Reliability Enhancement of Seismic Risk Assessment of NPP as Risk Management Fundamentals Part II: Quantifying Epistemic Uncertainty on Fragility Assessment using Expert Opinion

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 quantification. Upon these evaluation, the uncertainties are generally quantified based on statistical data, uncertainty analyses, engineering judgment and experience. Especially the epistemic uncertainty 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. 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 technique (KTT), all of which can be utilized for future fragility estimation.