(Reuters) - Dominion Resources officials said vibration from an August 23 earthquake in Virginia detected inside the two North Anna nuclear reactors caused the units to shut rather than a loss of power into the station, a Dominion spokesman said on Tuesday.
Reactors at the North Anna station, 12 miles from the epicenter of the historic East Coast earthquake, automatically shut and have remained so as inspectors looked for equipment or system damage. No major damage has been found, according to the company.
The 5.8 magnitude quake marked the first time an operating U.S. nuclear plant experienced an earthquake that exceeded its design parameters. Regulators are working to determine what Dominion will be required to do before operations resume at North Anna.
Unit 1 remains shut while Unit 2 has entered a planned refueling outage.
"Our investigation showed the units tripped before the loss of off-site power when multiple reactor sensors detected a slight power reduction in the reactors," said Dominion spokesman Rick Zuercher.
"The root cause team determined that this occurred as result of vibration in the reactor or the monitoring devices in the reactors, or both," Zuercher said. "There was no indication of damage."
Initially, Dominion said the reactors safely shut after the quake disrupted power flowing into the plant from the outside grid. Dominion said it notified the U.S. Nuclear Regulatory Commission of its latest finding.
Meanwhile, Dominion is waiting for the NRC to outline specific actions North Anna must take before the NRC approves a restart of the station.
On October 3, results from a special NRC inspection team will be presented at a public meeting in Mineral, Va. A final inspection report will be released later in October.