ML20085B774

Ro:On 71122,control Rod Drive Mechanism 39 Found Approx 4 Inches Below Remainder of Rods in Regulating Group 4.Caused by Improper Operation of Motor Brake.Motor Package Replaced

ML20085B774
Person / Time
Site:	Palisades
Issue date:	12/30/1971
From:	Sewell R CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 8307080378
Download: ML20085B774 (2)

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RFCULATCRY D December 30, 1971

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m a Dr. Peter A. Morris, Director Re: Docket No 50-255 Division of Reactor Licensing License No DFR-20 United States Ate =ic Energy Commission Washington, DC 20545

Dear Dr. Morris:

This letter is written to apprise you of an incident that occurred December 23, 1971, at the Palisades Plant, involving the motor package of control rod drive mechanism (CRDM) No 39 Plant Conditions at the Time of the Incident At the time the incident occurred, the reactor was critical at 2 x 10b power, primary coolant system temperature was 5280F, primary coolant system pressure was 2100 psig, three primary coolant pumps were running, and the boron concentration was 1215 ppm. Preparations were being made to test run the turbine-driven feed pumps. Reactor regulating Groups 1, 2 and 3 were withdrawn to their upper electrical limits (131 5 inches) and Group 4 (includes No 39) was being used for reactor regulation and was withdrawn to 122 inches.

Description of the Incident t

Regulating Group 4 was being withdrawn to increase reactor power.

A four-inch rod deviation alarm was received. Investigation revealed that CRDM No 39 was approximately four inches below the remainder of the rods in regulating Group h. Manual individual control was selected for CRDM No 39 CRDM No 39 responded intermittently to withdrawal and insert signals.

Control wiring connections were inspected and tested. All appeared satisfactory. It was concluded that the problem was in the CRDM motor package, and most likely due to intermittent or improper operation of the motor brake.

Corrective Action CRDM No 39 was designated inoperable because it would not respond consistently to control signals. Reactor regulating Group 4 was positioned to its upper electrical limit and the reactor regulation function was trans-fered to Group P (the four part-length rods). All CRDM except No 39 vere 8307080378 711230 4

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. Dr. Peter A. Morris 2 December 30, 1971 exercised by moving them approxirately one inch in the insert direction and then restoring the CRDM to their original position. All CRDM oper-ated normally, verifying the conclusion that only one CRDM was inoperable.

The reactor was shut down to the hot standby condition on December 26, 1971, to perfom maintenance on secondary system. During the shutdown, CRDM No 39 was tested for trip times and motor running cur-rents. The results of this testing were normal and the ir.temittent oper-ation could not be made to reoccur. ,,

The motor package was replaced with a spare unit. CRDM No 39 was again tested and tested satisfactorily.

The removed motor package was visually examined after removal.

The bearing surfaces of the brake appeared to be worn more than would be expected for the amount of operation experienced. No other abnomalities were observed. The motor package was shipped to Combustion Engineering for further inspection.

The motor package was disassembled and inspected by Combustion Engineering. These inspections revealed no defects in any portion of the motor package with the exception of the brake. The brake appeared to have been dragging. This dragging would be sufficient to prevent the motor package from rotating when it was energized as the motor develops very little torque. Test runs showed that the motor would not start if a finger was placed against the shaft.

Conclusions This incident and further incidents of this nature do not ccm-promise reactor safety because the CRDM is still capable of being tripped.

A failure in the motor package does not prevent proper functioning of the clutch unit. The de-energizing of the clutch separates the motor package frcm the remainder of the CRDM.

The effects of operation with one or more inoperable CRDM have be&n considered. Appropriate operating limitations are incorporated in the Technical Specifications to insure that hot channel factors and ejected rod worth limits are met. A similar occurrence could not go undetected be-cause of the two rod deviation alams. These alams alert an operator to a CRDM position deviation of greater than four inches or eight inches from the rcmainder of the CRDM group.

Yours very truly, Lfe/5e occf RBS/ map Ralph B. Sewell Nuclear Licensing Administrator

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