Modelling Isotope Distributions in Borecores

Summary

The distribution and possible mobilisation of natural radionuclides in the neighbourhood of a water-bearing fracture in rocks may provide valuable information for the understanding of transport mechansims thought to determine the long term behaviour of a radioactive waste repository.

A one-dimensional diffusion transport model of a three member decay chain including sorption (linear adsorption isotherm) is presented. The rock adjacent to a water-bearing fissure has been modelled as a porous, homogeneous, isotropic and infinite medium. A further assumption is that the bulk rock phase does not act as a radionuclide source and, hence, does not contain any minerals or/and impurities of the radionuclides of interest. From the isotopic concentrations in the water of the fracture, assumed to be in a steady state, boundary conditions are extracted. Initial conditions are specified, e.g. at the time when the fissure opened. We present analytical expressions and discuss diffusion from rock to fissure for the ²³⁸U - series members. We show results for two representative environments with respect to sorption (reducing, oxidising).