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At 1742 EDT on July 17, 2009, while attemp … At 1742 EDT on July 17, 2009, while attempting to restore normal alignment providing offsite power following repair of the 'D' Common Station Service Transformer (CSST), the transfer from the alternate to the normal power supply for the 6.9kV Shutdown Board 2B-B failed because of an apparent failure of the interlock that should have prevented closing the normal breaker until an undervoltage condition existed on the alternate feed. The emergency diesel generators received a blackout signal and all four diesel generators started. Loads transferred and shed properly, and all systems functioned as expected.</br>WBN (Watts Bar Nuclear) had been in LCO 3.8.1 condition A since 0032 EDT on July 16, 2009 due to the outage of the D CSST. Because loads did not transfer to the offsite power source, WBN remains in LCO 3.8.1 A which requires restoration of the offsite power supply by 0032 EDT 7/19/2009.</br>All systems are operating properly and the plant is stable. Troubleshooting and maintenance has begun and WBN anticipates return of the normal offsite power supply to the 2B-B Shutdown Board within the time required by technical specifications.</br>This event is reportable under 10 CFR 50.72(b)(3)(iv) as an event or condition that resulted in valid actuation of the emergency diesel generators.</br>The licensee has notified the NRC Resident Inspector</br>The 6.9kV Shutdown boards at Watts Bar are safety related busses. The electrical configuration prior to the event had the 1A-A and 2A-A Shutdown Boards aligned to their normal offsite power source. The 1B-B and 2B-B Shutdown Boards were aligned to an alternate offsite power source because their normal source, the 'D' CSST had been out of service for repairs. Following completion of the repairs to the 'D' CSST, the licensee attempted to restore normal offsite power to the 2B-B Shutdown Board from the 'D' CSST using a fast transfer from the alternate power supply. For unknown reasons, the normal supply breaker attempted to close onto the 2B-B bus before the alternate supply breaker had opened. This, in effect, would have paralled both the primary and alternate sources of power to the 2B-B bus. An interlock prevents paralleling these two sources and resulting in both supply breakers tripping and the bus being de-energized. This generated a blackout signal that started all four emergency diesels generators (EDGs). </br>All the EDGs started as required, only the 2B-B loaded onto its associated shutdown bus, as expected, because of the bus had been de-energized. The other busses (1A-A, 1B-B, and 2A-A) remained energized and the associated EDGs did not load. The 1A-A, 1B-B, and 2A-A EDGs were shutdown and returned to a standby condition. The 2B-B EDG continued to power the 2B-B Shutdown Board while the licensee investigated the fast transfer problem. </br>All systems functioned as required during this event except for the 2B-B fast bus transfer from the alternate to normal offsite power supply. alternate to normal offsite power supply.
21:42:00, 17 July 2009 +
45,210 +
00:26:00, 18 July 2009 +
21:42:00, 17 July 2009 +
At 1742 EDT on July 17, 2009, while attemp … At 1742 EDT on July 17, 2009, while attempting to restore normal alignment providing offsite power following repair of the 'D' Common Station Service Transformer (CSST), the transfer from the alternate to the normal power supply for the 6.9kV Shutdown Board 2B-B failed because of an apparent failure of the interlock that should have prevented closing the normal breaker until an undervoltage condition existed on the alternate feed. The emergency diesel generators received a blackout signal and all four diesel generators started. Loads transferred and shed properly, and all systems functioned as expected.</br>WBN (Watts Bar Nuclear) had been in LCO 3.8.1 condition A since 0032 EDT on July 16, 2009 due to the outage of the D CSST. Because loads did not transfer to the offsite power source, WBN remains in LCO 3.8.1 A which requires restoration of the offsite power supply by 0032 EDT 7/19/2009.</br>All systems are operating properly and the plant is stable. Troubleshooting and maintenance has begun and WBN anticipates return of the normal offsite power supply to the 2B-B Shutdown Board within the time required by technical specifications.</br>This event is reportable under 10 CFR 50.72(b)(3)(iv) as an event or condition that resulted in valid actuation of the emergency diesel generators.</br>The licensee has notified the NRC Resident Inspector</br>The 6.9kV Shutdown boards at Watts Bar are safety related busses. The electrical configuration prior to the event had the 1A-A and 2A-A Shutdown Boards aligned to their normal offsite power source. The 1B-B and 2B-B Shutdown Boards were aligned to an alternate offsite power source because their normal source, the 'D' CSST had been out of service for repairs. Following completion of the repairs to the 'D' CSST, the licensee attempted to restore normal offsite power to the 2B-B Shutdown Board from the 'D' CSST using a fast transfer from the alternate power supply. For unknown reasons, the normal supply breaker attempted to close onto the 2B-B bus before the alternate supply breaker had opened. This, in effect, would have paralled both the primary and alternate sources of power to the 2B-B bus. An interlock prevents paralleling these two sources and resulting in both supply breakers tripping and the bus being de-energized. This generated a blackout signal that started all four emergency diesels generators (EDGs). </br>All the EDGs started as required, only the 2B-B loaded onto its associated shutdown bus, as expected, because of the bus had been de-energized. The other busses (1A-A, 1B-B, and 2A-A) remained energized and the associated EDGs did not load. The 1A-A, 1B-B, and 2A-A EDGs were shutdown and returned to a standby condition. The 2B-B EDG continued to power the 2B-B Shutdown Board while the licensee investigated the fast transfer problem. </br>All systems functioned as required during this event except for the 2B-B fast bus transfer from the alternate to normal offsite power supply. alternate to normal offsite power supply.
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