ML20086L131

Ao:On 731204,scram & Group I Isolation Accidentally Initiated While Surveillance Testing of Main Steam Line Tunnel Temp Detectors in Progress.Caused by Failure of Switch 231D to Trip W/Companion Switches

ML20086L131
Person / Time
Site:	Peach Bottom
Issue date:	12/11/1973
From:	Cooney M PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC
To:	Giambusso A US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20086L132	List: ML20086L131 ML20086L134
References
NUDOCS 8402070408
Download: ML20086L131 (3)

Text

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. . O O PHILADELPHIA ELECTRIC COMPANY 2301 MARKET STREET PHILADELPHIA, PA.19101 12151 841 40oo December 11, 1973 Mr. A. Giambusso )

9-Deputy Director of Reactor Projects g n United States Atomic Energy Coninission I,

}f, , Ci Directorate of Licensing Washington, D. C. 20545 .

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Dear Mr. Giambusso:

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Reference:

Peach Bottom Atomic Power Station - Unit 2 Facility Operating License DPR-44 Docket No. 50-277

Subject:

Abnormal Occurrence Late on December 4, 1973, with the reactor operating at approximately 3% power, a scram and Group I isolation (closure of main steam isolation valves) was accidentally initiated while surveillance testing of the main steam line tunnel temperature detectors was in progress. The scram and isolation functions performed satisf actorily and plant response was as expected.

Subsequent review of the plant computer alarm typer output indicated that level switch 231D on the scram discharge volume had not operated at the same time as its other three companion switches. The 2310 switch had tripped 37 minutes later than the other identical de-vices. This was a repeated failure, as this same switch had failed to operate on two previous occassions. The previous f ailures had been reported on September 8,1973 and October 29, 1973 This failure was

, reported to Mr. Tom Shedlowsky, A.E.C. Region I, Regulatory Operations Office on December 5, 1973 In accordance with Section 6 7 2.A of the Technical Specifica-tions Appendix A of DPR-44 for Unit 2 Peach Bottom Atomic Power Station, this malfunction is being reported to the Directorate of Licensing as an Abnormal Occurrence. ,

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. O 2 O Investigation and Corrective Action Due to the repetitive nature of malfunctions on this partic-ular switch, the plant POR Commi ttee directed that the instrument designer be requested to come to the site to assist in the investigation and correction of the problem. The PORC further directed that the plant not be returned to power operation until the designer's appraisal had been received. The designer of this specific instrument arrived at the si te on December 6,1973. The switch was visually examined and tested in his presence.

The switch operated satisfactorily with its protective cover removed, but intermittently failed to operate with the cover installed.

The subtle difference was the length of wire between the terminal on the mercury bulb and the instrument internal terminal block. Since the mercury bulb mechanism is a low torque device, and the wire rubbed on the inside of the instrument protective case, intermi ttent or late operation was the result. The 231D switch had been supplied with a longer than normal section of wi re. This mercury bulb mechanism was replaced with an identical device with the correct wire length. The switch was tested satisfactorily under both conditions of cover in place and cover removed.

The designer was requested to inspect the remaining five (3 scram, I rod block, I alarm) identical switches on the scram dis-charge volume for any deficiency which might preclude the low torque device from operating.

Visual observation of the 231E switch (rod block function) indicated that a deformed contact on the mercury bulb rubbed against an actuator spring during the arc motion of the bulb. Operation was determined to be marginal, so the switch mechanism was replaced with an identical device whose mercury switch contacts were not bent.

Although the 231A switch (scram function) had always operated satisfactorily, it was visually observed to exhibit the problems asso-ciated with 231D and 231E, but to a far less observable degree. This switch mechanism was replaced with an identical device whose wiring was of proper length and mercury bulb contacts were not deformed.

In summation, three problems have been found and satisfac-torily resolved on the mercury switch mechanism.

1. Vendor supplied pivot ' adjustments too tight.

2. Vendor supplied wiring too long.

3. Vendor supplied mercury bulb contacts bent.

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O i l As proof testing of a satisf actory end-product, all six level i switches were surveillance tested and then the shutdown reactor was test scrammed twice. On all three occasions, all six switches actuated

{' properly.

i Safety Implications i

The scram discharge volune high level switches are used in a one of two twice logic to scram the reactor when the water level in the l

scram discherge volume reaches 50 gallons. These switches ensure a i

reactor shutouwn before the available volume of the Scram Discharge Volume is reduced to the point where the reactor could not be scrammed.

With the reactor at power this scram circuit would be activated if several control rods had a very high leakage from the reactor thr ough their scram exhaust valve to the header. Presently, the leakage from the reactor via this path is essentially zero. Since 3 of the 4 level switches which monitor this function were operable, a sudden increase in leakage would have successfully produced the required reactor scram.

The f ailure of an individual switch in this logic circuit does not '

have significant safety implications.

1 l Very truly yours,

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,'l lll-Inli t ye

'M. J. Cooney '

Ass't Gen't Superintendent Generation Division HJC:kam f cc: Mr. J. P. O'Reilly l Director. Region 1 '

j United States Atomic Energy Commission 631 Park Avenue King of Prussia, Pa. 19406 i

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