Solubility study of Ti,Zr-based ceramics designed to immobilize long-livedradionuclides

Aqueous alteration of five Ti-Zr-oxide-based ceramics containing elements s imulating long-lived radionuclides was studied experimentally by leaching a t 90 degreesC in deionized water for more than one year under conditions of high solid/liquid ratios. Four of these ceramics were synthesized by cold- crucible melting (two Synroc-like materials, one zirconia, and one aluminot itanate) and the fifth ceramic was a hot-pressed Synroc. Melted Synroc-like ceramics have the same major constitutive phases as hot-pressed Synroc, bu t crystal sizes are very different, millimetric as opposed to micrometric, respectively. After the first seven days of leaching, the alteration appeared to cease as solution concentrations for all of the constituent elements attained const ant values. The altered mass percentages determined from the release of Ca and Mo were less than 0.2% of the initial mass. Thermodynamic equilibrium c alculations using data or estimations for pure phases, or using a model of ideal solid solutions, showed that the cessation of the alteration cannot b e explained by the solubility limit of the primary phases of these ceramics . But, the data could be interpreted by the development of a passivating la yer of secondary phases, e.g., hydroxides. Examination of the altered surfa ces was carried out using SEM, XRD and XPS; however, the thickness of the a lteration layer, estimated as 3-5 nm, was below the resolution limit of the se techniques. Finally, despite the differences in the crystal size and the refore the amount of grain boundaries, and in the synthesis redox condition s, the leaching behaviors of melted and hot-pressed Synroc are the same for the present experiments.