Dissolution of uranium dioxide in nitric acid media: what do we know?

Abstract

Abstract. This article draws a state of knowledge of the dissolution of uranium dioxide in nitric acid media. The chemistry of the reaction is first investigated, and two reactions appear as most suitable to describe the mechanism, leading to the formation of monoxide and dioxide nitrogen as reaction by-products, while the oxidation mechanism is shown to happen before solubilization. The solid aspect of the reaction is also investigated: manufacturing conditions have an impact on dissolution kinetics, and the non-uniform attack at the surface of the solid results in the appearing of pits and cracks. Last, the existence of an autocatalytic mechanism is questionned. The second part of this article presents a compilation of the impacts of several physico-chemical parameters on the dissolution rates. Even though these measurements have been undertaken under a broad variety of conditions, and that the rate determining step of the reaction is usually not specified, general trends are drawn from these results. Finally, it appears that several key points of knowledge still have to be clarified concerning the dissolution of uranium dioxide in nitric acid media, and that the macroscopic scale which has been used in most studies is probably not suitable.

A method for phenomenological and chemical kinetics study of autocatalytic reactive dissolution by optical microscopy. The case of uranium dioxide dissolution in nitric acid media

“…As presented earlier [14], several experimental observations seem to indicate that the mechanism of the chemical reaction between uranium dioxide and nitric acid is autocatalytic. Nevertheless, the scale at which these observations were made implies that the conclusions drawn could result from the disturbance of other important phenomena like transport phenomena or bubbling [4].…”

“…Regarding the complication of the dissolution mechanism at the microscopic scale, the previously reported data, measured at a macroscopic scale [14], seem to be questionable.…”

“…The autocatalytic component of the dissolution reaction of uranium dioxide in nitric acid media has been widely documented in the literature [14]. The ratio of the volume of dissolution solution over dissolved amount of solid in the dissolution cells is an advantageous condition for studying this component under well known dissolution products concentrations and temperature conditions.…”

“…For this purpose, the stoichiometric equation (9), identified in a former paper as the most likely taking place [14], has been retained for the balance of the reaction:…”

“…An analysis of the state of knowledge of the dissolution reaction of uranium dioxide in nitric acid media [14] shows that despite the importance of this reaction in the hydrometallurgical reprocessing of spent nuclear fuels, its chemical and physico-chemical mechanisms remain poorly understood. The relationship between the fraction of dissolved solid, which can be linked more or less simply with the bulk concentration of the chemical elements composing it, and the chemical reaction kinetics requires the accurate knowledge of the surface of the dissolving solid and the reactivity of each element of this surface over time.…”