Study of the reduction of uranium from UF6 to UO2 by pyro-hydrolysis and action of dihydrogen

The core of a PWR reactor comprises many fuel pellet assemblies mainly made of uranium dioxide. The conversion of uranium hexafluoride, enrichment product, into uranium dioxide can be performed using various processes. Today, the dry way is the main technique used. Most of these techniques involve uranyl difluoride (UO2F2) as intermediate compound. Its conversion into UO2 is performed with an industrial furnace by heating at about 700 deg. C in the presence of water vapor and of a flow of reducing gas. This chemical reaction influences in a determining way the chemical and mechanical properties of the uranium dioxide powder. However, the action of these different factors (water, temperature and reducing agent) are badly known. The first study concerns the system UO2F2-H2O and has been carried out by X-ray diffraction, thermogravimetry and controlled kinetics thermal analysis. The stable phase in the normal conditions of temperature, pressure and hygrometry has been precised. The overall results allow to confirm the influence of the partial pressure of water vapor on the thermal dehydration, and in particular, on the number of steps identified during the transformation of the stable phase into anhydrous phase. Moreover, the existence of hydrates with a water content higher than the stable phase has been established. These ones have a clear bidimensional character. The effects of temperature, pressure and aging on each of these lamellar hydrates is reported. This has permitted to identify for the first time a new solid solution, named δ, which is the necessary preliminary step to the obtention of the anhydrous compound from the upper hydrates. The metastability of all these upper hydrates has been demonstrated. Several approaches have been considered to look for the intermediate compounds susceptible to intervene in the reduction of UO2F2 into UO2: the influence of temperature under vacuum, under ambient air and atmospheric pressure, under dihydrogen flow have been studied by X-ray diffraction. A common intermediate compound to all experiments has been identified: UO2F0.8, a high-temperature allotropic variant of a U3O8 iso-type. Two other intermediate compounds have been evidenced: UO2F1.6 and UO2F1.3. Therefore, the overall reaction steps of the UO2F2 transformation into UO2 in reducing environment seems to be established. (J.S.)