05000280/LER-2011-001
| Surry Power Station, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2802011001R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT On Saturday April 16, 2011, a Tornado Watch was issued by the National Weather Service (NWS) from 1217 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.630685e-4 months <br /> until 2100 hours0.0243 days <br />0.583 hours <br />0.00347 weeks <br />7.9905e-4 months <br /> EDT for Southeastern Virginia including Surry County, Virginia. NWS issued a Tornado Warning at 1811 hours0.021 days <br />0.503 hours <br />0.00299 weeks <br />6.890855e-4 months <br />.
At 1849 hours0.0214 days <br />0.514 hours <br />0.00306 weeks <br />7.035445e-4 months <br />, with Surry Power Station Unit 1 at 100% reactor power and Unit 2 at 98.3% reactor power, automatic reactor trips occurred on both units due to a loss of offsite power (LOOP) resulting from damage inflicted in the switchyard from a tornado. Damage to the switchyard resulted in a loss of power to the Reserve Station Service Transformers (RSSTs) [EIIS-EA-XFMR] and Station Service Buses [EIIS-EA-BU].
The operating teams promptly initiated the appropriate emergency operating procedures and completed the immediate actions with no discrepancies. Both teams initiated the abnormal procedure for the loss of power and transitioned to the Natural Circulation Cooldown procedure. A natural circulation cooldown was necessary due to loss of all Station Service Buses.
The plant responded to the reactor/turbine trip as designed. Emergency Diesel Generators (EDGs) [EllS-EK-DG] auto started and loaded, and the Anticipated Transient Without Scram Mitigation System Actuation Circuitry (AMSAC) armed and initiated. Due to the station blackout signal, the Circulating Water outlet motor operator valves (MOVs) throttled closed to approximately 25% open and the Station Blackout Diesel (AAC) automatically started. Auxiliary Feedwater (AFW) pumps [EllS-BA-P] started as designed. All three Emergency Service Water Pumps [EIIS-BI-P] were started per procedure to control intake canal level. The common emergency diesel generator, EDG #3, automatically loaded onto the Unit 1 J Emergency bus [EllS-EB-BU] leaving the Unit 2 J Emergency bus de-energized as designed. At 1917 hours0.0222 days <br />0.533 hours <br />0.00317 weeks <br />7.294185e-4 months <br />, EDG #3 was transferred to the Unit 2 J bus and the AAC diesel generator was aligned to the Unit 1 J bus, providing power to all emergency buses at 1922 hours0.0222 days <br />0.534 hours <br />0.00318 weeks <br />7.31321e-4 months <br />.
Following the reactor trips, Unit 1 RCS cooled down below the nominal temperature of 547°F to a minimum of 517°F, and Unit 2 RCS cooled down below the nominal temperature of 547°F to a minimum of 504°F. The cooldowns were due to the loss experienced further cooldown due to the inability to throttle AFW flow with the Unit 2 J bus initially de-energized.
A Notification Of Unusual Event (NOUE) was declared at 1855 hours0.0215 days <br />0.515 hours <br />0.00307 weeks <br />7.058275e-4 months <br /> due to loss of offsite power to both emergency busses on both units.