Integration of the reliability of passive system in probabilistic safety assessment

Probability Safety Assessment (PSA) of nuclear power plants has demonstrated its efficiency in decision-making process. But the treatment in PSA of safety passive systems, specially those implementing moving working fluid, is a difficult task because in addition to the mechanical failures of components, the failure of the physical process (e.g. natural circulation) has to be considered. The difficulty in the evaluation of the failure risk of the physical phenomenon lies in the great number of parameters that must be taken into account, in their associated uncertainties and in the limitations of physical modelling. We can note that in the existing PSA of future reactors equipped with passive systems, this risk of the physical process failure due to the uncertainties, is not at all taken into account. In this paper, we present a methodology to evaluate this risk of failure and to include it in a PSA. This evaluation is obtained by uncertainty analyses on thermalhydraulic calculations. As an example, a simplified PSA was carried out on a fictive reactor with two types of safety passive systems both in the primary circuit: Residual Passive heat Removal system (RP2) and a safety injection system consisting in accumulators and discharge lines equipped withmore » check valves. An accidental scenario has been analysed, starting with loss of electrical supply when the reactor is at full power. The failure analyses performed on this reactor have allowed the characterisation of the technical failures (on RP2 valves, tubes in RP2 exchanger and safety injection check valves) and the ranges of variation of uncertain parameters which influence the physical process. The resulting accidental scenario is presented in the form of a simplified event tree. The majority of the sequences of this event tree have been analysed by deterministic evaluations with envelope values of the uncertain parameters. For some sequences where the definition of envelope cases was impossible, basic events corresponding to the failure of the physical process have been added and uncertainty analyses have been performed to evaluate the corresponding probability of failure. For this purpose the thermal-hydraulic CATHARE code has been coupled to a Monte-Carlo simulation modulus. The failure probabilities obtained by these reliability analyses have been integrated in the corresponding sequences. This methodology allows the probabilistic evaluation of the influence of the passive system on an accidental scenario and could be used to test the interest to replace an active system by a passive system on specific situations. (authors)« less