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Japan Nuclear Safety Institute had recently reported the pioneering deterministic evaluation approach for nuclear power plant under seismic induced fault displacement. But the uncertainty of fault displacement based on probabilistic hazard analysis is described to be greater than that of other natural phenomena e.g. earthquake ground motions or seismic acceleration vibration in the report. Furthermore, for plant-wide risk assessment against fault displacement hazards beyond design basis displacement level, it is seriously necessary to promote a series of fundamental studies and develop the standard procedures regarding not only accident sequence analysis but also fragility analysis of buildings and structures as well as components and piping systems. Based on the above background, the objective of this study is focusing to obtain basic fragility data for the aleatory and epistemic uncertainties of structural responses for nuclear power plant buildings against fault displacement. A number of nonlinear soil-structure finite element analyses against relatively large fault displacement are performed with the randomness of soil and building material properties, the uncertainly of contact parameters relating to friction between soil and building, and also the uncertainty of fault hazards such as fault types and geometries. Their quantitative results for fragility data are shown in this paper.

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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, watertight 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.