Seismic Reviews at U.S. Nuclear Power Plants

Every U.S. nuclear power plant, even in areas with minimal seismic activity, is designed and 

built to withstand earthquakes and other natural hazards. The NRC has incorporated updated information 

and lessons from the 2011 Fukushima accident in asking U.S. plants to reassess their seismic situation.

Background

NRC regulations set out design, operation, and maintenance requirements for a nuclear plant’s 

safety-significant structures, systems, and components to keep working after most earthquakes. Plants 

use traditional engineering practices to add “safety margin” beyond those requirements. The NRC 

ensures its requirements are satisfied through the licensing, reactor oversight, and enforcement 

processes.

In the early 2000s the NRC received new reactor-related applications that included updated 

seismic information and analysis methods. The agency in 2005 began looking broadly at potential 

changes in earthquake hazards in the Central and Eastern United States (CEUS). Through this effort, the 

NRC, the Department of Energy and the Electric Power Research Institute developed an updated model 

for CEUS earthquake sources. This work was later incorporated into the NRC’s efforts to implement the 

lessons learned from the accident at Fukushima Dai-ichi.

Earthquake (or Seismic) Hazard

The NRC requires that everything in a U.S. reactor that can support a safety function must be 

designed according to: 

• the most severe natural phenomena historically reported for the site and surrounding area. This 

includes a margin for error to account for imprecise historical data; 

• appropriately combining the natural event with the effects of the plant’s normal and accident 

conditions; and 

• the importance of the safety functions.

An earthquake’s energy spreads 

out from the fault and moves the ground, 

as well as nuclear power plant structures 

on or in the ground. The movement’s 

strength and type depends on how the 

earthquake releases energy and on how the 

soil around a plant absorbs (or dissipates) 

the energy. Earthquakes can be measured 
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by both the frequency of the shaking and how strongly the ground accelerates, or moves at the plant. 

Different frequencies and strengths affect the plant’s buildings as well as the systems and components 

inside.

An earthquake energy’s frequency is measured in cycles per second (or Hz), and the acceleration 

is typically described as some fraction of the acceleration of Earth’s gravity (g), which is about 32.2 feet 

per second per second (ft/s2). Something accelerating at 0.15 g (15 percent of the acceleration of 

gravity) is moving at about 5 ft/s2.

Seismic Safety Assessment

Each operating U.S. nuclear power plant determined its expected ground motions with sitespecific information from historical earthquake catalogs and examination of local geology. Plant 

designers examined earthquake sources near a site. They used the largest quake from that sample to 

determine the site’s expected ground motion. The NRC also used the Individual Plant Examination of 

External Events Program to have existing plants examine potential vulnerability to larger earthquakes.

This examination focused on the plants’ available safety margins to ensure these margins, together with 

the plant’s accident management programs, continue to protect public health and safety. 

These “deterministic” approaches has been replaced with “probabilistic” analysis, which 

examines how all seismic sources and earthquake types can affect a site. This approach estimates how 

likely a given ground motion level is for a certain time period (such as the operating life of a reactor). 

The studies might conclude a site has a 10 percent chance of exceeding ground motion of 0.3 g 

(a “strong” quake) within 50 years. The study could also estimate the maximum ground motion expected 

in the next 10,000 years at the site. Any new nuclear plant the NRC licenses will use a probabilistic, 

performance-based approach to establish the plant’s seismic hazard and seismic design limits.

Evolving Knowledge about Earthquakes 

The CEUS is generally an area of low to moderate earthquake hazard with few active faults in 

contrast to the western United States. Even so, in 1811–1812, three major earthquakes (Magnitude 7 to 

7.7 on the commonly used Richter Scale) shook much of the CEUS. These earthquakes occurred near 

the town of New Madrid, Mo. In 1886, a large earthquake (Richter Scale magnitude of about 7) occurred 

near Charleston, S.C. This earthquake caused extensive damage and was felt in most of the eastern 

United States. Geologists are aware of these historic occurrences, and knowledge of such earthquakes 

was taken into account in plant design and analysis.
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The NRC regularly reviews new information on earthquake source and ground motion models. 

For example, the NRC reviewed updated earthquake information provided by applicants in support of 

Early Site Permits for new reactors. This additional information included new models to estimate 

earthquake ground motion and updated models for earthquake sources in seismic regions such as eastern 

Tennessee, and around both Charleston and New Madrid.

Analysis of these updates indicated slight increases to earthquake hazard estimates for some 

plants in the CEUS. The NRC also reviewed and evaluated recent USGS earthquake hazard estimates 

for the CEUS that are used for building code applications outside of plant licensing. These reviews 

showed that the estimated likelihood of earthquakes occurring at some current CEUS operating sites 

might be slightly higher than what was expected during design and previous evaluations.

NRC Response to Increased Estimated CEUS Earthquake Hazards 

The NRC began assessing the safety implications of increased plant earthquake hazards in the 

CEUS through a May 26, 2005, memorandum (available under Accession No. ML051450456 in the 

NRC Agencywide Documents Access and Management System) in which the staff recommended 

examining the new CEUS earthquake hazard information under the Generic Issues Program. The NRC 

staff identified the issue as GI-199 in a June 9, 2005, memorandum (ADAMS Accession No. 

ML051600272) and completed the screening analysis for the issue in January 2008. The staff then 

assessed available earthquake data and models.

The GIP confirmed that operating nuclear power plants were operating safely. The assessment 

also found that, although still small, some seismic hazard estimates have increased and warrant further 

attention. In September 2010, the NRC issued a Safety/Risk Assessment report (ADAMS Accession No. 

ML100270582) and an Information Notice (ADAMS Accession No. ML101970221) to inform 

stakeholders of the Safety/Risk Assessment results.

Post-Fukushima Seismic Re-Evaluations 

Following the events in March 2011 at Japan’s Fukushima Dai-ichi plant, the NRC considered 

how best to incorporate lessons from Fukushima into consideration of U.S. seismic issues. In February 

2012, the NRC, DOE and EPRI completed years of CEUS research by publishing an updated 

understanding of that region’s seismic sources. Shortly afterward, the NRC asked all U.S. nuclear power 

plants to develop a new ground motion response spectrum, or “hazard curve.” Western U.S. plants had 

to develop site-specific source models, while CEUS plants used the regional model.
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The plants compared the new hazard curve to their license’s original safe shutdown earthquake; 

if the new hazard was greater, additional actions were required. These actions could include examining 

if any immediate actions were necessary, determining seismic effects on spent fuel pools, and/or a 

detailed seismic risk evaluation. The detailed risk evaluations, where appropriate, are underway, and the 

NRC staff will review the evaluations and any identified corrective actions. More information on the 

NRC’s post-Fukushima seismic re-evaluation work is available on the NRC website.

June 2018