Dynamic reliability framework for a Nuclear Power Plant using dynamic flowgraph methodology

Abstract Passive safety systems are being considered in advanced reactor designs to provide inherent stability for the operation of the nuclear reactor. Passive shutdown is provided for guaranteed removal of decay heat under emergency conditions. The reliability of such systems should be very high. The static reliability assessment of such systems has been considered using various techniques such as fault tree analysis, failure mode effect analysis, and reliability block diagrams. Dynamic reliability methods are powerful mathematical frameworks capable of handling interactions among components and process variables explicitly. In principle, they constitute a more realistic modelling of systems for the purposes of reliability, risk and safety analysis. Although there is a growing recognition in the risk community of the potentially greater correctness of these methods, no serious effort has been undertaken to utilize them in industrial applications. The dynamic flowgraph methodology is an integrated methodological approach to modelling and analyzing the behavior of software-driven embedded systems for the purpose of reliability/safety assessment and verification. In the present work, dynamic flowgraph methodology has been used to analyze the Station Blackout Scenario for a Nuclear Power Plant. The benefits of the proposed method are brought out with respect to the traditional methods like fault tree analysis, which deteriorates in applicability with increasing system size and complexity and fails to accommodate the dynamics of the system. The proposed method has been validated on the passive residual heat removal system of pressurized heavy water reactor.