Technical Report NTB 85-40
Radionuclide Chain Transport in Inhomogeneous Crystalline Rocks:Limited Matrix Diffusion and Effective Surface Sorption
In Switzerland high-level radioactive wastes are planned to be deposited in deep-lying crystalline formations. The present paper describes the modelling approach for geospheric nuclide transport in inhomogeneous crystalline rocks in the framework of a HLW safety assessment. The processes taken into account are advection and dispersion in water conducting structures which consist of a network of either fractures or veins embedded in dykes or kakirite zones, respectively. Out of these structures nuclides may diffuse into a spatially limited zone of altered crystalline. In addition sorption of nuclides on the rock is taken into account. Diffusion out of the fractures and sorption in the altered rock may often be represented by an effective surface sorption on the fracture walls. For transport of the 237Np nuclide chain we present a sensitivity analysis taking a wide range of relevant parameters. The performance of the geologic barrier depends most critically on the extent of the altered zones.