Inspectors dispatched by the Nuclear Regulatory Commission arrived at Virginia’s North Anna nuclear power plant Tuesday on a three-week mission to assess any damage caused by last week’s earthquake, which was centered 12 miles from the dual-reactor facility.

On Friday, the plant’s operator, Dominion Virginia Power, informed the NRC that the ground motion produced by the 5.8 magnitude quake “may have exceeded” the amount of shaking the plant was designed to withstand.

That notice prompted the deployment of the NRC team, which will also focus on why the plant lost primary electrical power for several hours and how the plant’s operators responded to the situation, said Roger Hannah, a spokesperson for the NRC’s southeast region.

The inspection is unusual, said Scott Burnell, a spokesman for NRC. Such “augmented inspection teams” are dispatched to nuclear plants “infrequently. If I had to put a number on it I’d say once every couple of years.”

During the quake, 36 “scratch plates” at the facility recorded ground motion in three dimensions, said Richard Zuercher, a Dominion spokesman. Preliminary analysis of the plates performed by a seismologist contracted by Dominion showed “the plant may have exceeded design basis for ground-force acceleration,” Zuercher said.

The plant’s reactor containment buildings were built to withstand shaking equal to 12 percent of the force of gravity. Dominion said on the day of the quake that the plant would be safe up to a magnitude 6.2 earthquake. But the amount of shaking such a quake produces varies with distance to the epicenter, depth, and the type of rock the quake occurs in.

Zuercher would not provide the amount of shaking revealed by the preliminary analysis. “We want to know for sure what we have,” he said, adding that Dominion would make available a fuller analysis of the shaking by the end of the week.

“There’s been no serious damage to any safety-related equipment,” Zuercher said of the company’s inspections to date. “Everything is fine. We have had some minor damage — insulation on pipes, ceiling tiles in offices, cracks in walls that were really not structural.”

If the shaking did exceed what the plant was designed to withstand, it would mark the first such event at an operating reactor in the United States, said Edward Blandford, a reactor safety expert at the Center for International Security and Cooperation at Stanford University.

Blandford noted that nuclear plants add safety margins to their designs. “An elevator makes a good analogy,” he said. “If you exceed a 500-pound weight limit by 10 pounds it doesn’t suddenly fail.”

He pointed to a July 2007 earthquake in Japan that shook the Kashiwazaki-Kariwa nuclear power plant with a force 2 1 / 2 times greater than the facility was designed for. After inspections, the International Atomic Energy Agency concluded that the plant’s safety design margins prevented major damage. The plant was subsequently closed for 21 months for seismic upgrades.

The North Anna plant’s reactors remain shut down and are not generating electricity.

Dominion has not said when the plant might be operational again. Zuercher said the company has enough power available to continue uninterrupted service despite the loss of the 450,000-home generating capacity of the North Anna plant.

“Before we start up, we have to do a lot of work to understand what happened, to make sure everything is safe, and to provide that information to the NRC,” Zuercher said.

In 2010, the NRC updated its estimates of earthquake risks to all 104 nuclear reactors in the U.S. It listed the North Anna plant as the seventh riskiest. Each year, the NRC stated then, the plant’s two reactors face a 1 in 22,727 chance of nuclear core damage from an earthquake.

Meanwhile, one of two reactors at the Calvert Cliffs nuclear plant in Lusby, Md., is still powered down after Hurricane Irene damaged a transformer there this weekend. Constellation Energy Nuclear Group spokesman Mark Sullivan said two NRC representatives are overseeing the transformer repairs and that the reactor will be brought online when it is “fully safe.”