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Dominion reveals more about quake impact at North Anna in letters to NRC.
By RUSTY DENNEN
When the two nuclear reactors at North Anna Power Station automatically shut down moments after the Aug. 23 earthquake, operators initially thought it was because off-site power was lost.
In a letter to the Nuclear Regulatory Commission, Dominion power now says it was a combination of factors related to the magnitude-5.8 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 any changes or upgrades are needed before the units can be restarted.
The NRC will hold a public meeting at 1 p.m. Monday at the North Anna information center near Mineral to discuss the results of its own post-quake inspection. Dominion gave its version in a meeting with the agency earlier this month.
Both reactors at the plant on Lake Anna remain shut down as follow-up work and in-house inspections continue. The NRC and Dominion say Units 1 and 2 will remain idle until they are satisfied that the plant is safe to operate. Unit 2 is being refueled in the meantime.
In response to the NRC's request for more information, Dominion said there was a direct correlation between the reactors' shutdown and earthquake motion.
"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.
That topic was the subject of a protest at Dominion headquarters in Richmond Tuesday afternoon by Not on Our Fault Line, a newly formed watchdog group. The Louisa County group says if North Anna's existing units go back online, they should be upgraded to the higher seismic standards planned for a proposed Unit 3.
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.
Dominion said, "There is reasonable assurance that there was no significant physical or functional damage to the fuel."
The NRC also outlined a battery of tests and inspections that must be performed 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.
A task force looking into the disaster made a number of recommendations to the NRC, including some the staff said should be implemented quickly.
On the list: re-evaluation of plants' seismic and flood hazard protections; equipment inspections; maintaining safety during a prolonged loss of power; and strengthening emergency operating procedures and severe accident management guidelines.
Rusty Dennen: 540/374-5431