Field investigation studies, conducted in the context of safety analyses of deep geological repositories for nuclear waste, have pointed out that in fractured crystalline rocks sorbing radionuclides can diffuse surprisingly long distances deep into the intact rock matrix; i.e. much longer distances than those predicted by reactive transport models based on a homogeneous description of the properties of the rock matrix. Here, we focus on cesium diffusion and use detailed micro characterisation data, based on micro computed tomography, along with a grain-scale Inter-Granular Network model, to offer a plausible explanation for the anomalously long cesium penetration profiles observed in these in-situ experiments. The sparse distribution of chemically reactive grains (i.e. grains belonging to sorbing mineral phases) is shown to have a strong control on the diffusive patterns of sorbing radionuclides. The computed penetration profiles of cesium agree well with an analytical model based on two parallel diffusive pathways. This agreement, along with visual inspection of the spatial distribution of cesium concentration, indicates that for sorbing radionuclides the medium indeed behaves as a composite system, with most of the mass being retained close to the injection boundary and a non-negligible part diffusing faster along preferential diffusive pathways.

Open fractures are primary conduits for groundwater flow and thus constitute preferential transport pathways for dissolved contaminants. Fracture internal variability, caused, for example, by shearing, leads to significant variability in the in-plane groundwater velocity field (Egert et al., 2021;Zou et al., 2017), which in turn affects mass exchange processes with the stagnant water in the bordering porous rock matrix . Fracture filling minerals, that might have precipitated during past hydrothermal events, add more pieces to this already complex puzzle. In fact they might alter the local velocity field and react with some of the dissolved species. A thorough characterization of in-plane groundwater flow and reactive transport processes occurring at the scale of a single fracture has a direct impact on applications such as the remediation of a polluted site or the safety assessment study of a deep geological repository for nuclear waste.Groundwater channeling occurring at the scale of a single fracture was assessed by different experimental and numerical works. Brown et al. (1998) assessed in-plane channeling by injecting dye into a water-saturated precise replica of a natural fracture. The visual analysis of dye concentration shows the dye entering the fracture preferentially through a single large channel. The authors found the velocity contrast (i.e., the ratio between maximum and average velocity) to be within a factor of 5. Watanabe et al. (2009) performed experimental and numerical analyses on flow through shear (Mode II) fractures, which were generated by direct shear on granite under different constant normal load. Two different shear displacements (1 and 5 mm) were considered. The authors found that contact areas occupy around 30%-70% of the fracture plane, depending on the confining pressure. Numerical simulations, performed using the depth-average Reynolds equation, showed that only around 10%-30% of the non-contact areas contribute to fluid flow. More recently, Zou et al. ( 2017) used a laser-scanned real rock sample to assess the effect of roughness and variability in fracture aperture on radionuclide transport. The numerical models, which were based on the Navier-Stokes

No abstract

No abstract

No abstract