Kinetic study on dissolution of uranium oxides in a nitric acid solution

The dissolution of uranium dioxide, triuranium octaoxide and uranium trioxide was studied in nitric acid solutions of various concentrations at various temperatures. The dissolution rate, in general, increased in magnitude by the sequence of UO2 < U3O8 < UO3. Uranium dioxide obeyed a linear law in nitric acid of low concentration, while it obeyed a parabolic law at higher concentrations. If the dissolution occurred at temperatures higher than 350C or in a solution of higher concentration than 3N, triuranium octaoxide invariably obeyed a simple parabolic law. However, at low temperatures or at low concentrations, the dissolution proceeded through two distinct stages, which were separated by an intermediate induction period up to several hours. The first stage, in which the rate controlling step seemed to be the diffusion of a proton through the surface layer formed during the dissolution, obeyed a parabolic law. In contrast, the second stage obeyed a linear law which was interpreted by assuming a electrochemical mechanism. Finally, the dissolution of uranium trioxide obeyed a linear law, and in this case an interpretation was given on the basis of a different electrochemical mechanism. (auth.)