Resources & Stats

September 2011

Protection of nuclear energy facilities against earthquakes has been a frequent topic of conversation since the Aug. 23 quake that struck much of the East Coast. All nuclear energy facilities have specific seismic protection standards based on the historical earthquake activity in that area, plus an additional margin of safety.

No American nuclear energy facility has ever sustained significant damage from an earthquake. As a precaution, reactors are designed to automatically shut down safely and if such a disturbance results in a loss of electric power from the grid. Operators also can shut the reactor down manually as a safety precaution if conditions warrant.

Engineers carefully map existing fault lines before building major structures, such as nuclear energy facilities, dams and bridges, so they can design the structures to withstand potential ground forces from quakes. These forces have varying effects on systems and structures within a nuclear energy facility. Nuclear plants have safety margins for earthquakes that are beyond their design basis. This additional safety results from more robust plant design and construction than required in the design, improved data or analysis methods, and plant modifications made over time at a facility.

Like many scientific and engineering issues that span decades, there is new information emerging about earthquakes, particularly in the central and eastern regions of the United States. The industry and the independent U.S. Nuclear Regulatory Commission are evaluating this data and this fall will discuss steps that may be taken to update seismic criteria. “Although there is not an immediate safety concern, the NRC is focused on assuring safety during even very rare and extreme events. Therefore, the NRC has determined that assessment of updated seismic hazards and plant performance should continue,” the agency said in an FAQ about its seismic programs.

The nuclear energy industry and NRC have continuously reviewed seismic data and examined safety at U.S. reactors. We have learned new information about earthquakes from hundreds to thousands of years ago from geological studies. Also, there have been recent earthquakes in the Southern states and Gulf of Mexico that have been larger than previous estimates of the possible maximum quakes for those areas. Industry seismic models have been updated using this data. As new seismic data becomes available, utilities will perform the necessary safety reviews to maintain nuclear energy facilities with safe operating and shutdown capabilities. As the industry continues to study the issue, plants will make investments to provide further assurance that the plant and public safety will be protected from seismic events.

Preliminary analysis from Dominion Virginia Power’s North Anna Power Station, located 11 miles from the epicenter of the Aug. 23 earthquake, shows that the quake may have caused ground motion at frequencies that are greater than the plant’s licensing basis. Both reactors shut down automatically following the magnitude 5.8 Richter quake and are undergoing inspection by the U.S. Nuclear Regulatory Commission. Multiple inspections at the plant by Dominion Virginia Power have found no damage to the two reactors or safety equipment at the facility.

The industry assesses the impact of earthquakes on nuclear power facilities by measuring the ground motion produced by the event. Ground motion is typically reported in terms of peak ground acceleration. Nuclear energy facilities are designed to withstand peak ground acceleration in the range of 0.1g to 0.75g depending on the projected seismic hazard for each site.

Earthquakes cause ground motion over a range of frequencies. Lower frequency motions are more damaging to building and equipment than higher-frequency motions. Based on NRC reviews associated with early site permits for new reactors, the agency said it “is confident that the recent seismic data and models will show that increased estimates of the seismic hazards will occur primarily in the higher frequency motions. Accordingly, these increased estimates of seismic hazards would primarily have little impact on previous estimates of the potential damage to buildings and equipment.” This is because the safety margins built into nuclear energy facilities are expected to protect them against earthquakes that are stronger than the design basis for the plant.

Buildings and equipment at nuclear energy facilities are designed to absorb the energy from the largest historical earthquake near that site. Nuclear energy facility designers analyze equipment before installation to determine that it can withstand various motions expected at locations within the power plant.

Through testing, the industry and the NRC have identified some equipment that is susceptible to damage as a result of high frequency, rapid oscillation motion. However, this equipment is not used in systems or components that perform safety functions. State-of-the art electronic equipment is not affected by rapid motion (high-frequency) events. Other parts of the plant, such as steam generators and large pumps, are protected using giant shock absorbers that are designed into the facility during construction.