To define a strategy of mitigation for containerized hydrogen systems (fuel cells for example) against explosion, the main characteristics of flammable atmosphere (size, concentration, turbulence…) shall be well-known. This article presents an experimental study on accidental hydrogen releases and dispersion into an enclosure of 4 m3 (2 m x 2 m x 1 m). Different release points are studied: two circular releases of 1 and 3 mm and a system to create ring-shaped releases. The releases are operated with a pressure between 10 and 40 bars in order to be close to the process conditions. Different positions of the release inside the enclosure i.e. centred on the floor or along a wall are also studied. A specific effort is made to characterize the turbulence in the enclosure during the releases. The objectives of the experimental study are to understand and quantify the mechanisms of formation of the explosive atmosphere taking into account the geometry and position of the release point and the confinement. Those experimental data are analyzed and compared with existing models and could bring some new elements to improve them.