Sorption processes of radiocesium in soil and bedrock

摘要

Three recent studies on cesium sorption in soil and bedrock are reviewed. Cs-137, originating from fallouts of nuclear weapons tests and the Chernobyl accident was found to decrease in an exponential manner in forest soil, and the highest fraction in soil profiles was found in the organic layer. Also, the mineral layer below the organic layer contained a large fraction of cesium inventory but at depths below 20 cm only a very small fraction was observed. In the bedrock of Olkiluoto, where the final repository for spent nuclear fuel fromthe Finnish nuclear power plants will be constructed, mica mineral biotite plays the most important role in cesium sorption. The selectivity of biotite decreases in the order Cs> K> Na> Ca and the overall selectivity coefficient for Cs/Ca exchange was approximately five and seven orders of magnitude higher than those for Cs/Na and Cs/K exchange reactions, respectively. Ion exchange isotherms for Cs/Na and Cs/K exchange were modelled by assuming three different ion exchange sites: frayed edge sites (FES), basal plane sites and intermediate sites. The selectivity coefficients derived for these sites were successfully used to predict cesium sorption in a mica gneiss rock. Sorption of cesium in mineral soil layers from the Olkiluoto overburden were studied using three different approaches: model batch experiments, an in- situ method and calculations. All three approaches gave the same trend but the distribution coefficient values varied in range of one order of magnitude.