[en] Prediction of sodium leakage and combustion phenomena is important for the safety of liquid metal fast breeder reactors. In particular, small leakage is sensitive to obstacles and air flows. Falling liquid sodium is strongly deformed by separation and dispersion as well as various phenomena, such as combustion and accumulation, are combined. Since the existing finite difference methods need computational grids, it is so difficult to calculate separation and dispersion that small leakage of liquid sodium cannot be analyzed. MPS (Moving Particle Semi-implicit) method is a new numerical method, which is being developed in Univ of Tokyo. In the MPS method fluids are represented by macroscopic particles and governing equations are converted to equivalent particle interactions. Since computational grids are not necessary, separation and dispersion of fluids are easily calculated. Addition of combustion and accumulation will be easy as well because the fluid motion is just simulated by the particle motion. Thus, the objective of the present study is development of a numerical method to predict small size sodium leakage and combustion phenomena. In this paper, a model experiment using water and numerical calculations for this experiment are presented. (J.P.N.)