Early in the afternoon last August 23, people living near the small town of Mineral, Va., felt a 5.8 magnitude earthquake. That earthquake also hit the nearby North Anna Nuclear Power Station. The plant, operated by Dominion, automatically shut down and lost electrical power from off site. The plant staff declared an Alert, the second-lowest of four NRC emergency classifications, and shut down safely.
The NRC’s resident inspectors began reviewing the plant’s response to the event immediately. A broader NRC inspection, called an Augmented Inspection Team or AIT, was launched soon after the event and those inspectors began assessing Dominion’s actions following the earthquake and conducting an independent review.
That inspection team found that the plant staff responded appropriately. Although the ground motion exceeded some levels for which the plant was originally licensed, there was no damage to safety equipment and safety systems functioned during the quake. Another extensive NRC inspection was later completed as part of reviewing the plant’s readiness to restart.
The NRC staff began that technical review to evaluate Dominion’s post-quake inspections, steps required for restart and other long-term actions. The inspections and technical review led the NRC to conclude in November that both of the North Anna units could be operated following the earthquake without undue risk to the public. The NRC also issued a letter to Dominion that legally binds the company to its commitments, such as incorporating analyses and information from the quake into future plant license documents as well as continued reviews of equipment and systems.
While the North Anna Power Station survived an earthquake beyond its original design with no significant damage, the NRC continues to evaluate information from the earthquake and the plant staff has installed additional seismic and backup equipment. Despite the good news at North Anna, the NRC earthquake story does not end there.
Six years before the earthquake near North Anna, the NRC staff began evaluating updated seismic models and methods for analyzing earthquake hazards. That effort, which includes all nuclear plants, was rolled into the lessons learned from the event at Japan’s Fukushima Daiichi nuclear plant in March 2011. U.S. plants will use those updated methods to determine if additional improvements to reduce seismic risk at some plants are warranted.Roger Hannah Senior Public Affairs Officer Region II, Atlanta