Nuclear energy facilities are unyielding structures with multiple layers of safety designed and built into them. Federal regulations require that these plants be able to withstand extreme natural events that may occur in the region where they are located, including earthquakes, hurricanes, tornadoes and floods.
The independent U.S. Nuclear Regulatory Commission re-examines seismic safety at nuclear energy facilities as new information becomes available. The NRC has required the companies that operate U.S. nuclear plants to have seismic experts re-evaluate the potential earthquake impact at their sites using the latest data and methodologies.
The first part of the analysis was submitted to the NRC on March 31 for sites east of the Rocky Mountains; the analysis for western sites is due by March 12, 2015.
Although these studies are still under way, the NRC staff’s independent review found that seismic risk estimates are consistent with the agency’s conclusions from a 2010 study and that the plants are safe to continue operating.
Based on the results, some companies will perform more detailed evaluations of their plants’ ability to withstand stronger ground motion than they originally expected. In the meantime, the NRC has approved expedited evaluations that will reveal whether and where seismic safety should be enhanced in the near term. The results of the expedited evaluations are due by the end of this year.
Maintaining a High Standard for Seismic Safety
The industry’s highest priority is the safety of our workers, their families and others who live near our nuclear power plants. Since the future of nuclear energy depends on continued safe operation, nothing is more important. The nuclear energy industry has a simple approach to ensuring safety during extreme events: Expect the unexpected and prepare for it.
Federal regulations require that nuclear power plants be able to withstand extreme natural events that may occur in the region where they are located, including earthquakes, hurricanes, tornadoes and floods. The original seismic hazard estimates provided a baseline for nuclear plant design and construction. In licensing each facility, the NRC required that additional safety measures be built into the facility so that it can withstand even stronger ground motion than indicated by the data, and the industry’s standard practice is to add layer upon layer of safety so that a nuclear plant not only meets regulatory standards—it exceeds them.
Thus, our nuclear plants have a wide margin of safety between the ground
motions originally estimated for each site and what the facility is capability of withstanding. With new information about seismic hazards now in hand, the industry will conduct risk evaluations over the next few years to refine our knowledge about the seismic capabilities of our plants and identify ways to make them even safer.
Independent Federal Agency Reviews Earthquake Safety
The independent U.S. Nuclear Regulatory Commission re-examines seismic safety at nuclear energy facilities as new information becomes available. The NRC initiated its most recent review in light of new information indicating that the potential earthquake impact might be different from previous estimates for some locations in the central and eastern United States. The agency concluded in 2010 that all nuclear energy facilities are safe, but it is examining earthquake safety in more detail.
Earthquake safety standards are more stringent for nuclear energy facilities than for any other type of infrastructure. Models and methodologies developed expressly for these facilities are used to assess the types of ground motion they may experience. Between 2008 and 2012, the nuclear energy industry worked with the NRC and the Department of Energy to develop a new model to identify the potential sources of earthquakes for the central and eastern United States.
Earthquake safety drew increased attention in 2011 after a tsunami triggered by a powerful earthquake disabled safety systems at the Fukushima Daiichi nuclear power plant in Japan and led to an accident. Although it was flooding from the tsunami rather than shaking from the earthquake that led to the accident, the NRC folded its ongoing review of seismic safety into its post-Fukushima recommendations for U.S. reactors.
Industry Evaluations Use State-of-the-Art Methodologies
Seismic experts are re-evaluating earthquake protection at nuclear power plants using the latest available data and methodologies. The first part of the analysis was submitted to the NRC on March 31 for sites east of the Rocky Mountains; analysis for western sites is due by March 12, 2015. Different models must be developed for the western sites because of significant differences in geology between that region and the area east of the Rocky Mountains.
Although these studies are still under way, the NRC staff’s independent review found that seismic risk estimates for nuclear plants in this region are consistent with the agency’s 2010 conclusions and that the plants are safe to continue operating.
Based on the results of the studies, some companies will perform more detailed evaluations of their plants’ ability to withstand stronger ground motion than they originally expected. In the meantime, the NRC has approved expedited evaluations that will reveal whether and where seismic safety should be enhanced in the near term. The results of the expedited evaluations are due by the end of this year.
All US Nuclear Power Plants Built to Stringent Earthquake Safety Standards
All nuclear power plants in the United States are designed and built to stringent seismic standards appropriate for the region in which they are located.
The United States has averaged more than 3,000 earthquakes per year over the past 20 years, mostly in the mild to moderate range (magnitude 2.0 to 5.9), according to the U.S. Geological Survey. Magnitude is a measure of the energy released in an earthquake. Earthquakes exceeding magnitude 5.5 are relatively rare in the central and eastern United States. However, large earthquakes can and have occurred. These include earthquakes that exceeded magnitude 7 in the New Madrid, Mo., zone in 1811 and 1812 and at Charleston, S.C., in 1886. These earthquakes were taken into account when existing nuclear power plants were built.
One of the differences in seismic hazard between the West Coast and the region east of the Rocky Mountains is that ground motion is transmitted over greater distances in the East. The U.S. Geological Survey estimates that a magnitude 4.0 earthquake will be felt 60 miles away; a 5.5 earthquake can be felt as much as 300 miles away.
Experience Confirms Seismic Safety of Nuclear Energy Facilities
There have been few large earthquakes (magnitude 5.5 or greater) near nuclear power plants. The safety performance of these plants confirms the seismic ruggedness of these facilities.
The North Anna nuclear energy facility in central Virginia (2011). North Anna experienced a magnitude 5.8 earthquake—the second-largest earthquake recorded in the state—that originated 11 miles from the site. The closeness of the 2011 quake was significant because ground motion dissipates over time and distance from the source. Although the plant experienced stronger ground motion than had been predicted for the area, all systems were protected and performed as designed to maintain safety.
Chūetsu Offshore Earthquake (Japan, 2007). The Kashiwazaki-Kariwa plant experienced stronger ground motions than it was designed to withstand from a large earthquake, just offshore of Japan. All seven reactors were safely shut down as they were designed to do during an earthquake.
Great Japan Earthquake (2011). Several nuclear power plants on the east coast of Japan were affected by one of the largest earthquakes in recorded history. The Onagawa nuclear plant, closest to the source of the earthquake, safely shut down, and it even provided temporary shelter for displaced neighbors. All safety-related systems of the three reactors performed as designed. The Fukushima Daiichi and Daini plants also were structurally sound and were safe after the earthquake. However, the tsunami generated by the earthquake caused significant damage at both sites.
Experience demonstrates that the extensive safety features built into nuclear power plants to prepare for the unexpected ensures safety for workers and the public in the event of large earthquakes.