROJECT BWRVIP-05 REPORT (TAC NO. M93925)
Dear Mr. Terry:
l-By letter dated September 28,1995, as supplemented by letters dated June 24 and October 29, 1996, May 16, June 4, June 13, and December 18,1997, and January 13,1998, you submitted f the Electric Power Research Institute (EPRI) proprietary report TR-105697, "BWR Vessel and l Intemals Project [BWRVIP), BWR Reactor Pressure Vessel Shell Weld Inspection i Recommendations (BWRVIP-05)." The BWRVIP-05 report evaluates the current inspection l
requirements for the reactor pressure vessel shell welds in BWRs, formulates recommendations i for alternative inspection requirements, and provides a technical basis for these recommended i' requirements. It initially proposed to reduce the scope of inspection of the BWR reactor pressure vessel (RPV) welds from essentially 100 percent of all RPV shell welds to 50 percent of the axial welds and zero percent of the circumferential welds; however, as modified, it proposes to perform inservice inspections (ISI) on essentially 100 percent of the RPV axial shell welds, and essentially zero percent of the circumferential RPV shell welds, except for the intersections of the axial and circumferentia! welds. Approximately 2 - 3 percent of the circumferential welds will be inspected under this proposal. Revised criteria for the performance of successive and additional inspections are also recommended. A cost benefit study was performed to compare the existing vessel shell weld inspection requirements with the recommended inspection criteria.
The staff met with memt,ers of the BWRVIP and their consultants on several occasions to discuss issues related to the review of the BWRVIP-05 report. Additionally, by letter dated ,
April 18,1997, the BWRVIP requested to meet with the Commission on this issue. On May 12, 1997, the Commission was briefed by representatives of the BWRVIP and the staff on the issues related to the requirements for a full inspection of reactor pressure vessel shell welds.
On August 7,1997, the staff issued Information Notice (lN) 97-63, " Status of NRC Staff's Review of BWRVIP-05," regarding licensee requests for relief. IN 97-63 stated that the staff would l " consider technically-Justified requests for reliefs from the augmented examination in accordance with 10 CFR 50.55a(a)(3)(i),10 CFR 50.55a(a)(3)(ii), and 50.55a(g)(6)(ii)(A)(5) from BWR licensees who are scheduled to perform inspections of the BWR RPV circumferential shell welds during the fall 1997 or spring 1998 outage seasons. Acceptably-Justified requests would be considered for inspection delays of up to two operating cycles for BWR RPV circumferential g
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shell welds only. Licensees will sti:1 need to perform their required inspections of " essentially i 100 percent" of all axial welds." The acceptability of such requests was based on plant-specific
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information submitted by the licensee that demonstrated that the expected frequency of beyond-design-basis events, which appeared to dominate the estimated frequency of BWR RPV failure, 9g and the level of embrittlement of the RPV were low enough to assute low probability of vessel i
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! d l CarlTerry l failure during the period of relief. The staff issued schedular reliefs for inspections of the BWR RPV circumferential shell welds due during the fall 1997 outage season for four units who submitted technically Justified requests, and has issued schedular reliefs for two units during the spring 1998 outage season.
On August 14,1997, the staff forwarded to BWRVIP its independent safety assessment (ISA) of the BWRVIP-05 document. The staff's ISA was a multi disciplined, risk-informed review of the safety implications of reducing the inspection of the RPV shell welds as proposed in the
! BWRVIP-05 report. ISA provided a description of the techniques used by the several vendors to fabricate the reactor vessels, and discussions on the history of non-destructive examinations, of the two degradation mechanisms (fatigue and stress corrosion) which have the potential to initiate RPV cracking or to cause existing flaws to grow, and of the limiting transients of concem. ,
l The probabilistic fracture mechanics model was discussed and an integrated probabilistic i assessment was presented. Also transmitted with the ISA was additional guidance on what information the staff needed to assess plant-specific requests for relief from the inspection
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schedule in 10 CFR 50.55a(g)(6)(ii)(A).
f Further work was performed by both the staff and the industry to more fully assess the risk associated with beyond-design-basis events for both the axial and circumferential welds at fluence levels projected to be reached later in life at some plants. This additional work included
, (1) studies of potential precursor events in order to better quantify the potential for cold over-l pressure events in BWRs, (2) additional probabilistic fracture mechanics analysis to both understand the sensitivities to various parameters and to support an uncertainty analysis, and (3) assessment of the proposed changes in inspection requirements relative to the probability of vessel failure.
l On May 7,1998, the staff issued IN 97-63, Supplement 1, which informed BWR licensees that the staff was extending the period in which it would "... consider technically justified requests for !
relief from the augmented examination in accordance with 10 CFR 50.55a(a)(3)(i),
50.55a(a)(3)(li), and 50.55a(g)(6)(ii)(A)(5) from BWR licensees who are scheduled to perform ,
inspections of the BWR RPV circumferential shell welds during the Fall 1998 or Spring 1999 outage seasons. Acceptablyjustified relief would be considered for inspection delays of up to i two operating cycles for BWR RPV circumferential shell welds only. Licensees will still need to j perform their required inspections of " essentially 100 percent" of all axial welds."
1 l The staff has completed its review of the BWRVIP-05 report, as supplemented, and finds in the ;
enclosed Safety Evaluation (SE) that, due to differences in processes used to fabricate axial and circumferential welds and differences in processes used to fabricate the clad on axial and circumferential welds, axial and circumferential welds can not be considered the same population for the purpose of making statisticalinferences. Nonetheless, inspection of axial welds should provide some usefulinformation regarding development of service induce ,
degradation, should it occur.
The staff previously concluded that beyond design-basis events occurring during plant shutdown could lead to cold over pressure events that could challenge vessel ir;tegrity. The industry's response concluded that condensate and CRD pumps could cause conditions that could lead to ;
cold over pressure events that could challenge vesselintegrity. The BWRVIP's estimate of the frequency of over pressurization events that could challenge the RPV is 9.5 x 10d/yr for BWR-4 i h-
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4 and 9 x 10 /yr for BWRs other than BWR-4. Accounting for actual injections which were not included in the BWRVIP analysis, the staff conservatively estimates that the total frequency could be as high as 1 x 10'*/yr.
The BWRVIP 4 failure frequency for the limiting circumferential welds was 9.0 x 10*/yr
[(9.0 x 10 /yr event frequency for a BWR-3) x (1.0 x 104 conditional probability of failure)]. The l
limiting plant specific failure frequency for circumferential welds at 32 effective full power years was determined by the staff to be 8.2 x 10 4/yr [(1 x 10'8/yr event frequency) x (8.2 x 104 conditional probability of failure)). As depicted in NUREG 1560, Vol. I, core damage frequencies l (CDF) for BWR plants were reported to be approximately 10 /yr to 10"/yr. In addition, Regulatory Guide 1.154 indicates that PWR plants are acceptable for operation if the plant-specific analyses predict the mean frequency of through-wall crack penetration for pressurized thermal shock events is less than 5 x 104/yr. Since the failure frequencies for circumferential welds in BWR plants are significantly below the criteria specified in Regulatory Guide (RG) 1.154 l and the CDF of any BWR plant, and that continued future inspections would result in a negligible decrease in an already acceptably low value, elimination of ISI for RPV circumferential welds is justified.
l In Table 5-1 of your supplemental letter dated January 13,1998, concoming the BWRVIP's response to a staff's request for additionalinformation on the BWRVIP-05 report, the limiting conditional probability of RPV failure, or P(FIE), due to axial welds in boiling water reactors 4
(BWRs) is given as 1.55 x 10 for the Clinton plant (BWR/6) after 40 years of operation.
! Combined with the BWRVIP's estimate of the overpressure event frequency of 9.5 x 104/ year for BWR/4 and 9 x 10"/ year for BWRs other than BWR/4, the total RPV failure frequency for axial shell welds in the limiting plant is 1.4 x 104/ year. The staff recognizes there are conservatism in these analyses and the staff, in a request for additional information (RAl) dated June 8,1998, i
' requested that the BWRVIP provide a schedule for addressing the RAI questions. By letter dated July 23,1998, you provided your schedule for responding to this RAl.
BWR licensees may request relief from the inservice inspection requirements of 10 CFR ,
50.55a(g) for volumetric examination of circumferential reactor pressure vessel welds (ASME Code Section XI, Table IWB-2500-1, Examination Category B-A, item 1.11, Circumferential Shell Welds) by demonstrating: (1) at the expiration of their license, the circumferential h Olds satisfy the limiting conditional failure probability for circumferential welds in this evaluation, and (2) they have implemented operator training and established procedures that limit the frequency of cold over pressure events to the amount specified in this report. The staff will pursue clarification of this issue in future rulemaking.
The staff is processing a generic letter informing BWR licensees that relief from circumferential reactor pressure vessel weld examinations, per the proposal in the BWRVIP-05 report, will be considered on a plant-specific basis. This generic letter will be issued for public comment prior to the staff granting any final reliefs. It should also be noted that this safety evaluation is limited to the period of the current operating license. The requirements for inspection of circumferential reactor vessel welds during an additional 20 year license renewal period will be reassessed, on a plant specific basis, as part of any BWR license renewal application.
The staff requests that the BWRVIP review the enclosed SE, and incorporate the staff's conclusions into a revised BWRVIP-05 report so that it is acceptable for reference in plant-
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specific relief requests. Please inform the staff within 90 days of the date of this letter as to your proposed actions and schedule for such a revision.
l Please contact C. E. (Gene) Carpenter, Jr., of my staff at (301) 415-2169 if you have any further l questions regarding this subject.
Sincerely, .
u Gus C. Lainas, Acting Director
[ Division of Engineering
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Office of Nuclear Reactor Regulation
Enclosures:
As stated cc: See next page S
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Carl Terry specific relief requests. Please inform the staff within 90 days of the date of this letter as to your proposed actions and schedule for such a revision.
Please contact C. E. (Gene) Carpenter, Jr., of my staff at (301) 415-2169 if you have any further questions regarding this subject.
Sincerely, original signed by Jack Strosnider for:
Gus C. Lainas, Acting Director l
Division of Engineering Office of Nuclear Reactor Regulation
Enclosures:
As stated cc: See next page l Distribution:
SJCollins/FJMiraglia ACRS JTWiggins, R1 JPJaudon, Ril ATHowell, RIV File Center EMCB/RF PUBLIC GEGrant, Rill DOCUMENT NAME: G:\BWRVIP/BWRVIP05.SER
- See previous concurrence INDICATE IN BOX: "C" SCOPY W/O ATTACHMENT / ENCLOSURE. "E" SCOPY W/ATT/ ENCL "N"sNO COPY EMCB: LPM lE EMCB:ME E EMCB:SC E EMCB:(A)BC E SRXB:BC lE CECarpenter* g), BJElliot* KRWichman* EJSullivan* TECollins*
03/25/1998 V 03/25/1998 03/26/1998 05/14/1998 05/21/1998 mmmmmmE .
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M SPSB:(A)BC EMMEB:(A)BC E DET:D E DSSA:D E DE:(A)D E MPRubin* EHackett' LShao* GHolahan* GClainas*
05/28/1998 05/21/1998 05/26/1898 05/29/1998 05/26/1998 OFFICIAL RECORD COPY l
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! cc:
George Jones, Executive Chairman Dana Covill, Technical Chairman BWRVIP Assessment Task BWRVIP Assessment Task Pennsylvania Power & Light GPU Nuclear A6-1 1 Upper Pond Road Two North Ninth Street Parsippany, NJ 07054 Allentown, PA 18101 Joe Hagan, Executive Chairman Carl Larsen, Technical Chairman
, BWRVIP inspection Task BWRVIP Inspection Task l Entergy Yankee Atomic l P. O. Box 756 580 Main Street
- Waterloo Road Bolton, MA 01740 i Port Gibson, MS 39150 Paul Bemis, Executive Chairman Vaughn Wagoner, Technical Chairman l BWRVIP Integration Task BWRVIP Integration Task Washington Public Power Supply System Carolina Power & Light Company P. O. Box 968 One Hannover Square 9C1 North Power Plant Loop P.O. Box 1551 Richland, WA 99352-0968 Raleigh,NC 27612 Lewis Sumner, Executive Chairman John Wilson, Technical Chairman BWRVIP Mitigation Task BWRVIP Mitigation Task Southem Nuclear Operating Co. Clinton Power Station, M/C T-31C 40 invemess Center Parkway P.O. Box 678 Birmingham, AL 35201 Clinton,IL 61727 John Blomgren, Executive Chairman Bruce McLeod, Technical Chairman BWRVIP Repair Task BWRV!P Repair Task l Commonwealth Edison Co. Southem Nuclear Operating Co.
! 1400 Opus Place, Suite 600 Post Office Box 1295
- Downers Grove, IL 60515-5701 40 Invemess Center Parkway l Birmingham, AL 35201 '
i Bill Campbell, BWRVIP Vice Chairman Warren Bilanin, EPRI BWRVIP l
Raleigh, NC 27612 Mitigation Manager i
Ken Wolfe, EPRI BWRVIP Robert Carter, EPRI BWRVIP Repair Manager Assessment Manager Electric Power Research Institute Greg Selby, EPRI BWRVIP P. O. Box 10412 i
inspection Manager 3412 Hillview Ave.
! EPRI NDE Center Palo Alto,CA 94303 P. O. Box 217097 1300 W. T. Harris Blvd.
Charlotte, NC 28221 l
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