WASHINGTON — Regulators weighing whether to allow the restart of a nuclear power plant shut down by the earthquake last August in Virginia have some new information to work with: The plant was shut down not by a power outage, as first thought, but by vibration from the quake, the plant operator now says.
What happened at the North Anna nuclear power station, just 12 miles from the epicenter of the 5.8 magnitude quake felt all along the East Coast, marked two important firsts for an operating U.S. nuclear plant:
- The first time one experienced an earthquake that exceeded its design parameters.
- The first time an earthquake caused an automatic shutdown.
"Our investigation showed the units tripped before the loss of off-site power when multiple reactor sensors detected a slight power reduction in the reactors," Reuters quoted Dominion Resources spokesman Rick Zuercher as saying Tuesday.
"The root cause team determined that this occurred as result of vibration in the reactor or the monitoring devices in the reactors, or both," Zuercher said. "There was no indication of damage."
The Nuclear Regulatory Commission tells NBC News that the plant's systems reacted as they should have.
But it is also weighing what Dominion needs to do before restarting the plant.
On Monday, the results of a NRC inspection team will be presented at a public meeting in Mineral, Va., the town closest to the epicenter.
A final NRC inspection report will be released later in October.
The NRC in August released a draft review stating that North Anna's two reactors are among 27 in the eastern and central U.S. that might need upgrades based on new seismic data.
"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.
The Union of Concerned Scientists, a group that tracks nuclear energy issues, says the shutdown raises questions about plant safety and earthquakes.
The NRC has argued for years that plants are able to withstand bigger quakes than they are designed for, said UCS nuclear expert Ed Lyman. That might prove true at North Anna, but what about a bigger quake there — or elsewhere in the country at another plant?
The U.S. is also vulnerable to a situation like that of Japan's Fukushima nuclear plant, which lost power in the quake and tsunami there on March 11. The long power outage led to core meltdowns at three reactors.
U.S. plants only have to maintain backup power capability for four hours, Lyman noted, arguing that the Fukushima experience shows the minimum should be days if not weeks.
The Virginia quake, coming so soon after the Japan crisis, "does heighten attention to seismic (risk), no question about it," Tony Pietrangelo, senior vice president for the Nuclear Energy Institute, said when the NRC issued its review. But he said the lack of serious damage shows "there's a lot of margin built into these plants."
The NRC has said it plans to order all U.S. plants later this year to update their earthquake risk analyses, a complex exercise that could take two years for some plants to complete.
The NRC review, initiated well before the Japan disaster, marks the first complete update to seismic risk in years for the nation's 104 existing reactors.Story: Earthquake threat to nuclear reactors far higher than realized
The draft review noted that the seismic upgrades could be costly, both for the industry and the NRC, which would have to increase its staff and add specialized expertise.
Still, the NRC said the science on seismic hazards has improved significantly since most plants were built at least three decades ago, making a review appropriate.
Pietrangelo said he hoped regulators would not try to force changes too quickly. "We don't want to do it twice. We want to do it once and get it right," he said.
NBC News' Tom Costello as well as Reuters and The Associated Press contributed to this report.