Diffusion of ⁶⁰Co(II), ¹⁵⁴Eu(III) and ¹³⁴Cs(I) in Opalinus Clay: results from in-diffusion measurements combined with a high-resolution abrasive method

Document available in extended abstract form only. Diffusion parameters for strongly sorbing tracers in argillaceous rocks are rare because such measurements are difficult. Due to a strong interaction of the tracers with the equipment, initial and boundary conditions are ill defined and this makes a proper modelling of the results difficult. Moreover, the strong sorption of radionuclides on the clay causes high diffusion gradients over the filter discs that are commonly used to confine the clay samples. A new type of diffusion cell was developed that used radially perfused filters to circumvent the strong diffusion gradients over the filters. Moreover, the diffusion cells were made of PEEK to lower the sorption of the tracers on the equipment. The diffusion of ⁶⁰Co(II), ¹⁵²Eu(III) and ¹³⁴Cs(I) in Opalinus Clay was studied by in-diffusion experiments. The diffusive tracer profiles in the rock were determined using a high-resolution abrasive peeling method. The tracer concentrations of the radionuclides in the grinding swarf were measured directly via γ-spectrometry. The diffusion of ¹³⁴Cs(I) showed a normal diffusion profile that could be described using one single effective diffusion coefficient and one sorption value similar to the one measured on intact Opalinus Clay samples. In the case of ⁶⁰Co(II) and ¹⁵²Eu(III), anomalous diffusion profiles were obtained. Both tracers diffused faster than expected based on known sorption values and assuming an effective diffusion coefficient similar to the one of HTO. Moreover, the profiles could only be modelled when more than one diffusion path, each of them characterized by an effective diffusion coefficient and a sorption value, was assumed. The reason for this anomalous diffusion behaviour is not understood at the moment. (authors)