ML20056H475
| ML20056H475 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 08/27/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20056H469 | List: |
| References | |
| GL-88-01, GL-88-1, NUDOCS 9309090396 | |
| Download: ML20056H475 (3) | |
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ENCLOSURE
- i g UNITED STATES i[i g1Tv Wj' j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION DISPOSITION OF FLAW IN HIGH PRESSURE CORE SPRAY N0ZZLE SAFE-END WELD NIAGARA M0 HAWK POWER CORPORATION NINE MILE POINT NUCLEAR STATION. UNIT 2 DOCKET NO. 50-410
1.0 INTRODUCTION
By letter dated July 8,1993, Niagara Mohawk Power Corporation (NMPC or the licensee) requested NRC staff approval of a change to its previously approved approach for dispositioning a flaw in the weld joining the high pressure core spray (HPCS) nozzle safe end to the safe-end extension. NMPC had intended to replace the safe-end extension during the third refueling outage to remove the fl aw.
During the first refueling outage, ultrasonic testing identified an indication in the weld joining the HPCS nozzle safe end to the safe-end extension. The licensee concluded that the indication was from the original fabrication, resulting from solidification shrinkage cracking in the weld. The indication was evaluated as a flaw 0.15 inches deep (17.6% of the wall thickness) and 1.9 inches long (6.3% of the circumference). NMPC applied the mechanical stress improvement process (MSIP) to improve the residual stress distribution near the flaw and eliminate the potential for flaw growth. Reinspection subsequent to MSIP showed a flaw size of 0.35 inches deep (41% of the wall thickness) maximum, and 3.4 inches long (11.3% of the circumference). The licensee attributed the increase in ultrasonic response of the flaw to increased ultrasonic reflectivity resulting from the HSIP.
Ultrasonic inspections were again performed during a mid-cycle outage during the second fuel cycle and during the second refueling outage. The mid-cycle inspection indicated a maximum flaw depth of 0.32 inches (38% of the wall thickness) and a length of 3.3 inches (11% of the circumference). Inspection during the second refueling outage revealed a flaw depth of 0.25 inches (29%
of the wall thickness) and r. length of 3.3 inches (11% of the circumference).
The last two ultrasonic inspections have revealed no growth in the flaw, unlike the results of the first post-MSIP inspection. NMPC believes that MSIP stabilized the flaw.
In view of the favorable ultrasonic inspection results and further analysis, NMPC is proposing to not replace the safe-end extension during the third refueling outage as originally planned. The licensee is proposing to ultrasonically inspect the flaw during the upcoming third refueling outage and at each refueling outage thereafter. If ultrasonic inspection shows growth at any time, i.e., to a depth greater than 41% of the wall thickness and/or
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9309090396 930827 PDR ADOCK 05000410-P -
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I length greater than 11.3% of the weld circumference, NMPC would implement a weld overlay repair. The licensee considers the risks associated with replacement of the safe-end extension, such as radiation exposure, to not be justified.
2.0 DISCUSSION AND EVALIJATION The staff finds NMPC's proposal to be acceptable for the following reasons:
A. The licensee evaluated the post-MSIP residual stresses in the axial direction based on field measurements of the circumferential pipe contraction. The evaluation showed that these stresses are compressive on the inner half of the wall thickness, extending well beyond the depth on the flaw, even when including welding residual stresses, seismic, and operating loads. The licensee's evaluation is supported by independent tests by Argonne National Laboratory on similar pipe. The analysis showed that the flaw should not grow. In spite of the analysis results, NMPC will monitor the flaw each refueling outage.
B. NMPC is confident that the indication is no deeper than 41% of the wall thickness, no longer that 11.3% of the circumference, and not growing in depth or length. This is supported by the following:
- Flaw depth measurement results have varied from 41 to 29% of wall thickness. This variation is not unexpected for this narrow flaw.
Electric Power Research Institute (EPRI) staff have reviewed the data and concurred that the variation in results is reasonable.
- The previous ultrasonic inspections, when combined with the planned inspection during the third refueling outage, cover a significant period of time, i.e., approximately 22,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation, or 2 complete fuel cycles.
- Previous ultrasonic inspections were performed with examiners, equipment, and procedures qualified at the EPRI Nondestructive Examination Center on samples with actual flaws with known depths.
C. The maximum size of the flaw falls well within the limit of 0.6 times the wall thickness specified in the American Society of Mechanical Engineers (ASME) Code.
D. The affected weld will remain classified as Category F according to Generic Letter (GL) 88-01, "NRC Position on IGSCC in BWR Austenitic Stainless Steel Piping," and NUREG 0313, Rev. 2, " Technical Report on Material Selection and Processing Guidelines for BWR Coolant Pressure Boundary Piping." According to the GL, Category F weldments are to be inspected every refueling outage.
The licensee also submitted its repair plan for approval. NMPC designed a full structural overlay meeting the guidance of GL 88-01 and the requirements of Section XI of the ASME Code. For overlay design purposes the flaw depth
4 was assumed to extend through wall and the length was assumed to extend 360 degrees around the circumference. Accordingly, the weld overlay design is independent of the size of the indication.
3.0 CONCLUSION
The staff has concluded that NMPC's proposal for continued inspection and, if necessary, repair of the flaw in the weld joining the HPCS nozzle safe to safe-end extension is acceptable. The weld overlay repair plan meets the guidance of GL 88-01 and the requirements of the ASME Code,Section XI, and is, therefore, also acceptable.
Principal Contributor:
M. Banic Date: August 27, 1993
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