Development of the seismic fragility assessment methodology for seismic isolated NPP structures

Abstract

As demonstrated by the 2011 East Japan Great Earthquake, earthquakes are a major risk contributor to nuclear power plants (NPPs). Seismic isolation is one of the most effective alternatives for enhancing the seismic safety of NPPs. When we consider seismic isolated NPP structures, we should consider the seismic risk of a seismic isolated NPP structure. Unlike conventional structures, NPPs do not see improved seismic safety only by the reduction of seismic ground motion. In the case of conventional structures, seismic isolation systems should be considered for only a decrease of acceleration responses. In the case of seismic isolated NPP structures, however, because isolation displacement is an important issue for the seismic safety of an NPP structure, seismic risk should be considered. When considering the seismic risk of a seismic isolated NPP structure, seismic PRA methodology can be used. A seismic PRA procedure can be divided into a seismic hazard analysis, a seismic fragility analysis, and an accident sequence analysis. The additional seismic hazard analysis is not needed compare to the conventional NPPs. Seismic fragility should be reconsidered because of the change of input seismic motion. An accidence sequence analysis also should be reconsidered. In this study, seismic fragility assessment methodology for seismic isolated NPP structures are considered. For the assessment of seismic fragility of seismic isolated NPP structures, target structures, systems, and components were selected. Failure mode and criteria were also taken into account. For the assessment of seismic fragility of seismic isolation devices, previous ultimate test results were reviewed. As a result, seismic fragility assessment methodologies are proposed in this study.