Last week’s central Virginia earthquake jolted huge concrete containers holding spent nuclear fuel at the North Anna power plant in Louisa County, shifting some containers one to four inches, said the plant’s operator, Dominion Virginia Power.

The containers, called casks, each weigh 115 tons and are filled with bundles of uranium dioxide fuel rods that no longer generate enough heat to produce electricity.

The plant houses 53 casks on two concrete pads. Of the 27 casks on one pad, 25 shifted during the magnitude 5.8 earthquake that struck Aug. 23 about 12 miles south of the plant, Dominion spokesman Richard Zuercher said.

“They are safe and remain intact,” he said. “They are designed not to fall over and they didn’t fall over.”

Sensors installed to detect escaping helium gas, which fills the casks, indicated no leaks, Zuercher added.

The North Anna nuclear power plant is shown on the morning after an 5.8-magnitude earthquake centered in Mineral, Va., eight miles from the plant. (John McDonnell/THE WASHINGTON POST)

The casks, which look like concrete silos, sit on two secure pads outside the two-reactor power plant, which was shut down pending inspections by a special team sent this week by the Nuclear Regulatory Commission.

Each of the 27 casks on one pad is at capacity, holding 32 fuel rods.

On a second storage pad, 26 newer storage casks did not shift, Zuercher said. Some concrete flaked off the outside of some newer casks, which sit horizontally instead of vertically like the older casks. Thirteen of those casks hold fuel; the rest are empty.

The containers that moved were licensed and began taking fuel in 1998.

“This indicates that reactors that have these dry casks in these earthquake-prone areas, they’re going to have to do more to protect them from ground motion,” said Robert Alvarez of the Institute for Policy Studies, who has extensively studied nuclear waste storage. “One thing is to bolt them to the pads, and that’s not a Home Depot-type job. The pads themselves also need to be examined to see if they’re durable enough.”

Zuercher said that Dominion is conducting its own facility inspections alongside the NRC team. “We have not found anything significant, nothing that affects nuclear safety,” he said. “Both units are offline and in cold shutdown.”

On Aug. 26, Dominion notified the NRC that the earthquake may have shaken the facility more than it was designed to handle. An analysis of “shake plates” that record ground motion is to be completed Friday, Zuercher said, and the company will then determine whether the ground motion exceeded the plant’s design.

As at many U.S. nuclear plants, when fuel rods at North Anna lose power, they are moved to an indoor spent-fuel pool. After cooling for several years, workers move the rods to the so-called dry-storage casks, which are outdoors and cooled by natural air circulation.

Fuel rods remain radioactive for thousands of years as they slowly decay.

Dry casks were designed for temporary storage, Alvarez said, but they have become de facto long-term waste warehouses because the United States has not built a permanent waste repository.

According to the NRC, 55 sites in the United States have nuclear waste in dry-cask storage, including two sites in Virginia and one in Maryland, at the Calvert Cliffs facility in Lusby.

The federal government in 1986 began planning to move the nation’s nuclear waste to a permanent repository at Yucca Mountain in Nevada. But President Obama defunded the partially built project and declared it closed.

Lawsuits arguing for and against completing the Yucca Mountain facility are wending their way through federal courts.

A report released in April by the Government Accountability Office estimated that $15 billion has been spent on the attempt to find a location and build a facility to house spent nuclear fuel.

Until such a facility is built, waste will continue to accumulate at the nation’s 104 nuclear reactors.