During the synthesis of hydrogen by methane steam reforming, mixtures composed of H2, CH4, CO and CO2 are produced in the process. In this work, the explosion reactivity of these mixtures, on the basis of detonation cell size and laminar flame speed, is calculated using a reactant assimilation simplification and a kinetic approach. The detonation cells width are calculated using the Cell_CH Kurchatov institute method and the laminar flame velocities are calculated with Chemkin Premix using different detailed chemical kinetic mechanisms. These calculations are used to define if these mixtures could be considered having a medium or a high reactivity for risk assessment in case of leak in the hydrogen plants.