PERFORMANCE OF THE NORTH ANNA NUCLEAR POWER FACILITY DURING THE 2011 MINERAL, VIRGINIA, EARTHQUAKE – A REGULATORY PERSPECTIVE

The North Anna Nuclear Power Station (NANPS) was the first ever commercial nuclear power facility in the United States to experience a safe shutdown as a result of earthquake-induced ground motion. This facility, located about 22 km northeast of the epicenter of the moderate (moment magnitude 5.8) 2011 Mineral earthquake, comprises two nuclear power reactors that began operation in 1978 and 1980. Although duration of the ground motion was relatively short (i.e., 2 to 4 seconds at different elevation levels in the facility), response spectra developed from recorded ground motions affecting the NANPS facility at the level of the reactor containment basemat showed a modest exceedance of the Safe Shutdown Earthquake in both the vertical and north-south horizontal directions. However, because there was no observed damage to safety-related structures, systems and components (SSCs), performance of the safety-related SSCs during the earthquake demonstrated that the original seismic design was sufficient for the NANPS to accommodate the ground motion without any damage resulting in undue risk to public health and safety or the environment. Three months of extensive inspections of the facility conducted by the utility and NRC after occurrence of the earthquake confirmed that safety-related SSCs at the plant experienced no detectable damage. Only minor damage of some ancillary structures that were not safety-related was observed (e.g., wall cracks and displacement of dry storage casks), and the NRC allowed the facility to restart operation. In addition, no field investigations performed by the utility and independent researchers or field observations made by NRC staff identified a causative surface fault related to the Mineral earthquake either at or immediately adjacent to the plant site. Therefore, the initial conclusion of the NANPS licensee that no capable tectonic structures occur at Units 1 and 2 remains valid. Assessing the existence of a capable tectonic structure at Units 1 and 2, using the term “capable fault” as historically defined in Title 10 of the Code of Federal Regulations (10 CFR), Part 100, Appendix A, was an important part of the deterministic assessment of geologic and seismic criteria for nuclear power plants at the time site characterization was conducted for those two units by the NANPS licensee.