Radionuclide transport modeling in fissured zones and channels

Radionuclides escaping from a repository for high level nuclear waste in crystalline rock may eventually be carried by the flowing water in fissure zones. In such zones the rock is broken in blocks of varying sizes and shapes. Also, the water velocity may vary very much along the flow path. The integrated finite difference method (IFDM) is proposed for numerical calculations of radionuclide transport in such zones. A method for lumping blocks of various sizes and shapes into a single PSEUDOBODY is tested by comparing it with an exact analytical solution which can account for the diffusion into blocks of any size distribution. The errors obtained in using the PSEUDOBODY-approach are found to be small. Furthermore, a method for determining an average Peclet number in a strongly varying velocity field is tested and found to give comparatively small errors. Recent investigations give strong indications that water flows in largely isolated channels in fissured rock. The second part of the paper sets out to investigate the radionuclide transport in channels including matrix diffusion. It is shown that a cylindrical hole in a matrix is much more effective, per unit contact area, than a flat surface. Comparison of diffusion from cylindrical and slit-formed channels shows that the differences in interfacial flux are minor. A slit-formed channel may, therefore, be approximated by a cylindrical one. Using the integrated finite difference method a number of radionuclide transport calculations are performed for the case of flow and dispersion in a cylindrical channel coupled to diffusion and sorption in the matrix