Reliability of passive systems: an outline

Passive systems are embedded into the nuclear technology safety and design. In relation to safety, accumulators are one example of vital passive systems strictly needed to mitigate consequences of Large Break LOCA. In relation to design, the configurations of both PWR and BWR primary system is based on natural circulation: the mutual positions of core and steam generators in the case of PWR and the elevation of the feedwater nozzle in the BWR vessel are determined to ensure (at least) removal of decay heat when active systems – noticeably, centrifugal pumps – are not available. Immediately after the Chernobyl accident in 1986, the passive systems received new attention by industry and scientists. Therefore, the importance of passive systems is well established in nuclear technology. Here one may add that passive systems include components like valves, pipes, heat exchangers and phenomena like draining of a tank, formation of incondensable gas bubbles, stratification and natural circulation. The discussion about reliability of passive systems shall include the components and the phenomena: a more detailed list of subjects can be found in the call for participation to the session. Special focus is given to the reliability of thermal-hydraulic phenomena expected during the operation of passive systems. The topic of reliability of phenomena occurring in passive systems was introduced before the year 2000, merging ideas from PSA and thermal-hydraulics.