Immobilisation of actinide simulants in cement
The current UK strategy for radioactive waste management is to permanently store the waste in an underground repository. Final disposal of the radwaste may then be preceded by chemical conditioning and physical encapsulation. The objective of this work was to determine the extent of actinide immobilisation in cement. Since actinides are hazardous and costly to study directly, a chemical analogue approach to studying actinide immobilisation was adopted. Th(IV), Ce(III, IV) and Eu(III) were chosen as actinide simulants and their suitability assessed by a critical review of the literature. Ca(OH)2 and C-S-H dominate the observed chemical properties of the aqueous phase in cement. As they are of such importance, it was these cement components which were used to investigate the reaction of the simulant elements with cement. The phases found to be predicted were ThO2, ThSiO4, Eu(OH)3, Ca2Eu8(SiO4)6O2, CeO2, CeSiO4 and Ca2(SiO4)6O2. CeSiO4 and Ca2Ce8(SiO4)O2 are newly reported phases, produced by hydrothermal synthesis. Rietveld refinement confirmed CeSiO4 to have the zircon structure, with space group 141/amd and cell parameters a = 6.9564(3) A, c = 6.1953 (4) A. Ca2Ce8(SiO4)6O2 exhibits the apatite structure, with space group P63/m and cell parameters a = 9.4343(3) a, c = 6.8885(4) A. Preliminary solubility studies were carried out on all of the solubility-limiting phases. Phase impurity, poor crystallinity and incongruent solubility of phases hindered the generation of solubility product data. Nevertheless, these phases have naturally occurring analogues which are known to be environmentally stable and have low solubilities. On the basis of the experimental results obtained, it may be concluded that cement has the potential to be a very effective immobilisation matrix for actinide elements. Recommendations for future experiments using active elements are discussed.