ML20086E713

AO 74-04:on 740223,MSIV Rv 2-71B Failed to Open at Required Setpoint.Indirectly Caused by Approved vendor-recommended Mods to Certain Relief Valve Internals.Procedure Revised

ML20086E713
Person / Time
Site:	Vermont Yankee
Issue date:	03/01/1974
From:	Riley B VERMONT YANKEE NUCLEAR POWER CORP.
To:	US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20086E718	List: ML20086E713 ML20086E724
References
AO-74-04, AO-74-4, VYV-3179, NUDOCS 8312070496
Download: ML20086E713 (3)

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SCVENTY GCVEN OROVC STRCCT RUTI, AND, VI;1t BIO NT 03701 VYV-3179 r>. o. nOx i s7 VERNON, VERMONT oSa54 March 1, 1974 A

Director 4Y '

Directorate of Licensing -N' ,

United States Atomic Energy Cc=tission Washington, D.C. 20545  ; ,

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• e REFElm:;CE: Operating License DPR-28  ;

Docket. 1:o. 50-271 F x'L' c o'

Abnormal Occurrence 1:o. A0-74-04 \ '

n'[tg. .;Ws Gentlemen:

As defined by Technical Speelfications for the Vermont Yankee I;uc1 car Powcr Station, Section 6.7.B.1, we are rcporting the following Abnormal Occurrence as A0-74-04.

At approx!nately 1008 on February 23, 1974, while performing a Main Steam Isolation Valve full closure test as a part of the plant startup test program, one of the four Main Steen Relief Valven, RV 2-71B, failed to open at Its required setpoint as referenced in Technical Specifications, Section 2.2.B. It was determined that this condition was in violation of Technical Specifications, Section 1.0.A.1, in that it caused a safety system setting less conservative than the limiting setting established in Section 2. The reactor was in a scrammed condition at the time of the incident and had been operating at approximately 93%

power preceding the occurrence.

Valve RV 2-71B was r,ubsequently manually opened through its Control Room panel mounted switch at approximately 700 psig reactor pressure. The manual opening confirmed valve operability in that mode of operation.

The maximum reactor pressure attained throughout the transient was approximately 1104 psig which was well uithin the design and Technical Specification pressure limits.

A plant cooldown was initiated and ir. mediate action was undertaken to determine the caw:e of and correct the valve problem prior to resuming plant operation.

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• TION l Directorate of Licensing l March 1, 1974 l Page 2 On February 25, 1974, a drywell entry was made to investigate the cause of the valve problem. A pressure test was performed which revealed the valve lift point to be 1120 psig. The pilot valve bonnet cap was match-marked and removed in order to permit adjusting of the valve litt point. The .alve was adjusted to lif t at 1080 psig, retested, and the bo. met cap was replaced.

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At this time, it was obt,erved that ttie bonnot cap was "rochinn" in a hori7nntal pinne rather than displaying fifm flange alignment. Upon investigating, it was determined that the dowel pin, which is an integral part of t.he bonnet cap and extends through the set point adjusting nut to prevent it from changing position, was bearing on the set pressure adjustment spring causing spring compression. It should be noted that additional spring compression causes the valve to lift. at a higher set point.

To correct this problem, the dowel pin was shortened approximately 3/16" to prevent the pin from protruding through the set point adjusting nut. Subsequent positive flange alignmcot was proven when repeated efforts t.o " rock" the bonnet cap proved unsuccessful. After replacing the bonnet cap, the valve was tested three times and each test verified l that the lift po3nt was 1080 psig. l This problem had not been previously experienced at Vermont Yankee.

The reason the problem occurred this time is attributed to the following factors. Certain internals of the relief valves were modified per approved vendor recommendation and direction during the fall 1973 reconstitution outage. These modifications resulted in, among other things, the final position of the adjusting nut being closer to the cover plate than had existed before, ubich effectively resulted in the 7

dowel pin being too long. Also, the existing procedure allowed dctermination of final lift pressure with the cover on or off.

The three iemaining relief valves were inspected to determine the j dowel pin position relative to the set pressure adjustment spring. This inspection indicated that in all cases sufficient clearance did exist.

A reason for this is the fact that these valves are set to lift at a higher pressure which requires the set point adjusting nuts to be threaded further clockwise into the bonnet. Thus, the dowel pins are prevented f rom extending through the adjusting nuts and bearing on the adjustment springs.

Valve RV 2-71B was declared operable at 1920 on February 25, 1974,.

and a reactor startup commenced at 0130 on February 26, 1974.

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O D VERMONT YANKCE NUCLEAlt POWCR CORPORATION 111 rectorate of Licensing March 1, 1974 l' age 3 The plant Operations Review Committee meeting to review the incident agreed that the corrective measure taken was appropriate. In addition, it was recommended that the procedure used in t esting Main Stean Relief Valves be revised to:

1. Require final lift point testing after valve assembly subsequent to any maintenance having been performed.

2. Insure adequate dowel pin engagement with the set point adjusting nut on reinstallation of the pilot valve bonnet cap.

Very truly yours, VER\l0NT YANKEE NUCLEAR PO'AER CORPORATION D ')

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ll.U. Riley ~ ~A Plant Superintendent UFC/kbd

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