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Category:NRC Preliminary Notification of Event/Occurrence
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March 26, 2008 PRELIMINARY NOTIFICATION OF EVENT OR UNUSUAL OCCURRENCE - PNO-III-08-002 This preliminary notification constitutes EARLY notice of events of POSSIBLE safety or public interest significance. The information is as initially received without verification or evaluation, and is basically all that is known by the Region III staff on this date. Facility Licensee Emergency Classification Byron Unit 2 _X_ Notification of Unusual Event Exelon Generation LLC ___ Alert Byron, IL ___ Site Area Emergency Docket: 50-455 ___ General Emergency License: NPF-66 ___ Not Applicable
SUBJECT:
BYRON UNIT 2 NOTICE OF UNUSUAL EVENT DUE TO LOSS OF OFFSITE POWER TO VITAL BUSSES DESCRIPTION:
At 6:34 p.m. CDT on March 25, 2008, one of the Byron Unit 2 electrical transformers failed due to a Phase C overcurrent condition causing a loss of power to both Unit 2 vital 4160 VAC busses. Unit 2 remained essentially at full power during the electrical transient. Unit 1 was shutdown for a refueling outage and was not affected by the electrical transient. Both Unit 2 Emergency Diesel Generators (EDGs) started and supplied the 4160 VAC vital busses as designed.
The licensee declared a Notice of Unusual Event (NOUE) at 6:49 p.m. CDT per Emergency Action Level MU1, "Loss of all offsite power to essential busses for greater than 15 minutes."
By 8:05 p.m., the licensee cross-tied the Unit 2 4160 VAC vital busses with the Unit 1 vital busses and secured the Unit 2 EDGs in accordance with existing procedures. The licensee continues its investigation of the root and contributing causes of the loss of the electrical transformer on Unit 2. The Byron Senior Resident responded to this event and was onsite shortly following the licensee's declaration of the NOUE. The NRC entered Monitoring Mode at 7:40 p.m. on March 25, 2008, and staffed the Regional Incident Response Center (IRC). During this event, the resident inspectors continuously monitored the licensee's activities surrounding this event from the plant control room, and the region continuously manned the IRC. The State of Illinois was informed. The information in this preliminary notification has been reviewed by the licensee. This information is current as of 5:00 a.m. CDT on March 26, 2008. CONTACT: Richard Skokowski (630) 829-9620
