Frequently Asked Questions:
Another Look at Seismic Safety for Nuclear Energy Facilities
June 2014
1. Why are nuclear energy companies re-evaluating the seismic hazards for their facilities?
Nuclear power plants are very robust 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.
Nuclear plants have been generating electricity for more than 40 years in the United States and globally,
but only in the past 10 years have moderate to large earthquakes occurred in close proximity to a few
reactors. In all cases, nuclear plant structures and systems that were required to maintain the plants in a
safe condition have performed as designed. Nonetheless, the NRC has required 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 to the NRC by March 12, 2015.
2. What conclusions can we draw about seismic safety?
Although the seismic studies are still under way, the NRC staff’s independent review found that seismic risk
estimates for nuclear plants east of the Rocky Mountains are consistent with the agency’s 2010 conclusions
and that the plants are safe to continue operating.
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, nuclear plants have a wide margin of safety between the ground motions originally
estimated for each site and what the facility is capable 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.
3. What are the next steps?
In themselves, the new seismic hazard estimates submitted to the NRC in March provide no safety
information about a facility. They are estimates of ground motion the site could experience, but they do
not provide information about how the plant would perform under those circumstances. Depending on the
results of the seismic calculations, some companies will perform more detailed evaluations of their plants’
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ability to withstand stronger ground motion than they originally expected.
The NRC divided the nuclear plants in the central and eastern United States into three groups for purposes
of seismic evaluations. This reflects the relative priority for conducting the evaluations and what the NRC
believes is the appropriate allocation of limited staff resources and seismic expertise. Plants in the first
group are expected to conduct a seismic risk evaluation and submit it to the NRC by June 30, 2017;
evaluations for the second group are due by Dec. 31, 2019. The NRC said it will decide later which plants
in the third group need to complete a risk evaluation, which will be due by Dec. 31, 2020.
In the meantime, the NRC has approved expedited evaluations using guidance developed by the Electric
Power Research Institute. These expedited evaluations will reveal whether and where seismic safety
should be enhanced in the near term.
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