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| Entered date | Event description | |
|---|---|---|
| ENS 43676 | 29 September 2007 02:19:00 | On September 28, 2007, while reducing power for a planned refueling outage with the reactor at approximately 30 percent power in MODE 1, an unplanned actuation of the reactor protection system occurred. At 2232 a fault pressure trip signal was received on the A Startup Transformer (SUT), causing a loss of power to Aux Buses D, A & C electrical buses as well as the A-SA safety bus. The loss of A & C buses initiated the RCP underfrequency trip which tripped the Reactor and all three RCPs as designed. The A Diesel Generator automatically started and reenergized bus A-SA as designed. The auxiliary feedwater system actuated as expected due to undervoltage on the A-SA safety bus and loss of the main feedwater pumps. All control rods inserted on the reactor trip. The operations staff responded to the event in accordance with applicable plant procedures. The plant stabilized at normal operating no-load reactor coolant system temperature and pressure following the reactor trip. Steam generator water levels are being maintained using auxiliary feedwater. All emergency core cooling system equipment is available. The plant electrical system is being restored at this time. The A SUT remains out of service. The cause of the loss of power from A SUT is under investigation. This condition is being reported as an unplanned reactor protection system actuation and specified system actuation in accordance with 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50.72(b)(3)(iv)(A). The NRC Resident Inspector was notified of this event by the licensee. The plant was in natural circulation for approximately 1 hour. The Main Steam Isolation Valves (MSIVs) were manually isolated per procedure due to loss of EHC indication. Presently the B RCP has been restored to service, MSIVs are still closed, and the motor driven auxiliary feedwater pumps are feeding the Main Steam Generators. There are not any leaking steam generator tubes. The A EDG will be secured after backfeeding of the deenergized buses have been established. |