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 "pyrohydrolysis pro cess" is currently a preferred route for the preparation of UO2 from enrich ed uranyl hexafluoride UF6. In this technology, the reaction is carried out under flow of dihydrogen (for its reducing function) and in the presence o f water. Between the gaseous fluoride UF6 and the ceramic UO2, uranyl diflu oride, UO2F2, is the solid intermediate, that contains the oxygen, but in w hich uranium is not yet reduced. The study of the interactions between this intermediate and wafer is of aca demic 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 hygrome try, temperature or pressure and gravimetric analysis. The samples have been conditioned in the form of thin films by crystallizat ion of a saturated solution. Weight loss from this saturated solution shows stabilization at ca. UO2F2 . 1.6 H2O. An increase of the hygrometry up to 90 % causes a fast increase of the weight up to the composition UO2F2 . 4 H 2O. The mathematical deconvolution of the hydration curve indicates the sup erposition of three arctangent functions that show the coexistence of the f ollowing hydrate phases : UO2F2 . (1,6-2,5) H2O, UO2F2 . 3,5 H2O, UO2F2 . 4 H2O. The hydration reactions occur simultaneously as confirmed by radiocry stallographic analysis of a sample of the same nature. If the hygrometry is increased up to 100 %, the uranyl difluoride tetrahydrate accepts one addi tional mole of water before becoming deliquescent. The regular shifting of the angular positions of the diffraction peaks at the passage of UO2F2 . 4 H2O to UO2F2. 5 H2O indicates the existence of a solid solution between thi s two limiting compositions. The heating of a sample of uranyl difluoride tetrahydrate enables us to obs erve the succession of the less hydrated phases described above, then the a ppearance of anhydrous UO2F2. Curiously, under these conditions, the stable phase beta (the composition of which can vary between UO2F2 . 1.2 H2O and UO2F2 . 1.55 H2O) is not observed and the thermal dehydration from upper hy drates down to the anhydrous compound occurs through a new phase termed del ta. In this phase, the structure of the "pillars" formed by wafer molecules separating the layers of UO2F2 is particularly stable. The departure of th e water corresponds to a reduction of the number of pillars but does not af fect the internal arrangement of the pillar. A strictly reversible phenomen on is observed during controlled hydration of anhydrous uranyl difluoride. The absorption and desorption of water appear as absolutely symmetrical rev ersible phenomena. This argues in favour of the strongly lamellar character of the hydrates of uranyl difluoride like anhydrous UO2F2, the structure o f which is layered.