**1. Introduction**

Radioactive waste storage at great depths is likely to produce gas, mainly di-hydrogen, due to water radiolysis and corrosion. This will lead to gas transport through storage structures, such as concrete tunnels, or surrounding rocks—often clay rocks. This is why numerous studies were conducted by ONDRAF/NIRAS (in Belgium) or Andra (in France) in order to characterize the gas transfer properties of dry or partially saturated concrete or of its host rock. Gas-permeability tests are often carried out with a significant pressure gradient, i.e., with quite a high injection pressure—a few MPa, for example. It must be nevertheless mentioned that the gas pressure would slowly increase in the storage structure at the beginning of its production. As a consequence, and for the purpose of further simulations, it seems important to take this phase into account and to measure the respective role of permeation and diffusion in the gas-transfer process. Diffusion, which occurs when there is a gas concentration gradient, has in fact to be taken into account during the low-gas-pressure phase. Hence, the scope of the present experimental study was to design a gas-diffusion device and to evaluate the effective diffusion coefficient of the concrete intended to be used by ONDRAF/NIRAS (Organisme national des déchets radioactifs et des matières fissiles enrichies—in Belgium) for its storage tunnels. This measurement, conducted alongside 'traditional' permeability tests, demonstrates that a large proportion of gas transport may be occurring due to diffusion under a very low pressure injection.
