Earthquake may have exceeded nuclear plant's design - USA Today (blog)

A nuclear power plant has reported that last week's 5.8-magnitude earthquake may have created stronger shaking that it was designed to withstand, prompting the Nuclear Regulatory Commission to send additional inspectors to the Virginia facility this week.

"We're perplexed," said Jim Norvelle, spokesman of Dominion, which operates the North Anna nuclear plant, located about 10 miles from the quake's epicenter in Mineral, Va. He said the plant's engineers had estimated that the two reactors, which went online in 1978 and 1980, could withstand quakes of between 5.9- and 6.2-magnitude.

That estimate is now in question. The difference may be lost in the translation from magnitude to ground motion. Although earthquakes are routinely measured in magnitude, or energy released, the NRC requires the nation's 104 nuclear reactors to withstand a predicted level of ground motion, or acceleration — something calculated as a g-force. The NRC has cautioned that it's not easy to convert the two.

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Norvelle said last week that the North Anna plant, which has been shut down since the quake, was designed to withstand ground motion of 0.12g where the foundation is rock and 0.18 where it's soil. What was the quake's ground motion under the facility?

"That data are so preliminary that we're not going to discuss them," he said in an interview Monday, adding the plant may be able to do so by week's end. He said the plant has more than a dozen sensors, and it's not received data yet from all of them.

He hastened to add that no "significant" damage has been found yet at the plant, which shut down automatically last Tuesday after it lost off-site power for seven hours. During that time, backup diesel generators kicked in to keep the radioactive core cool.

On Friday, Dominion notified NRC that "the seismic activity potentially exceeded the Design Basis," and on Monday, the NRC announced that it is sending an "augmented inspection team."

"There doesn't appear to be any damage to safety systems," said regional NRC spokesman Roger Hannah. But if the quake's ground motion did exceed the plant's design basis, he said the facility will have to examine what can be done to rectify the situation.

30 Aug, 2011

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