For prevention and mitigation of the containment failure during severe accident, the study is focused on the severe accident phenomena, especially, the ones occurring inside the cavity and is intended to improve existing models and develop analytical tools for the assessment of severe accidents. The natural circulation models in the MAAP and MELCOR were compared and the limitation and differences of these codes were investigated. The calculation of the local hydrogen concentration distribution in Ulchin 3, 4 containment was performed as part of hydrogen combustion study and the flame velocity model was examined. Explosive Fuel-Coolant Interaction, i.e. steam explosion model was reviewed and a debris entrainment model was investigated. Through international program of PHEBUS-FP and participation in the software development, sensitivity study for the oxidation model of the ICARE2 code was performed. CONTAIN and MELCOR codes were continuously updated under the cooperation with USNRC and problem areas of the code were reported through the MCAP program. (Author).