The construction of metal fast reactors main vessel’s roof consist of argon cavity. Since the behavior patterns of aerosol particles may be different under compact space conditions, the three-dimensional model of gas space and the force model of aerosol particles are developed through CFD software. By solving conservation equations, the flow field distribution within the gas space and the deposition results of aerosol particles are obtained. The simulation results show that the flow field distribution in small spaces is more sensitive to temperature differences. With the increasing of temperature difference, the natural circulation could be significantly enhanced. Based on the effect of the drag force and thermophoresis, aerosol particles are more likely to remain suspended rather than settle as well as promoting depositing towards vertical walls.