New conceptual models for the resaturation of bentonite

Abstract Bentonite plays an important role in the design of underground repositories for radioactive waste. It is envisaged to serve as a buffer around the waste canisters and as sealing material in the emplacement boreholes and drifts. One of the main tasks of the bentonite is to reduce and delay water flow from the host rock in order to postpone a contact of water with the waste canisters as long as possible. The resaturation of the initially air-dry bentonite can be a very complex phenomenon being influenced by hydraulic, mechanic, thermal and chemical processes. These processes are coupled by a series of parameters and equations of state. Some of them are highly nonlinear and some of them are not yet completely understood. However, in the simple case of one-dimensional, isothermal resaturation under atmospheric pressure, the uptake of liquid water can be described by a Fickian type law with a constant “diffusion” coefficient. The relationship is generally accepted for this type of resaturation problem since the reported values for this coefficient lie within a rather narrow bandwidth. It has to be pointed out, though, that this Fickian type relationship is purely empirical. In the present paper two conceptual models—called the advection model and the vapour diffusion model—are therefore proposed as an alternative to the empirical approach. They explain the uptake of liquid water and water vapour, respectively, by three basic processes—Darcy flow, binary gas diffusion and hydration—and need only the related parameters. The advection model has already been tested against an water uptake experiment. It reproduces the measured water content as well as the uptake rate deduced from the calibrated empirical diffusion law. The validity of the vapour diffusion model and its importance for isothermal conditions is under investigation. So far, both models have the potential to be extended to physically more complex situations rather easily.