The methods implemented by EDF-SEPTEN (Electricité de France - Department of Thermic and Nuclear Studies and Project) of Basic Nuclear Installations (BNI) in Nuclear Electricity Power Stations (NEPS) requires the implementation of pressurized hydrogen. Within the scope of a study on safety, hypothetical leaks are investigated, leading to the formation of explosive atmosphere (ATEX); the intention here is to evaluate their dangerousness. The present publication deals with the establishment of a simplified model in order to determine the XLEL [distance on the axis of the leak jet, where a concentration is reached equal to the Lower Explosive Limit (LEL)] and VATEX (volume of the locus of the spatial points where the concentration is greater than or equal to LEL) parameters for typical H2-N2 mixture leaks.