ML20059M149

Special Rept,On 900918:shim 1 Relay Failed to Release Completely Upon Manual Scram.Caused by mercury-wetted- Contact Relay in Manual Scram Circuit of Shim.Relay Replaced & Relays in Sys Checked & Found to Operate Normally

ML20059M149
Person / Time
Site:	National Bureau of Standards Reactor
Issue date:	09/25/1990
From:	Rowe J COMMERCE, DEPT. OF
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9010030196
Download: ML20059M149 (2)

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, UNITED STC.TES DEPARTMENT OF COMMERCE

. . . Nationd inetitute of Stenderde and Technology t ) cav - e ro w v.% ecaca  ;

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! (301) 975 6210

, FTS 879-6210 FAX 921 9847 ,

i september 25, 1990 l i

V.S. Nuclear Regulatory Commission 1 Attention: Document Control Desk k'ashington, DC 20555 Subj ec t: Shim #1 Relay, Docket No. 50 184 Centlemen:  ;

l On Tuesday morning, September 18, 1990, there was a routine shutdown to allow maintenance en the cryoger.ic facility. Routine shutdown is accoroplished by driving the shim arms to about 12 degrees, which is at the

, edge of the shock absorber, and then manually scraimning the rest of the way.

The scram cuts off current to the clutch allowing it to disengage and +he shim to fall. If the clutch is not released, a rundown signal associated with the scram drives the shims in. -

Two hours later, fo11owing completion of the maintenance, an- attempt was i made to restart the reactor, but there was insufficient reactivity to i

override xenon. ~ k' hen the reactor was chut down again, shim No. 1 failed to

( release completely upon' manual scram from 12 felling to about 9 . From l

there it drove in to abbuti-6 at which time the manual scram button was ,

depressed again and held resulting in the shim dropping to the bottom.

During this period, it.was noted that the current'to the clutch of Shim No.

I did not cut off 'even after the reactor keys,were removed. The manual scram button was depressed again, and the current did cut off. The problem was traced to a mercury wetted-contact . relay in'the manual scram circuit of Shim No. 1, the contacts of- which failed, tio = break completely. The '

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. o Page 2 manual scram circuit is completely independent of the nuclear logic scram.

The relay -was replaced and the shim operated normally. This type of= relay.

has proved to be extremely reliable over many years. All oth:t .similcr relays in the systes were checked and were found to operate norarily.

Sincerely,-

./ ,l

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, Michael Rowe Chief, Reactor Radiation Division cc: Project Inspector U.S. Nuclear Regulatory Commission Region 1 475 Allendale Road King of Prussia, PA 19406 Project Manager Non Power Reactor Directorate U.S. Nuclear Rtgulatory Commission k'ashington, DC 10555 1

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