Experimental facilities for the safety analysis of nuclear systems and new technologies,having hydrogen safety as the main subject of research, have been operated since severalyears at the CEA (Commissariat a l'Energie Atomique).

It is well known that, for practical reasons, experimental investigation on hydrogensafety issues often adopts mixtures of di erent gases. Helium is commonly used as asubstitute for hydrogen.

This paper deals with the phenomenological analysis of light gas dispersion phenomenaof interest in the eld of hydrogen safety.

First, a simple analytical methodology is proposed to identify the relevant parametersin a di use leak scenario and to guide the scaling of experiments using helium as asubstitute for hydrogen.

Involving typical scales of the experiments performed at the CEA/LEEF, examples ofapplications of this methodology are then proposed.

In one of the considered cases, it is also demonstrated that the dynamic phenomenainvolved in the dispersion of a cloud of hydrogen are of the same order than in a cloud ofhelium having the same molar concentration.