Water saturation and retention of hydrophilic clay buffer—microstructural aspects

Abstract The microstructure of dense smectitic-rich buffer clay confined in deposition holes and embedding hot rad-waste containers is not homogeneous. The rate of hydration depends on the transient structure and the water pressure. At low pressures and thermal gradients, capillary forces in wider void channels initiate the wetting while water subsequently migrates from these channels into the denser aggregates with the tension of the interlamellar ion/water medium as driving force. This negative pressure cannot be distinguished from the “osmotic” potential by measurements, which only give a measure of the total hydration potential. The transport process can be described as one of diffusion. At high pressures, water penetrates wider void channels to several centimeters depth, but this process stops soon because the channels behind the waterfront become closed by the expansion of the clay aggregates. Further hydration follows the same procedure as at low water pressure. In general, therefore, hydration of blocks of very dense smectite clay can be described as a diffusion process. Drying of the buffer clay takes place close the hot canisters and causes fracturing and continuous channels, which host water vapor. Water that is initially sorbed on the external surfaces of the stacks lamellae is vapourized, which causes migration of interlamellar water to the channels to an extent that depends on the retention potential. Equilibrium is established between these water phases. The rate of drying is quick because macroscopic fractures are frequent and the distance from them to adjacent interlamellar space is very short.