The seismic Soil-Structure Interaction (SSI) analysis of nuclear facilities is usually performed in frequency domain where the response of the soil-structure system’s components is modeled as linear. The linear response assumption may be appropriate for small- to medium-intensity seismic events; however, the Seismic Probabilistic Risk Assessment (SPRA) of nuclear facilities requires the study of the system’s response under large seismic events with a very small annual exceedance probability as well. It is known that under such large events the response of the soil may be highly nonlinear, gapping and sliding may occur at the soil-structure interface, and structural components may experience damage; consequently, the equivalent-linear treatment of the soil, ignoring the interface nonlinearities, and linear modeling of structural components in frequency domain may not capture the physical response of the system. These nonlinearities however can be modeled in the time domain.

In this study the seismic response of a typical Pressurized Water Reactor (PWR) containment building under a relatively large seismic event is analyzed using three different approaches: linear frequency domain, linear time domain, and nonlinear time domain. The time domain response history analyses are performed in LS-DYNA while the frequency domain analysis is performed in SC-SASSI using the direct method. SC-SASSI is an in-house version of SASSI program developed at SC Solutions. It is shown that the obtained seismic responses, presented as in-structure response spectra at different locations within the containment building, are similar for both linear time domain and frequency domain analyses. The soil material and interface nonlinearities are then introduced into the time domain model to perform the third analysis. It is shown that the seismic response of the nonlinear system deviates from what is obtained via linear analyses. Linear analyses may either under-predict or over-predict the response depending on the frequency range of interest. The observed discrepancies are also dependent on the site soil and structural properties of the SSI system as well as the characteristics of the ground motion under study.