ML20155E191
| ML20155E191 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 11/02/1998 |
| From: | Hood D NRC (Affiliation Not Assigned) |
| To: | Mueller J NIAGARA MOHAWK POWER CORP. |
| Shared Package | |
| ML20155E194 | List: |
| References | |
| TAC-M99720, NUDOCS 9811040230 | |
| Download: ML20155E191 (5) | |
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UNITED STATES g
j NUCLEAR REGULATORY COMMISSION 2
g WASHINGTON, D.C. 20066 0001
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- November 2, 1998 Mr. John H. Mueller Chief Nuclear Officer Niagara Mohawk Power Corporation Nine Mile Point Nuclear Station Operations Building, Secorvi Floor P.O. Box 63 Lycoming, NY 13093
SUBJECT:
NRC STAFF SAFETY EVALUATION OF REQUEST TO EXTEND REINSPECTION INTERVAL FOR CORE SHROUD VERTICAL WELDS, NINE MILE POINT NUCLEAR STATION, UNIT NO.1 (TAC NO. M99720)
Dear Mr. Mueller During the 1997 refueling outage at Nine Mile Point Nuclear Station, Unit 1 (NMP1), Niagara Mohawk Power Corporation (NMPC) performed core shroud inspections in accordance with the BWR Vessel and Intemals Program (BWRVIP) document BWRVIP-07, " Guidelines for Reinspection of BWR Core Shrouds," dated February 1996 and found cracking in the vertical welds. NMPC also found that shroud stabilizer assemblies (" tie rod") nuts had lost some preload and that the lower wedge retainer clips on three of four stabilizer assemblies had experienced damage, including one that had actually broken off. NMPC modified the lower wedge retainer clip design to restore the stabilizer assemblies to the intended function previously approved by
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the NRC staff. The repair modification was considered as an altemate repair under the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code)Section XI definition of repair or replacement pursuant to 10 CFR 50.55a(a)(3)(i) and was approved by the NRC staff in a letter and safety evaluation (SE) dated May 8,1997. In addition, NMPC proposed that reinspection of the shroud vertical welds be performed after 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> of hot operation.
This interval,10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> of hot operation, was based upon a fracture mechanics analysis of the most limiting case of vertical weld V-9. The allowable flaw size for weld V-9 was determined by using the conservative linear elastic fracture mechanics (LEFM) analysis. The 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> of hot operation was calculated using a bounding crack growth rate of 5 x 10-5 inch / hour. By letter dated April 8,1997, NMPC stated that additional analyses were planned, and would be submitted to the NRC staff, to justify extension of the reinspection interval for the shroud vertical j
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The testing and analyses results reported in NMPC's letters dated September 30,1997, and January 30, February 27, September 21, and October 22,1998, show that the characteristics of the observed cracking were consistent with the features of intergranular stress corrosion
- N cracking (IGSCC). Further, the estimated fluence at the remaining uncracked ligaments was shown to be less than 5 x 102 2
n/cm at the end of the current operating cycle (cycle 13) extended to 14,500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br />.
By letter dated February 27,1998, NMPC submitted a request to extend the reinspection interval for the NMP1 core shroud vertical welds from 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> to 14,500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br /> of hot operation. To support the request, the submittalincluded the results of the metallurgical evaluation of two boat n J. v 3 1 9811040230 981102 PDR ADOCK Of!OOO220 f ps)gf
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l samples, fluence measurements and calculations, and reassessment of the crack growth for welds V 9 and V-10 with the application of a stress intensity factor (K)-independent crack growth rate of 2.2 x 10-5 inch / hour.
I The NRC staff has completed its review of the extension request and supporting information and finds the proposed extension to be acceptable. The NRC staff's related SE is enclosed. Based upon a review of the results of the metallurgical testing of the two vertical weld boat samples, as well as the fluence measurements and neutron flux calculations, the NRC staff has determined that the use of a K-independent crack growth rate of 2.2 x 10-5 inch / hour given in BWRVIP-14 for the calculations of crack growth in shroud vertical welds is appropriate for application to NMP1 because all the conditions stipulated by the NRC staff regarding the use of the lower crack growth rate have been satisfied. Therefore, the NRC staff concludes that reasonable assurance exists that NMP1 can be safely operated for a period of 14,500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br /> before reinspecting the core shroud vertical welds because the structuralintegrity of the vertical welds will be maintained with safety margin specified by the ASME Code. Therefore, the NRC staff also concludes that a mid-cycle reinspection (after 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> of hot operation) of the shroud vertical welds is not necessary since a mid cycle reinspection would result in hardship without a compensating increase in the level of quality and safety.
This approval of NMPC's request to extend the core shroud reinspection interval at NMP1 does not affect the NRC staff's earlier letter and SE dated May 8,1997, which approved a design modification of a 1995 shroud repair pursuant to 10 CFR 50.55a(a)(3)(i).
During its review of the extension request, the NRC staff has considered comments received from members of the public and local officials during an NRC meeting with the public in Oswego, New York, on September 24,1998. The comments expressed a preference that the reinspection occur once the 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> of hot operation has been reached, primarily because of the higher assurance afforded by actual observation compared to reliance upon engineering calculations.
As discussed in the enclosed SE, the NRC staff has concluded that reasonable assurance exists that NMP1 can be safely operated with the current operating cycle extended to 14,500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br /> before reinspection of the vertical welds. Therefore, the mid-cycle reinspection at 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> is not necessary as it would result in a hardship without a compensating increase in the level of quality and safety.
This completes the NRC staff's efforts under TAC No. M99720. If you have questions, please contact me by phone on (301) 415-3049 or by electronic mail at dsh@nrc. gov.
Sincerely, hv-k t
Darl S. Hood, Senior Project Manager Project Directorate 1-1 Division of Reactor Projects - l/II Office of Nuclear Reactor Regulation Docket No. 50-220
Enclosure:
Safety Evaluation cc w/ encl: See next page
- J. Mutlltr November 2, 1998 samples, fluence measurements and calculations, and reassessment of the crack grow *h for welds V-9 and V-10 with the application of a stress intensity factor (K)-independent crack growth rate of 2.2 x 10'5 inch / hour.
The NRC staff has completed its review of the extension request and supporting information and i
finds the proposed extension to be acceptable. The NRC staff's related SE is enclosed. Based upon a review of the results of the metallurgical testing of the two vertical weld boat samples, as well as the fluence measurements and neutron flux calculations, the NRC staff has determined that the use of a K-independent crack growth rate of 2.2 x 10-5 inch / hour given in BWRVIP-14 for the calculations of crack growth in shroud vertical welds is appropriate for application to NMP1 because all the conditions stipulated by the NRC staff regarding the use of the lower crack growth rate have been satisfied. Therefore, the NRC staff concludes that reasonable assurance exists that NMP1 can be safely operated for a period of 14,500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br /> before reinspecting the core shroud vertical welds because the structuralintegrity of the vertical welds will be maintained with safety margin specified by the ASME Code. Therefore, the NRC staff also concludes that a mid-cycle reinspection (after 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> of hot operation) of the shroud vertical welds is not i
necessary since a mid-cycle reinspection would result in hardship without a compensating increase in the level of quality and safety.
This approval of NMPC's request to extend the core shroud reinspection interval at NMP1 does i
not affect the NRC staff's earlier letter and SE dated May 8,1997, which approved a design modification of a 1995 shroud repair pursuant to 10 CFR 50.55a(a)(3)(i).
During its review of the extension request, the NRC staff has considered comments received from members of the public and local officials during an NRC meeting with the public in Oswego, New York, on September 24,1998. The comments expressed a preference that the reinspection occur once the 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> of hot operation has been reached, primarily because of the higher assurance afforded by actual observation compared to reliance upon engineering calculations.
As discussed in the enclosed SE, the NRC staff has concluded that reasonable assurance exists that NMP1 can be safely operated with the current operating cycle extended to 14,500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br /> before reinspection of the vertical welds. Therefore, the mid-cycle reinspection at 10,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br />
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is not necessary as it would result in a hardship without a compensating increase in the level of quality and safety.
This completes the NRC staff's efforts under TAC No. M99720 If you have questions, please contact me by phone on (301) 415-3049 or by electronic mail at dsh@nrc. gov.
Sincerely, ORIGINAL SIGNED BY:
Dari S. Hood, Senior Project Manager Project Directorate 1-1 Division of Reactor Projects - t/II Office of Nuclear Reactor Regulation Docket No. 50-220
Enclosure:
Safety Evaluation cc w/ encl: See next page DISTRIBUTION:
See next page DOCUMENT NAME: G:\\NMPi\\NM199720.LTR To receive a copy of this document, indicate in the box: "C" = Copy without attachment / enclosure "E" = Copy with attachment / enclosure "N" = No copy 0FFICE PM: POI-1 lEl LA: POI 1@y l
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$8ajwa ML WAME DHood/rst. td b SlittleW DATE 11/ t /98 11/fl/98 11/ L /98 Official Record Copy
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l DISTRIBUTION-Docket File.
W. Koo
'PUBUC R. Hermann '
l PDI-1 R/F E. Sullivan J. Zwolinski N. Sheehan, Region I S. Bajwa L. Doorflein, Region i S. Little L. Lois D. Hood T. Collins T. Harris (e-mail SE only to TLH3)
C. E. Carpenter R. Caruso OGC K.Kavanagh ACRS I
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l John H. Mueller Nine Mile Point Nuclear Station l-
- Niagara Mohawk Power Corporation Unit No.1 cc:
i Regional Administrator, Region l Warren Bilanin, EPRI Task Manager
' U.S. Nuclear Regulatory Commission 3412 Hillview Avenue l
475 Allendale Road Palo Alto, CA 94303 King of Prussia, PA 19406 Resident inspector Robin Dyle, Technical Chairman U.S. Nuclear Regulatory Commission BWRVIP Assessment Task P.O. Box 126 Southem Nuclear. Operating Company j
Lycoming, NY 13093 Post Office Box 236 40 invemess Center Parkway Charles Donaldson, Esquire Birmingham, AL 35201 Assistant Attomey General New York Department of Law 120 Broadway New York, NY 10271 Mr. Paul D. Eddy State of New York Department of Public Service Power Division, System Operations 3 Empire State Plaza Albany, NY 12223 Mr. F. William Valentino, President New York State Energy, Research, and Development Authority Corporate Plaza West 286 Washington Avenue Extension Albany, NY 12203-6399 Mark J. Wetterhahn, Esquire Winston & Strawn 1400 L Street, NW Washington, DC 20005-3502 Gary D. Wilson, Esquire Niagara Mohawk Power Corporation 300 Erie Boulevard West Syracuse, NY 13202 Supervisor l
Town of Scriba Route 8, Box 382 Oswego, NY 13126 I -
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