This paper summarises the basic concepts of models used for the evaluation of the safety margins in a nuclear containment, focusing on the prediction of condensation/evaporation phenomena and of natural circulation fields inside the containment atmosphere. To allow the prediction of these phenomena, emphasised in the safety of new advanced reactors that heavily rely upon passive cooling systems, it was necessary to develop a computer code, called FUMO, and to validate it against a wide spectrum of experimental data. The significant results of this assessment activity, using experimental tests carried out in facilities having different volumetric scale, and considerations about the possibility of transferring the acquired knowledge from experimental facilities to a real plant are discussed. This work demonstrates the importance of models that predict local parameters and reduce the discrepancy with respect to the experimental data. As result, FUMO allows to perform realistic predictions of the safety margins for DBA or SA sequences inside containment systems of present or advanced NPPs generation.