21 mai 2015
Alors que ces nouveaux onduleurs ont été installés il y à 8 ans, les calculs de température du local, liées au fonctionnement de ce matériel n’ont pas été mis à jour. C’est au cours de vérification complètes des analyses de risques que cette découverte a été faite. Le scénario postulé est celui de perte des lignes extérieures ou de perte de réfrigérant primaire suivi d’un dysfonctionnement du diesel de secours d’urgence. Ces nouveaux onduleurs chauffent beaucoup trop et leur mise en route menace les deux alimentations en courant continu ainsi que les chageurs de batteries qui, dans certaines conditions n’auraient plus fonctionné. Des mesures de contournement provisoires ont été adoptées en attendant la réparation définitive.
Type : BWR Mark3 - Puissance : 3 091 - Premières divergence : 10 / 1985 -
Available in english only
Event Number : 51083
Facility : RIVER BEND - 4 State : LA
Unit : [1] - RX Type : [1] GE-6
Event Date : 05/21/2015 - Event Time : 03:09
Emergency Class : NON EMERGENCY 10 CFR Section : 50.72(b)(3)(v)(A) - POT UNABLE TO SAFE SD
Initial PWR : 100 % Current PWR : 100 %
Event Text
DC ELECTRICAL ROOM HEAT LOAD CALCULATIONS NOT UPDATED AFTER NEW INVERTERS INSTALLED
"At 0309 CDT on May 21, 2015, with the plant operating at 100 percent power, the shift manager was notified of a condition that could potentially lead to the failure of safety-related inverters in the DC electrical distribution system. This condition was identified while reviewing completed probabilistic risk analysis (PRA) calculations. It was discovered that control building ventilation system heat load calculations were not updated when new AC/DC inverters, with significantly higher heat output, were installed in 2007. Since the heat load calculations were not updated, it was not realized at the time that, under post-accident conditions involving a specific single failure, the DC electrical equipment rooms (i.e., inverters and battery chargers) could exceed their design basis ambient temperature, potentially leading to the failure of the inverters or battery chargers, and the loss of DC power in both divisions of the distribution system. The postulated scenario is the loss of offsite power (LOP)/loss of coolant accident (LOCA), followed by the failure of a single emergency diesel generator. If within 20 minutes prior to the onset of the event, the control building ventilation system had been shifted to the division in which the emergency diesel generator successfully started, the chiller in that division would, by design, be prevented from re-starting for as much as 20 minutes by its anti-recycle feature. (That feature limits successive starts of a chiller to prevent over-heating the motor.) The other chiller in that same division must be manually aligned for service, so it is not assumed to be available for these purposes. During the period in which the anti-recycle timer is running, no chiller would be in service, and the actual heatup rate of the inverters/battery charger could lead to room temperatures in excess of their design basis assumptions. As stated above, this condition (no ventilation cooling) would only exist for a maximum of 20 minutes from the time the associated divisional chiller was started, concurrent with a LOP/LOCA and a single failure. After expiration of the 20 minute timer, the chiller would perform the required function and lower room temperatures as expected. For best estimate PRA purposes, River Bend assumes an equipment survivability criteria of exposure to temperatures above 122 degrees F, but below 150 degrees F, for up to four hours. This is based upon the industry Station Blackout guidance of NUMARC 87-00, which was endorsed by the NRC as the basis for industry response to the Station Blackout Rule. This condition exists only after planned chiller swap and can be mitigated by opening the door to the inverter room should a LOP/ LOCA occur prior to the expiration of the 20 minute timer. Measures are in place for a dedicated operator to perform this function pending a modification to resolve the issue.
"This condition is being reported in accordance with 10 CFR 50.72(b)(3)(v) as the potential loss of the safety function of the DC electrical distribution system."
The licensee has notified the NRC Resident Inspector.
https://www.nrc.gov/reading-rm/doc-collections/event-status/event/2015/20150522en.html#en51083
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