This paper deals with the reliability assessment of passive systems that have been developed in the recent years by suppliers, industries, utilities and research organisations, aiming at plant safety improvement and substantial simplification in its implementation. The present study is concerning the Passive Decay Heat Removal Systems which use, for the most part, a condenser immersed in a cooling pool. The content of the paper reports the reliability study of the Isolation Condenser system foreseen for the advanced Boiling Water Reactors for the removal of the excess sensible and core decay heat from the Boiling Water Reactor by natural circulation. Furthermore an approach aiming at the thermal hydraulic performance assessment (i.e. the natural circulation failure evaluation) from the probability point of view is given. The study is not plant specific related but is pertaining to the conceptual design of the foregoing system.