Time-variant consideration of parameters dependence-based reliability of passive systems: Synopsis and proposed framework

Passive safety systems are adjudged inherently safe, simple and economical; however, they are associated with some uncertainties. The uncertainties are due to passive systems reliance on physical principles which are often characterized by low driving forces, flow instabilities and thermal-hydraulic (t-h) parameters uncertainties. To ensure that passive systems fulfill their missions satisfactorily when deployed, their reliability, which translates to satisfactory performance and safety whenever required, must be ascertained by realistic and dependable approaches. The issue of dependence of reliability on critical parameters and time-variant attribute of reliability in practice are often neglected in the existing t-h reliability methods. Few of the existing t-h reliability methods already considered critical parameters dependence while none has considered both parameters dependence and time-variant nature of t-h reliability concurrently. To lay a strong background, this study firstly presents an overview of the developed parameters dependence-based t-h reliability methods and their underlying principles for simplified natural circulation systems. Since passive systems behaviours are time-variant in reality, the basics of time-variant reliability were discussed. Furthermore, a proposed framework for integrating the time-variant concept into the existing parameters dependence-based t-h reliability of passive systems was presented. Finally, an approach to concurrent consideration of time-variance and parameters dependence of t-h reliability was proposed.