ML20084D211

AO 50-237/75-36:on 750606,MSIV-1C 10% Closure Limit Switch Failed.Caused by de-energized 10% Closure Relay.Switch Reassembled W/Less Torque Applied to Switch Contact Assembly Screws.Switch Operated Successfully

ML20084D211
Person / Time
Site:	Dresden
Issue date:	07/03/1975
From:	Stephenson B COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
410-75, 7353, AO-50-237-75-36, NUDOCS 8304080497
Download: ML20084D211 (2)

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One First'Wational Plaza. Chicago, lihnois Address Reply to: Post Office Box 767 l

Chicago, Illinois 60690 l

BBS Ltr. #410-75 Dresden Nuclear Power Sta.ti R. R. #1 k.J j

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July 3, 1975 g

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Mr. James G. Keppler, Regional Director

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A U. S. Nuclear Regulatory Co:=ission 799 Roosevelt Road

'*N Glen Ellyn, Illinois 60137

SUBJECT:

REPORT OP UNUSUAL EVD'T PER SECTION 6.6.C OF T'4E TECHNICAL SPECIFICATIONS FAILURE OF MSIV-1C "d,10% CLOSURE" LIMIT TJITCH

References:

1) Regulatory Guide 1.16 Rev. 1 Appendix A

2) Notification of Region III of U. S. Nuclear Regulatory Co:rraission Telephone:

P. Johnson, 1400 hours0.0162 days <br />0.389 hours <br />0.00231 weeks <br />5.327e-4 months <br /> on June 6, 1975

3) Drawing Number 12E2464 Report Number:

50-237/75-36 Report Date: July 3, 1975 Occurrence Date: June 6, 1975 Facility: Dresden Nuclear Power Station, Morris, Illinois IDENTIFICATION OF OCCURRENCE While testing the " n10% closure" of the main steam isolation valves (MSIV's) on Unit-2, a half-scram was received on channel "B".

_ CONDITIONS PRIOR TO OCCURRENCE Unit-2 was at a steady-state power level of 1650 IGt and 525 me.

DESCRIPTION OF OCCURRWCE At 0036 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> on June 6, 1975, while MSIV-1A was being tested for the "fi10% closure" surveillance, a half-scram was received on channel "B" of the reactor protection system. At this point, the surveillance was terminated and MSIV-1A was returned to the open position.

8004080497 750703 PDR ADOCK 05000237 7353 JUL' 7 P3

Mr. Jamea G. Kepp

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July 3, 197$

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DESIGNATION OF APPARD7P CAUSE OF OCCURRENCE (Component Failure)

Investigation revealed that the "$10% closure" relay on MSIV-10 was de-engerized because the limit switch had remained open electrically after the previous

"$10% closure" test. The limit switch had functioned properly during five

" $10%' closure" tests since being installed on May 19, 1975 ANALYSIS OF OCCU:iRECE The " $10% closure" limit switch failed in the " fail-safe", or open position, initiating a scram signal in the "B" channel of the reactor protection system.

Any scram signal in the "A" channel would have immediately triggered a scram; therefore, plant personnel and the public were not endangered by this occurrence.

CORRECTIVE ACTION On June 14, 1975, a drywell entry was made to inspect the 'C MSIV " 610% closure" limit switch. The switch cover was removed and the contacts were found to

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be binding. The switch was reassembled with less torque applied to the switch contact assembly screws and the switch operated satisfactorily.

j Two problems which have occurred with these switches have been:

1) Warping of bakelite switch contact assembly

2) Operating arm switch assembly slippage The electrical maintenance department has ordered new nuclear grade switches of the same type which should eliminate both problems. Replacement will take place during the next outage as time permits.

I FAILURE DATA This type of limit switch has had a history of failures at Dresden as rqported in lotters U-3-72-7, U-3-72-12, and U-3-73-6.

The switch is a snap-lock type SL3-11W, manufactured by Namco.

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