ML20154C218

Discusses Impact of Prolonged Low Power Operation on Reliability of Check Valves as Result of 851121 Water Hammer Event at San Onofre Unit 1.Review of Plant Configuration to Preclude Similar Occurrence Requested

ML20154C218
Person / Time
Site:	Wolf Creek, San Onofre, 05000000
Issue date:	09/06/1988
From:	Oconnor P Office of Nuclear Reactor Regulation
To:	Withers B WOLF CREEK NUCLEAR OPERATING CORP.
References
NUDOCS 8809140279
Download: ML20154C218 (4)

Text

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UNITED $TATES f

NUCLEAR REGULATORY COMMISSION

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September 6, 1988 Docket No. 50-482 Mr. Bart D. Withers President and Chief Executive Officer Wolf Creek Nuclear Operating Corporation Post Office Box 411 Burlington, Kansas 66839

SUBJECT:

THE IPPACT OF PROLONGED LOW POWER OPERATION ON THE RELIABILITY OF CHECK VALVES l

Reference:

Letter from M. O. Pedford to NRC, "Investigation Report, San Onofre Unit 1 Water Hamer Event of November 01, 1985 " dated April, 1986.

Check valves are used in many systems important to plant safety and reliability.

Failure of check valves can lead to overpressurization of low-pressure systems, water harrer, steam binding, and extensive danage to other components at nuclear power plants.

Recent industry experience showed that accelerated wear and failure of certain check valves could occur due to continued undesirable flow operations. The pririary cause of this prenature degradation is continuous fluctuation of the check valve disc when it is not firinly held against the backstop by the fluid forces pushing on it. The position of the check valve disc can be deternined by a balance of fluid lifting force and the gravitational force acting on the disc and hinge arm weights. Low flow velocities are not sufficient to lift l

the disc through its full stroke and hold it firinly in a stable position against a stop. With each minor flow pulsation, the disc then fluctuates causing prer.ature wear of moving parts. This instability if coupled with close proximity upstream flow disturbance such as elbows, tees, control valves and pumps could cause rapid wear and in some cases even failure of the valves.

An analysis (Reference 1) was perfomed to identify the root causes of check valve failures during the Noverber 21, 1985, water hamer event at San Onofre Nuclear Generation Station. Unit 1.

That analysis concluded that the failure of the five failed check valves was caused by repeated hansnering of the disc stud and the stud nut against the backstop due to insufficient disc lifting force of the low flow rate during reduced power operation. The plant had been operating at reduced power and the reduced feedwater flow, coupled with a high level of turbulence due to increased throttling of the flow control valve, resulted in an arplified oscillation and accelerated degradation. Eventually it led to failures of the check valves.

Based on the above discussion, it is reasonable to conclude that certain check valves are vulnerable to damage caused by prolonged reduced flow operation.

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Mr. Bart D. Withers Therefore, I arn advising you of this concern and requesting that you review your plant configuration for possible inpact of prolonged low flow operation on the reliability of check valves.

This letter requires no fornal response.

Sincerely,

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Paul W. O'Connor, Project Manager Project Directorate - IV Division of Reactor Projects - !!!,

IV, Y ard Special Projects cc: See next page i

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t Therefore, I am advising you of this concern and requesting that you review t

your plant configuration for possible impact of prolonged low flow operation on i

the reliability of check valves.

l This letter requires no formal response.

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Paul W. O'Connor, Project Manager i

Project Directorate - IV 3

Division of Reactor Projects - III, IV, V and Special Projects r

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Mr. Bart D. Withers Wolf Creek Generating Station Wolf Creek Nuclear Operating Corporation-Unit No. I CC:

Jay S11 berg, Esq.

Mr. Gerald Allen Shaw, Pittinan, Potts & Trowbridge Public Health Physicist 1800 M Street, NW Bureau of Air Quality & Radiation Washington, D.C.

20036 Control Division of Environinent Chris R. Rogers, P.E.

Kansas Department of Health Manager, Electric Departsnent and Environment Public Service Conenission Forbes Field Building 321 P. O. Box 160 Topeka, Kansas 66620 Jefferson City, Missouri 65102 Mr. Gary Boyer, Plant Manager Regional Administrator, Region !!!

Wolf Creek Nuclear Operating Corp.

U.S. Nuclear Regulatory Comission P. O. Box 411 799 Rcosevelt Road Burlington, Kansas 66839 Glen Ellyn, Illinois 60137 Regional Administrator, Region !Y Senior usident Inspector / Wolf Creek U.S. Nuclear Regulatory Comission c/o U. S. Nuclear Regulatory Comission Office of Executive Director i

P. O. Boy 311 for Operations Burlington, 8:ansas 66839 611 Ryan Plaza Drive. Suite 1000 i

Arlington, Texas 76011 Mr. Robert Elliot, Chief Engineer Utilities Division Mr. Otto Maynard, Manager Licensing Kansas Corporation Comission Wolf Creek kuclear Operating Corp.

4th Floor - State Office Building P. O. Box 411 Topeka, kansas 66612-1571 Eurlington, kansas 66839 i

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