The accident of Fukushima Nuclear Power Plant (NPP) underlines the necessity of conducting probabilistic safety assessment (PSA) of an NPP against the hazards caused by external events of natural origin and their combinations. One of the important tasks of an external event PSA (EE-PSA) of an NPP is determination of component fragility. The fragility is the probability of its reaching a limit state condition on a particular level of demand caused by the hazard. Fragility is expressed in terms of the median capacity D_m, random variability β_R and uncertainty β_U in the median capacity. D_m is defined as median ground acceleration capacity A_m, wind velocity capacity V_m and water height capacity H_m for seismic, wind and flood (inundation) hazard respectively. For seismic fragility, there are direct and indirect methods to calculate fragility parameters A_m>, β_R and β_U. 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 V_m, β_R and β_U are estimated for different SSCs using design data (e. g. wall thickness, reinforcement, and anchorage) and variability associated with the wind response and capacity. Flood fragility is derived from flood response (e.g. inundation depth) and capacity (e.g. 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 D_m, β_R and β_U of NPP components for use in PSA against seismic ground motion, wind and flood hazard. The paper also briefly discussed the external event PSA of multiple NPP units in a site for natural hazards like earthquake, wind and flood.
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