ML20206F365

Safety Evaluation Re Proposed Repair of Core Spray safe- Ends,During Current Refueling Outage.Plant Can Be Safely Returned to Power Operation After Satisfactory Completion of Core Spray safe-end Repairs

ML20206F365
Person / Time
Site:	Vermont Yankee File:NorthStar Vermont Yankee icon.png
Issue date:	06/16/1986
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20206F279	List: ML20206F275 ML20206F365
References
NUDOCS 8606240440
Download: ML20206F365 (2)

Text

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i p keg UNITED STATES  !

y e p, NUCLEAR REGULATORY COMMISSION 5 :j WASHINGTON, D. C. 20555

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO THE REPAIR OF CORE SPRAY SAFE ENDS FOR FACILITY OPERATING LICENSE NO. DPR-28 VERMONT YANKEE NUCLEAR POWER CORPORATION VERMONT YANKEE NUCLEAR POWER STATION DOCKET N0.: 50-271

1.0 INTRODUCTION

j By letters dated May 5 and June 2, 1986, Vermont Yankee Nuclear Power Corporation (the licensee) proposed repair of the core spray safe ends.

i During the current refueling outage, ultrasonic and liquid penetrant l examinations were performed on the core spray buttered nozzle-to-safe end

- welds in accordance with I&E Notice 84-41. Cracking was found in the Inconel 182 butter of both core spray nozzle-to-safe end welds (NSA and NSB). The cracking was assumed to be IGSCC (Intergranular Stress Corrosion Cracking).

, Liquid penetrant examinations revealed many short and tight indications on the

outside diameter surface of the butter, which were parallel or skewed to the g nozzle / butter interface. Some of these indications were rejectable by ASME

>> Code Section XI (standard). The ultrasonic examinations also indicated a ny crack-like indications (circumferential and axial) in the buttered regions of the nozzles. The licensee proposed to weld overlay repair both cracked core

!. spray nozzle-to-safe end welds with Inconel 82.

i 2.0 EVALUATION The Structural Integrity Associates (SIA) designed the overlay for the licensee. As the core spray nozzles are made of low alloy steel, it is essential to ensure that the nozzles are not embrittled during weld overlay repairs. Therefore, the Inconel 82 overlay is designed to have two portions:

The lower portion of the overlay located on top of the low allow steel nozzle, consisting of three layers with a minimum thickness of 0.125 inch, is designed to eliminate any embrittlement in the nozzle. To deposit this portion of the overlay, the licensee proposed to use a butter-temper process, which is an extension of the pressure vessel repair procedure described in ASME Code Case N-432. This butter-temper welding process, as shown in the EPRI sponsored B&W development work, is capable of eliminating any possible embrittlement in low alloy steel. The licensee indicated that the effectiveness of this butter-temper process will be demonstrated on a mockup prior to using it in the weld overlay repairs. The upper portion of the overlay is designed to be a full structural overlay, meeting the requirements in the ASME Code,Section XI, IWB-3640. The minimum overlay thickness of the upper portion not including

( B606240440 860616 hDR ADOCK 05000271 PDR

_2 the butter-temper layers is designed to be 0.34 inch. After completion of the overlay repairs, ultrasonic examinations will be performed on the overlays using qualified personnel and EPRI recommended procedures to ensure the bonding and structural integrity of the overlays.

The licensee also indicated that the repairs overlays will be inspected during the next refueling outage and the detailed inspection plan will be submitted for NRC review three months prior to the scheduled outage.

We have reviewed the licensee's repair proposal as discussed above. We find that the proposed weld overlay design and repair methodology for the two cracked core spray nozzle-to-safe end welds meet the ASME Code and NRC staff requirements. Although the licensee proposed weld overlay repair is acceptable on an interim basis of one fuel cycle, we consider a permanent solution is more reassuring in maintaining the integrity of the nozzlos and the reactor pressure vessel. Therefore, we recomend that the licensee seriously consider replacing both core spray safe-ends during the next refueling outage when the schedule allows.

3.0 CONCLUSION

We conclude that the Vemont Yankee plant can be safely returned to power operation for the upcoming fuel cycle after the satisfactory completion of the proposed core spray safe-end repairs.

Principal Contributor: W. Koo Dated: Ju9 16,1986

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