ML19260B184

LER 79-152/01T-0:on 791116,nonconservatism Found in Error Analysis of Nuclear Instrumentation Reactor Rate Trip Circuit.Caused by Improper Treatment of Factors Affecting Accuracy of Reactor Protection Rate Trip Circuitry

ML19260B184
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-79-152-01T, LER-79-152-1T, NUDOCS 7912070386
Download: ML19260B184 (2)

Text

NRC FORM 366 U. S. NUCLEAR REGULATORY COMMISS;ON (7 77) -

LICENSEE EVENT REPORT (PLEASE PRINT OR TYPE ALL REQUIRED INFORMATION)

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• t e,, 1 0 3 l exists in the error atC mis of the Nuchar Instrum atation Reactor Rate Tein ci rcini t -

o 4 l Since April 1979, plant operating conditions were administrative 1y r,nineninaA *a pro- I O s Iclude the consequences of this event. This event is rer,ortable as per T.S. 6.4.1_ R _ rt 1 0 s l The health and safety of the general public were unaffected by thic: evane- I o 7 l .

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CAUSE DESCRIPTION AND CORRECTIVE ACTIONS h i O lThe cause of this event was improper treatment of error analysis concernine cancelln- I i i l tion of errors in circuitry which preceeds a dif ferentiator in the rare trin cirr>>tery 1 i 2 IThe corrective action on an interim basis will procedurally limit reactor acerations I i,,3; lto conditions which preclude any consequences of this event. I i 4 I I 7 8 9 80 ST S  % POWER OTHER STATUS . IS O RY DISCOVERY DESCRIPTION 1 5 Wh l 0l 0 l 0 ]@l NA l [D_J@l Notification from NSSS Vendor l A TlVITY CO TENT RELE ASED OF RELEASE AMOUNT OF ACTIVITY LOCATION OF RELEASE gl NA NA l 7

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NAME OF PREPARER n C3 - * ;- 1 A - PHONE: ^^',_ ; ;

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

Attachment:

Page 1 of 1 Docket No. 50-338 Report No. LER 79-152/01T-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 error analysis assumptions used to determine the accuracy of the Nuclear Instrumentation Reactor Rate Trip circuitry. The present allowance for instrument inaccuracy in the Power range negative rate trip is 0.5% as per T.S. table 2.2-2. This allowance was determined assuming that all circuitry inaccuracies preceeding the rate trip differentiator cancel. Further analysis eveals that the ef fect of errors associated with precision, hysteresia, repeatability, etc., was greater than originally assumed. Prelimina y analysis which accounts for this consideration resulted in an error of about 1 to 1.8%.

Probable Ccnsequences of Occurrence In reference to LER 79-041/01T-0, since April 1979 the unit was administra-tively limited to operations to preclude the e5?ect of an event in which a single dropped rod would result in 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 an initial operating license.

Cause of Occurrence The cause of this occurrence was determined to be improper treatment of factors which af fect the accuracy of the Reactor Protection Rate Trip Circuitry.

Immediate Corrective Action The i= mediate 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% Pcver: No change from current rod insertion limit.

Automatic Rod Cnntrol O to 90% Power: No change from current rod insertion 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 effects on load rejection transients. This action will ulticately minimize the impact of the problem in the setpoint error analysis by mitigating the results if the reactor fails to trip.

Scheduled Carrective Action Scheduled corrective action is to implement the commitment referenced above and to re;'ew for possible implementation any long term solutions which may be more desirable.

Actions Taken to Prevenc Recurrence -

No actions are required.