The impact of a NaNO3 plume in a deep geological repository on the diffusion of dissolved gases in clay

In the framework of the geological disposal of high and intermediate-level long-lived radioactive waste, the Boom Clay is considered as a potential host formation in Belgium. Boom Clay has suitable characteristics that limit the mass transport of dissolved species. Since gas generation in geological repositories is unavoidable, its dissipation needs to be evaluated. This study focuses on the gas diffusion properties in the case of a geochemical perturbation of the near field of Boom Clay, caused by NaNO3 leaching from Eurobitumen bituminised waste. The diffusion of 4 gases (He, CH4, Xe, and C2H6) was measured in intact and chemically (with NaNO3) disturbed samples and the change in diffusion coefficients after perturbation was assessed. Intact and disturbed samples were petrographically and petrophysically characterised. Results of this study show that most of the clay properties are not altered by the chemical perturbation. The parameter which changed significantly is the specific surface area, which decreased 40–60 % in the chemically disturbed samples. Water transport (hydraulic conductivity) is not influenced by the perturbation. For diffusivity of dissolved gases and HTO, the differences are not significant as they are within the uncertainty boundaries. Molecular dynamics simulations in montmorillonite revealed that the clay interlayer might potentially shrink or even collapse due to the chemical and osmotic potential differences induced by geochemical perturbation (NaNO3). The reduced volume of the interlayer porosity did not impact the diffusion of dissolved gases. The key message of this study is that a perturbation of clay with 1 M NaNO3 will not significantly alter the transport properties of dissolved gases in the near field. This information is of relevance for the development of a geological disposal concept of bituminised waste in Boom Clay.