Existence, characterization and decomposition of uranyl difluoride hydrates

The heart of a pressurized water nuclear reactor is composed of assemblies of fuel rods of principally uranium dioxide powder. The 'pyro-hydrolysis process' is currently a preferred route for the preparation of UO2 from enriched uranyl hexafluoride UF6. In this technology, the reaction is carried out under flow of dihydrogen (for its reducing function) and in the presence of water. Between the gaseous fluoride UF6 and the ceramic UO2, uranyl difluoride, UO2F2, is the solid intermediate, that contains the oxygen, but in which uranium is not yet reduced. The study of the interactions between this intermediate and water is of academic interest but is clearly of importance to the industrial process. The aim of this work was to specify the conditions of existence of the hydrates of uranyl fluoride and the reaction mechanism leading from the hydrates to anhydrous UO2F2, precursor in the industrial reaction. The study has been performed using a combination of X-ray diffraction as a function of hygrometry, temperature or pressure and gravimetric analysis. Several hydrated phases have been characterized: two solid solutions, the compositions of which can vary respectively between UO2F2.5 H2O and UO2F2. 4 H2O and between UO2F2. 2.5 H2O and UO2F2.1.6 H2O and one hydrate of defined composition UO2F2.3.5 H2O. Thermal dehydration from the higher hydrates shows the existence of a new phase termed δ, the structure of which is similar to that of UO2F2.3.5 H2O. Hydration of the anhydrous compound under a controlled atmosphere enables us to observe this phase and confirms the strongly lamellar character of these hydrates. (authors)