Seismic Qualification of Nuclear Plant Components Using PSHA-Based Spectra

With the prospect of a revival of nuclear power industry after a long hiatus, there is an emphasis on designing the next breed of nuclear plants in the US using seismic spectra derived from a probabilistic seismic hazard analysis (PSHA). The methods available in guidance documents to establish Safe Shutdown Earthquake (SSE) spectral shapes at a site using PSHA invariably show that the risk-based spectra have high peaks, high zero period accelerations (ZPA), and significant energy content at higher frequencies when compared to the previous deterministic spectra at the same site. It is well known that earthquakes in Central and Eastern United States (CEUS) will typically contain some high frequency energy. While the early site permit applications and reactor supplier’s design certifications for new plants are expected to use the PSHA-based spectra for their seismic design, existing nuclear plants designed to deterministic spectra may also need to be reviewed for the probabilistic seismic spectra at their sites. This paper considers the implications of a probabilistic hazard spectrum for the seismic qualification of equipment and components for an operating plant and suggests a procedure for conducting a review. Amplification of ground spectra through nuclear plant structures and other intervening systems such as a piping system or an electrical cabinet are calculated using conventional linear dynamic analysis methods in much the same way as was done in the past for high frequency hydrodynamic loads in the Boiling Water Reactor (BWR) containments. Electrical and mechanical equipment, including devices such as relays that may be sensitive to high frequency vibratory loads are evaluated. While the spectral peaks at equipment mounting location are high at higher frequencies, the damage potential is considerably low. For an existing plant, a limited review of the previous seismic analyses and testing with the redefined seismic spectra concludes that the previous design has sufficient seismic margin. Implications of the PSHA based spectra for seismic qualification of equipment for new plants is not expected to be as severe as once believed. Additional assurance of safety can be obtained by updating or conducting a plant-specific seismic probabilistic risk analysis.