Pressure building during the early stages of gas production in a radioactive waste repository

In a radioactive waste repository, hydrogen may be produced by anoxic corrosion of the metallic components, and by water radiolysis. The design of deep geological repositories presently envisaged in many countries tends to inhibit the migration of every chemical species coming from the repository, by means of several concentric watertight barriers. This tightness, which is desirable for other reasons, might prevent hydrogen evacuation and cause a pressure rise in the near-field. Can this pressure break the barriers? In the first stage of the process, the hydrogen dissolves in the porewater. It is shown in the present paper that in the vicinity of a steel surface embedded in clay, the pressure rise due to the production of dissolved hydrogen may be large already (a few MPa), and may represent a significant fraction of the barrier resistance limit. In the second phase of the process, a gas bubble may form, causing a further pressure increase. The problem of gas production in geological repositories is treated here as a special case of thermo-hydro-chemico-mechanical coupling in a porous medium. The time needed for the bubble to form depends on the nature of the metal, and on the characteristics of the barrier. The purpose of this paper is to give orders of magnitude for the time and space scales associated with hydrogen production in the near-field of a deep repository, before the formation of a gaseous phase. The conditions under which a gas phase forms are also discussed.