Seismic fragility analysis of SMRs considering intensity-dependent SSI: A multi-level conditional spectrum approach

Conventional seismic fragility analysis for nuclear facilities is typically based on a single reference earthquake (RE), implicitly assuming that response and fragility characteristics calibrated at the reference level remain applicable across seismic intensity levels. This assumption may be inadequate for small modular reactors (SMRs), whose seismic response can be strongly influenced by intensity-dependent soil–structure interaction (SSI). This study presents a multi-level conditional-spectrum (CS)-based seismic fragility approach that accounts for intensity-dependent SSI. The seismic hazard curve from probabilistic seismic hazard analysis (PSHA) is discretized into multiple hazard levels, and CS-consistent ground-motion suites are selected at each level. SSI analyses using motion-specific strain-compatible soil properties are then performed to evaluate fragility parameters at each hazard level, which are synthesized into a single global fragility function compatible with seismic probabilistic risk assessment (SPRA). The approach is demonstrated using a representative SMR structure and two nuclear equipment components. Results show that the median seismic capacity and associated dispersions vary with hazard level because of intensity-dependent SSI effects and changes in floor-motion demand. For the case considered, the traditional single-RE analysis yielded higher HCLPF point estimates than the proposed multi-level analysis. The proposed approach provides a hazard-consistent framework for explicitly representing hazard-level-dependent seismic demand and fragility while retaining a global fragility representation compatible with SPRA.