Steady-state radionuclide transfer from a cylinder intersected by a fissure

Abstract

The steady-state concentration of a diffusing nuclide from a waste cylinder has been obtained from the exact analytical solution. Two phenomena which have been neglected by the previous investigators include the effect of axial diffusion in the fissure, and the effect of radial diffusion in the porous rock. Our numerical results indicate that, for a waste-cylinder radius of 15 cm and fissure width of 2 cm, one can neglect diffusion in the axial direction in the fissure, i.e., assume a uniform concentration in the axial direction. The steady-state concentration at the fissure/rock interface is greater than or equal to99.9 percent of the concentration at the fissure's center line. Hence, one can use a simplification, i.e., that the concentration of the nuclide in the fissure in the transverse direction (z direction) is uniform. On the other hand, radial diffusion in the porous rock is very important and should not be neglected. At steady-state about 3 times more mass is lost from the waste cylinder to the rock than is lost to the fissure. Furthermore, results indicate that close to the waste cylinder approximately 100 times more mass is transported in the radial direction in the porous rock adjacent to the fissuremore » than is diffused across the fissure/rock interface. Therefore, neglect of radial diffusion in the porous rock will cause underestimation of the dissolution rate. 11 refs., 8 figs., 1 tab.« less