Reactive transport model for bentonites in COMSOL multiphysics: Benchmark and validation exercise

At present, the deep geological repository concept for spent nuclear fuel is considered the most reliable and safe technique for the permanent disposal of this type of waste. One of the many safety elements used is an engineered barrier made of compacted bentonite. This material allows the encapsulated waste to be isolated from the host rock. Therefore, there is great interest in a detailed study of the behavior of bentonites to different changes in the composition of the surrounding groundwater. In this context, this work presents a new reactive transport model for bentonites implemented in the COMSOL Multiphysics platform. The model contemplates a non-simplistic geochemical system composed of 42 species and 4 minerals. Reactive transport involves the diffusive-dispersive-advective processes defined by the Nernst Planck equations for two overlapping modeling levels (macro- and microstructural) to simulate the behavior of double-porosity media. The uniqueness of this model is that the system of equations used to calculate the chemical speciation problem and the advective-diffusive-dispersive transport can be integrally solved in COMSOL. The model has been satisfactorily verified and validated using the benchmark exercise consisting of the simulation of the multicomponent advective-diffusive column experiment conducted on a compacted bentonite core extracted from a field experiment (LOT project) in the Äspö Hardrock laboratory (Sweden).