[en] One step of the conversion of gaseous UF₆ to solid UO₂ by dry route is the formation of particles of UO₂F₂ in a triple coaxial jet UF₆/N₂/H₂O. The characteristics of resulting powder have an influence on the properties of final particles of UO₂, and then on the quality of pellets of nuclear fuel. So a good control of this step of the process is of interest. This study deals with an experimental investigation and a modelling of the influence of various parameters on particles obtained by reaction in a turbulent coaxial jet. For example, the influence of absolute and relative velocities of gases on particle size distributions has been investigated. Two kinds of experimental studies have been undertaken. First, the development of mixing layers in the near field of the jet has been evaluated with temperature measurements. Then, particle size distributions have been measured with a turbidimetric sensor, for particles obtained by hydrolysis of gaseous metallic chlorides (SnCl₄, TiCl₄) in double and triple coaxial jets. A model has been proposed for mixing of gases and growth of particles. It takes into account the development of mixing layers, meso-mixing, micro-mixing and growth of particles through agglomeration. The influence of operating parameters, especially velocities, on experimental results appear to be different for TiCl₄/H₂O jets and SnCl₄/H₂O jets. In fact, a comparison of theoretical and experimental results shows that particles obtained by hydrolysis of TiCl₄ seem to grow mainly through agglomeration whereas another growth phenomenon may be involved for particles obtained by hydrolysis of SnCl₄. (author)