ML19260B186

LER 79-153/01T-0:on 791116,nonconservatism Found in Method Used to Generate Power Distributions & Rod Worths for Dropped Rod Events.Caused by Improper Analysis of Reactivity Distributions & Rod Worths

ML19260B186
Person / Time
Site:	North Anna
Issue date:	11/30/1979
From:	Cartwright W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML19260B179	List: ML19260B178 ML19260B184 ML19260B186
References
LER-79-153-01T, LER-79-153-1T, NUDOCS 7912070388
Download: ML19260B186 (2)

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NRC FORM,366 U. S. NUCLE AR REGULATORY COMMISSION (7 77)

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75 80 DOCKET NUM EH E. VENT DATE 14 REPOHT D ATE EVENT DESCRIPTION AND PROB ABLE CONSEQUENCES h o 2 l On 11/16/79 station personnel were informed by Westinghouse that a nonconserva tisn I g 3  ; in the method used to generate power distributions and rod worths for dropped rod I ggg,,; events were discovered. New analyses were performed using a more conservative reacti-I o s l vity feedback model which indicated that certain multiple rod drops may not result in 1 0 s Ia reactor trip. This event is re ,ortable as per T.S. 6.9.1.8.H. The health and l o 7 l safety of the general public were unaffected by this event. I O 8 l l DE CODE SUSC E COYPONENT CODE SUBCOdE SU E 7

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CAUSE DESCRIPTION AND CORRECTIVE ACTIONS h

, g The cause of this event was determined to be improper analysis of reactivity distribu- l

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, , gwill procedurally limit reactor operations to conditions which preclude any consequen- l

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Virginia Electric and Power Company North Anna Power Station, Unit #1

Attachment:

Page 1 of I Docket No. 50-338 Report No. LER 79-153/01T-0 Description of Event On 11/16/79 with the unit in the refueling mode, Station personnel were informed by Westinghouse that a nonconservatism was found in the method used to generate power distributions and rod worths for dropped rod events. A new analysis technique using a frozen feedback and full feedback of rod worths to model a dropped rod eve e indicates that in certain ca'ses double rod drops will not trip the reactor. The model used prior to this event considered only the full feedback effect.

Probable Consequences of Occurrence In reference to LER's 79-041/01T-0 and 79-152/0lT-0, since April 1979 the unit was administratively limited to operations to preclude the ef fect of an event in which a single dropped rod would result in a subsequent rod bank withdrawal causing a power overshoot and DNBR limit violation. This action also provided protection against this event and therefore the health and safety of the general public were unaffected. A similar problem exists for Unit 2 which is awaiting initial operating license.

Cause of Occurrence The cause of this occurrence was determined to be improper analysis of dropped rod reactivity worths under dynamic conditions.

Immediate Corrective Action The immediate corrective action was to send a letter of commitment to the NRC to implement the following procedural solution when applicable.

Manual Rod Control 0 - 100% Power: No change from current rod insertion limit.

Automatic Rod Control 0 to 90% Power: No change from current rod instertion limit.

90 to 100% Power: D bank greater than 215 steps.

The benefit of this solution is that it prevents a power overshoot for rod drop events and will minimize the effects of load rejection transients.

Scheduled Corrective Action Scheduled corrective action is to implement the commitment referenced above and to review for possible implementation any long term solutions which are more desirable.

Actions Taken to Prevent Recurrence No actions are required.

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