Passive systems and 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 example. A step-wise, 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 an attempt is made to use 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 strictly needed and not yet formulated; (c) reliability of operation, once a target mission is assigned, may reveal not unit; (d) systems implying the use of active components like pumps shall not be avoided in future designed/built nuclear reactors