The composite containment proposed aims to cope with beyond-design-basis accidents. The goal is to restrict the consequences of severe core meltdown accidents to the reactor plant. One essential of this new concept is passive decay heat removal from the containment by natural air convection. Experimental and calculational results obtained up to now with the passive containment cooling program suggest that in the composite containment of a 1300-MW(electric) pressurized water reactor, the decay heat can be safely removed by natural air convection. Detailed experimental investigations and large-scale tests envisaged will complement the results and provide the database for the design of the containment and further development of multidimensional computer codes.