Establishing Knowledge Base for Fragility Assessment of Npp Using Expert Opinions

Abstract eng:
The assessment of seismic safety of nuclear power plant facilities has been performed by identifying and quantifying uncertainties in seismic probabilistic risk assessment (SPRA). The level 1 PRA consists of three steps to assess an annual core damage frequency (CDF); seismic hazard evaluation, fragility evaluation of buildings and equipment, and system analyses. For the evaluation process, all uncertainties are classified into either aleatory or epistemic uncertainty for their practical treatment for better understanding and quantification of them. Upon these evaluation, the uncertainties are generally quantified based on statistical data, uncertainty analyses, engineering judgment and experience. The past PRAs for NPPs show that there are much more uncertainty for risk assessment against external events. Especially the epistemic uncertainty in the seismic PRA is difficult to quantify, and it often and perhaps only relies on expert judgment in earthquake engineering. Therefore, in this study, systematic evaluation of the epistemic uncertainty on the seismic fragility of structures and equipment is studied and implemented for a model NPP in Japan. There are two expert groups formed in this project: experts in the field of buildings and soil ground (CE experts) and experts in the field of pipe and equipment (ME experts). Each group conducted a pilot study on the use of expert opinion elicitation assisted by technical integrators for uncertainty evaluation in fragility analyses. Along with ample results from relevant sensitivity analyses conducted, elicited opinions are carefully treated and classified into several specific areas and integrated into the form of knowledge tree (KT), all of which can be utilized for improving fragility estimation. In the three-year project, the standard procedure to evaluate epistemic uncertainty in the seismic fragility assessment of NPPs has been developed using experts judgements. For formal elicitation procedures, the SSHAC(Senior Seismic Hazard Analysis Comittee) procedure is well known and has been often used in the probabilistic seismic hazard analyses. However, no similar study has been reported for the fragility assessment so far. Effectiveness and shortcomings of the proposed method should be verified through further applications to real NPP structures to improve the method.

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