Radionuclide Transport in Fast Channels in Crystalline Rock

Recent field investigations in crystalline rock give strong indications that water flows in largely isolated channels in fissured rock. The present paper sets out to investigate radionuclide transport in such channels including diffusion into the rock matrix. It is shown that the uptake from a cylindrical hole in a matrix is much more effective, per unit contact area, than a flat surface (semi-infinite solid). 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. Diffusional transport in the matrix is then treated as one dimensional instead of two dimensional, which gives a considerable numerical simplification. A simple estimate of the “penetration depth” into the cylindrical region, i.e., the location of the diffusional front, is proposed. By 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.