North Anna restart likely to take days

Tuesday's earthquake apparently shook small protective devices at the North Anna Power Station enough to shut down the plant's two nuclear reactors, officials said.

Dominion Virginia Power would not say exactly when it expects the reactors — representing nearly 13 percent of the state's electric generating capacity — will start producing power again, but it will likely take days, the company said.

Offsite power to the two nuclear reactors at the North Anna plant in Louisa County apparently was disrupted when relays on three large electrical transformers were shaken by the quake, ultimately causing the reactors to shut down automatically, utility officials said Wednesday.

"In the earthquake, the shaking actually opened the contacts" of the switches in the plant transformers, said David A. Christian, CEO of Dominion Generation. "The transformers were intact. They were not, in fact, damaged."

However, the relays' opening interrupted electrical circuits carrying power from off the site to run the two reactors' cooling systems. Loss of the power in turn tripped the reactors offline.

The relays are relatively small switches used to protect the electrical transformers from damaging overloads.

Power to the reactors was off for about 10 seconds before backup diesel generators picked up the required electrical loads to run the reactors' critical cooling systems, said Dan Stoddard, senior vice president of nuclear operations for Dominion.

Stoddard, during an interview at the North Anna plant site Wednesday, added that operators in the power station's control room were reaching for switches to manually shut down the reactors when the reactors automatically shut down.

He added that the safety of the reactors was never in jeopardy. No release of radioactive material occurred beyond the minor releases associated with normal station operations, the company said.

Several aftershocks felt in the region did not affect the station, Dominion Virginia Power said.

"Because the nuclear plants are the lowest-cost source of generation for our customers, we're making all efforts to return the units to service as soon as possible," Christian said, "yet being careful to be thorough and diligent in our post-event inspections to have absolute assurance of operational safety."

Dominion Virginia Power ended the plant's "unusual event" condition Wednesday at 1:16 p.m. after completing inspections of equipment most susceptible to seismic activity. Unusual events are the least serious of four U.S. Nuclear Regulatory Commission emergency classifications.

"We haven't found anything of significance," Christian said.

Earlier Wednesday, the company ended its alert condition, the second to least serious of the four Nuclear Regulatory Commission classifications, after starting a reactor cooling pump for each of the two North Anna nuclear units.

The 980-megawatt reactors were cooled by natural circulation and four diesel-powered emergency pumps while the reactor coolant pumps were not running.

The North Anna units were designed to withstand a magnitude-6.2 earthquake. The earthquake at 1:51 p.m. Tuesday, centered about 11 miles from the plant, had a magnitude of 5.8, according to the U.S. Geological Survey.

Tuesday's earthquake would have had to unleash at least four times more energy than it did to equal the theoretical earthquake the North Anna station was designed to withstand.

The NRC requires that nuclear-power plants be built to take into account the most severe natural phenomena historically reported in a nuclear plant's area.

Though a magnitude-5.8 quake would appear to be approaching the magnitude-6.2 quake that was the basis for the plant's design, the earthquake magnitude scale is logarithmic.

That means that an increase of just one whole unit in an earthquake's magnitude represents about 32 times more energy, explained Harley Benz, scientist in charge at the U.S. Geological Survey's National Earthquake Information Center in Golden, Colo., on Wednesday.

The difference of 0.4 of a unit in magnitude between the actual and the design-basis quake works out to four times more energy than Tuesday's earthquake released, Benz said.