Seismic Isolation of the IRIS NSSS Building

The safety-by-designTM approach adopted for the design of IRIS resulted in the elimination of some of the main accident scenarios classically applicable to PWRs and to the reduction of consequences of the remaining classical at-power accident initiators. As a result the Core Damage Frequency (CDF) from at-power internal initiating events was reduced to the 10-8/ry order of magnitude, thus elevating CDF from external events (seismic above all) to an extremely significant contributor. The safety-by-designTM approach was exported to the design of the IRIS Nuclear Steam Supply System (NSSS) building for reducing the impact of seismically induced scenarios. The small footprint of the IRIS building makes the isolation of the entire nuclear island relatively easy and economically competitive. The isolated NSSS building dramatically reduces the excitation perceived by all the main IRIS components. This solution is also contributing to standardization, leading to a single design compatible with a variety of seismic conditions. The conceptual IRIS seismic isolation system is herein presented, along with a selection of the results of preliminary analyses. The approach adopted for the computation of the fragility analysis of the components is also briefly summarized, focused on the analysis of the isolators themselves.