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The energy transfer mechanism from the electric spark to the reactive solid sample in powder form has been determined. Only a small portion of the solid is heated to the explosion critical temperature through a resistive heating by electric current. In fact, hot spots develop at the thinnest part of the solid, i.e. at intergrain contact points. We studied the evolution of thermal diffusivity and of sensitivity to electric spark as a function of powdered HMX granulometry. It allowed us to specify the respective importance of the grain size and shape in this evolution.