Immobilization of actinide wastes in ceramics for long-term disposal

Glass can undergo devitrification with time and the incorporation of actinides with their alpha-decay in MeV-range due to compacted alpha-recoiled irradiation over 10^6 years storage leads to expansion and phase transformation of nuclear waste glasses. Moreover, the maximum tolerable amount of actinides that can be incorporated in glass is approximately 5%. The development of ceramics adapted for nuclear-waste immobilization is the focus of this dissertation. The yttria-stabilized zirconia ceramic (ZrY)O_x is a promising candidate material for both inert matrix fuel and waste form due to its high solubility of actinides, high chemical durability and exceptionally high stability under irradiation over a vast temperature range. Cubic materials swell isotropically when damaged by radiation under different strains. This leads to less microcracking in a repository. They are also capable of forming isomorphous solid solutions with actinides. In order to obtain fully stabilized solid solution with cubic symmetry, yttria oxide was chosen as a stabilizer for zirconia ceramics and ceria oxide and neodymia oxide were chosen as stimulant wastes in the current study. A simple fabrication method was chosen for the synthesis of powders. The cubic yttria-stabilized zirconia matrix with different ceria contents (0-25 wt%) and neodymia (0-100 wt%) contents was synthesized using the co-precipitation method. The mild attrition method was used to dry powders at 110°C or calcine powders at 600°C. Both acetone and propanol milling solvent media were used to get active fine powders for neodymia ceramics. Then, the different characterizations of powders were performed using the following methods: agglomerate size and distributions of powder by optical microscopy and granulometry, properties of the powder and its structural appearance by XRD, phase transition and structural formation of the powder with a temperature gradient by TG/DSC, surface area and crystalline size of the powder by BET. After these initial investigations into the characteristics of the powders, pellets with different ceria and neodymia contents, were compacted using isostatic pressure of 229 MPa - 1019 MPa. Then, they were sintered at a temperature of 1600°C. Some physical and mechanical properties of ceria ceramic and neodymia ceramic pellets were investigated before chemical durability tests, were carried out. The chemical durability of the final products was subsequently investigated using static and dynamic leaching tests, according to MCC-1 and MCC-5 with different leachants at interesting pHs. The influence of the different fabrication processes on the leachability of fabricated products was investigated and evaluated. The results from these chemical durability tests were then used to determine the effect that different structures have a stability of the ceramic materials