Solution composition and particle size effects on the dissolution and solubility of a ThO₂ microstructural analogue for UO₂ matrix of nuclear fuel

The objective of this study was to investigate the dissolution rate of ThO₂ which was synthesised to approximate, as closely as possible, the microstructure of UO₂ in a nuclear fuel matrix. The optimal sintering temperature for ThO₂ pellets was found to be 1750, which produced pellets with a microstructure similar to UO₂ nuclear fuel pellets, with randomly oriented grains ranging in size from 10 to 30μm. Dissolution was conducted using ThO₂ particles of different size fractions (80 to 160μm and 2 to 4mm) in the presence and absence of carbonate, in solutions with pH from 2 to 8 and at 80. Dissolution rates were calculated from Th released from the solid phase to solution. Particles of ThO₂ were also leached with 1M HNO₃ at 80 in order to investigate the morphological changes at the particle surfaces. The concentration of Th was found to be ≥10^-9mol/L at pH≤4, lower than the theoretical solubility of crystalline ThO₂. At higher pH values, from 4 to 8, the measured concentrations (10^-10 to 10^-12mol/L) were between the theoretical solubility of ThO₂ and Th(OH)₄. Grain boundaries were shown to exert an influence on the dissolution of ThO₂ particles. Using high resolution aqueous solution analysis, these data presented here extend the current understanding of Th solubility in solution.