BYRON - UNUSUAL EVENT DUE TO LOSS OF OFFSITE POWER GREATER THAN 15 MINUTES

At 1101 EST, Byron Unit-2 experienced a reactor trip due to RCP undervoltage. All rods fully inserted, MSIV's were manually closed and decay heat is being removed by Auxiliary Feedwater pumps running and steam leaving via atmospheric relief valves. The unit is currently in a natural circulation cooldown with the diesels supplying station emergency loads. Licensee will be cooling the plant down to Mode 5.

At 1118 EST, Byron declared an Unusual Event due to a loss of offsite power on Unit 2 from a faulted Station Auxiliary Transformer (SAT). The faulted SAT caused both 6.9 kV and 4.1 kV bus voltage to drop. Smoke was observed coming from the SAT with no visible flames being apparent. This caused bus loads to trip without a complete loss of ESF busses 241 and 242. These buses were manually disconnected from the SAT, which transferred the load to the emergency diesel generators 2A and 2B. Both diesel generators started and loaded without incident. Offsite assistance was requested from the local fire department as a precaution.

The licensee is also declaring notification for 10 CFR 50.72(b)(3)(v)(D) and 10 CFR 50.72(b)(3)(xi).

Unit 1 is not being affected by this event and remains at 100% power.

The licensee has notified the NRC Resident Inspector.

Notification Date: 01/30/2012
Notification Time: 11:39 [ET] 10:39 [CST]
Event Date: 01/30/2012
Event Time: 10:01 [CST]
Last Update Date: 01/30/2012

Region: 3 State: IL
Unit: [ ] [2] [ ]
RX Type: [1] W-4-LP,[2] W-4-LP
NRC Notified By: LEO WEHNER
HQ OPS Officer: JOHN KNOKE