Passive safety systems, proposed for advanced reactor design, are based on the inherent properties of physical processes as, for instance, natural circulation, gravity and heat conduction. However, their performance arouses a lot of significant uncertainties, mostly related to the thermal hydraulic phenomena upon which systems of interest depend, which affect considerably the expected behavior of the plant in accident conditions. The content of this paper presents a methodological and structured procedure to address the uncertainties related to passive safety functions: from a methodical view it is more appropriate to base the uncertainty analysis on passive functions instead of passive systems. The matter is treated with reference to a passive 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 main lines. Starting point of the analysis is the identification of the main sources of physical failure: hazard identification used qualitative methods, as FMEA (Failure Mode and Effect Analysis) and HAZOP (Hazard and Operability study),are utilized and the relative outcomes compared in order to assess the potential failure modes and the consequences associated with passive system operation. Main purpose of the work is the identification and evaluation, on a qualitative basis, of the sources of uncertainties related to passive system performance, in terms of parameters, which are critical for the passive function performance/reliability, and which are direct indicators of the failure causes. Finally a discussion on how to deal with the uncertainties is given: this concerns both their assessment in probabilistic terms and the determination of those contributors whose uncertainty has a significant impact on the passive system behavior.