ML19312C851

AO-270/75-18:on 750825,loss of Required Overlap Between Operating Control Rod Groups.Caused by Dropping Group 7 Rods & Withdrawing Group 6.Power Supplies Examined & No Problems Identified.Required Overlap Immediately re-established

ML19312C851
Person / Time
Site:	Oconee
Issue date:	09/08/1975
From:	DUKE POWER CO.
To:
Shared Package
ML19312C847	List: ML19312C846 ML19312C851
References
NUDOCS 8001130005
Download: ML19312C851 (2)

Text

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9 DUKE POWER COMPANY OCONEC UNIT 2 ijeport No.: A0-270/15-18

_lje po r t Dqtel September 8, 1975 Occurrence Dat e: August 25, 1975 racilityi Oconec Unit 2, Seneca, South Carolina Identification of Occurrence: Loss of required overlap between operating control rod groups Conditions Prior to occurrence: Unit at 95 percent full power Description of Occurrence:

On August 25, 1975, while the unit was operating at 95 percent full pover with control rod Group 7 at approxitately 18 percent withdrawn, the Group 7 rods dropped into the core. 'Ihe Integrated Cor.t rol System (T CS) ir.:-

mediately withdrew Group 6 to 100 percent withdrawn position to maintain power level. The dropping,of the Group 7 control rods, followed by the withdrawal of the Group 6 centrol rc.ds by the ICS, resulted in the loss of the required 25 1 5 percent overlap between the operating control rod groups. The Control Operator immediately placed the Control Rod Drive System in manual control and resequerced Groups 6 and 7 to establish the required overlap. The 25 5 percent overlap requitecent was not met during a period of approximately five rainutes.

Designation of Apparent Cause of C currence:

The apparent cause of this occurrence was the dropping of Group 7 control rods and subsequent withdrawal of Group 6 by the ICS. The Control Rod Drive System and Group 7 power supply were e::atined to determine the cause of the Group 7 rod drop, and no problems were identified.

Analysis of Occurrence:

The regulating control rod group sequence is such that when a group is either less than 25 percent withdrawn or above 75 percent' withdrawn, two adjacent regulating groups are operated in overlapped fashion since the reactivity worth of a group is low at the upper and lower part of the group. Technical Specification 3.5.2.5b specifies an overlap of 25 5 percent between two operating groups. Ucwever, there is no core safety significance associated with the mininum overlap requirement, rather the l ccacern is for an overlap greater than 30 percent. Following the Group 7 l rod drop, the overlap becar.e 0 percent until the r2 quired overlap was l reestablished after five minutes. This reductirn in overlap did not l j create any unsafe conditions, and no core protection limits were approached. i This incident did not affect the health and safety of the public. 1 b l 80013gonog

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Correetiver Actlon:_

I i The 2515 percent overlap was in=ediately reestablished by reali;ning t control rod Groups 6 and 7 to appropriate positions.

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