An investigation by the Nuclear Regulatory Commission found that some seismic equipment briefly stopped working at Virginia's North Anna nuclear plant during an earthquake in 2011.
No damage occurred, reactor safety systems performed as designed, and both units at the Dominion plant shut down automatically during the 5.8 magnitude quake and power outage. But for eight seconds while emergency diesel generators powered up, part of the control room seismic monitoring equipment shut off because it was not connected to an uninterruptible power supply, the NRC reported in an information notice released Tuesday. Operators declared an alert in response to the earthquake, but the NRC reported the equipment outage prevented them from promptly accessing measurements of the shaking. A measurement of the quake's intensity would be used to determine if precautions such as shutting down the reactors were required, had the units not tripped automatically, according to the agency.
Later, information on the peak shaking period was retrieved and analyzed from a battery-powered Kinemetrics tri-axial seismic time history accelerograph. Nonetheless, the NRC issued the information notice to make other plants aware of the power supply issue potentially affecting other seismic equipment. No action on their part is required. The notice also noted that Kinemetrics SMA-3 monitors at the Surry nuclear plant, also in Virginia, did not trigger following the quake near North Anna. According to the NRC, troubleshooting found that all three sensor masses for the Kinemetrics triggering unit were misaligned and could not trigger at the required level of ground movement.