ML20247M585
| ML20247M585 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 07/27/1989 |
| From: | Weigand J VERMONT YANKEE NUCLEAR POWER CORP. |
| To: | NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| References | |
| REF-PT21-89, REF-PT21-89-114-000 BVY-89-72, IEIN-85-059, IEIN-85-59, IEIN-88-085, IEIN-88-85, PT21-89-114, PT21-89-114-000, NUDOCS 8908020155 | |
| Download: ML20247M585 (4) | |
Text
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. ;YERMONT YANKEE L
NUCLEAR POWER CORPORATION 1
.c Ferry Road, Brattleboro, VT 05301-7002 ENGINE IN OFFICE 580 MAIN STREET l
DOLTON, M A 01740 l
(508)779 6711 July 27, 1989 U.S. Nuclear Regulatory Commission Office of Inspection & Enforcement Region I'-
475 Allendale Road King of Prussia, PA 19406 l
l Attn:
Regional Administrator
Reference:
a)
License No. DPR-28 (Docket No. 50-271)
Dear Sir:
Subject:
Notification of Potential Existence of a l
Defect in Accordance with 10CFR21.21 l
In accordance with the provisions of 10CFR Part 21, Section 21, we are j
hereby notifying you that we have obtained information indicating that a defect
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may exist with respect to a valve stem manufactured by Walworth, and replace-
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ment parts manufactured by Aloyco for Walworth.
Enclosure I to this letter documents the details of this notification.
Should you have any questions regarding this matter, please contact us.
Very truly yours, j
i VERMONT YANKEE NUCLEAR POWER CORPORATION j
- 2..-.,
. Gary Weige l
President and
'ef Executive Officer
/dm cc: USNRC Document Control Desk, NRR USNRC Resident Inspector, VYNPS l
N 8906020150 890727
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'PDR ADOCK 05000271 4
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S PDC l
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ENCLOSURE I PART 21 REPORT COMPANY INFORMING Vermont Yankee Nuclear Power Corporation THE COMMISSION RD 5, Box 169 Ferry Road
-Brattleboro, Vermont 05301-FACILITY Vermont Yankee Nuclear Power Station PO Box 157 Governor Hunt Road Vernon, Vermont 05301 FIRMS SUPPLYING Walworth Company
& Crane Aloyco COMPONENT 1400 W. Elizabeth Avenue 12 East Devonwood Linden, New Jersey 07036 Romeoville, Ill.
60441 Ebasco Services Inc.
2 World Trade Center New York, New York 10048 NATURE OF DEFECT The gicbe valve stem failed on April 5, 1989, at the transition between the shaft and stem flange. Evaluation has concluded that the failure of the stem is the result of several factors. These factors are improper heat treatment of the 410 SS alloy used in the stem which increased its susceptibility to stress corrosion cracking (the material property changes that occurred when the shaft was' forged were not recognized by Walworth) presence of sharp stress riser at point of high stress in stem; mismatched stem and disc nut; and incorrect orien-tation of the valve.
The mismatched stem and disc part problem was due to part number control difficulties between Walworth and Aloyco. This occurred when Alyco replaced Walworth as the firm supplying the components.
In 1988, when the stem installed in 1976 failed, the parts were replaced with a stem f rom pre-Aloyco (Walworth) supply and disc nut from Aloyco supply.
The difference in parts resulted in a smaller surface area for contact between the stem and disc nut.
The broken stem made the valve inoperable which is a degradation of an essential safety related component, V10-89A. This could have led to exceeding safety limits by isolating the cooling water flow through the RHR Heat Exchanger and thereby eliminating RHR cooling. None of the factors contributed to the stem failure involved a major deficiency in design, construction, inspection, test, or operation. Of the factors leading to stem failure in the valve:
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improper control of heat treating of certain alloys; sharp stress risers in cri-tical locations; mismatch of parts; and improper orientation of the valve, none of these alone is a major deficiency.
It was also discovered replacement. stems bought for V10-89A&B were incorrect dimensionally. They had a larger diameter and an excessive length.
DATE ON WHICH DEFECT WAS DETECTED A potential Part 21 Report Evaluation was initiated on June 21, 1989.
l NUMBER OF COMPONENTS AT FACILITY Vermont Yankee has two valves of this design (V10-89A&B). V10-69A was the L
only valve to have a stem from a pre-Aloyco supply and disc nut from a post-Aloyco supply.
CORRECTIVE ACTIONS 1.
The larger shaft was modified by machining a radius at the inside notch of the stem flange thereby reducing the stress concentration.
2.
The disc nut was bored to provide only.015" diametrical clearance from the stem. The reduced clearance does not adversely affect the operation of the j
valve and provides greater contact surface for load distribution.
l 3.
The Limitorque operator stem nut, valve bonnet, packing follower and stainless junk ring were all bored to fit the larger diameter shaft. The valve was reassembled, tested and returned to service.
4.
As an additional precaution, the time delay permissive relays were changed to allow the valve to position itself off its seat before the RHR service l
water pumps actually start. This action will reduce the pressure surge and l
impact loading on top of the disc and stem from the pump start.
1 LONG-TERM CORRECTIVE ACTIONS l
1.
Vermont Yankee will either revise stem ordering information to require a l
radiused corner on the stem or revise appropriate plar.t documents to indi-cate that Vermont Yankee will modify replacement stems before use.
2.
Vermont Yankee has directed Yankee Atomic Quality Assurance Department to review their vendor audit program relative to special process control at the sub-vendor level and, if necessary, revise the auditing program.
l 3.
Vermont Yankee Procurement Procedures will be revised to include a require-l ment for a vendor surveillance to be performed to verify consistency in part i
numbering in the event of a change of ownership takes place.
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4 Vermont Yankee has also requested Yankee Atomic to develop for review a program to addr;ss material and design concerns related to the use of high strength steels.
5.
Vermont Yankee will perform ultrasonic testing on the V10-89A stem on a bi-I monthly basis until the orientation of the valve is corrected. This j
testing method has the capability of detecting any indications of a stem l
crack in its early stages.
l 6.
Vermont Yankee will evaluate the need to inspect / replace valve components in the V10-898. Any corrective action warranted will be completed no later than the 1990 outage.
7.
The V10-89A valve will be rotated to the correct orientation at the first convenient opportunity, which is currently scheduled during the next I
refueling outage.
RELATED ADVICE 1ermont Yankee reconmends other facilities be aware of the possible sy..orgistic effect whan several seemingly unobtrusive factors are combined.
These factors being improper control of heat treating of certain alloys, sharp stress risers in critical locations, mismatch of parts, and improper installa-tion of components. Additional information regarding stress risors and improper heat treatment can be obtained from NRC Information Notices 85-59 and 88-85.
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/dm l