Verification of matrix diffusion by means of natural decay series disequilibria in a profile across a water-conducting fracture in granitic rock

According to current Swiss concepts for the disposal of radioactive waste, emplacement in deep rock formations will ensure that only small (or negligible) concentrations of radionuclides will ever reach the surface and so enter the biosphere. It is considered likely that, in the formations of interest to the Swiss program (e.g. granitic rocks, argillaceous sediments), the rock will be fractured (even at repository depth) and that advection along the fractures will be the dominant mode of groundwater flow, and hence transport of any radionuclides released from the repository. Data from a rock drillcore, taken perpendicular to a water-bearing fracture in crystalline rock, clearly indicate water-rock interactions in and around the fracture. Although there is evidence of microfracturing at some distance from the main fracture, and therefore potential advective flow across the entire shear zone, simple calculations indicate that transport of Ra-226 in the vicinity of the main fracture can be described simply by matrix diffusion. This is a useful simplification for safety assessment studies and the consequences to the estimated radionuclide retardation in the vicinity of the fracture are discussed with respect to the probably over-conservative assumptions made in Nagra's safety assessment model base case