After closure, the V3 seals that are placed in the access shafts to seal the repository from the surface environment saturate relatively rapidly, due to their high suction and the flow from more permeable rock formations above down the shafts and into the seals. Once saturated, the sealing elements have a low permeability to both water and gas (permeability to water of 10^-19 to 10^-18 m²). This largely prevents further water flow down the shafts into the as yet unsaturated underground structures of the repository. It also largely prevents the migration of repository-generated gas from the underground structures of the repository towards the surface environment (see the discussion of the fate of repository-generated gas in Section 6.2.3). This is relevant to radiological safety, as repository-generated gas can transport ¹⁴C, mainly in the form of methane (¹⁴CH⁴).