Time-Domain Seismic Response Analysis of Pile-Founded Nuclear Power Plants Considering Soil and Soil–Structure Interface Nonlinearities

Soil–structure interaction critically governs the seismic safety of the nuclear power plant (NPP) structure, with pile foundations inducing complex soil–pile–structure interaction (SPSI). We propose a coupled stiffness matrix method–finite-element method scheme for time-domain nonlinear SPSI analysis, simulating wave propagation through nonlinear soils, nonlinear interfaces, and three-dimensional refined pile-supported NPP structure models (including equipment and coolant tanks). Code verification and model validation confirm reliability. Simulations of the pile-founded NPP structure on soil cushions reveal that nonlinear effects at the soil–structure interface significantly alter acceleration spectra and displacement responses of containment vessels and internal structures. Soil nonlinearity amplifies high-frequency components (>5 Hz), thereby increasing equipment vulnerability—a fact that requires critical attention. The reason may be that soil nonlinearity triggers more intense nonlinear behavior at the foundation–soil interface, such as gapping and sliding. Pile foundations attenuate high-frequency responses and reduce superstructure displacements. However, the presence of the superstructure amplifies pile bending moments (241% at the pile head) and shear forces (224% at the pile head). Pile diameter and pile spacing show a significant effect on the response of containment vessels and piles. This work demonstrates that omitting nonlinearities in the SPSI effect would lead to nonconservative designs, providing validated modeling strategies for coastal NPP structure resilience.