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.

Nuclear
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.