Retention of Cs in Boom Clay: Comparison of data from batch sorption tests and diffusion experiments on intact clay cores

For performance assessment analysis of a high-level waste and spent fuel (HLW/SF) disposal in a geological formation, retention data are needed. In most cases, retention data are derived from batch sorption experiments on dispersed systems but the applicability of these data for the real compacted state is still a point of debate.Cesium retention onto Boom Clay was measured by batch sorption tests on dispersed systems and by diffusion experiments on intact clay cores and the sorption parameters obtained from both methods were compared. In a dispersed system, the concentration dependent uptake of cesium onto Boom Clay is described using the generalised 3-sites cation exchange model for illite developed by Bradbury and Baeyens, [Bradbury, M., Baeyens B., 2000. A generalised sorption model for the concentration dependent uptake of cesium by argillaceous rocks. Journal of Contaminant Hydrology 42, 141–163].In the compacted state, long-term (13.7 years) diffusion experiments resulted in reproducible and robust values for the apparent diffusion coefficient (Dapp). The determination of retardation factors suffered from a large uncertainty making a good comparison troublesome. But the retardation factors obtained from batch sorption data tend to be higher than those obtained by migration experiments. A chemical coupled transport simulation for the Cs migration in Boom Clay based on the ion-exchange model was used for a sensitivity analysis on the effect of increased pore diffusion coefficients and decreasing available sorption sites. This analysis suggests, irrespective of the underlying hypothesis, that in a compacted system not all sorption sites are available. For Cs retention in Boom Clay, conversion of batch sorption data to compacted systems cannot be applied in a straightforward way.