Fukushima Lessons: Updating Earthquake Hazards at U.S. Nuclear Plants

Scott Burnell
Public Affairs Officer
 

The NRC is examining new earthquake-related information from U.S. nuclear power plants, and we’re making that information available over the next week or so. We’d like to summarize how we got here and what the next steps are.

seismicgraphicNuclear power plant designs set a basic standard for reactors to completely and safely shut down after an earthquake, based on site-specific information. Plant construction methods and other design factors add to a reactor’s capacity to safely withstand stronger motions than what the basic design describes.

The end of March marked an important milestone for our post-Fukushima activities. We received 60 reports from central and eastern U.S. nuclear power facilities updating the seismic hazard at their individual reactors. The NRC staff is making these reports available through its normal process. The NRC will post each plant’s report on the agency website’s Japan Lessons-Learned Activities page.

We will require the same updates of the three western power plants (Palo Verde in Arizona, Columbia Generating Station in Washington, and Diablo Canyon in California), but delayed by one year because of the more complex geology in that region of the country. Each western plant is individually looking at the seismic sources and local ground motion characteristics that could affect it. This Senior Seismic Hazard Analysis Committee process will inform the overall seismic hazard reassessment that the western plants are completing.

Our staff will spend the next month going over the submissions carefully, checking for errors, before confirming which plants will be required to do more extensive analysis of their ability to respond safely to a significant earthquake event.

These reports mark the first step in a comprehensive process to keep safety at U.S. plants up-to-date with the latest understanding from the earth sciences on the processes that create earthquakes in the U.S.  In 2012, the Department of Energy, Electric Power Research Institute and the NRC joined forces to update the “seismic source model” for the central and eastern U.S. This was based on a new understanding of what creates earthquakes on the North American tectonic plate, with a focus on the New Madrid fault zone near St. Louis, the Charleston fault zone near Charleston, S.C., and other updated information.

The data on seismic sources will be used in conjunction with a ground motion model for the central and eastern U.S. as well as data from individual plants on the localized geology, topography, soil cover, and other data to create a picture of the “ground motion response spectra” for each plant. This new ground motion response spectrum at each plant will be compared with that developed in the past to see if the new data suggests the plant could see higher ground motions than previously thought. If that is the case, the plant will be considered to have “screened in” to further detailed seismic hazard analysis.

Those plants that “screen in” will be required to do a seismic “probabilistic risk assessment” or a seismic “margin analysis” to evaluate in detail how the existing plant structures and systems would respond to the shaking from the range of earthquakes that could affect the plant based on our current understanding of seismic sources. This assessment is extensive, involving experts from a variety of fields, and will require at least 3 years to complete. Once these assessments are complete, the NRC will decide if significant upgrades to plant equipment, systems, and structures are required.

In the meantime, to ensure that the plant is safe, the NRC requires that by the end 2014, plants have reported the interim actions they will take to ensure the safety of the plants before the assessment is complete. Such measures could include re-enforcing existing safety-significant equipment or adding equipment.

It’s important to remember that significant earthquakes at central and eastern U.S. plants are unlikely. But it is our job to ensure that these plants are ready for all that nature might throw at them. And it is our job to keep up with the changes in the science to ensure that plants are as safe as they can be.