Dissolution of thorium-uranium mixed oxides in concentrated nitric acid

Thorium-uranium mixed oxides, (Th, U)O2, of different ratios of the heavy metals were prepared by sol-gel technique. The oxides, well characterized by physical and chemical methods, were subjected to disslolution with an excess of boiling nitric acid for 140 h. Mixed oxides with thorium concentrations up to 60% have been dissolved almost completely. At higher thorium concentrations, the fraction of the dissolved oxides decreases sharply and pure ThO2 is not dissoluble at all. By extrapolation from the dissolution behavior of pure ThO2 with small crystallite size, speculations about the possible dissolution mechanism have been made. (orig.)