DISSOLUTION OF THO2-BASED OXIDES IN NITRIC-ACID SOLUTIONS AS ELEVATED-TEMPERATURES

Highly-dense spherical particles of thorium-based oxides, ThO2 and (Th, U)O2, prepared by the sol-gel method were subjected to dissolution with nitric acid containing 0 approximately 0.05 mol/l NaF at high temperatures above 120-degrees-C. The dissolution rate depended upon temperature, fluoride concentration and UO2 content. High-temperature in the range of 120 approximately 200-degrees-C enhanced the dissolution of the ThO2-based fuels. At low temperatures and/or low UO2 concentrations, insoluble tetrafluoride precipitates were formed on the particle surfaces and they resulted in the decrease of the dissolution rates. In the present study, the apparent activation energies for the high-temperature dissolution were obtained.