Nuclear Power Plant Design and Seismic Safety Considerations

Anthony Andrews
Specialist in Energy and Defense Policy

Since the March 11, 2011, earthquake and tsunami that devastated Japan’s Fukushima Daiichi nuclear power station, the seismic criteria applied to siting commercial nuclear power plants operating in the United States have received increased attention; particularly the Nuclear Regulatory Commission’s (NRC’s) 2010 reassessment of seismic risks at certain plant sites.

Commercial nuclear power plants operating in the United States vary considerably, as most were custom-designed and custom-built. Boiling water reactors (BWRs) directly generate steam inside the reactor vessel. Pressurized water reactors (PWRs) use heat exchangers to convert the heat generated by the reactor core into steam outside of the reactor vessel. U.S. utilities currently operate 104 nuclear power reactors at 65 sites in 31 states; 69 are PWR designs and the 35 remaining are BWR designs.

One of the most severe operating conditions for a reactor is a loss of coolant accident (LOCA), which can lead to a reactor core meltdown. The emergency core cooling system (ECCS) provides core cooling to minimize fuel damage by injecting large amounts of cool, borated water into the reactor coolant system following a pipe rupture or other water loss, and (secondarily) to provide extra neutron poisons to ensure the reactor remains shut down. The ECCS must be sized to provide adequate make-up water to compensate for a break of the largest diameter pipe in the primary system (i.e., the so-called “double-ended guillotine break” (DEGB)). However, the NRC considers the DEGB to be an extremely unlikely event. Nevertheless, even unlikely events can occur, as the combined tsunami and magnitude 9.0 earthquake that struck Fukushima Daiichi proves.

U.S. nuclear power plants have designs based on Deterministic Seismic Hazard Analysis (DSHA). Since then, Probabilistic Seismic Hazard Analysis (PSHA) has been adopted as a more comprehensive approach in engineering practice. Consequently, the NRC is reassessing the probability of seismic core damage at existing plants.

In 2008, the U.S Geological Survey (USGS) released an update of the National Seismic Hazard Maps (NSHM). USGS notes that the 2008 hazard maps differ significantly from the 2002 maps in many parts of the United States, and generally show 10%-15% reductions in spectral and peak ground acceleration across much of the Central and Eastern United States (CEUS), and about 10% reductions for spectral and peak horizontal ground acceleration in the Western United States (WUS). Seismic hazards are greatest in the WUS, particularly in California, Oregon, and Washington, as well as Alaska and Hawaii.

In 2010, NRC published its GI-199 Safety/Risk Assessment; a two-stage assessment of the implications of USGS updated probabilistic seismic hazards analysis in the CEUS on existing nuclear power plants sites. NRC does not rank nuclear plants by seismic risk. NRC’s objective in GI-199 was to evaluate the need for further investigations of seismic safety for operating reactors in the CEUS. The data evaluated in the assessment suggest that the probability for earthquake ground motion above the seismic design basis for some nuclear plants in the CEUS, although still low, is larger than previous estimates. In late March 2011, NRC announced that it had identified 27 nuclear reactors operating in the CEUS that would receive priority earthquake safety reviews.


Date of Report: May 2, 2011
Number of Pages: 34
Order Number: R41805
Price: $29.95

Follow us on TWITTER at http://www.twitter.com/alertsPHP or #CRSreports

Document available via e-mail as a pdf file or in paper form.
To order, e-mail Penny Hill Press or call us at 301-253-0881. Provide a Visa, MasterCard, American Express, or Discover card number, expiration date, and name on the card. Indicate whether you want e-mail or postal delivery. Phone orders are preferred and receive priority processing.