The seismic performance study of the base isolated nuclear power plants (NPP) has gained importance after the recent devastating earthquake events. For past few decades, the soil structure interaction (SSI) approach has been applied to structures to investigate the effects on the nonlinear response of the structure. The present work focuses on the performance of the rigidly fixed base isolated NPP reactor building compared to the one considering nonlinear pile-soil interaction. The NPP model is supported by deep pile foundation with 20m depth. Free site response analysis of two soil sites are considered to evaluate the dynamic behavior of the NPP models due to different geotechnical characteristics of stiff and soft soil strata. Two sets of Tohoku earthquake ground motions of short- and long-period; are selected and applied to the NPP model as input motions. The selected ground motions are spectrally compatible based on the guidelines provided by U.S Nuclear regulatory commission (USNRC). The numerical results of the seismic analysis are presented in terms of the lateral nodal displacement, the shear forces and the spectral acceleration of the NPP stick model. The obtained results indicate under the short-period inputs that the base isolated NPP model considering pile foundation show higher responses than the rigidly fixed base isolated NPP, while they are less under the long-period ground motions.
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