Local inflow to bentonite - solution with simple diffusion/swelling model

Document available in extended abstract form only. During the planning process of deep repositories of spent nuclear fuel we have to consider number of coupled thermo-hydro-mechano-chemical (THMC) processes which will run between natural and engineered barriers. In the Swedish repository concept KBS-3V (deep repository in crystalline rock with horizontal tunnels and vertical deposition holes sealed with bentonite), one of the processes important for long-term repository stability and function is possible hydration through individual fractures to a part of the bentonite barrier with the effect of non-uniform swelling and resulting stresses and deformations. We simulate such phenomena with numerical model. The model concept and data are partly based on the currently starting BRIE experiment (Bentonite Rock Interaction Experiment) in the Aespoe hard rock laboratory in the hydrogeological conditions of planned repository. The experiment will serve for understanding the exchange of water across rock-bentonite interface and predictions of the wetting of the bentonite buffer and is going to provide measured data to compare with results of numerical simulations, in particular within the Task Force EBS activity. In addition to the experiment we also model the swelling related to local hydration. In the experiment and model concept, the vertical borehole is filled with bentonite and water inflow is through single horizontal fracture, Included inflow across the interface rock-bentonite is considered to have only limited influence on bentonite saturation due to faster hydration through open fracture. Hydration process in bentonite, usually described by generalized Darcy's law for unsaturated conditions, is possible to solve using (nonlinear) diffusion equation. This simplification is directly derived from Darcy's law and it is referred with two different nonlinear diffusion coefficients dependent on degree of saturation. The numerical simulation has been done in ANSYS multi-physical simulation software (Ansys 2010). The nonlinear diffusion model was represented as heat conduction in analogy and the swelling as the thermal expansion, within a nonlinear elastic model representing changes of stiffness with water content. We solved several variants of the diffusive hydration models, first simple 1D and 2D axisymmetric testing models without influence of saturation across rock-bentonite interface were used for comparison with reference and analytical solutions. Results were in good correspondence. Next simulations with 2D axisymmetric models with the tunnel, the fracture, and surrounding rock were evaluated and saturation behaviour depending on the time and the position in the deposition hole (axial and radial profiles) were obtained. It was necessary to calibrate the permeability (and consequently the 'diffusion' coefficients) to reference computation. For the reference permeability from literature, the hydration of bentonite proceeded too quickly across the rock-bentonite interface and the hydration through the fracture had only limited importance. The bentonite hydration process is followed by the volume change (swelling). In the next phase nonlinear mechanical process is simulated based on the calculated hydration process (specifically volumetric expansion of bentonite material caused by its saturation). We investigate an influence of the bentonite swelling pressure on stress field in bentonite and rock with possible question of strength. Problem was tested using 2D axisymmetric model with mentioned geometry but in fact there is an air gap between rock and bentonite in deposition hole which allows bentonite swelling in all directions and possibly to seal the gap. For such conditions we solved the process as coupled contact problem