Performance assessment of a passive system as a non-stationary stochastic process

In this paper, an ongoing research topic related to NPP safety, as the reliability of passive systems, is addressed under the focus of a specific facet, i.e. the time aspect.A methodology for the assessment of time-specific reliability of thermal–hydraulic passive systems under time varying conditions is proposed. The time variant circumstances refer to the system required opposed to expected conditions, in terms of characteristic performance parameters, relevant to the system mission. The emphasis is basically on time domain parameter behaviour and parameter uncertainties, related to passive system operation modelled as a non-stationary stochastic process. The simulation of a non-stationary stochastic process with respect to its probability distribution functions is illustrated. For this purpose, various relationships are developed for simulation of a non-stationary stochastic process. They are based on modelling of its time-dependent amplitude probability density, expressed analytically.

► Passive systems relying on natural circulation are addressed.
► An approach for their reliability evaluation is proposed.
► The methodology addresses the estimation of time-dependent failure probability considering the variability associated with time-dependent parameters.
► An illustrative example is presented.
► Needs for future research are outlined.