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The Channel Network model and its computer implementation, the code CHAN3D, are presented for simulations of fluid flow and transport of solutes. The tool may be used to simulate and interpret field experiments of flow and transport on a large or small scale. It also may also be used for performance and safety assessments of repositories for nuclear and other hazardous wastes, e.g., chemical wastes. From observations in the field it is deduced that the flow and transport take place in a three‐dimensional network of connected channels. The channels have different properties and are generated in the model from observed stochastic distributions. This allows us to represent the large heterogeneity of the flow distribution commonly observed in the field. Solute transport is modeled considering advection and rock interactions such as matrix diffusion and sorption within the interior of the rock. For repository conditions the main contributions to the dispersion of solutes come from the large variation in the flow field (channeling) and the rock interactions. Objects such as fracture zones, tunnels, and release sources can be incorporated in the model. In addition, a methodology of how data may be obtained for the model is described. Data may be obtained from borehole and laboratory measurements.

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The spatial porosity and mineral distribution of geological materials strongly affects transport processes in them.X-ray micro computed tomography (X-μCT) has proven to be a powerful tool for characterizing the spatial mineral distribution of geological samples in 3-D. However, limitations in resolution prevent an accurate characterization of pore space especially for tight crystalline rock samples and 2-D methods such as C-14polymethylmethacrylate (C-14-PMMA) autoradiography and scanning electron microscopy (SEM) are needed.The spatial porosity and mineral distributions of tight crystalline rock samples from Äspö, Sweden, and Olkiluoto, Finland, were studied here. The X-μCT were used to characterize the spatial distribution of the main minerals in 3-D. Total porosities, fracture porosities, fracture densities and porosity distributions of the samples were determined using the C-14-PMMA autoradiography and characterization of mineral-specific porosities were assisted using chemical staining of rock surfaces. SEM and energy dispersive X-ray spectroscopy (EDS) were used to determine pore apertures and identify the minerals. It was shown that combination of the different imaging techniques creates a powerful tool for the structural characterization of crystalline rock samples. The combination of the results from different methods allowed the construction of spatial porosity, mineral and mineral grain distributions of the samples in 3-D. These spatial distributions enable reactive transport modeling using a more realistic representation of the heterogeneous structure of samples. Furthermore, the realism of the 2 spatial distributions were increased by determinig the densities and porosities of fractures and by the virtual construction heterogeneous mineral distributions of minerals that cannot be separated by X-μCT.

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.