Nuclear power plant design and construction ensure the plants can withstand powerful earthquakes. Plant designs include a detailed evaluation of potential earthquake-induced ground motion at the site. This is followed by thorough analysis and testing of the plant structures, systems and equipment, using simulated earthquake-induced vibrations.
July 2012 Key Facts - The independent U.S. Nuclear Regulatory Commission (NRC) requires that nuclear power plants be able to withstand extreme natural events that may occur in the region where they are located, including earthquakes, tsunamis, hurricanes, tornados, fires and floods.
- Nuclear power plant designs are based on thorough seismic analyses and testing. Before a plant is built, detailed analyses are made of earthquake potential at a site to show that the facility can withstand earthquakes.
- Sensitive monitoring instruments at each nuclear energy facility detect earthquakes. Operators are required to shut down a plant if ground motion exceeds a specified level.
- The nuclear energy industry is dedicated to learning all relevant lessons from the 2011 nuclear accident in Japan and is taking steps to make already safe facilities even safer.
Designed to Withstand Maximum Projected Earthquake While the Richter magnitude scale is the most familiar earthquake measurement, it only calculates the energy released by an earthquake, not other important parameters, such as effects on buildings. The NRC requires that reactors be able to withstand a certain level of ground motion specific to a given site, not a specific magnitude on the Richter scale.
Before a nuclear energy facility is built, engineers calculate the potential for earthquake-induced ground motion at the site and review the effects of historical earthquakes whose epicenters are within 200 miles. Earthquakes that have occurred within 25 miles are studied in greater detail. Local geology also is taken into account. For example, a site that features clay over bedrock will respond differently to ground motion than a hard-rock site. Engineers use this research to determine the maximum potential earthquake for a particular site and include an extra margin of safety in the design of safety systems at nuclear energy facilities.
If an earthquake occurs, the NRC requires operators to shut down a reactor when the ground motion crosses a threshold well below the maximum for which the facility was designed. Each plant has instruments to record earthquake-induced motion to determine whether the reactor is required to shut down. Once a reactor is shut down, the owner must perform extensive inspections to evaluate the impact of the earthquake and the condition of the safety systems and equipment. NRC approval is required before operators can restart the reactor.
When a rare 5.8-magnitude earthquake shook the East Coast in 2011, Dominion’s North Anna nuclear power station in central Virginia, located just 11 miles from the epicenter, shut down safely. Because the earthquake exceeded the intensity specified in North Anna’s operating license, Dominion was required by the NRC to demonstrate that the earthquake did not damage the facility’s safety structures or systems before it was granted permission to restart. After 80 days and more than 110,000 hours of inspections and analysis, the NRC authorized the facility to restart. The agency concluded that “safety system functions were maintained” and “reviews of the plant equipment, systems and structures did not reveal significant damage.”
Ongoing Study of Earthquake Safety for Nuclear Plants As estimates of potential earthquakes are refined, nuclear power plant owners have taken steps to make the facilities even safer. A 2005 NRC study found that earthquake hazard estimates were higher than previous evaluations had determined for some plants in the central and eastern United States.
1 The study prompted the agency to launch a study to better understand the safety margin at these plants. In January 2012, the NRC, the Electric Power Research Institute and the U.S. Department of Energy released a new seismic model that will help nuclear energy facilities determine how to better prepare for extreme events.
2 The Pacific Gas and Electric Co. and Southern California Edison are conducting studies of earthquake potential in the Pacific Ocean near the PG&E Diablo Canyon nuclear energy facility and SCE’s San Onofre Nuclear Generating Station. SCE is collaborating with the Scripps Institution of Oceanography at the University of California San Diego.
Nuclear Energy Industry Responds to Fukushima After the 2011 accident at the Fukushima Daiichi nuclear power plant, operators of U.S. reactors conducted detailed inspections to ensure that all safety equipment was fully operational. The reviews included evaluations of safety and emergency response systems as well as assessments of staffing and safety programs.
The inspections focused on issues that presented major challenges in Japan, including:
- protecting against extreme natural events
- ensuring that critical equipment can withstand an earthquake
- maintaining cooling capability to safeguard reactor fuel and used fuel stored on site
- mitigating the effects of flooding on systems inside and outside the plant
- effectively managing the loss of all electric power.
Operators of U.S. nuclear power plants began making safety enhancements before the NRC issued post-Fukushima requirements. The industry and the NRC are in general agreement on near-term action. The industry another layer of portable safety equipment and strategies that can be used to respond to a variety of natural events. The approach will place emergency equipment at strategic locations on a plant site, supported by additional equipment that is pre-staged off site.
1 Fact Sheet on Seismic Issues for Nuclear Power Plants, U.S. Nuclear Regulatory Commission. Retrieved from http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/fs-seismic-issues.html, June 2012. (U.S. Nuclear Regulatory Commission 2012)
2 New Seismic Model Will Refine Hazard Analysis at U.S. Nuclear plants. Retrieved from http://pbadupws.nrc.gov/docs/ML1203/ML120330098.pdf, June 2012. (U.S. Nuclear Regulatory Commission 2012)