A hard stop will be required for seismically isolated nuclear power plants (NPPs) in the United States to eliminate the seismic isolation system from accidents sequences that could lead to core damage or large release of radiation. The isolation system will be analysed, designed and tested for two levels of seismic hazard: a ground motion response spectrum+ (GMRS+) with a return period of 10,000 years and an extended design basis (EDB) GMRS with a return period of 100,000 years. Response-history analysis for ground motions consistent with these spectra will establish the clearance to the hard stop (CHS), which is required to be the greater than the 99th (90th) percentile displacement for GMRS+ (EDB GMRS) shaking. Alternate representations of seismic hazard (uniform hazard response spectrum, conditional mean spectrum, conditional spectra) are considered for the site of the Diablo Canyon Nuclear Generating Station and distributions of isolation-system displacement are calculated. The 90th percentile displacement for EDB GMRS shaking dictates the calculation of the CHS. Seismic hazard for isolated NPPs should be defined using a uniform hazard response spectrum with due consideration of the differences in the amplitude of the orthogonal horizontal components of ground shaking.
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