Passive systems in nuclear thermal hydraulics

Passive systems are in use within nuclear technology, noticeably those systems, which are capable of transferring thermal power from a heat source to a sink with the use of energy coming from gravity: Natural Circulation inside the vessel for Boiling Water Reactors (BWR) and between vessel and steam generators for Pressurized Water Reactor (PWR) constitutes noticeable examples. A stepwise, somewhat fashion-type, renewed interest followed, after the three major nuclear accidents in 1979, 1986 and 2011. The words thermal-hydraulic passive systems, design and safety, open to a myriad of research and application activities, which without surprise may appear contradictory and, at least, not converging into a common understanding. In the present paper, we used the word reliability in order to select a space in the design and safety assessment and to derive agreeable outcomes for the technology of passive systems. The key conclusions are: (a) Passive systems are not the panacea for protecting the core of nuclear reactors in each foreseeable accident condition. (b) Specific licensing rules are necessary. (c) Reliability of operation, for any assigned target mission, may reveal less than one. (d) Systems implying the use of active components like pumps shall not be discarded in future designed/built nuclear reactors.