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When the two nuclear reactors at North Anna Power Station automatically shut down moments after the Aug. 23 magnitude 5.8 earthquake, operators thought it was because off-site power was lost.

In a letter to the Nuclear Regulatory Commission (NRC), Dominion power now says it was a combination of factors related to the tremor that took the station offline. That was among a long list of questions the NRC asked as the federal agency and Dominion delve deeper into what happened, and whether they need any changes or upgrades before they restart the units.

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Both reactors at the plant on Lake Anna remain shut down as follow-up work and in-house inspections continue. The NRC and Dominion said Units 1 and 2 will remain idle until everyone thinks the plant is safe to operate. In the meantime, they are refueling Unit 2.

In response to the NRC’s request for more information, Dominion said there was a direct correlation between the reactors’ shutdown and earthquake motion.

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“The units tripped seconds before we lost off-site power,” said Richard Zuercher, spokesman for Dominion’s nuclear operations. “Both reactors shut down as designed when multiple reactor sensors detected a slight power reduction as a result of the vibrations in the reactor or monitoring devices.”

Dominion soon discovered that ground motion briefly exceeded the plant’s design limit, but said there was no damage to safety or operating systems.

The NRC also asked Dominion whether there was any damage to fuel in the reactors. The fuel is thumbnail-size uranium dioxide pellets loaded into metal alloy rods that compose fuel assemblies.

“There is reasonable assurance that there was no significant physical or functional damage to the fuel,” Dominion said.

The NRC also outlined a series of tests and inspections that must occur on both units before and after startup.

The NRC is stepping up scrutiny of the nation’s 104 commercial nuclear reactors in the wake of the Fukushima Dai-ichi disaster in Japan. The March 11 earthquake and tsunami severely damaged several reactors at the plant on Japan’s northern coast, contaminating workers and the surrounding countryside.

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