Dominion Virginia Power (NYSE: D) told the Nuclear Regulatory Commission (NRC) that multiple inspections have found no significant damage to equipment at the two-unit, 1,800 MW North Anna Power Station resulting from the 5.8 magnitude earthquake on Aug. 23. Both nuclear reactors shut down automatically and remain out of service.
Dominion said that North Anna, like other U.S. nuclear power stations, was hardened against earthquakes and other potential hazards in the 1990s. An analysis at that time demonstrated that it could successfully withstand, without significant damage to safety systems, a quake much larger than the one recorded in Central Virginia.
In a presentation to NRC staff, Dominion executives said seismic instruments showed the quake caused accelerations at some frequencies greater than the station's design basis, but that no significant damage had been seen to any nuclear structures, equipment, pipes, valves, pumps, the Lake Anna dam or any safety-related equipment.
"We are seeing exactly what independent seismic experts have told us to expect: minor damage such as insulation that was shaken off some pipes, electrical bushings that will be replaced and some surface cracking on non-seismic qualified walls," said Eugene Grecheck, vice president-Nuclear Development. "Still, we will not restart the units until we have demonstrated to ourselves and the NRC that it is safe to do so.”
While some of the Aug. 23 earthquake's vibrations briefly exceeded the station's licensing design basis at certain frequencies, none exceeded the level that the station subsequently demonstrated it could withstand.
Dominion said all tests and repairs will be completed on Unit 1 by the latter part of September. Unit 2 is going into a planned refueling outage and its restart will be based on that schedule.
Earthquake vibrations are evaluated on a spectrum of 1 Hertz (Hz) to 100 Hz both horizontally and vertically. The station's licensing design basis is 0.12g (peak ground force acceleration) at 100Hz. However, the design accelerations are significantly higher than 0.12g in the critical range of 2Hz-10Hz, where most earthquake damage is likely to occur.
Station instruments showed that vertical motion and motion along one horizontal direction very briefly exceeded the licensing design accelerations in the 2 Hz to 10 Hz range – by approximately 12 percent on average in horizontal direction and by about 21 percent on average in the vertical direction.
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