The Nuclear Regulatory Commission (NRC) has sent a more proficient inspection team to Dominion’s quake-hit North Anna nuclear power plant in Mineral, Va., to further investigate effects of the Aug. 23 5.8-magnitude quake whose epicenter was only five miles away from the twin-reactor station in Mineral, Va., after Dominion told the NRC that initial reviews determined the ground motion resulting from the quake may have exceeded the plant’s design basis.

The federal regulatory body said in a statement on Monday that it had dispatched an Augmented Inspection Team (AIT) to review Dominion’s response and plant staff activities to check equipment.

Dominion declared an alert (the next to lowest of the four NRC emergency classifications) on Aug. 23 after the quake rattled Central Virginia and sent tremors that caused nuclear plants in locations as far away as North Carolina and Michigan to declare unusual events. Offsite power was restored to North Anna later on Aug. 23. As designed, four diesel generators supplied power to the power plant while offsite power was unavailable, but one of the four generators was later taken offline to repair a generator coolant leak. Dominion reported that a fifth generator at the station was activated to replace the fourth generator until offsite power was restored. The faulty diesel generator has now been repaired.

Both units are in cold shutdown, Dominion’s Manager of Nuclear Public Affairs Richard Zuercher told POWERnews today. After preliminary findings from North Anna’s auxiliary building showed that the quake “potentially exceeded the seismic design basis for the station,” the company submitted a report to the NRC. An auxiliary building at a nuclear power plant is usually located near the reactor containment structure and houses auxiliary and safety systems, including radioactive waste systems, chemical and volume control systems, and emergency cooling water systems.

The determination is in line with the NRC’s preliminary independent analyses, the regulator has said, though it notes that data is still being collected and analyzed to determine the precise level of shaking that was experienced at key locations within the North Anna facility.

Dominion also said it is still analyzing data for both the auxiliary and containment buildings. “We are currently evaluating more seismic data and should know by the end of this week whether the peak ground acceleration frequencies at North Anna were within the design of the station, or if they exceeded it,” Zuercher said. It is “premature to speculate on what may or may not happen” if Dominion finds that ground frequencies exceeded the plant’s seismic design. “We are focused on completing inspections of station equipment,” he said. “We won’t restart the unit until we are sure it is safe to do so.”

Neither North Anna nor Dominion’s other Virginia nuclear station at Surry was impacted by Hurricane Irene. Zuercher said Surry operated safely at 100% during the storm.

The NRC said it had initially dispatched a seismic expert and another structural expert to assist the agency’s resident inspectors on site. “Further reviews indicated that additional inspection is warranted. The NRC inspection has now been officially classified as an AIT or Augmented Inspection Team,” it said.

An AIT is formed by the NRC to review more significant events or issues at NRC-licensed facilities. The AIT will include technical experts from NRC headquarters in Rockville, Md., as well as two additional inspectors from the regional office in Atlanta.

“The fact that we’re sending an AIT should not be interpreted to mean that Dominion staff responded inappropriately or that the station is less safe as a result of the quake,” NRC Region II Administrator Victor McCree noted in a statement. “An AIT provides us with the resources needed to completely understand all the effects at North Anna and gather important information for the NRC’s continuing evaluation of earthquake risk at all U.S. nuclear plants.”

The team is expected to hold an exit meeting with the company upon completion of the inspection to discuss its preliminary findings and issue a written report 30 days after completion of the inspection.

Sources: POWERnews, Dominion, NRC