By Ayesha Rascoe
WASHINGTON (Reuters) - The historic earthquake that shut Dominion Resources Inc's North Anna nuclear plant in Virginia last week may have shaken the facility more than it was designed to withstand, the U.S. nuclear regulator said on Monday.
The Nuclear Regulatory Commission said it had sent a special inspection team to the plant rocked by the 5.8-magnitude quake, after initial reviews from Dominion indicated the ground motion may have exceeded North Anna's design parameters.
The plant cannot be restarted until the operator can show no "functional damage" occurred to equipment needed for safe operation, the NRC said.
"The company and the NRC will continue to carefully evaluate information to determine if additional actions may be necessary," the regulator said in a statement.
It will probably take about three or four weeks before the team's preliminary findings are released, NRC spokesman Roger Hannah said. He would not speculate on how long the plant might be closed, saying it would depend on the team's assessment.
If it does turn out that the quake exceeded North Anna's design specifications, Hannah said the team's analysis could find that the plant could withstand quakes stronger than what was originally intended.
But the team could also call for changes such as back fitting or reinforcing equipment for more powerful earthquakes.
"It could be they would be shut down for a while, we just don't know at this point," Hannah said.
MORE KNOWN MIDWEEK
Dominion said the North Anna reactors, which entered service in 1978 and 1980, were designed for an earthquake of up to 6.2 magnitude, but the NRC does not use that scale to measure seismic design specifications. Instead, the commission looks at ground-motion measurements.
Dominion spokesman Rick Zuercher said on Monday that more will be known by midweek about whether the quake exceeded the station's design as further analysis is conducted on seismic plates from the station's containment building.
Zuercher said physical inspections of the plant have found no major damage beyond cracks in office building walls, some broken tiles, loose insulation on pipes and small damage to the main transformer area where power is sent to the grid.
"We welcome the team to the site and will be sharing information with them," Zuercher said.
The NRC has been reviewing the ability of U.S. plants to cope with disasters after an earthquake and tsunami nearly led to a complete meltdown at Japan's Fukushima nuclear complex earlier this year --- the world's worst nuclear crisis since Chernobyl.
An NRC task force set up in the aftermath of the Japan crisis urged a shift in the NRC's safety regime that would force plants to plan for catastrophes far worse than design specifications allowed for, as well as require that companies assess seismic and flooding hazards at plants every 10 years.
Critics have urged the NRC to move more quickly to adopt these changes to ensure plants are prepared for disasters, with last week's earthquake further fueling these calls.
The earthquake "does change the equation," said Jim Warren, executive director of NC Warn, a North Carolina climate and nuclear watchdog group. Warren said if last week's quake did exceed North Anna's design specifications, that will bolster the task force's recommendation about changing the NRC's safety regime.
NRC Region II Administrator Victor McCree stressed that the NRC's decision to send a team to the plant does not mean that Dominion has "responded inappropriately or that the station is less safe as a result of the quake."
McCree said in a statement that the team will help the commission understand the effects of the quake on North Anna and gather information that will help the NRC's evaluation of earthquake risks at all U.S. nuclear plants.
(Additional reporting by Eileen O'Grady and Timothy Gardner; Editing by Bob Burgdorfer, Alden Bentley and Dale Hudson)