ML19322C145

RO 287/76-08:on 760609,control Rod Overlap in Excess of Tech Specs Limit Due to Dropped Group 7 Control Rod.Caused by Parallel Group 7 Control Rod Power Supply Failure.Blown Fuse & Failed Transistor Caused Power Failure.Parts Replaced

ML19322C145
Person / Time
Site:	Oconee
Issue date:	06/23/1976
From:	DUKE POWER CO.
To:
Shared Package
ML19322C133	List: ML19322C132 ML19322C145
References
RO-287-76-08, RO-287-76-8, NUDOCS 8001090558
Download: ML19322C145 (2)

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$ DUKE POWER COMPANY OCONEE UNIT 3 Report No.: RO-287/76-8 7g Report Date: June 23, 1976 Occurrence Date: June 9, 1976 Facility: Oconee Unit 3, Seneca, South Carolina Identification of Occurrence: Control rod overlap in excess of Technical Specification limit due to a dropped control rod group Conditions Prior to Occurrence: Unit at 100 percent full power Description of Occurrence:

On June 9, 1976, during full power operation of Oconee Unit 3, the Group 7 control rods dropped into the core resulting in a control rod overlap condition of less than 25% t 5%. This overlap is required by Oconee Technical Specification 3.5.2.5.b. The control operator promptly placed the control rod drive system in manual control and decreased reactor power to 88 percent full power with Group 6 control rods at 80 percent withdrawn to re-establish the required overlap. Control rod Group 7 was then placed on auxiliary power and reactor power was increased to 97 percent full power while maintaining the required overlap. Repairs to control rod Group 7 were completed within four hours and the group was returned to its normal power supply.

Apparent Cause of Occurrence:

This incident resulted from the failure of both parallel Group 7 control rod power supplies. One power supply was inoperable due to a blown fuse and a tripped breaker. No apparent cause for this failure was determined. The other power supply failure was due to a failed transistor in the A phase output of the programmer feeding the gate drives. As a result of the failure of these power supplies, the Group 7 control rods dropped into the core when it became necessary to energize the A phase.

Analysis of Occurrence:

This incident resulted in control rod Group 7 dropping into the core and a condition of approximately 18 percent control rod overlap existing for approximately five minutes until Group 6 control rods were re-positioned and the required overlap was re-established. This reduction in control- rod overlap did not affect the safe operation of the reactor and no ' core  ;

protection limits were approached. It is therefore concluded that this occurrence did not affect the health and safety of the public.

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8001.090 [ N 1

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Corrective Action:

Within four hours following the incident, the faulty programmer and fuse were replaced, the breaker reset, and the Group 7 normal power supply returned to service and verified operable.

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