ML20086E408

AO 74-15:on 741202,scram Discharge Vol High Level Scram Switch 3-231A Would Not Actuate Respective Scram Relay. Caused by Fissure in Switch Casement.Switch Replaced

ML20086E408
Person / Time
Site:	Vermont Yankee
Issue date:	12/03/1974
From:	Riley B VERMONT YANKEE NUCLEAR POWER CORP.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
AO-74-15, VYV-3474, NUDOCS 8312070095
Download: ML20086E408 (3)

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TY VERMONT YANKEE NUCLEAR POWER CORPORATION'

• d SCVENTY SCVCN OROVC STRcCT  ;

Ru rLAxo, Vsusoxr osvoi ""

VYV-3474 P. O. DOX 157

' VERNON, VERMONT c5354 n: MM December 3, 1974 y~ .

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Director Directorata of Regulatory Operations, Regfon I United States Ato ic Energy Co :ission 631 Park Avenue King of Prussia, Fennsylvania 19406 FIFERENCE: Operating License DPR-28 Docket ::c. 50-271 Abnorcal Occurrence No. A0--74-15 (24 Hour Report)

Dear Sir:

As defined by Technical Specifications for the Vermont Yankee huclear Pa.eer L tica, Sectic: 5.7, we cre reporting tha foll owin e Abnormal Occurrence as AC-74-15 and confirming our telephone conversation with your office.

At approxicately 1700 on December 2, 1974, while performing a Reactor Protection Systa= response tine test, it was discovered that Scram Discharge Volume High '.evel scraa switch 3-231A would not actuate its respective scran relay. Technical Specifications Section 3.1 requires that there be a sinicu: cf two operating instrument channels per trip systen for this scran functicn. In the case of the scran discharge volume high 3cvel scran function circuitry, only two instrument channels per trip systen exist and tnerefore the failure of enc of the switches results in a failure to teet the require:ents of the Technical Specifications. The plant was in a cold shutdczn condition at the time of the discovery of this switch failure.

The switch was replaced with a new one and satisfactorily tested tithin an hour and the remaining three switches were satisfactorily tested.

Very truly yours,

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8312070095 741203 Q s 7 Ig PDR ADOCK 05000271 _

D 7 O Bradford W. Riley Plant Superintendent WPn/kbd ID'p>9pt$R!

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t)pFY SENT REGION #  %

(~') A3:;0ML OCCURRE::CE REPORT f)

, %j v Report Nn-?m r : 50-271/74-15 '

.4 Rogort Date: December 12, 1974 Oceqrrence Late: December 2, 1974 Faciliti: Vercent Yankee, Vernon, Vermont Identificatirn of Occurrence _

s Failure of Scram Discharge Volute High Level scram switch to actuate. '

Conditions Prior to Occurredce Plcnt in cold shutdown for a refueling outage.

Description of Occurrence At approxicately 1700 on December 2,1974, while performing a Reactor Protection Syste= response time test, it was discovered that Scram Discharge Volut.e High Level scraa switch 3-231A would not actuate its respective scram relay. Technical Specifications Section 3.1 requires that there he a mininum of two operating instrutent channels per trip systen for this scram function.

In the case of the scran discharge volume high level scram functicn circuitry, caly two instrument channels per trip system exist and therefore, the failure _

of one of the switches results in a failure to meet the requirements of the Technical Specifications.

Desirnation of Apparent Cause of Occurrence Mercury switch 3-231A was discovered to have a fissure in its encaserent. ~

lt was concluded that the cercury became contaninated by oxidation. The oxidation of the cercury, in conjunction with nornal arcing across the switch contacts, caused a permanent conducting path to exist which failed the switch in the closed position.

Analysis of Occurrence Tha Scram Disenarge Volume utilizes two (2) level actuated marcury switches in each of its two (2) trip systems. Switch 3-231C, located in the s::a trip system as 3-231A, and switches 3-231B and 3-231D, located in the other trip systca, actuated satisfactorily during the test. A scram signal would have been generat'ed under actual Scram Discharge Volume high level ccaditions. The last successful test of switch 3-231A was conducted on October S prict to the shutdown for refueling. Based on the availability of the redundant instrucent, the failure of switch 3-231A did not present any safety implication. An investigation of the circuit design revealed that a suitch designed for thirteen (13) amperes is used in a circuit limited to five (5) angeres by fuse protection. It was concluded that the switch is rated properly for this circuit application and its design did not contribute to the failure described.

12742

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00PY SENT REGION i

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,  %.l (O Corrective Action ~

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The faulty switch was replaced and satisfactorily tested on December 2 1974. The failure of this switch is considered to be an isolated incident of component nalfunction.

_ Failure Data External Float Chamber SL-305-E3 cercury switching station switch -

Part No. 3-146-A2, Robertshaw Controls Company. No previous problems ' '

associated with this switch. ,,

Bradford W. Riley Plant Superintendent g E'JB/kbd p-o r

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