Floor Response Spectrum of Nuclear Power Plant Structure Considering Soil-Structure Interaction

Abstract

Floor response spectrum (FRS) of nuclear power plant (NPP) structure should be calculated to be used in seismic design or safety assessment of the structure and the system. A lot of NPP structures are on a rock site, but there is a case when the structure is constructed on a soft soil site. When the structure is on a soil site, soil-structure interaction (SSI) effect largely affects the seismic response of the structure, which in turn affects the shape of the resultant FRS. In this paper, effects of the SSI on the FRS are numerically investigated by comparing two models with different soil profiles. The SSI analysis is done in the frequency domain, which is less computationally expensive than the time-domain analysis. In order to perform frequency-domain SSI analysis, a widely used software for SSI analysis, ACS SASSI, is used. The SSI analysis by ACS SASSI is based on a substructure method. In this method, a free-field analysis and a structural analysis are separately performed. Impedance and transfer function can be obtained by free-field analysis which are required for structural analysis. As a numerical model for the analysis, a simplified beam-stick model based on an in-service reactor containment building structure in South Korea is used. The design response spectrum from USNRC R.G. 1.60 scaled to 0.1 g is applied as an input motion for the seismic analysis. From the analysis, floor response spectra of the structure are developed at four different heights including bottom and top of the structure. As a result, the SSI effect results in frequency shifts and amplitude changes of the floor response spectra of the structure. Frequency of the peak acceleration becomes smaller when SSI effects are considered. And the peak acceleration of the FRS at the bottom node is amplified due to the SSI effect; on the other hand, those at the higher levels are reduced.