The migration of radioactive caesium and strontium through a bentonite-like clay
thesisposted on 20.06.2014, 15:07 by Stephen J. Pendleton
A number of leaks of radioactive liquors to ground are known to have occurred from several plant buildings, vaults and disposal trenches within and around the Separation area at the Sellafield nuclear site over the past 50 years1. The most significant of these leaks have occurred from the Magnox Silo, the Caesium Extraction Plant, the Magnox Reprocessing Pump House, the Sludge Storage Tanks, the Burial Pits and the Medium Active Evaporation and Thermal Denitration Plant. The radioactive contamination will be accompanied by other components of spent fuel reprocessing, including inorganic salts from neutralised acids, solvents and other organic compounds along with the typical contaminants commonly associated with large industrial activity such as heavy metals, fuel, oils, degreasing agents etc. The research in this thesis describes the effects of common industrial contaminants on the behaviour of Cs and Sr sorption to bentonite and montmorillonite clay minerals. Batch sorption experiments of Cs and Sr uptake onto bentonite and montmorillonite in a number of complex systems were investigated as follows: 1. Initial batch sorption experiments investigating the normal behaviour of the clay minerals. 2. Introduction of anthropogenic organic ligands EDTA, NTA and picolinic acid into the system and their influence on Sr and Cs uptake. 3. Time dependent studies investigating the effect that residence time of the anthropogenic organic ligands has on the sorption properties of montmorillonite and bentonite. 4. Quaternary systems where simulant Magnox sludge equilibrated water, at a number of concentrations, was introduced to ternary systems containing anthropogenic organic ligands. 5. Time dependent studies investigating the effect of hydrocarbons TCE, toluene and naphthalene on the uptake of Cs and Sr to bentonite and montmorillonite. The batch sorption experiments show that the presence of anthropogenic organic ligands reduces the uptake of Cs and Sr by both montmorillonite and bentonite. It is also shown that the presence of simulant Magnox sludge in quaternary systems can also reduce the uptake of Cs and Sr, with significant reductions in sorption observed for Sr sorption in EDTA quaternary systems. Further, the uptake of Cs and Sr to bentonite and montmorillonite appears to decrease with increasing anthropogenic organic ligand residence time. This suggests that the ligands are altering the surface of the clay minerals reducing the number of available binding sites. The influence of hydrocarbons shows a significant decrease in sorption after 3 months for Cs and Sr sorption to montmorillonite. There were no further significant changes for all other hydrocarbon systems investigated.