A computer model is described that simulates the effects of releasing molten debris into a gas-filled container. This work is motivated by studies of direct containment heating due to the dispersal of debris produced in certain nuclear reactor accident scenarios. The model consists of a finite difference scheme for the gas flow coupled with a Lagrangian particle transport algorithm. It computes the transport of the debris through the gas and evaluates radiative and convective heat transfer effects. It also accounts for the chemical reaction of the debris with the oxygen in the atmosphere, including the concurrent heat release. The computer code is used to simulate experiments in the Surtsey Direct Heating Test Facility. Computational results are compared with those obtained from experiments with small and large debris input mass. It is shown that the simulation of configurations with large debris mass can be improved with better submodels to describe the debris behavior. The description of the interaction of the debris with the container walls is of particular importance.