ML20207G274

Safety Evaluation Concluding That Firstenergy Flaw Evaluation Meets Rules of ASME Code & That IGSCC & Thermal Fatigue Crack Growth Need Not Be Considered in Application

ML20207G274
Person / Time
Site:	Perry
Issue date:	06/07/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20207G272	List: ML20207G274
References
NUDOCS 9906110083
Download: ML20207G274 (2)

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1 UNITED STATES 3

j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20056-4001

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION l

PERRY NUCLEAR POWER PLANT. UNIT 1 FEEDWATER NOZZLE WELD POST REPAIR FLAW EVALUATION FIRSTENERGY NUCLEAR OPERATING COMPANY DOCKET NO. 50-440 l

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1.0 INTRODUCTION

By FW Nozzle to safe-en|letter dated April 28,1999]], FirstEnergy Nuclear Operating Company (the licensee) submitted its flaw evaluation for the detected flaws on the feedwater nozzle weld overlay for the Perry Nuclear Power Plant. The material for the weld overlay is Alloy 52, which is very ductile. The flaws were detected during the post repair examination of the weld overlay which was applied to the flawed weld connecting the feedwater nozzle to safe-end. The licensee intended to demonstrate, through an analytical flaw evaluation, that the Unit could be operated without repair of the overlay.

2.0

' EVALUATION 2.1 Licensee Based on the characterization of the post repair ultrasonic examination result for the weld overlay, the licensee performed a stress evaluation using the finite element method (FEM). The FEM model approximates the lengths and widths of alllack-of-fusion (LOF) and lack-of-bonding (LOB) areas of the weld overlay. The licensee used this model to calculate the stress intensities for three critical load paths and compared these values with the allowable stresses specified in NB-3200 of Section lli of the American Society of Mechanical Engineers (ASME) Code for the normal and upset (1.5 S ) and faulted (3.6 S.) conditions. The licensee's results indicate that margins exist on stresses for the three load paths under the above-mentioned loading conditions. Hence, the weld overlay meets the ASME code requirements, including that of the ASME Code Case N-504.

Next, the licensee performed a Section XI flaw evaluation using the Appendix C methodology (limit load analysis) for the flawed weld overlay. The evaluation considered three cases, each with a different flaw geometry. The bounding case assumed that the original safe-end weld was completely severed and a portion of the 360-degree crack extended into the weld overlay to encompass all LOF indications. The bounding model assumed that the remaining ligament of this poition of the weld overlay is 0.18-inch. Based on the loads and stresses used in the design of the weld overlay, the licensee calculated the safety factors (using Appendix C) for the three cases mentioned above for both the upset and the faulted conditions. For each case, the calculated safety factors are greater than the safety factor of 2.77 for the upset condition and 1.39 for the faulted condition specified in the ASME Code. Hence, the weld overlay meets the 9906110083 990607 PDR ADOCK 05000440 P

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2-ASME Code,Section XI, requirements. Since all requirements in the ASME Code are satisfied,

- the licensee concluded that the existing weld repair is justified for continued operation.

2.2 NRC Staff The licensee's FEM model is axisymmetric and is, therefore, conservative because all seven LOF flaws, with the maximum circumferential dimension of 101-degrees (28 degrees to 129 degrees), and the LOB flaw of 132-degrees (14 degrees to 146 degrees), are modeled as 360-degree flaws. Further, according to the schematic of the FEM model, the staff determined that the mods! is adequate for the stress evaluation. Since the calculated stresses are less than the allowable stresses specified in the ASME Code for the three critical load paths and for the e

normal, upset, and faulted conditions, the staff agrees with the licensee's conclusion that the repaired weld overlay meets the ASME Code,Section XI, requirements.

The staff evaluated the licensee'sSection XI flaw evaluation. Conventionally, Appendix C is used to calculate the allowable flaw size applying the Code specified safety factors. The calculated allowable flaw size is then compared to the current flaw size. The licensee, however, calculated the safety factors using the current flaw size, and then compared the calculated safety factors to the Code specified safety factors. The staff considered these two approaches equivalent. The licensee's evaluation indicates that the calculated safety factor for the bounding case is 3.36 for the upset condition and 1.50 for the faulted condition, exceeding the Code specified safety factor of 2.77 for the upset condition and 1.39 for the faulted condition. The staff considered these narrow margins acceptable because the limiting case of using a completely severed safe-end to represent a flaw about 22.5 degrees long is very conservative. Hence, the staff agrees with the licensee's conclusion that the repaired weld overlay meets the ASME Code,Section XI, requirements.

The licensee did not mention the crack growth due to intergranular stress corrosion cracking (IGSCC) and thermal fatigue. The staff confirmed that the coolant does not reach the LOF and LOB flaws, and that IGSCC does not need to be considered. Further, because of the thermal sleeve, the coolant between the safe-end and the upper thermal sleeve is almost at a steady state. Therefore, the thermal fatigue growth effect can be neglected.

3.0 CONCLUSION

The staff has reviewed the licensee's submittal and has determined that the flaw evaluation meets the rules of the ASME Code and that IGSCC and thermal fatigue crack growth need not be considered in this application. Since the calculated safety factors are greater than the Code specified safety factors, the staff determined that continued operation for Perry without repair of the overlay is acceptable for at least two cycles. However, the repaired weld overlay is subject to reinspection in accordance with the inspection schedule of NUREG-0313, Revision 2, for boiling water reactor coolant pressure boundary piping.

Principal contributor: S. Sheng, NRR Date: June 7, 1999 -

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