The migration of uranium through Clashach Sandstone: the role of low molecular weight organics in enhancing radionuclide transport

The migration of uranium through siliceous porous media has been investigated by conducting a series of core flood experiments in conjunction with the development of predictive coupled chemical transport models. Initial investigations made during the programme have demonstrated that under oxidising conditions, at pH≈5–6 with NaCl concentrations ≈0.3 mol dm−3, uranium is strongly retarded by adsorption to a Clashach Sandstone matrix. Sorption has been shown to be largely reversible, dependent on the absolute concentrations involved and on the finite sorptive capacity of the rock. Comparative experimental and modelling studies have also demonstrated the sorptive behaviour of the Clashach Sandstone to be close to that of amorphous silica. Recent work has focused on assessing the role of EDTA and cellulose degradation products in enhancing uranium mobility by complexation/chelation. Laboratory scale migration experiments involving radio-labelled EDTA and saccharic acid have been supported by batch sorption studies to investigate the behaviour of the U–EDTA-silica and U–saccharic acid-silica systems. In order to obtain unequivocal comparisons of the rates of uranium migration in the presence and absence of the organic complexant, parallel column floods have been carried out such that intact cores of Clashach Sandstone have been flooded with waters from the same reservoirs at carefully controlled flow rates. Geochemical modelling techniques have been used to assess the chemical speciation of the waters, to describe the sorption behaviour observed in the batch sorption studies and, where appropriate, derive model specific data to allow predictive coupled chemical transport modelling of the migration experiments. It is shown that, for a pre-equilibrated uranium-organic system, the use of conventional techniques encompassing complexation in the aqueous phase and reversible adsorption of organically-bound species would give a reasonable approximation of the system. Conversely, where uranium is introduced ahead of the organic plume, re-mobilisation is severely constrained by slow desorption kinetics. The results point to the need to consider the sequence of radionuclide interactions with organic waste components in formulating models used in performance assessment.