Component fragility for use in PSA of nuclear power plant

The accident of Fukushima nuclear power plant (NPP) underscores the necessity of conducting probabilistic safety assessment (PSA) of an NPP against the hazards caused by external events of natural origin. One of the important tasks of an external event PSA (EE-PSA) of an NPP is determination of component fragility. The fragility of a component is the probability of its reaching a limit state condition on a particular level of demand caused by the hazard. The seismic fragility of component is expressed by means of a double lognormal model with three parameters - the median ground acceleration capacity Am, random variability βR and uncertainty βU in the median capacity. There are direct and indirect methods to calculate fragility parameters Am, βR and βU. Direct method involves either analysis or testing. Indirect method is experience based method that maximizes the use of past experience in conjunction with plant walk down. The parameters for wind fragility Vm, βR and βU are estimated for different SSCs using design data (e.g. wall thickness, reinforcement, and anchorage) and variability associated with the wind response and capacity. The parameters for flood fragility Hm, βR and βU are also estimated for different structures, systems and components (SSCs) using design data and variability associated with the flood response such as inundation depth and capacity such as installation height. However, it generally shows more cliff-edge shape without any special barriers (e.g. protective walls, water-tight doors and sealing countermeasures). It means that flood fragility is highly path-dependent and thus flood hazard is influenced by barriers' fragility. The paper reviews the available open literature and summarizes the development of methods for deriving the fragility parameters of NPP components for use in PSA against seismic ground motion, wind and tsunami induced flood hazard. The paper also briefly discusses the component fragility for external event PSA of multiple NPP units at a site for natural hazards like earthquake, wind and flood. The state-of-the-art of seismic fragility analysis for NPP component is in matured level, while that of flood hazard in early stage of development and wind hazard in nascent stage.