ML19305C414

LER 80-005/01T-0:on 800311,during Review of Safety Analysis, Discovered Incorrect Assumption Was Made in Analysis of Mode 5 Boron Dilution Event.Caused by Failure to Consider Operation W/Rcs Drained to Centerline of Hot Leg

ML19305C414
Person / Time
Site:	Millstone
Issue date:	03/21/1980
From:	Gurney P NORTHEAST UTILITIES
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
Shared Package
ML19305C412	List: ML19305C410 ML19305C414
References
LER-80-005-01T, LER-80-5-1T, NUDOCS 8003280513
Download: ML19305C414 (2)

Text

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DATE EVENT DESCHIPTION AND PRO!!ABLE CONStoOENCES

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Paul V. Gurney E-W-M DE NAME OF PREPAHER pgggg; {

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ATTACHMENT LER 80-05/1T-0 Millstone Unit 2 Docket No. 50-336 Event Description During a review of the safety analysis it was discovered that an incorrect assumption had been made in the analysis of the boron dilution event. The analysis of this event while in Mode 5 (cold shutdown) had assumed a full reactor coolant system and a 1% shutdown margin. This resulted in a time to becone critical of 20 minutes f which satisfied the requirement for the 15 minute maxinum time assumed for the operators to recognize the situation and take action. However, the analysis had not considered the fact that operation is possible with the reactor coolant system drained to the centerline of the hot leg. Including this reduced volume of reactor coolant to be diluted in the analysis results in a time to reach criticality of less than 15 minutes.

Cause Description and Corrective Actions At the reduced system volume the time to reach criticality iuring the Mode 5 dilution event would have been less than the 15 i..inutes allowed for the operator to tenninate the event. By increasing the required shutdown margin in Mode 5 to 2%, a time to criticality of greater than 15 ninutes is reestablished. Until the necessary Technical Specification changes can be approved, the unit will administrative 1y maintain a shutdown margin in excess of 2% in Mode

5. In the past it has been standard practice to maintain the Mode 4 shutdown boron concentration requirement while in Mode 5, which provides reasonable assurance that a greater than 2% shutdown margin has been maintained during previous Mode 5 operation.

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