While the use of seismic base isolation for traditional structures and bridges are more common, this is not the case for NPP. However, several new projects are being designed and built with isolators. The purpose of this paper is to present the work carried out by NECS for EDF and CEA, to study and design NPPs equipped with different type of isolators. For this purpose several material constitutive models have be developed and implemented in CodeAster, the finite element software created by EDF. The studies presented concern the ASTRID and EPR nuclear power plants, for which 3D models of the entire nuclear island were created. Soil structure interactions are taken into account, and floor response spectra estimated by time history analyses. In cases when the behavior of base isolation devices is not linear, nonlinear resolution algorithm is used. These results allowed well understanding the seismic behavior of superstructures, basemats, and soil pressure. For these two projects, results indicate complex phenomenon such as horizontal and vertical movement interactions, secondary peak floor response spectrum, and non-homogeneous base isolator loading in the presence of poor soil conditions. These were possible thanks to the use of more appropriate modeling techniques which are detailed in this paper.
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