Chemical Evolution of Bentonite Buffer in a Final Repository of Spent Nuclear Fuel During the Thermal Phase

Finnish spent nuclear fuel disposal is planned to be based on the KBS-3V concept. Within this concept, the role of the bentonite buffer has been considered to be central. The aim was to model the evolution of a final repository during the thermal phase (heat-generating period of spent fuel), when the bentonite is partially saturated at the beginning, and the rock matrix surrounding it is fully saturated. It is essential to study how temperature affects saturation and how both of these affect the chemistry of bentonite. In order to make the modelling more concrete, an experimental case was adopted: the Long Term Test of Buffer Materials (LOT) A2-parcel test at the Äspö Hard Rock Laboratory (HRL) in Sweden. In the A2-parcel the MX-80 bentonite was exposed to adverse (120-150oC) temperature conditions and high-temperature gradients. The test parcel diameter was smaller than in the KBS-3V concept to speed up the saturation. Different kinds of thermodynamic and kinetic properties of minerals cause a redistribution of phases inside the bentonite. For example, according to laboratory tests, gypsum seems to dissolve and anhydrite seems to precipitate near the heater-bentonite interface. Also incoming groundwater affects the bentonite porewater and its properties. These changes may affect the mechanical properties of bentonite and it has to be clarified if these phenomena have to be taken into account in safety assessment. The applied model is a coupled thermo-hydro-chemical model, which means that all the mechanical alterations and effects are not considered. The purpose of the model was first to obtain similarity to the results compared to the experiment, and thus, the time frame was limited to 10 years (the LOT A-2 parcel test lasted approximately 6 years). The system is simplified to 1-D in order to reduce the computational work, which is significant mostly due to complex chemical calculations. TOUGH and TOUGHREACT was applied to model the reactive unsaturated transport processes in 1-D and the grid was pitched at uniform intervals. The results may be used to gain knowledge of the bentonite evolution during the thermal phase, and after a good match with experiment the modelling can be continued until the end of the thermal phase for thousands of years.