A compound seismic screening index for global nuclear power plant sites combining probabilistic rock-PGA hazard with vs30-based site conditions

Abstract Seismic safety assessment of nuclear power plant (NPP) sites depends critically on the simultaneous characterisation of ground motion intensity and the capacity of the local soil to amplify that motion. While global probabilistic seismic hazard models and global shear-wave velocity (Vs30) compilations are now available, no prior study has combined these two open global datasets into a single, reproducible screening metric spanning the multi-country reactor-unit inventory. Deriving site-specific ground motion for an individual plant by convolving rock-outcrop hazard with site-response analysis is, by contrast, well-established regulatory practice. This paper introduces a Compound Seismic Screening Index (CSSI), a dimensionless 0–10 relative score that combines two independently normalised components: the 475-year return-period peak ground acceleration (PGA) on reference rock, extracted from the Global Earthquake Model (GEM) Ground Shaking Hazard Model version 2023 at 3-arc-minute resolution, and the inverted normalised Vs30 as a site-condition proxy. A higher score indicates greater regional hazard and/or softer, more amplifying ground. The CSSI is a relative screening tool and does not incorporate fragility, exposure, plant design level, safety-system robustness, soil–structure interaction, or multi-hazard probability; the Vs30-based amplification factor and surface PGA are reported separately as a descriptive site-response illustration and do not enter the index. The index is applied to 759 reactor units in a March-2024 snapshot of the GeoNuclearData v0.17.17 database, covering operational, shutdown, under-construction, and planned units. Results show that 50.7% of sites fall in NEHRP Class C (very dense soil, 360 < Vs30 ≤ 760 m/s) and 38.1% in Class D (stiff soil, 180 < Vs30 ≤ 360 m/s); crucially, every site in the Very High hazard tier (PGA > 0.3 g) is on NEHRP Class C or D soil rather than Class B rock, indicating that the most seismically exposed facilities are also subject to soil amplification. The global mean CSSI is 3.52 (standard deviation 2.11). Japan records the highest national mean CSSI (6.19) and hosts 19 of the 20 highest-ranked individual sites, with the Tokai and Hamaoka clusters reaching CSSI values of 9.91 and 9.88 respectively. Among reactor types, the four Advanced Boiling Water Reactor (ABWR) units show the highest mean CSSI (7.58); this reflects their siting at soft-soil Japanese locations rather than any property of the reactor technology. Planned and shutdown facilities show slightly higher median CSSI than operational plants, although the distributions overlap substantially and the difference is not tested for significance. The CSSI framework is transparent, reproducible, and globally scalable, providing a comparative summary of regional hazard and site-condition proxy information across the analysed inventory. It can be used to identify sites whose relative position changes when regional hazard is considered alongside Vs30, but its prioritisation utility is not independently validated here. It is not a design-basis or adequacy assessment.