1er novembre 2017
De mauvais calibrages de fusibles ont été identifiés : un défaut sur le câble associé aurait pu affecter le primaire du transformateur qui alimente en 4 000 volts les quatre cartes d’arrêt. Ceci provoquerait le déclenchement des moteurs de charge en 4 kV, la déconnexion des alimentations électriques extérieures ainsi que le démarrage des générateurs diesels d’urgence. Ce 4 kV distribue les pompes de retrait de chaleur résiduelle du coeur, les pompes d’injection d’eau dans le coeur et les pompes d’alimention en eau de refroidissement d’urgence. Les câbles d’alimentation en 4 kV traversent des zones sensibles aux incendies qui menacent, de fait, des éléments d’arrêt du réacteur. Des mesures compensatoires de surveillance d’incendie ont été prises. Ce problème a engendré la dégradation sérieuse d’une barrière de sécurité importante.
Type : Fukushima 1 (BWR Mark 1) - Puissance : 3 458 MWth - Première divergence : 8 / 1976 -
Available in english only
Event Number : 53049
Facility : BROWNS FERRY - 2 State : AL
Unit : [3] - RX Type : [3] GE-4
Event Date : 11/01/2017 - Event Time : 14:25 [CDT]
Emergency Class : NON EMERGENCY 10 CFR Section : 50.72(b)(3)(ii)(B) - UNANALYZED CONDITION
Initial PWR : 100 % Current PWR : 100 %
Event Text DESIGN DEFICIENCIES IDENTIFIED DURING ENGINEERING REVIEW
"At 1425 [CDT] on November 1, 2017, Operations was notified of a condition affecting Unit 3 4kV Shutdown Boards 3EA, 3EB, 3EC, and 3ED. It was discovered that multiple potential transformer (PT) primary fuses are GE type EJ1 size 0.5 AMP which does not coordinate with the PT’s secondary fuses.
"A fault on the associated cable could clear the primary PT primary fuses for the 4kV Shutdown Board. This would result in the board tripping 4kV motor loads, disconnecting from Off-site power and connecting to the Emergency Diesel Generator. However, since the PT fuse is cleared, the under-voltage trips on the 4kV motors would remain in if there is no Common Accident Signal (CAS) present. The 4kV motor loads include Residual Heat Removal (RHR) Pumps, Core Spray (CS) Pumps, Residual Heat Removal Service Water (RHRSW) Pumps, and Emergency Equipment Cooling Water (EECW) pumps.
"Review of NFPA 805 analyses show the cables for all four U3 4kV Shutdown Boards are routed in Fire Area 03-03 and Fire Area 16. Therefore a fire in either area could result in a loss of all four U3 4kV Shutdown Boards motor loads. Cables for 4kV Shutdown Board 3EA and 3EB are both routed in Fire Area 21 which could result in a loss of both Division I Shutdown Board motor loads.
"Compensatory fire watch measures have been established.
"This event requires an 8 hour report in accordance with 50.72(b)(3)(ii)(B), ’Any event or condition that results in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety.’
"The NRC Resident Inspector has been notified.
"CR 1354129 was initiated in the Corrective Action Program."
https://www.nrc.gov/reading-rm/doc-collections/event-status/event/2017/20171102en.html
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