ML20045C973
| ML20045C973 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 06/22/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20045C969 | List: |
| References | |
| NUDOCS 9306250171 | |
| Download: ML20045C973 (4) | |
Text
,
ENCLOSURE
[fa atog'o UNITED STATES g
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NUCLEAR REGULATORY COMMISSION g
c WASHINGTON, D. C. 20555
\\.....p SAFETY EVALVATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AN EVALUATION OF A FLAW INDICATION IN WELD KC-32 NIAGARA M0 HAWK POWER CORPORATION NINE MILE POINT UNIT 2 DOCKET NO. 50-410
1.0 INTRODUCTION
In a letter dated April 24, 1992, Niagara Mohawk Power Corporation (the licensee) submitted for staff review and approval the results of an ultrasonic (UT) examination during the second refueling outage of a flaw in the reactor pressure vessel high pressure core spray nozzle safe end to safe end extension weld (KC-32) at Nine Mile Point Nuclear Station, Unit 2 (NMP-2). The licensee proposed at that time to restart NMP-2 for the third fuel cycle without repairing the flaw and to either (1) submit justification for continuing operation until the third refueling outage, three months prior to exceeding 9700 hours0.112 days <br />2.694 hours <br />0.016 weeks <br />0.00369 months <br /> of operation in the third fuel cycle, or (2) repair or replace the weld prior to exceeding 9700 hours0.112 days <br />2.694 hours <br />0.016 weeks <br />0.00369 months <br /> of operation in the third fuel cycle.
In a letter dated March 16, 1993, supplemented with a letter dated April 30, 1993, the licensee submitted a justification for continuing operation until the third refueling outage with a total cycle operating time not to exceed 11,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
2.0 DISCUSSION During an October 1990 inservice inspection of NMP-2, a circumferential indication was found in the 10-inch reactor pressure vessel high pressure core spray nozzle safe end to safe end extension weld. The initial UT examination of the weld indicated a flaw depth of 17.6% of wall thickness and a length of 6.3% of the weld circumference.
Stress improvement utilizing a Mechanical Stress Improvement Process (MSIP) was performed on the weld after the flaw was discovered.
A subsequent UT examination indicated a flaw depth of 41% of the wall thickness and a length of 11.3% of the weld circumference.
The licensee then committed to perform UT examination of the indication midway through the second fuel cycle, and submitted the results of this examination to the NRC in a letter dated September 16, 1991. The results of the mid-cycle examination showed a maximum indication depth of 38% of the wall thickness and a length of 11.0% of the weld circumference. This slight decrease in indication size demonstrated that the MSIP had maintained the weld in compression and little or.no crack growth had occurred.
62h h10 PDR p
4 i On April 24, 1992, the licensee submitted the results of the UT examination performed during the second refueling outage. This examination showed a flaw depth of 29% of the wall thickness and a length of 11.0% of the weld circumference.
At that time the licensee proposed to restart NMP-2 for the third fuel cycle without repairing the weld and to either (1) submit justification for continuing operation until the third refueling outage, three months prior to exceeding 9700 hours0.112 days <br />2.694 hours <br />0.016 weeks <br />0.00369 months <br /> of operation in the third fuel cycle, or (2) repair or replace the weld prior to exceeding 9100 hours0.105 days <br />2.528 hours <br />0.015 weeks <br />0.00346 months <br /> of operation.
In a letter dated March 16, 1993, the licensee submitted a justification for continuing operation through to the third refueling outage.
In the original fracture mechanics analysis, submitted on June 28, 1991, the licensee applied the 5% uncertainty associated with UT examination to the analytically predicted crack size.
Since uncertainty is associated with the UT sizing of the flaw and not the analysis, and the most recent analysis applied the uncertainty to the value cotained from the UT measurement of the flaw.
The results of the most recent analysis were submitted to the NRC staff in a letter dated March 16, 1993, and were proposed as a basis for continuing to operate the unit through to the third refueling outage. The results of the analysis predicted a maximum crack depth of 59.97% of wall thickness for an operating period of 10,900 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.4245e-4 months <br />.
This result meets the wall thickness requirement of 60% contained in Section XI of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code.
In a letter dated April 30, 1993, which supplemented the March 16, 1993, letter, the licensee informed the NRC of their plans to extend the current (third) fuel cycle from 13,900 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.4245e-4 months <br /> to a maximum of 11,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
The licensee's fracture mechanics analysis showed that the flaw depth would not reach the Code limit of 60% of wall thickness until a minimum of 11,080 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br /> i
of operation.
The staff has reviewed the licensee's fracture mechanics analysis, and performed an independent crack growth calculation. The staff finds that the influence functions used by the licensee in the calculation of the stress intensity factors were based on a model incorporating the ectual crack length.
This methodology, described in "Circumferential Cracks in Pressure Vessels and Piping - Volume 11, ASME PVP Vol. 95, 1983," is less conservative than that i
recommended by the NRC.
The method recommended by the NRC, which assumes a 360' circumferential crack, is detailed in NUREG 0313, Revision 2, " Technical l
Report on Materials Selection and Processing Guidelines for BWR Coolant Pressure Boundary Piping," and was published in 1988.
The staff's analysis, using the NUREG as guidance, calculated a maximum operating time significantly lower than that derived from the licensee's analysis.
In view of the uncertainties in the crack growth analysis as well as in the UT technique, it is prudent to employ a conservative methodology for the crack growth evaluation. Therefore, the licensee's fracture mechanics analysis will not be accepted as justification for extended operation.
. However, the staff finds the licensee's request to operate NMP-2 until-the third refueling outage with a total cycle operating time not to exceed 11,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> to be acceptable. This is based on the considerations that this weld was subjected to MSIP and that the results of the subsequent UT examinations have shown no apparent growth of the flaw.
3.0 CONCLUSION
S Based on a review of the licensee's submittals as discussed above, the staff concludes that the structural integrity of the nozzle safe-end weld KC-32 would be maintained during the current fuel cycle as the growth of the flaw would not be significant. Therefore, NMP-2 can be safely operated until the third refueling outage with a total cycle operating time not to exceed 11,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
Principal Contributor:
Cheryl Beardslee Date:
P S
Mr. B. Ralph Sylvia June 22, 1993 l
l l
maintained during the current fuel cycle, and that operation of NMP-2 for up to 11,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> in the current fuel cycle is acceptable.
i This concludes the staff's efforts under TAC No. M86013.
Sincerely, Original Signed By:
John E. Menning, Project Manager Project Directorate I-l Division of Reactor Projects - I/II Office of Nuclear Reactor Regulation
Enclosure:
Safety Evaluation cc w/ enclosure:
See next page DISTRIBUTION:
Docket File NRC & Local PDRs PDI-l Reading SVarga JCalvo RACapra CVogan JMenning CBeardslee, 7/D/4 l
OGC l
ACRS (10)
Plant File CCowgill, RGN-1 crnet PDI-1:LA PDI-1:PM PDI-1:D CVoganI'-
JMenningkV1 RACapra W WE 6 t*/93
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DATE
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OFFICIAL RECORD COPY FILENAME: G:\\NMP2\\NM286013.LTR
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