Hydrogeochemical evolution of the bentonite buffer in a KBS-3 repository for radioactive waste. Reactive transport modelling of the LOT A2 experiment

The Swedish Nuclear Fuel and Waste Management Company (SKB) is conducting a series of long term tests at the Äspö Hard Rock Laboratory to assess the behaviour of the bentonite buffer under conditions similar to those expected in a KBS-3 repository for high level nuclear waste. The LOT A2 experiment consists of a vertical borehole with a central heater inside a copper tube surrounded by compacted bentonite. During four years, the temperature of the copper tube was maintained at 130 °C, while the bentonite was progressively water saturated by the injection of groundwater. During this period, physical and hydro-geochemical data were collected. By using the code TOUGHREACT, a model was made to simulate the processes of solute transport which control the chemical and the mineralogical distribution observed in the bentonite at the end of the test. Additionally, a series of sensitivity analyses was performed to assess the influence of key parameters controlling the thermal-hydro-geochemical processes. Numerical results indicate that, within the first year, the heated bentonite blocks are completely water saturated, which agree with the measured data. The simulated transport of chloride, the dissolution/precipitation of Ca sulphate minerals, and the cation redistribution in the montmorillonite interlayer also agree with data measured at the end of the experiment.