By Roberta Rampton
WASHINGTON (Reuters) - Last month's record earthquake in the eastern United States may have shaken a Virginia nuclear plant twice as hard as it was designed to withstand, a spokesman for the nuclear safety regulator said on Thursday.
Dominion Resources told the regulator that the ground under the plant exceeded its "design basis" -- the first time an operating U.S. plant has experienced such a milestone -- but said its seismic data from the site showed shaking at much lower levels than those reported by the U.S. Geological Survey.
Both the company and the Nuclear Regulatory Commission have not yet found any signs of serious damage to safety systems at the North Anna nuclear plant, and the company said it is eager to resume operations once inspections and repairs are complete.
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 North Anna quake shows the need for the nation's 104 aging reactors to reevaluate earthquake risks using up-to-date geological information, said Majid Manzari, an engineer at George Washington University who studies quake impacts.
"The implications of exceedance could be disastrous," Manzari said. "I would say these studies have to be done as soon as possible."
Expensive "backfits" to North Anna or any other U.S. plant are not a given from this exercise, Manjari said.
But a former chairman of the NRC said he expects the broad review ultimately will impact most nuclear plants along the U.S. East Coast.
"I think what the East Coast earthquake demonstrated is the design parameters might be changing," said Dale Klein, a mechanical engineer at the University of Texas.
SHAKEN, BUT NOT BROKEN
Japan's nuclear disaster at the Fukushima Daiichi plant -- overwhelmed six months ago by an earthquake and tsunami -- put quake risks at the forefront.
Then the historic August 23 earthquake hit the United States. Its epicenter was 12 miles from the North Anna plant, which shut down as it was designed to do.
The regulator's preliminary analysis is based on USGS data, collected about 30 miles away. It continues to inspect the site and expects to issue a final report in mid-October.
"We are currently thinking that at the higher frequencies, the peak acceleration was around 0.26" g, which is a unit of gravity that measures the impact of shaking on buildings, said Scott Burnell, an NRC spokesman.
The plant was designed to withstand 0.12 g of horizontal ground force for parts that sit on rock, and 0.18 g for parts that sit on soil, Burnell said.
Dominion's sensors recorded average horizontal ground force of 0.13 g in an east-west direction and 0.175 g in a north-south direction, officials said.
The levels were not high enough to be expected to cause significant damage, and inspections have borne that out thus far, Dominion said.
Dominion's analysis will be considered as the NRC does its analysis of what the company will be required to do to restart operations, Burnell said.
UNCLEAR WHAT NRC WILL REQUIRE
In an interview last week, NRC Chairman Gregory Jaczko told Reuters it was unclear what the plant would need to show to resume operations because it is the first time an operating plant has sustained a beyond-design-basis quake.
There could be new requirements stemming from the incident or from the NRC's broader review of earthquake risks, and plant operators will need to assess the costs to ensure they're worth it, said Ed Batts, a partner at law firm DLA Piper.
"You shake something really hard, and it's not designed to be shaken that hard -- it doesn't mean that it's broken," he said.
The incident helps make the case for new-generation nuclear plants, which have additional safety features, Batts argued.
"If you can have a car from 2011 vs. a car from 1978, what are you going to put your toddler in?" Batts said.
(Additional reporting by Eileen O'Grady in Houston and Timothy Gardner in Washington; Editing by Jim Marshall, Sofina Mirza-Reid and Bob Burgdorfer)