Event Number: 47805 – NOTICE OF UNUSUAL EVENT DUE TO DUAL UNIT LOSS OF OFFSITE POWER
At 2003 EDT on 4/4/12 Catawba Unit 1 automatically tripped due to a loss of offsite power. The licensee declared a Notice of Unusual Event due to entry into EAL 4.5.U.1, ‘AC Electrical Power from All Offsite Sources Has Been Lost for Greater Than 15 Minutes.’ Power is still available from onsite sources. All four Emergency Diesel Generators [EDG] started and powered their respective safety buses.
“The EAL poses no threat to the safety of the public.”
The Unit 1 control rods fully inserted, there is no known primary to secondary leakage, and the auxiliary feedwater water system automatically started.
The licensee notified the NRC Resident Inspector
Notified: DHS SWO, FEMA, DHS NICC, USDA, HHS, DOE, EPA, and Nuclear SSA via email.
* * * UPDATE AT 0150 EDT ON 04/05/12 FROM DARRYL HELTON TO S. SANDIN * * *
Catawba Nuclear Generating Station exited the Unusual Event at 0137 EDT after restoring offsite power at 0135 EDT to the Unit 1A and Unit 2B essential busses. The EDGs powering these busses have been secured and returned to Standby.
The Unit 1B and Unit 2A essential busses remain energized by their respective EDGs. The licensee does not have an estimate as to when offsite power will be restored for these busses.
Additional information provided as follow-up includes:
4-hour notification per 10CFR50.72(b)(2)(iv)(B) due to the Unit 1 Reactor Protection System [RPS] actuation (Rx Trip),
4-hour notification per 10CFR50.72(b)(2)(xi) for offsite notifications to the States of North and South Carolina and the counties of York, Gaston and Mecklenburg,
8-hour notification per 10CFR50.72(b)(3)iv)(A) due to the Unit 1 Rx Trip, Unit 1 Auxiliary Feedwater automatic start, Unit 1A and 1B and Unit 2A and 2B EDG auto starts.
8-hour notification per 10CFR50.72(b)(3)(v)(B) due to the loss of Residual Heat Removal [RHR] cooling on the initial loss of power. Unit 2 RHR was restored in less than three (3) minutes when the essential busses were re-energized by the EDGs.
The licensee informed the NRC Resident Inspector.
A loss of offsite power during calm weather should be very unusual, however this year we have seen Byron,WolfCreek, and now Catawba suffer from unexplainable events.
Offsite Power was restored within a few hours. What makes this occurrence unusual is that on 4/2/12, an Inverter 2KUI had a fire. Such a fire could have disabled equipment that would start Emergency Diesels. Because of the coincidence, this is a more severe occurrence.
The electrical fire affected Control and Instrument Power to many plant systems.
To date there has been no Root Cause for the Loss of Offsite Power.
The accompanying diagram is Catawba Unit 1 offsite power to onsite intertie.