Study of UO2F2 reduction. Influence of temperature, steam, hydrogen and hydrogen fluoride

The PWR nuclear reactor core contains fuel pellets assemblies containing uranium dioxide. The conversion of the enrichment product, uranium hexafluoride, in UO2 is carried out by the 'dry way' process. The uranyl difluoride, UO2F2, intermediate agent of this technique is reduced in UO2 in the industrial process with turning furnace at 700 C, in presence of steam and dihydrogen. The influence of these factors, still badly known, is determining for the chemical quality and the physico-chemical properties of fuel. We have then studied the role of these factors on the reduction of UO2F2 by thermal gravimetric analysis and X-ray diffraction. The influence of hydrogen fluoride has been specified; this one is produced in a non negligible quantity in the furnace and is capable of modifying the equilibria. When UO2F2 annealings are carried out at different temperatures on steam and in conditions near those of the thermodynamic equilibria, the reaction mechanism includes three steps: 1)hydrolysis of UO2F2 in UO3 2)thermal dissociation of UO3 in UO2.9 and 3)thermal decomposition of UO2.9 in U3O8. In this case, the increase of the steam partial pressure favours the formation of crystallized phases and amorphous UO3 crystallize in its β allotropic phase. Under steam, in presence or not of HF and in conditions near those of the industrial process, the reactional process is carried out in two steps: hydrolysis of UO2F2 in UO2.9 and thermal decomposition of UO2.9 in U3O8. Under humid H2, it is the chemical reduction of UO2.9 in U3O8 that is observed. The reaction is delayed under HF compared with those carried out under steam whereas it is strongly accelerated under H2. A kinetic study of the UO2F2 pyro-hydrolysis in U3O8 and the UO2F2 reduction in UO2 under dihydrogen have allowed to establish the existence of a diffusion kinetic step in ashes. (O.M.)