ML20085B419

Ao:On 720602,control Rod Drive Mechanism 5 Failed to Drop. Caused by Clutch Faces Not Separating Due to Prior Racheting Operation.Clutch Assembly Replaced & Racheting Procedures Modified to Include Replacing Clutch

ML20085B419
Person / Time
Site:	Palisades
Issue date:	06/12/1972
From:	Haueter R CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:	Oleary J US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20085B411	List: ML20085B419
References
NUDOCS 8307080131
Download: ML20085B419 (3)

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.J a oenerai on.ces: 2i2 west u.chigan Avenue. Jackson Machigan 4 92O1. Area Code 517 7R8-0550 June 12,1972 Mr. John F. O' Leary, Director Re: Docket No 50-255 Directorate of Licensing License No DPR-20 US Atomic Energy Commission Washington, DC 20545

Dear Mr. O' Leary:

This letter is written to apprise you of a malfunction that occurred June 2,1972, at the Palisades Plant, pertaining to the clutch assembly of Control Rod Drive Mechanism (CRDM) No 5 After the safety check sheets had been completed, it was ob-served that CRDM No 5 had not dropped from the Lower Electrical Limit (LEL) which should have occurred due to an inadvertent reactor trip during the safety system check. The test panel trip switch was placed in the clutch power "off" position and CRDM No 5 did not respond. CRDM No 5 was then raised to 8 inches and again to 9 inches and did not re-spond when the test switch was actuated. CRDM No 5 was withdrawn above the LEL and successfully tripped when the drive was run back in to the LEL with the test switch in the "on" position.

CRDM No 5 was successfully drop-tested five times and a torque trace was performed with no abnormalties noted. The investigation of CRDM No 5 did not indicate the cause for misoperation; therefore, it was determined that additional drop tests would be performed. CRDM No 5 was

, fully withdrawn and would not drop when the test switch was operated.

~ [l The operation of the No 5 motor drive was observed at the reactor head s and it was visually verified that the clutch faces did not separate.

.j_ Plant personnel verified that the voltage to the drive was correct and 3g that the test switch was operable. Plant personnel then removed the control cable from the drive package and the CRDM clutch still failed

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to separate. The CRDM clutch lower jaw was tapped with the handle of {

a screwdriver and the jaws disengaged, dropping the rod.

i The CRDM No 5 drive package was removed aqi,j;ha-elutch assembly replaced with a spare unit. This drive package was reinstalled and Isuccessfully drop-tested six times and the clutch separation was normal in all respects. The CRDM torque trace was also normal. l

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Mr. John F. O'Lu.ry 2 Docket No 50-255

. June 12, 1972 The faulty clutch assembly was disassembled and inspected at the plant. This inspection did not reveal the cause for the improper i operation of the clutch; therefore, it was decided to ship the unit back to Combustion Engineering for a detailed examination.

The clutch assembly was inspected by Combustion Engineering at their Windsor facility and this detailed inspection did not indicate an obvious cause for the failure to disengage on June 2,1972.. The clutch assembly was then subjected to over 150 trips which included different i operating temperatures and loads and included misoperation as described below. This clutch assembly operated satisfactorily in all respects.

This clutch assembly was subjected to one unusual operating condition at the Palisades Plant immediately before it failed to dis-engage, which was a ratcheting operation that occurred when the drive mechanism was inadvertantly driven against the upper mechanical hard-stop during post-maintenance. checkout due to a limit switch-synchro assembly being out of adjustment. The CRDM are normally prevented from driving into the hardstop by an Upper Electrical Limit (UEL). This is the most severe operation which the clutch can experience and testing programs have demonstrated that repeated extensive ratcheting can ul-timately cause failure. We, therefore, conclude that the ratcheting operation, although the detailed inspection could not definitely cor-roborate it, was the cause of the malfunction. Driving a CRDM into the upper hardstop is normally prevented by two means, the synchro system.and a limit switch. Both of these systems are checked out prior to operation.

The ratcheting of a CRDM is a very unlikely occurrence and because this is the only abnormal operation that CRDM No 5 had experi-enced prior to the misoperation, we must conclude at this time that this caused the failure to disengage. We will institute a procedure that will require replacement of a clutch assembly that has been sub-jected to ratcheting. The clutch mechanism will be carefully inspected, refurbished if necessary and tested prior to reuse.

During a reactor trip signal, the CRDM receive a " Rod Rundown" signal which would drive in any rod whose clutch does not separate.

This backup feature will assist the clutch jaws in disengaging, in addition to driving the rod to the " full in" limit.

We have concluded that the misoperation of CRDM No 5 was

probably caused by the ratcheting of the clutch assembly because the drive was driven into the upper hardstop. In the future, if a drive -

is subjected to ratcheting, the clutch assembly will be replaced.

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-Mr. John F. O'Le y 3 Docket No 50-255 June 12,1972 We further concluded that the reactor could be operated safely and, therefore, it was returned to service.

Yours very truly, Robert L. Haueter (Signed)

RLH/ map Robert L. Haueter Electric Production Superintendent -

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