Uncertainty analysis in the nuclear industry: Analytical unavailability modelling incorporating ageing of safety components

There is a growing interest in analyzing the possibility for current nuclear power plants operation extension. In that sense, life management programs, considering safety components ageing, are being developed and employed. On the other side, the large uncertainties of the ageing parameters as well as the uncertainties associated with most of the reliability data collections are widely acknowledged.This paper deals with uncertainty analysis associated with specific ageing rates database. The analysis is conducted using an analytical unavailability model applied for a selected safety system in a nuclear power plant. The most important problem is the immense uncertainty associated to the component ageing data sets as well as the lack of the very data in general, which would correspond to the more detailed modelling of ageing.New probability distributions, encompassing the ageing rates available in the considered data set, are suggested. The obtained results indicate the extent to which the uncertainty of the considered ageing data set, given the inherently assigned probability distribution, influences the performed unavailability calculations. Additionally, comparative analysis regarding the insights gained out of the application of the suggested probability distributions is conducted.

► Analysis of existing equipment ageing rates uncertainty propagation for NPPs.
► Calculation of uncertainty propagation using a Monte Carlo approach.
► Non-linear functions of parameters with large uncertainties are very sensitive on the parameter distribution shape.
► Not well defined distribution for the input parameter may lead to dangerous under-estimations of the reactor safety.
► New probability distributions suggested for the existing highly uncertain ageing rates.