Thermodynamic study of U-Fe-O ternary system

This work presents the results of a three-stage study performed on the U-Fe-O system. During the first stage, the isothermal section at 1300 K was studied. The phase equilibrium between the uranium and iron oxides at 1300 K was first determined. Mixtures of uranium and iron oxides, placed in vacuum-sealed quartz tubes are annealed one week at 1300 K and quenched in water. The X-ray Diffraction, Scanning Electron Microscopy and microprobe analysis provided essential results. Analysis of the nature of the equilibrium phases revealed the existence, at that temperature, of the ternary compound UFeO4. Moreover, microprobe analysis of the samples has shown low solubility of the iron in the uranium oxides and negligible solubility of the uranium in iron oxides. A thermal gravimetric study was secondly conducted to determine the oxygen potentials of the three-phase equilibrium based on the UFeO4 ternary compound. Thus, it was established that at 1322.5 K+/-3, U3O8-Fe2O3-UFeO4 are in equilibrium for a pressure of 4.10-4 +/-0.3 bar. The oxygen potential of this three-phase equilibrium was measured over a temperature range between 1280 and 1325 K. This thermodynamic data, combined with the phase diagram data obtained from the study of the isothermal section will enable us to determine the projection of the U-Fe, U-O and Fe-O binary systems in the ternary system on experimental data. Finally, the ternary oxide UFeO4 was characterised. The enthalpy of formation by dissolution in lead metaborate at 988 K was measured and is -25.2+/0.6 kJ/mol. The references used are the base compounds used for the synthesis of the ternary compound: Fe3O4 and U3O8, The specific heat of UFeO4 was also determined, from 523 to 1050 K by dropping calorimetry. (author)