Modelling the Long Term Sorption Diffusion Experiment (LTDE-SD) at Äspö HRL using a microtomography-based inter-granular network

The in-situ diffusion experiment, LTDE-SD, performed at the Äspö Hard Rock Laboratory (HRL) in Sweden, has pointed out that in fractured crystalline rocks strongly sorbing radionuclides (e.g. Cs-137) can travel surprisingly long distances driven by molecular diffusion. These anomalous penetration profiles are being assessed by an international team of experimentalists and modellers in the framework of the SKB Task Force GWFTS (Groundwater Flow and Transport of Solutes; www.skb.se/taskforce). Here, we use a grain-scale transport model to offer a quantitative explanation for these anomalous profiles. The diffusion-available pore space is mapped from micro computed tomography data and the resulting inter-granular network is used to carry out High Performance Computing (HPC) reactive transport calculations. In these calculations, reactive mineral grains (i.e. biotite grains) are also mapped and considered in the calculations as sorbing surfaces, which are described using a mechanistic cation exchange model. The model is analogous to one developed earlier, but with the main difference that a single micro-fracture, which is evident from a visual inspection of the considered rock sample but not captured during grain segmentation, is included here. Diffusion simulations of both a sorbing (i.e. Cs-137) and a non-sorbing tracer have been carried out using the open-source, massively parallel flow and reactive transport code PFLOTRAN. The calculations were performed on the JUQUEEN and JURECA supercomputers at the Jϋlich Supercomputing Centre (JSC). Our work shows that the interplay between micro-fractures, which constitute preferential diffusive pathways, and the sparsely distributed sorption sites can explain the anomalous patterns observed from the experimental data