A Review: Passive System Reliability Analysis – Accomplishments and Unresolved Issues

*Correspondence: Arun Kumar Nayak, Reactor Design and Development Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India e-mail: [email protected] Reliability assessment of passive safety systems is one of the important issues, since safety of advanced nuclear reactors rely on several passive features. In this context, a few methodologies such as reliability evaluation of passive safety system (REPAS), reliability methods for passive safety functions (RMPS), and analysis of passive systems reliability (APSRA) have been developed in the past.These methodologies have been used to assess reliability of various passive safety systems. While these methodologies have certain features in common, but they differ in considering certain issues; for example, treatment of model uncertainties, deviation of geometric, and process parameters from their nominal values. This paper presents the state of the art on passive system reliability assessment methodologies, the accomplishments, and remaining issues. In this review, three critical issues pertaining to passive systems performance and reliability have been identified. The first issue is applicability of best estimate codes and model uncertainty. The best estimate codes based phenomenological simulations of natural convection passive systems could have significant amount of uncertainties, these uncertainties must be incorporated in appropriate manner in the performance and reliability analysis of such systems. The second issue is the treatment of dynamic failure characteristics of components of passive systems. REPAS, RMPS, and APSRA methodologies do not consider dynamic failures of components or process, which may have strong influence on the failure of passive systems.The influence of dynamic failure characteristics of components on system failure probability is presented with the help of a dynamic reliability methodology based on Monte Carlo simulation. The analysis of a benchmark problem of Hold-up tank shows the error in failure probability estimation by not considering the dynamism of components. It is thus suggested that dynamic reliability methodologies must be integrated in passive systems reliability analysis to have a true estimate of system failure probability, and hence the reliability.Third issue is the treatment of independent process parameters variations in passive system reliability analysis. Certain process parameters such as atmospheric temperature can vary with time. Performance of some passive safety systems depends on this parameter. However, the present methodologies do not consider this dynamic variation from the nominal values and hence introduce a subject of discussion.