The 5.8-magnitude earthquake last month in Virginia caused about twice as much ground shaking as a nearby nuclear power plant was designed to withstand, according to a preliminary federal analysis.
Parts of the North Anna Power Station in Mineral, Va., 11 miles from its epicenter, endured jolts equal to 26% of the force of gravity (0.26g) from some of the vibrations unleashed by the quake, said Scott Burnell, spokesman of the Nuclear Regulatory Commission.
An NRC document says the reactors' containment structure was built to withstand 12% of the force of gravity (0.12g.) Dominion, the plant's operator, says parts of the plant can handle up to 0.18g.
"It's the things inside the buildings that may have been shaken more than the design called for," Burnell said, adding the buildings themselves appear to have been less affected. He said the analysis is based on a seismograph reading taken about 30 miles away by the U.S. Geological Survey.
Whatever the final numbers on shaking or ground motion, the plant withstood the jolts, Burnell said, indicating there's a "great deal" of safety margin. "That margin was certainly enough for North Anna this time."
Then "maybe you shouldn't rely on the margin," said Edwin Lyman at the Union of Concerned Scientists, a non-profit critical of nuclear energy. "The jury is still out," he said, on whether the plant was adequately designed.
The two reactors at the North Anna plant, which began operation in 1978 and 1980, have remained closed since the Aug. 23 quake. They automatically shut down after losing off-site power. Backup diesel generators kept their cores cool until electricity was restored several hours later.
Dan Stoddard, Dominion's senior vice president of operations, said Friday that initial readings from the facility's scratch plates and other monitors indicate its shaking during the quake exceeded its design, but he declined to give numbers. Dominion officials plan to brief the NRC today on those findings.
Stoddard said the quake caused "no significant damage" to the facility, based on Dominion's ongoing inspections. He said there were some cosmetic cracks in concrete and floor tiles and 25 of 27 vertical casks holding spent-fuel assemblies moved up to 4.5 inches.
"The plant is safe," he told reporters, adding all radiology readings have been normal. He said it will be operational again once inspections are complete. The NRC sent in an augmented inspection team that's slated to continue its work through next week.