In order to render nuclear power plants a larger seismic margin from design earthquakes and standardize the seismic design procedure for different locations with various seismic fortification intensities, the base isolation technique that has been successfully applied in traditional civil engineering, is proposed for nuclear power plants. Considering the seismic demand from installed facilities and pipes within the plants, a three-dimensional base isolation technique is developed in this study. It was first examined by simplified single-degree-of-freedom models to search for the suitable parameters for the base isolation layer. It is found that the vertical frequency of the base-isolated plant shall be larger than l.0Hz to avoid the dominated rocking mode. A set of time history analyses were then conducted to further explore the damping effect of the base isolation layer on the structural response indices. It is observed that the damping within the reasonable range, commonly less than 30%, is helpful to suppress structural displacement, velocity and acceleration.
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