Advanced seismic probabilistic risk assessment using nonlinear soil-structure interaction analysis

The objective is to provide advanced seismic probabilistic risk assessment (SPRA) methods with the goal of removing large uncertainties, to the extent possible, and to provide "best estimate" seismic risk numbers. The concern is that large uncertainties in traditional SPRAs will mask other potential sources of risk and focus disproportionate time and money on mitigating seismic risk. This paper is not proposing to change the process for characterizing the seismic hazard at a given nuclear power plant site. However, there are two potential areas to remove conservatism in the SPRA process (again we want best estimate risk numbers with appropriate treatment of uncertainties so that other risks, such as risk offlooding, are not masked). One source of conservatism is in the seismic fragility approach, which comes primarily from assuming that the structure response scales linearly with ground motion level. This source of conservatism can be removed by using nonlinear soil- structure interaction (NLSSI) analysis to explicitly model the interaction between the soil and structure (i.e. the structure will slide during larger ground motions). The second source of conservatism is the response of the soil will be nonlinear for larger ground motions and NLSSI will account for this.