This phenomenon involves heat and mass transfer from the containment atmosphere towards the surrounding structures. This phenomenon would occur in existing reactors in case of a coolant release into the containment. It also occurs in advanced designs where containment surfaces are cooled externally, usually by natural mechanisms. Good examples are the designs of the AP series by Westinghouse, where the steel containment is cooled externally by water flowing on its exterior surface from a reservoir above the containment, and by ascending air driven by bouyancy.

Steam condensation is largely affected by conditions which can be split into two groups depending on the relevance of the physical dimensions of the system. The ‘scale-independent factors’ are variables like the fraction of non-condensables, the pressure, the gas composition and so on, the effect of which could be well investigated through separate effect tests. The ‘scale-dependent factors’ are those phenomena that require to be investigated in actual or scaled geometries (i. e. Integral Effect Tests) since physical dimensions largely influence their quantitative effect. Examples of this kind are the natural convection process at both sides of the metallic structures and the potential gas stratification.