By Paula C. Squires
Officials of Dominion Virginia Power and the U.S. Nuclear Regulatory Commission met Thursday to review multiple inspections that have been done at the North Anna Power Station in Louisa County since an Aug. 23 earthquake tripped both the station’s 980-megawatt reactors offline. The meeting at NRC’s headquarters in Rockville, Md., also focused on what needs to happen going forward for the nuclear plant to be restarted.
The meeting was open to the public and webcast over the Internet. Judging from the line of questioning from NRC officials, extensive safety verifications on everything from underground piping to the effect of the quake’s vibrations on the station’s spent fuel pool will need to be done before a restart could be considered. “We have lots of questions. We need to understand the extent of the fuel assessment inspections that were performed,” said one NRC official, who noted that any damage or deflection could affect the heat flux in the pool.
In a presentation to NRC staff, Dominion executives said seismic instruments showed that the 5.8 magnitude earthquake 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,“ Eugene Grecheck, vice president of nuclear development, said in a press release that Dominion issued after the meeting. “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, Dominion said all tests and repairs will be completed on Unit 1 by late September. Unit 2 will go into a planned refueling outage, and its restart will be based on that schedule.
While some of the earthquake’s vibrations briefly exceeded the station’s licensing design basis at certain frequencies, none exceeded the level that the station demonstrated it could withstand, Dominion said. The vibrations were strong enough to cause 25 spent fuel storage casks, each weighing 115 tons, to move on their concrete pads from an inch to 4 ½ inches.
The station’s design basis is 0.12 g (peak ground force acceleration) at 100 Hz. However, Dominion said that design accelerations are higher than 0.12g in the critical range of 2Hz-10Hz where most earthquake damage usually occurs. Dominion said instruments showed that vertical motion and motion along one horizontal direction exceeded 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. According to Dominion, 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. U.S. commercial nuclear power stations were designed based on West Coast U.S. seismic assumptions, which have different characteristics from known Eastern U.S. quakes.The NRC has 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. The 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 the NRC said there is adequate protection at all plants.
It plans to hold another public meeting in Louisa County near the power station in late September to discuss its preliminary inspection results, with a final report expected by mid-October.
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