ML20086E754

AO 74-02:on 740131,core Spray Sys Valve CS-11A Failed to Open When Control Switch Placed in Open Position.Caused by burned-out Valve Motor.Motor Replaced

ML20086E754
Person / Time
Site:	Vermont Yankee
Issue date:	02/07/1974
From:	Riley B VERMONT YANKEE NUCLEAR POWER CORP.
To:	US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20086E756	List: ML20086E754 ML20086E763
References
AO-74-02, AO-74-2, VYV-3156, NUDOCS 8312070509
Download: ML20086E754 (2)

Text

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O, b' d VERMONT YANKEE NUCLEAR power CoRPORATrON SCVcNTY ScVcN OROVE STRcCT l(UTIaWD, VIUntONT 03701 VYV-3156 P. o. nox is7 VERNON, VCRMONT 05354 February 7, 1974

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Director f' D

• Directorate of Licensing United States Atomic Energy Commission

• _g Washington, D.C. 20545 { FE8121974 Et A,* cpg g REFE C CE: Operating License DPR-28 $,$ 9 6 b 1 Occ r nce No. A0-74-02 3 Centlemen:

As defined by Technical Specifications for the Vermont Yankee Nuclear Power) Station, Section 6.7.B.1, we are reporting the following Abnormal Occdrrence as A0-74-02.

At approximately 2215 on January 31, 1974, while performing routine surveillance on the Core Spray System, valve CS-llA failed to open when the control switch was placed in the open posit, ion.

This constitutes a failure of a safety system component and !is thereby an abnormal occurrence per Technical Specifications Section 1.0.A.4. The reactor was at approximately 97% power at the time of this incident.

ValveCS-llAwasthenmanuallyopenedtothefull'openjposition.

Since this valve will open in response to a Core Spray System initiation signal and is normally left open, this Core Spray subsysted was then considered operable. ThevalveoperabilityportionofthclCoreSpray surveillance testing was completed and the system was aligned to tho' I

standby mode.

It was determined that the valve failed because the v,alve motor burned out. The motor f ailed because the key that holds the drive -l motor gear and engages the drive gear became loose and jammed in the -l

, drive gear. The motor was ret. laced and the key was locked. in place n J

with a set screw by drilling and tapping a hole in the gea'r hub cnd

'" l providing a recess in the key. This set screw and the one that goes .)

'l through the gear hub and seats on the' motor drive shaft'are recessed l

; into the hub and a circumferential groove is provided in' which a j locking wire was placed to prevent the set screws from backing out.

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February 7, 1974 j

Page 2 i The valve was thon returned to service and upon continuance of the surveillance testing of the Core Spray Syr. tem at approximately 2225 on 2/1/74, which was i$terrupted by the valve failure the night before, while operating valve CS-11A in the closed direction, the motor again burned out after the valve was fully closed. The valve was then opened manually to the full open position. An investigation of the valve drive rmchanisu revealed that a bolt in t he torque switch asserably became loose and caused the torque switch to fail when the valve closed. The motor was replaced and the loose holt was retightened and a commercial liquid thread lock was app 11ed to the threads t o prevent the bolt i

fren loosening again. The valve was then successfully tested and returned to service.

The Plant Operations Review Com.nittee raet to review the incident and concluded that in our experience mechanims of thic type have  :

operated trouble free over a long period of time and that this was an isolated incident of component failure and therefore no further action is required. It was also concluded that the valve would have operated as designed if an actual Core Spray system initiate signal was present because the failure of the torque switch would only fail to chut the valve drive notor off and would not have prevented it from operating.

Very truly yours, VERMONT YANKEE NUCLEAR POWER CORPORATION x

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D. U . uQ B.W. Riley _ m Plant Superinter.Jent '

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