Diffusion of gases in partially saturated clay - based materials
In many countries in Europe, clay-based materials are considered for safe disposal of high and intermediate level radioactive waste, either as an important component in the the engineered barrier system, or as geological formation to host the repository. Furthermore clays under consideration all have high sorption capacity for many radionuclides (Altmann et al., 2012; Maes et al., 2008; Maes et al., 2004), low hydraulic conductivity (Enssle et al., 2011; Yu et al., 2013) and interesting self-sealing properties (Van Geet et al., 2008; Zhang, 2013). As these clay-based materials have a low water permeability, diffusion is the main transport mechanism. Hence, a good knowledge of relevant diffusion coefficients is important to assess the safety function of these clay materials. In literature, mainly diffusion coefficients for radionuclides have been reported as part of the inventory. However, next to the radionuclide inventory, also gases are released (mainly generated by anaerobic metal corrosion) and knowledge of the diffusion coefficients of dissolved gases are also important because they are an essential parameter for evaluating the balance between gas generation and gas dissipation by diffusion. When the latter is not balanced, a free gas phase will form, which leads to gas pressure build-up, which might influence the barrier function of clay materials. Recently, SCK-CEN established a unique, large data set of diffusion coefficients of several gases (binary gases H2/N2, noble gases and light hydrocarbons) for different materials (Boom Clay, Eigenbilzen Sands, Opalinus Clay, Callovo-Oxfordian Clay, bentonite, shale, cement paste and concrete). All diffusion measurements were performed on fully saturated samples, with gases dissolved in water. However, at certain stages in the repository evolution, unsaturated conditions may exist. The diffusion coefficients of gases in clay host rocks and buffer material at partially saturated conditions are currently unknown. Given the fact that the transport of gases has a large influence on the evolution of waste repositories, several waste management organisations (WMO’s) expressed the need to assess the effect of desaturation on diffusive transport of gases within the first European joint project on radioactive waste management and disposal (EURAD). The PhD will be executed in the international context of the EC project, with close collaboration between SCK-CEN and the Paul Scherrer Institute (PSI, CH) and other partners involved in EURAD. The main aim of the PhD is to investigate how desaturation influences the diffusive properties of the samples for gases. This will be accomplished by performing diffusion experiments on different clay-rich materials (e.g. Boom Clay, Opalinus Clay, bentonite, sand-bentonite mixtures, …) at different degrees of saturation (but still highly saturated to avoid a continuous gas pathway), using different gases (He, Ne, CH4, C2H6). Different methods to perform transport experiments under desaturated conditions exist (e.g. osmosis method, saturated salt method …), but none of them has been used to measure diffusivity of dissolved gases. Diffusivity data will be complemented with a full petrophysical characterisation of all samples in order to assess the effect of changing petrophysical properties on diffusivity, and to compare the observations to those of fully saturated samples.