Passive systems noticeably those which are capable of transferring thermal power from a heat source to a
sink without the use of energy which is not coming from gravity are in use of nuclear technology since the
pioneering design of reactors. They received a step-wise, fashion-type, renewed interest following 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
