Post 23 August 2011 Mineral, Virginia, Earthquake Investigations at North Anna Nuclear Power Plant

Abstract

An earthquake with a magnitude ( M w) 5.8 occurred near Mineral, Virginia, on 23 August 2011. This earthquake is one of the largest, instrumentally recorded seismic events to occur east of the Rocky Mountains. The North Anna nuclear power station (NANPS) is located about 18 km from the epicenter. The plant and local communities were both impacted by the earthquake, and NANPS is the first U.S. commercial nuclear power plant that experienced an automatic safe shutdown from an earthquake, wherein the nuclear reaction was stopped in a controlled manner by inserting the control rods into the reactor. Immediately after the earthquake, the U.S. Nuclear Regulatory Commission (NRC) dispatched several teams, including an Augmented Inspection Team and other teams of seismologists and engineers, to investigate the earthquake’s impact on the two nuclear reactors at the site. This paper reports the findings of the investigations and lessons learned from the earthquake’s impact on NANPS. The earthquake occurred in Mineral at approximately 13:51 hours, with the mainshock occurring at shallow depth about 6 km from the surface. The earthquake was associated with a reverse fault and the fault probably strikes in the north or northeast direction, according to the U.S. Geological Survey’s focal mechanism solution (USGS, 2011). However, it is uncertain which fault actually caused the earthquake. A few aftershocks persisted, the largest having a magnitude of 4.5. All of the shocks, including the mainshock, were inside the Central Virginia seismic zone (CVSZ). The largest historical earthquake known to have occurred in the CVSZ before 2011 is the magnitude 4.8 (estimated because no seismic monitors were available at that time) earthquake that occurred in 1875 (USGS, 2011). NANPS (see Fig. 1, inset) has two pressurized‐water reactor (PWR) units that began operation in 1978 and 1980. Each unit has an output core power of approximately …