Results and interpretation of bentonite resaturation experiments with liquid water and water vapour

Most countries favour compacted air-dry bentonite for engineered barriers in final repositories for nuclear waste. While the hydrophilic properties of bentonite appear to be ideally suited for minimising any contact of water with the waste the process of resaturation is not fully understood yet. In order to investigate the dynamics of water uptake several resaturation experiments with liquid water as well as with water vapour have been performed. A series of tests concerning the uptake of liquid water in compacted MX-80 bentonite samples has been completed recently. The experiments provide uptake rates and moisture distributions as a function of time. Analogous experiments with water vapour are presently running. Some first results are presented. The data gained is used to check new conceptual models which explain resaturation by flow of liquid water and by diffusion of water vapour, respectively. In the new models the effects of hydration on the local pore water content and the change of porosity corresponding to the amount of hydrated water are considered. The results strongly suggest that vapour diffusion plays a significant - if not dominant - role in the resaturation process of bentonite.