This paper presents the study performed within the framework of a European Project called Reliability Methods for Passive Safety Functions (RMPS). Its objective is to propose a specific methodology to assess the passive system thermal-hydraulic (T-H) reliability. The methodology is tested on an example of industrial passive system: the Isolation Condenser System (ICS). The T-H calculations are performed using the RELAP5, ATHLET and CATHARE computer codes. In this paper, the present state of the methodology and its application to the example are described. Owing to the recent start of the project, only results concerning the first objective of the project (identification and quantification of the sources of uncertainties and determination of the important variables) are presented. A sensitivity analysis was carried out on 69 computation results performed with the RELAP5 code. This first analysis highlights the significant parameters influencing the performances of the ICS and shows a non-monotonous behavior of the system.