Advanced ceramic wasteforms for the immobilisation of radwastes

Recent progress in the synthesis, characterisation and radiation damage behaviour of advanced ceramic wasteforms for the immobilisation of actinides and halide radionuclides is reviewed. A systematic methodology is described to probe the structure and evolution of the radiation damaged structure of model wasteform materials, combining ex-situ ion beam irradiation of bulk ceramics with X-ray Absorption Spectroscopy (XAS), to quantify damage induced changes in element speciation. The defect chemistry and crystal structure of cerium brannerite, Ce_0.975Ti₂O_5.95, was clarified by Rietveld analysis and defect energy calculations, combined with careful investigation of the phase diagram. Formation of oxygen vacancies at the 01 site, charge compensated by Ce vacancies, relived considerable coulombic repulsion and structural strain associated with short 01-01 contacts forming the shared edge of neighbouring TiO₆ polyhedra. The rapid synthesis of Pb₅(VO₄)₃l, a potential immobilisation host for iodine radioisotopes, was achieved in an open container by microwave dielectric heating of a mixture of PbO, Pbl₂, and V₂O₅ at a power of 800 W for 180 s (at 2.45 GHz). The resulting ceramic bodies exhibited a zoned microstructure, differentiated by inter-granular porosity and phase assemblage, as a consequence of the inverse temperature gradient characteristic of microwave dielectric heating.