Nuclear power reactor containments are equipped with safety systems which protect the containment integrity under various accident conditions. The focus of this phenomenon is the natural circulation cooling and heat transfer in various containment passive cooling systems under accident conditions to remove the energy out of the containment by natural circulation and condensation heat transfer. Typical systems are the tube condensers such as the passive containment cooling system (PCCS) and external air cooling system or external liquid film cooling and internal condensation of steam in the containment by natural circulation. The major purpose of these containment systems is to protect the containment under both design basis accidents and severe accidents involving serious core damages and to prevent the significant release of radioactive materials to the atmosphere. These systems are required to remove the load on the containment from the LOCAs and other accidents by removing the heat but containing the mass within the structure. Most of load comes from the released steam from the primary coolant system due to the LOCA or venting of the pressure relief valves. The major part of the non-condensable gases consists of the original containment atmosphere such as air or nitrogen, however with the core damage, hydrogen or fission gases can be also released into the containment atmosphere. The thermal-hydraulic phenomena of importance are tube surface condensation with non­condensable gases, natural circulation of steam and non-condensable gases, degradation of condensation by the accumulation of non-condensable gases and purging of non-condensable gases from condenser systems. The passive containment cooling system can be vertical or horizontal tube condensers in external water pool, exposed condenser tube system in the containment cooled by natural circulation water through the tubes from the external pool or by external air circulation and others.