Uncertainty and sensitivity analyses for age-dependent unavailability model integrating test and maintenance

The interest in operational lifetime extension of the existing nuclear power plants is growing. Consequently, plants life management programs, considering safety components ageing, are being developed and employed. Ageing represents a gradual degradation of the physical properties and functional performance of different components consequently implying their reduced availability. Analyses, which are being made in the direction of nuclear power plants lifetime extension are based upon components ageing management programs. 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 addresses the uncertainty and sensitivity analyses conducted utilizing a previously developed age-dependent unavailability model, integrating effects of test and maintenance activities, for a selected stand-by safety system in a nuclear power plant. The most important problem is the lack of data concerning the effects of ageing as well as the relatively high uncertainty associated to these data, which would correspond to more detailed modelling of ageing.A standard Monte Carlo simulation was coded for the purpose of this paper and utilized in the process of assessment of the component ageing parameters uncertainty propagation on system level. The obtained results from the uncertainty analysis indicate the extent to which the uncertainty of the selected component ageing data set influences the performed unavailability calculations on system level, as well as they present sensitivity insights on the equipment. Sensitivity analyses were additionally conducted. The obtained results indicate sensitivity insights associated to the coded Monte Carlo simulation itself as well as component ageing effects sensitivity judgements related to the selected system unavailability calculation.

► Application of analytical unavailability model integrating T&M, ageing, and test strategy.
► Ageing data uncertainty propagation on system level assessed via Monte Carlo simulation.
► Uncertainty impact is growing with the extension of the surveillance test interval.
► Calculated system unavailability dependence on two different sensitivity study ageing databases.
► System unavailability sensitivity insights regarding specific groups of BEs as test intervals extend.