[en] This thesis is in the frame of study of the energy conversion system for ASTRID project. The objective of this thesis is to provide a numerical tool that enables to describe the structure of under-expanded gas jet as a function of the flow rate of the gas leak. In a gas jet submerged into a liquid, there is a layer of liquid droplets issue from the entrainment by the gas and a layer of gas bubbles. The Baer-Nunziato model is then chosen as the starting model. In addition, the viscous diffusion, the momentum exchange between the two fluids are taken into account in the present work. The developed model and its numerical schemes are implemented in the CANOP software, which enables to generate the adaptive mesh refinement and to compute in parallel. The capability of the code to reproduce under-expanded gas jets is validated. The structure of the monophasic jet is consistent with the empirical correlations; the viscous diffusion shortens the size of the Mach disk due to the change of flow behavior inside nozzle. For a two-phase jet, the empirical correlations obtained from monophasic jets are not suitable anymore. This numerical tool remains to be improved in some directions, for example, to take the distribution of non-uniform bubbles and break-up of particles owning to the shock waves into consideration. (author)