NRC ramps up inspection at Va. nuclear plant; earthquake may have exceeded plant’s design base – Signs That Utilities Knew Of Shortfall

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WASHINGTON — The Nuclear Regulatory Commission is sending additional inspectors to the Virginia nuclear power plant closest to the epicenter of last week’s earthquake.

Tuesday’s earthquake may have exceeded the design basis for seismic events at the North Anna Power Station in Mineral. That’s according to the Nuclear Regulatory Commission, in an event report this morning. Tuesday’s earthquake was a 5.8 magnitude quake.

The plant was designed to withstand a magnitude 6.2 earthquake. But monitoring equipment at the plant showed that the quake, at higher frequencies, may have exceeded that design.

“Scratch plates” that record ground motion at Unit 1 were sent to a lab for analysis. An NRC spokesman this morning said the agency is monitoring the post-quake analysis.

The company and the NRC have said there was no damage to critical components of the plant. Meantime, the two reactors at North Anna are shut down as an in-house evaluation continues. There’s no word on how long the reactors will be shut down.

The only known damage from the earthquake was in an on-site substation. A failure there cut power to the main station when the earthquake hit at 1:51 p.m. yesterday. The two reactors automatically shut down, as designed.

North Anna was designed and built in the 1970s. In 2005, NRC staff recommended that the agency take a new look at the seismic design basis of all older US plants, given new findings about seismic hazards in the last several decades.

The staff report admitted, however, that a great deal of data on how plants were actually built was missing, and NRC is in the process of gathering that data for eastern and Midwestern plants, a process slated to go into 2012. Only then does NRC plan to decide whether action is needed.

But after an earthquake and tsunami hit the Fukushima Daiichi plant in Japan in March, NRC performed special inspections to assess US plants’ capabilities to withstand similar assaults. At North Anna and other stations, that inspection found some equipment important to fighting fires and withstanding floods that was not designed as earthquake-resistant. All operators involved, including Dominion, have been evaluating how to respond to the NRC findings.

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