The incorporation of advanced reactor design passive systems, to perform safety-related functions in the event of transients and accidents, requires their evaluation within the accident scenario assessment process. The objective of the present study is the development of a consistent approach for the introduction of passive safety systems into an accident sequence, usually defined by fault tree and event tree models, in the fashion of and in combination with an active system or a human action. With reference to the thermal-hydraulic passive systems (e.g. natural circulation systems), in addition to the component failures (i.e. mechanical and electrical faults), the impairment of the physical principle upon which the system relies, deserves special consideration. This makes the relative assessment process different as regards the system model commonly adopted in the fault tree approach (i.e. exponential failure model). For the thermal-hydraulic passive system, since the failure process is driven mainly by the occurrence of the phenomenological failure modes, each pertinent basic event will be characterized by defined critical parameters (e.g. non-condensable fraction) that are expected to drive the failure mechanisms. An application of this approach is presented, with reference to a system designed for decay heat removal of advanced Light Water Reactors, relying on natural circulation and provided with a heat exchanger immersed in a cooling pool, acting as heat sink, and connected to the pressure vessel via steam and condensate lines.