Dominion Virginia Power informed the U.S. Nuclear Regulatory Commission yesterday that multiple detailed inspections have found no significant damage to equipment at North Anna Power Station from the Aug. 23 earthquake.
North Anna, like other U.S. nuclear 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 at its headquarters in White Flint, Md., Dominion executives said seismic instruments showed that the quake caused accelerations at some frequencies greater than the station's design basis, but that no significant damage has 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. Based on results to date, we believe 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."
While some of the Aug. 23 earthquake's vibrations very briefly exceeded the station's licensing design basis at certain frequencies, none exceeded the level that the station subsequently demonstrated it could withstand.
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.
Independent seismic experts retained by the company have concluded that the earthquake caused only minimal damage at the station, in part, because the duration of the strong vibrations was extremely short. The entire quake lasted about 25 seconds. However, in this event the peak motion lasted only 3.1 seconds.
The 1,800-megawatt, twin reactor nuclear power station shut down automatically following the 5.8 Richter quake. Both units remain temporarily out of service. There continues to be no threat to the public.
All U.S. commercial nuclear power stations were designed based on West Coast U.S. seismic assumptions, which have certain different characteristics from known Eastern U.S. quakes.
The NRC noted that it had reviewed and evaluated recent U.S. Geological Survey earthquake hazard estimates for the Central and Eastern U.S. that are used for building code applications outside the licensing of nuclear stations.
"These reviews showed that the seismic hazard estimates at some current Central and Eastern U.S. operating sites may be potentially higher than what was expected during design and previous evaluations, although there is adequate protection at all plants," the NRC stated.
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