The International Project on Innovative Nuclear Reactors and Fuel Cycle (INPRO) initiated a collaborative project to contribute to an international consensus on the definition of the reliability of passive systems that involves natural circulation, and a methodology to assess this reliability. Different reliability methodologies available in Member States were used to assess the performance and reliability of the passive decay heat removal system of the French gas cooled fast reactor (GFR) design for various transients. This publication is the final report of this project and summarizes the results of the participants’ work concerning the evaluation of the performance and reliability of the passive decay heat removal system of the GFR design. The report also presents a unified definition of the reliability of thermohydraulic passive system, and the possibility of unifying the features of the methodologies in order to develop a generic methodology