6 avril 2015
Lors de l’analyse du rapport de protection incendie, a été découvert un temps d’isolement trop long de ce réservoir. Suite à des défaillances multiples liées à la survenue d’un incendie, ce défaut pourrait entraîner, en plus d’un fort débit de liquide de refroidissement, un excès de température et de pression sur ce réservoir. Ces dépassements entraîneraient, à leur tour, un risque de perte des pompes de charge. Alors que les vannes devraient se fermer en 4 minutes, elles ne le feraient qu’en 70.
Type PWR - Puissance : 3 459 MWth - Première divergence : 01/1996
Available in english only.
Event Number : 50960
Facility : WATTS BAR - State : TN
Unit : [1] - RX Type : [1] W-4-L
Event Date : 04/06/2015 - Event Time : 17:00
Last Update Date : 04/06/2015 Emergency Class : NON EMERGENCY 10 CFR Section : 50.72(b)(3)(ii)(B) - UNANALYZED CONDITION
Initial PWR 100 %
Current PWR : 100 %
Event Text
UNANALYZED CONDITION DUE TO VOLUME CONTROL TANK ISOLATION TIME ANALYSIS ERROR
"On April 6, 2015, Watts Bar determined that the current Unit 1 Fire Protection Report (FPR) analysis for 10 CFR 50, Appendix R contained a non-conservative time for isolation of Volume Control Tank (VCT) following a postulated fire in room 737.0-A1A. Due to multiple fire-induced failures, Component Cooling System (CCS) cooling of letdown flow to the VCT will be lost in conjunction with increased Reactor Coolant System (RCS) injection flow through Reactor Coolant Pump (RCP) seals and Boron Injection Tank (BIT) flowpaths. As a result, VCT temperature and pressure would increase, which could cause RCP seal damage and loss of RCS inventory, and net positive suction head (NPSH) to the Centrifugal Charging Pumps (CCPs) could be lost. In this postulated fire scenario, Thermal Barrier Cooling is also not available due to fire-induced failures. The current FPR analysis assumes a time of 70 minutes for closure of VCT outlet isolation valves. Preliminary analysis performed by TVA showed that VCT outlet isolation is required in approximately 4 minutes.
"As a result, Watts Bar Unit 1 is in an unanalyzed condition. This condition significantly degrades plant safety because operator action to isolate the VCT in the event of a postulated fire in room 737.0-A1A would not have been performed in time to prevent RCP seal damage and loss of RCS inventory and eventual loss of NPSH to the CCPs.
"Watts Bar is utilizing previously established fire watches in the affected fire areas as a compensatory measure.
"This issue is being reported under 10 CFR 50.72(b)(3)(ii)(B), ’unanalyzed condition that significantly degrades plant safety.’
"The Watts Bar NRC Resident Inspector has been notified."
https://www.nrc.gov/reading-rm/doc-collections/event-status/event/2015/20150407en.html
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