ML19317F441

RO 270/77-17:on 761227,quadrant Tilt Exceeded Tech Spec Limit.Caused by Failure of Electrical Component Supplying Control Rod Group 6 & 7 Drive Assemblies.Components Replaced & Tilt Closely Monitored

ML19317F441
Person / Time
Site:	Oconee
Issue date:	01/20/1977
From:	DUKE POWER CO.
To:
Shared Package
ML19317F437	List: ML19317F433 ML19317F441
References
RO-270-77-17, NUDOCS 8001140679
Download: ML19317F441 (2)

Text

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DUKE POWER COMPANY OCONEE UNIT 2 Y

• Keport Nr.: s R0--270/ 76-17 Report Date_: January 20, 1977 Occurrence Date: December 27, 1976 Facility: Oconce Unit 2, Seneca, South Carolina Identification of Occurrence: Quadrant t11t limit exceeded Conditions Prior to Qccurrence: Unit at 100% full power Description of Occurrence:

On December 27, 1976, the reactor protective system on-line channel check was performed on Ocons e Unit 2. This test involvec tripping of the AC supply breaker to one of the two redundant parallel control rod drive power supplies. When the AC breaker for the B control rod drive power supply was tripped, group 5 rods partially fell into the core, stopping in a scattered pattern. The group 7 rods also dropped from approximately 10% withdrawn after 6 momentary pause. Due to the assymetric configuration of the group 6 control rods, an indicated tilt of 9.1% was observed on the out-of-core detectors. This tilt exceeded the limits of Technical Specifi-cation 3.5.2.4 for actual incore tilt and hence, a reactor shutdown was initiated. Approximately six minutes later the reactor tilt was restored within limits; howpver, it was discovered that rod 6 in group 6 was not properly responding. Two minutes later the reactor was manually tripped from 40% full power.

Apparent Cause of Occurrence:

The apparent cause of this incident was the failure of sen ral electrical components supplying the control rod drive assemblies. Tho control rod drives are six phase star connected stators connected for aperation in a pulse stepping mode. To obtain control rod motion, a rotat!nn madnetic flux field is created by alternately energizing the windings. Two windings must be continuously energized to raaintain control rod position.

When'the B power supply breaker was tripped, two of six phases each of groups 6 and 7 were rendered inoperable due to component failures. There-fore, the rods in these two groups started. falling when the control system started to command rod motion and at least two motor phases were not energize 5. The B power supply breaker was reclosed in approximately 2 seconds; thereby stopping the further dropping of group 6 and 7 rods.

This incident did not affect the reactor protective system's ability to trip the control rods. This is because the RPS employs two redundant, independent ' trip mechanisms to ensure that all six phases of the control rod drive assemblies are de-energized and proper rod motion is assured.

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. - Analysis of Occurrence:

This occurrence was caused by the dropping of group 6 and 7 control rods and resulted in the core tilt exceeding the limits specified in Technical The limits on quadrant power tilt set forth in Specification 3.5.2.4(c).this specification have been established with cor.aidera ef fects of tod bowing and fuel densification to prevent thetilt linear heat from ex-rate peakfag increase associated with a positive quadrantthe maximum linear ceeding r,11owable limits. It has been established that A conser-heat rate prior to this occurrence was approximately 11.0 kw/f t.

transient vative analysis of the maximum linear heat rate during this indicates that a maximum heat rate of 13.2 kw/ft could have been achieved.

This is well below the established limits on linear heat rate.

Additionally, as described above, this occurrence did not affect the ability of the reactor protective system to function properly, if required.

It is therefore concluded that the health and safety of the public was not affected by this occurrence.

Corrective Action:

The control rod drive power supply components were replaced and preperrod Control operation of the control rod drive system was demonstrated.

The determination of 6, group 6 was exercised and performed properly.

the *; actor tilt was identified as being caused by the dcopped controlIt rods and the tilt was closely monitored during the reactor restart.

is concluded that all systems performed properly, i

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