Nuclear Power Plant Design And Seismic Safety Considerations

The earthquake and subsequent tsunami that devastated Japan's Fukushima Daiichi nuclear power station and the earthquake that forced the North Anna, VA, nuclear power plant's temporary shutdown have focused attention on the seismic criteria applied to siting and designing commercial nuclear power plants. Some Members of Congress have questioned whether U.S nuclear plants are more vulnerable to seismic threats than previously assessed, particularly given the Nuclear Regulatory Commission's (NRC's) ongoing reassessment of seismic risks at certain plant sites. The design and operation of commercial nuclear power plants operating in the United States vary considerably because 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 are BWR designs. One of the most severe operating conditions a reactor may face 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 water containing boron (borated water slows the fission process) into the reactor coolant system following a pipe rupture or other water loss. The ECCS must be sized to provide adequate makeup water to compensate for a break of the largest diameter pipe in the primary system (i.e., the socalled "double-ended guillotine break" (DEGB)). The NRC considers the DEGB to be an extremely unlikely event; however, even unlikely events can occur, as the magnitude 9.0 earthquake and resulting tsunami that struck Fukushima Daiichi proves. U.S. nuclear power plants designed in the 1960s and 1970s used a deterministic statistical approach to addressing the risk of damage from shaking caused by a large earthquake (termed Deterministic Seismic Hazard Analysis, or DSHA). Since then, engineers have adopted a more comprehensive approach to design known as Probabilistic Seismic Hazard Analysis (PSHA). PSHA estimates the likelihood that various levels of ground motion will be exceeded at a given location in a given future time period. New nuclear plant designs will apply PSHA. In 2008, the U.S Geological Survey (USGS) updated the National Seismic Hazard Maps (NSHM) that were last revised in 2002. 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). Spectral acceleration refers to ground motion over a range, or spectra, of frequencies. Seismic hazards are greatest in the WUS, particularly in California, Oregon, and Washington, as well as Alaska and Hawaii. In 2010, the NRC examined the implications of the updated NSHM for nuclear power plants operating in the CEUS, and concluded that NSHM data suggest that the probability for earthquake ground motions may be above the seismic design basis for some nuclear plants in the CEUS. In late March 2011, NRC announced that it had identified 27 nuclear reactors operating in the CEUS that would receive priority earthquake safety reviews.

This Safety Guide provides recommendations on a generally accepted way to design a nuclear power plant so that an earthquake motion at the site will not jeopardize the safety of the plant. It also gives guidance on a consistent application of methods and procedures for analysis, testing and qualification of structures and equipment so that they meet the safety requirements covering the design of nuclear power plants, safety assessments for the design and the regulatory issues concerned with the licensing of plants.

This guide was originally issued as Safety Guide No. 50-SG-S2. It provides details on the design of a nuclear power plant such that earthquakes at the site determined according to Safety Guide No. 50-SG-S1 will not jeopardize its safety.

This Safety Guide provides recommendations on the seismic safety evaluation of nuclear installations. It addresses all types of new and existing nuclear installations. This Safety Guide presents three assessment methodologies: the deterministic approach, generally known as seismic margin assessment (SMA), seismic probabilistic safety assessment (SPSA), and a combination of SMA and SPSA known as ‘probabilistic safety assessment (PSA) based SMA. This Safety Guide provides specific recommendations on applying a performance-based graded approach to the seismic safety evaluation of nuclear installations other than nuclear power plants. It also covers the relation between seismic safety margins and defense-in-depth (DiD) level 3 and level 4. For new nuclear installations, this Safety Guide provides recommendations to assess adequacy of seismic margin to avoid cliff edge effects considering DiD level 3 and level 4. This publication is intended for use by regulatory bodies, operating organizations, and designers of nuclear installations.

Experience shows that an assessment of the seismic capacity of an existing operating facility can be required for a number of reasons, for example identification of potential seismic vulnerabilities based on operating experience events or the periodic safety review programme. This publication covers the seismic safety evaluation programmes to be performed on existing nuclear installations in order to ensure that the required fundamental safety functions are available, with particular attention to the safe shutdown of reactors. It includes lessons learned based on the IAEA Action Plan on Nuclear Safety following the Fukushima Daiichi accident, and updated methodologies for seismic safety evaluation of nuclear installations.