Effect of seismic hazard definition on isolation-system displacements in nuclear power plants

The effect of seismic hazard definition on the distribution of isolation-system displacements in a nuclear power plant (NPP) are studied in the paper and recommendations are made for design practice. The NPP is considered to be located at Diablo Canyon in California, a site of high seismic hazard, and is horizontally isolated using Friction Pendulum™ (FP) seismic isolation bearings. Four descriptions of seismic hazard are investigated: uniform hazard response spectrum (UHRS), conditional mean spectrum (CMS), conditional spectra (CS), and UHRS-MaxMin. Uniform hazard response spectra are derived by probabilistic seismic hazard analysis and are the traditional description of seismic hazard in the nuclear industry in the United States. The UHRS is used to characterize the effects of design basis shaking but its ordinates across a wide range of period do not represent shaking associated with one ground motion set. The CMS and CS are derived from a UHRS and better characterize the effects of shaking from one ground motion set. The UHRS-MaxMin definition is also based on the UHRS but explicitly recognizes differences between motions in the orthogonal horizontal directions. To investigate the utility of alternate descriptions of seismic hazard, the macro model of a seismically isolated NPP is subjected to ground motions consistent with the four definitions and for two intensities of earthquake shaking: design basis (DB) and beyond design basis (BDB) shaking as defined in the forthcoming seismic isolation NUREG. The coefficient of friction at the sliding surface is defined using two models: 1) Coulomb, and 2) p-T-v model that updates the coefficient of sliding friction at each time step as a function of axial pressure, temperature and sliding velocity. The key results of the study, which are broadly applicable to sites of lower seismic hazard and other nonlinear bearings (e.g., the lead-rubber bearing), are: 1) the seismic hazard definition should account for differences between the amplitude of ground motions in the principal horizontal directions, 2) the displacement capacity of an NPP isolation system is controlled by the 90th percentile BDB shaking displacement, for a given hazard definition, and 3) the coefficient of friction at the sliding surface of a single-concave FP bearing should be defined using a p-T-v model because the standard Coulomb model may be inadequate for high values of axial pressure and nominal coefficient of sliding friction.