Last week's central Virginia earthquake caused 25 spent-fuel storage casks — each weighing 115 tons — to move on their concrete pad at Dominion Virginia Power's North Anna nuclear power plant.
The shifting of these massive casks holding used nuclear fuel was the first caused by an earthquake in the U.S., according to the U.S. Nuclear Regulatory Commission.
None of the metal cylinders was damaged and no radiation was released, Dominion Virginia Power said.
Like hockey pucks on a jostled tray, the 16-foot tall casks shifted from an inch to 4½ inches, utility company spokesman Rick Zuercher said.
"They just moved because of the vibration," Zuercher said. "They remained upright and fully intact."
Federal regulators will examine data from the spent-fuel storage area as part of the inspection of the plant that the NRC began Tuesday, said Scott Burnell, a spokesman with the federal agency's headquarters in Rockville, Md.
"The information available indicates the shifting did not affect safety in any way," Burnell said. "It is an instance of an event we had not previously seen, so were trying to learn as much as possible."
Used nuclear fuel rods from commercial atomic power plants are highly radioactive.
Dominion Virginia Power has 27 of the massive TN-32 storage casks standing vertically outdoors on the concrete pad. The casks, made from thick steel, are not fastened to the ground, being held in place by gravity.
Concrete bunkers for other used-fuel containers stored horizontally at the Louisa County power station experienced "cosmetic" damage, he said. "None of these moved."
NRC regulations specify the spacing of the vertically set dry casks from one another mainly to ensure workers are not exposed to unexpected levels of radiation, Burnell said.
"If the spacing shifts a slight amount, the dose shifts a very slight amount," he said. "Those doses have to be taken into account. The doses would be very low in any case."
The NRC requires a nuclear plant's assemblies of spent fuel rods to be cooled in water pools for at least five years before being transferred to dry casks.
Spent-fuel storage in casks is considered safe, the NRC said. Such storage systems are designed to resist floods, tornadoes, projectiles, temperature extremes and other unusual scenarios, the federal agency said.
During the past 20 years, the NRC said, there have been no radiation releases affecting the public, no radioactive contamination, and no attempts to sabotage spent-fuel casks or their storage sites across the U.S.
"I am a strong advocate of accelerating the transfer of irradiated fuel from spent fuel pools to dry casks," said nuclear engineer David Lochbaum with the Union of Concerned Scientists.
Last week's magnitude-5.8 earthquake shook protective electrical devices at the North Anna Power Station strongly enough to cause the plant to shut down automatically, the first time this has occurred in the United States.
The power station remained out of service Wednesday as company and NRC officials continued detailed inspections of its two 980-megawatt reactors and other station structures and systems.
"To date, there has been no significant earthquake-induced damage in safety-related structures or systems important to safety and shutdown cooling," Zuercher said.
The two spent-fuel storage pads cover 11.4 acres at North Anna — one pad holds the 27 cylinders and the other has the concrete bunkers for the horizontal storage containers. The first container was placed in storage in July 1998.
The 27 vertically stored casks are each 16 feet tall and 8 feet in diameter. Each has steel walls 18 inches thick.
The containers hold 32 fuel assemblies and weigh 115 tons when loaded with the used-fuel assemblies.
Also, some spent fuel at North Anna is stored horizontally in concrete bunkers, using 50-ton metal containers that are 16 feet long and 6 feet in diameter. These are placed in concrete modules 19 feet high, 10 feet wide and 21 feet long. The modules have 3-foot-thick concrete end walls, and each holds a single container.
Nuclear plant operators originally anticipated that spent fuel would be reprocessed, with usable portions of the fuel recycled and the rest disposed of as waste.
However, commercial reprocessing was never successfully developed in the United States, and no permanent waste repository has yet been developed. Utilities have looked to the dry-cask system to increase spent-fuel storage capacity.
"I'd prefer to see emptier spent-fuel pools and more dry casks at the (nuclear plant) sites," Lochbaum said. "It's the best way to manage the spent-fuel storage risk."