Radioactive waste repositories are constructed underground below ground water level. Main engineered barrier of the repository consists of structural walls, backfill mortar, and consolidated wastes. Cementitious materials are mainly adopted for the engineered barrier. Gas (hydrogen or methane) generation due to corrosion and microbial degradation is predicted, since the waste consists of metals like steel and organic material. It is important issue to define gas transport mechanism in cementitious materials in order to assess the gas release influence on the engineered barrier and pressure rise in the repository. Although cementitious materials have enough permeability to transport gas in dry condition, it is expected to be impermeable or so in saturated condition, in which repository is placed. In this paper, water and gas permeability both in dry and saturated conditions are investigated. The pressure in the vault, and gas transport mechanism is predicted. Certain threshold pressure exists, and it can be recognized that gas permeability is dependent on water saturation degree at a pressure greater than the threshold pressure. Loading pressure of 0.7 MN / m2 is needed for the flow of gas in saturated concrete. Intrinsic gas permeability of 6.5 × 10−19 m2 is obtained, which is very small value to keep the low pressure inside the vault. Consequently, it is predicted that gas cannot be transported through the intact concrete, however, it can be transported through defective zone which is developed during construction, and so on.