Gas generation and migration in Boom Clay, a potential host rock formation for nuclear waste storage

For more than 20 years, the Boom Clay layer has been investigated as a potential host formation for the disposal of high-level (HLW) and medium-level radioactive waste (MLW). Experimental work regarding gas generation and migration associated with a hypothetical waste repository in the Boom Clay has been carried out in an integrated approach in several Belgian and European projects. This paper describes the resulting current understanding of these issues for the considered host formation. Hydrogen is generated by the anaerobic corrosion of the waste canisters. There is an experimental evidence that the presence of methane forming bacteria in the Boom Clay (and sulfate-reducing bacteria to a lesser extent) can reduce the gas source term, as the bacteria use four molecules of hydrogen in their metabolic reactions to form one molecule of methane. Numerical modelling indicates that if the gas generation rate in the disposal gallery becomes higher than the rate of the dissolved gas evacuation by diffusion (advection is negligible) into the host formation, the gas pressure increases. This gas pressure build-up may lead to contaminated water being expelled into the clay. Indeed, a gas phase could be formed and its pressure could exceed the local total stress, leading to a mechanical deformation of the rock and to the formation of a preferential pathway for the migration of gas and possibly contaminated water. To study the potential effects of the mechanical deformation of clay on radionuclide transport, a long-term in situ gas migration experiment through an artificially created pathway in the Boom Clay was carried out. The experiment was followed by a tritiated water injection test in order to detect any local change in the radionuclide transport properties of the host rock. Current results indicate that the geological formation possesses good macroscopic self-healing properties since the experimental results were in good agreement with calculations in which the permeability and the dispersion coefficient for the undisturbed clay was used.