A simple numerical model has been developed to analyze the effects of releasing molten debris into the Containment Vessel (CV) which is filled with air in certain severe reactor accident scenarios. This model predicts the time dependent heat up of a gas atmosphere resulting from the gas-driven dispersal of high temperature debris droplets. The debris deoplet phase is tracked in the Lagrangian frame and the gas phase is described by using single-volume lumped parameter approach. It computes the transport of debris particles/droplets through the air in the containment by simplified debris pathway and evaluates heat exchanges between two phases. It also takes into account for the chemical reaction of the debris with the oxygen and the formation of aerosol by the vaporization of metal from the surfaces of droplets. This numerical model is used to simulate the Sandia National Laboratory (SNL) SURTSEY Direct Coantainment Heating tests. Computational results are compared with those obtained from experiments. It has agreements qualitatively with the experimental data.