Seismically Isolated Nuclear Power Plants: Is Soil–Structure-Interaction Analysis Needed?

This study assesses the need for soil–structure interaction (SSI) analysis of surface- or near-surface mounted, seismically isolated nuclear power plants (NPPs). The current rules and guidance for SSI analysis of NPPs are based on the legacy assumption that reactor buildings are stiff and heavy: the two key attributes needed for significant SSI on soil sites. Reactor developers in the United States are considering seismic isolation as a design feature to reduce the impact of the seismic load case and to enable standardization. The substantial reduction in lateral stiffness associated with the introduction of horizontally flexible isolators at the base of a reactor building led to the hypothesis that SSI has no meaningful effect on seismic acceleration and displacement demands on structural components and equipment in surface- or near-surface-founded, base-isolated NPPs. Herein, reactor buildings and their safety-class equipment, the supporting soil domains, and nonlinear isolation systems are explicitly modeled and analyzed to judge whether the hypothesis is correct. An extensive set of response-history analyses was performed for 945 combinations of (1) three fundamentally different, surface mounted reactor buildings, (2) five horizontal isolation systems with a range of linear and bilinear properties, (3) nine seismic inputs covering a range of frequencies and amplitudes of shaking, and (4) seven generic soil profiles that cover a range of sites across the United States and were a part of the Design Certification Documents that enabled the KEPCO APR1400 to be certified for use by the United States Nuclear Regulatory Commission under the 10CFR Part 52 licensing framework. The peak resultant horizontal displacements of the isolation systems, peak resultant horizontal accelerations in each reactor building, and in-structure horizontal and vertical acceleration response spectra were essentially identical with and without considerations of SSI, confirming the hypothesis.