Seismic Risk Assessment of Safety-Critical Nuclear Facilities for the Purpose of Risk-Informed Periodic Reevaluation

Following U.S. Department of Energy Order 420.1 C for the mitigation of natural phenomena hazards, such as earthquakes, to nuclear facilities through periodic reassessments, Idaho National Laboratory (INL) has developed the Seismic Hazard Periodic Re-Evaluation Methodology (SHPRM). The SHPRM involves seven criteria that evaluate changes to the seismic hazard at a site due to changes in the input models/data over time. Should these changes to the seismic hazard result in an increase in the design or licensing-basis ground motion of the facility from that which the facility was designed for, the SHPRM includes a criterion for reevaluating the facility risk objectives. While the criteria corresponding to the reevaluation of the seismic hazard and the design basis have been previously demonstrated and published, there is currently no guidance on reevaluating seismic risk for the purpose of SHPRM. This paper complements the published reports and papers on the application of SHPRM by demonstrating the risk objectives criterion for a generic nuclear facility (GNF), thereby closing the loop on the application of the SHPRM. The GNF is assumed to be located at the INL site and designated as a Seismic Design Category-3 facility as per American Society of Civil Engineers (ASCE)/Structural Engineering Institute (SEI) 43-05. The demonstration includes a risk assessment for a baseline seismic hazard calculated in 2006 and an updated seismic hazard calculated in 2015. After presenting the baseline and the updated seismic hazard curves at this site, the state-of-practice methodology for calculating fragility functions for the facility is presented, along with the fragilities calculated for the GNF. Employing a fault tree analysis using the INL in-house seismic analysis and risk assessment software MASTODON, the seismic risks of collapse of the GNF for the baseline and updated seismic hazards are computed to be 5.27E-05 and 5.2E-06, respectively. The results show that not only the reevaluated seismic risk is smaller, but more importantly, that it meets the risk objectives set by ASCE/SEI 43-05.