||In order to assess performance and safety of nuclear waste long-term repository, varied damage and failure scenarii are investigated by the French Agency for Nuclear Waste Management (Andra), in partnership with academic experts. In particular, anaerobic corrosion of carbon steel canisters, coupled to radiolysis of water and organic waste, may induce hydrogen gas production [1,2]. Gradually, gas pressure may increase first, inside disposal pits, for example, at the interface between waste metal canisters and host rock and, subsequently, inside the repository tunnel. Aside from diffusion phenomena, whenever the capillary threshold for gas passage is reached, hydrogen gas leakage may occur through the whole structure, including the geological clay layer. One main issue is to identify the threshold pressure when gas starts to migrate into claystone and what mechanisms are involved in this gas passage.