Development and applications of the channel network model for simulations of flow and solute transport in fractured rock

The Channel Network model and its computer implementation, the code CHAN3D, for simulations of fluid flow and transport of solutes have been developed. The tool may be used for performance and safety assessments of deep lying repositories in fractured rocks for nuclear and other hazardous wastes, e.g. chemical wastes. It may also be used to simulate and interpret field experiments of flow and transport in large or small scale. Fluid flow and solute transport in fractured media are of interest in the performance assessment of a repository for hazardous waste, located at depth in crystalline rock, with potential release of solutes. Fluid flow in fractured rock is found to be very unevenly distributed due to the heterogeneity of the medium. The water will seek the easiest path, channels, under a prevailing pressure gradient. Solutes in the flowing water may be transported through preferential paths and migrate from the water in the fractures into the stagnant water in the rock matrix. There, sorbing solutes may be sorbed on the micro surfaces within the matrix. The diffusion into the matrix and the sorption process may significantly retard the transport of species and increase the time available for radionuclide decay. Channelling and matrix diffusion contribute to the dispersion of solutes in the water. Important for performance assessment is that channeling may cause a portion of the solutes to arrive much faster than the rest of the solutes. Simulations of field experiments at the Aespoe Hard Rock Laboratory using the Channel Network model have been performed. The application of the model to the site and the simulation results of the pumping and tracer tests are presented. The results show that the model is capable of describing the hydraulic gradient and of predicting flow rates and tracer transport obtained in the experiments. The data requirements for the Channel Network model have been investigated to determine which data are the most important for predictions. The data needed and how model parameters may be obtained, in order to use the model for site analysis, are shown. The predictive features of the Channel Network model have been investigated. It was found that the model, with a limited amount of data, is capable of accounting for the uneven flow distribution commonly found in fractured rock and can model matrix diffusion and sorption. Possible approaches to develop the model further are presented. To develop a tool for performance assessment CHAN3D was integrated with a near field code. In the performance assessment calculations it was found that the coupled models can be used as an efficient tool to simulate release from a repository and the transport to a recipient. It may also be used to study the impact of various factors which may affect the design of the repository