POST EARTHQUAKE INVESTIGATIONS AT THE NORTH ANNA NUCLEAR POWER PLANT

A magnitude 5.8 (Mw) earthquake occurred near Mineral, Virginia, United States on August 23^rd 2011. This earthquake is one of the largest recorded seismic events occurring east of the Rocky Mountain since seismic instrumentation was deployed. North Anna nuclear power station is located about 18 km from the epicenter. Like the local community, the plant was also impacted by the earthquake vibration, and it is the first US commercial nuclear power plant in which a seismic trip automatically triggered initiating a safe shutdown* of the reactors, as a result of earthquake ground motions. Immediately after the earthquake, the US Nuclear Regulatory Commission dispatched an Augmented Investigation Team to the site to inspect the earthquake impact to the nuclear power plant. The team conducted walkdowns to the structures, systems and components (SSC) both inside and outside the reactor containment buildings. The investigators checked various seismic instruments located at different floor levels and reviewed corresponding seismic recordings. The investigators also interacted and interviewed a large number of staff engineers working at the plant. Based on the investigations, the team concluded that the earthquake ground motion exceeded the SSEs at the plant site at the frequencies important to SSCs, but no significant damages to safety related SSCs were observed because large seismic margins were embedded in the original reactor design.