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 w ith nitric acid containing 0 approximately 0.05 mol/l NaF at high temp eratures above 120-degrees-C. The dissolution rate depended upon tempe rature, fluoride concentration and UO2 content. High-temperature in th e range of 120 approximately 200-degrees-C enhanced the dissolution of the ThO2-based fuels. At low temperatures and/or low UO2 concentratio ns, insoluble tetrafluoride precipitates were formed on the particle s urfaces and they resulted in the decrease of the dissolution rates. In the present study, the apparent activation energies for the high-temp erature dissolution were obtained.