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The sorption characteristics of U and Cs on bentonite and montmorillonite were investigated in batch experiments to understand the near-field behaviour in geological nuclear repositories. The pH dependence and the retention of Cs are small and only caused by cation exchange. For the U sorption, maxima were found between pH 5.5 to 6.0 and at pH > 10.5. The linear sorption isotherm indicates only one binding type.
The retention of Cs in the concentration range between 3·E-9 and 1·E-4 mol/L on bentonite and montmorillonite (KD between 30 and 189 mL/g, retention from 7 to 31 %) is low compared to U (KD between 430 and 31500 mL/g, retention from 50 to 100 %)

This work is supported by the European Commission (NF-PRO C2-ST-C-01)