ROCKVILLE, Md. Oct. 21, 2011. The chief nuclear officer of Dominion today told the U.S. Nuclear Regulatory Commission that the North Anna Power Station successfully withstood an earthquake without any significant damage because its robust design included multiple, designed-in safety margins.
David A. Heacock, president and chief nuclear officer of Dominion, said the lack of damage from the Aug. 23 earthquake clearly demonstrated that North Anna's true seismic capability is greater than design basis of the station.
"The plant tells the story," Heacock said. "We have gone over North Anna very systematically---every safety system, structure and component---and found no safety-related functional damage. The seismic, thermal and mechanical stress margins designed in to all safety-related piping, equipment and structures made it more than able to withstand this earthquake. These margins, in effect, add up to form multiple layers of strength and safety.
"The station has proven itself through this experience and the results have been verified through extensive inspection, testing and analysis," he said.
Heacock told the NRC that both North Anna units are ready to restart and resume safe operation once the agency completes its independent review, analysis, and on-site inspections and grants permission. At the NRC's request, the company has agreed to perform additional seismic analysis on certain components after restart to quantify and further demonstrate they can meet specific seismic requirements.
Dominion has spent approximately $21 million on its inspection, testing and analysis program, including repairs, since the earthquake occurred.
"Public safety is always first on our minds," Heacock said. "Dominion is fully prepared to undertake this additional analysis and incorporate it into the station's licensing basis."
Both units at North Anna shut down automatically and safely at 1:51 p.m. on Aug. 23 when the earthquake struck Central Virginia. The epicenter was about 11 miles from the station and approximately 4 miles underground. Both units were at full power when the event occurred.
"The short duration of intense motion from the quake was fully within North Anna's capability to absorb and safely shut down without significant damage," Heacock said. "While the quake lasted about 25 seconds, only 3.1 seconds of intense motion occurred. The station could have withstood significantly more."
Heacock said North Anna's designers anticipated significantly greater stresses on safety-related systems than occurred during the quake. While the station may have briefly experienced accelerations that exceeded seismic accelerations to which it was originally licensed, its multiple layers of strength and safety provided a safety margin that was not exceeded.
The scope of the more than 100,000 person-hours expended in support of inspections, testing and analyses conducted by Dominion went above and beyond NRC regulatory guidance to make sure the station was undamaged and ready to resume safe operations. Dominion must demonstrate to the NRC prior to restart that no "functional damage" occurred that would prevent the units from performing safely during normal and emergency operations.
Dominion is one of the nation's largest producers and transporters of energy, with a portfolio of approximately 28,200 megawatts of generation, 11,000 miles of natural gas transmission, gathering and storage pipeline and 6,300 miles of electric transmission lines. Dominion operates the nation's largest natural gas storage system with 947 billion cubic feet of storage capacity and serves retail energy customers in 15 states.