“The event that occurred at North Anna had high acceleration spikes but did not have sufficient duration of energy to cause any damage,” Grecheck said. “We know that there is [a safety] margin in the plant.”
At the meeting, NRC staff members questioned whether Dominion had sufficiently inspected the two nuclear cores, which are loaded with radioactive uranium fuel rods.
“We have seen no anomalies or issues with the core,” said Larry Lane, Dominion’s site vice president for the North Anna plant, adding that core inspections would continue.
Lane said the Unit 1 reactor could restart Sept. 22, and Unit 2, which is being refueled as scheduled, could restart Oct. 13. During the quake, the two reactors at the plant automatically shut down.
The company cannot restart the reactors until it gets NRC approval, and the commission gave no hints as to when approval would come. An NRC inspection team is amid a three-week inspection of the facility.
Dominion requested Thursday’s meeting with the NRC to share the company’s latest data on the earthquake, information the company had promised to make public. Jack Grobe, a deputy director at NRC’s Office of Nuclear Reactor Regulation, told Dominion officials to expect “a series of meetings” to review safety data, including public meetings at the North Anna plant in Louisa County, although no dates have been set.
He also told Dominion officials to expect continued probing of the plant’s safety. “I guarantee you, you’re going to get a lot of, ‘Did you think of this? Did you look at that?’ ” Grobe said.
North Anna is the first nuclear reactor to shut down after an earthquake in the 53-year history of commercial nuclear power in the United States.
The NRC is reviewing seismic risks to 27 of the country’s nuclear power plants, including North Anna. The agency ordered the review after the March 11 earthquake and tsunami in Japan, which triggered large releases of radiation at that country’s Fukushima Daiichi nuclear power plant.
The dual-reactor at the North Anna facility experienced “strong” ground motion in the north-south direction for about three seconds during the August quake, said Eric Hendrixson, director of nuclear engineering for Dominion.
The company’s analysis of data from seismographs at the plant showed that the facility experienced “cumulative absolute velocity” — a measure of the total amount of shaking over time — in excess of the so-called “design basis” for the plant, Hendrixson said.
Hendrixson emphasized that this shaking was below a threshold at which the facility’s two nuclear containment structures would experience damage.
The company’s analysis confirms the NRC’s. Also Thursday, the NRC said analysis of seismic monitors “dozens of miles away” from the plant maintained by the U.S. Geological Survey showed “probable acceleration” beyond the plant’s design basis.
NRC spokesman Scott Burnell said the USGS monitors estimated shaking at 26 percent of the force of gravity, whereas the facility’s containment buildings were designed to withstand shaking of 18 percent of the force of gravity.
Nuclear safety experts say nuclear plants are built with a large safety margin beyond the design basis.