Measurement of the equilibrium oxygen, carbon and uranium activities associated with the uranium oxycarbide phase

Abstract

The composition of uranium oxycarbide in equilibrium with pure liquid uranium and appropriate oxygen partial pressures (10 −25 > po 2 [ atm] > 10 −35) has been determined by chemical and X-ray analyses for the temperatures 1473 °K, 1573 °K and 1648 °K. The oxygen partial pressures were established using an isopiestic technique in conjunction with the calciumcalcium oxide equilibria. A similar investigation along the oxycarbide-uranium dioxide phase boundary involved isothermal equilibration with uranium-gold alloys to establish the relevant uranium activities and oxygen partial pressures. The experimental values were analysed by treating the uranium oxycarbide phase as a pseudo-binary solid solution of UO-UC, and it was concluded that a simple regular solution model adequately described the data. Calculations based on this model provided information about the free energy of formation of UO ( ΔG° = −121 200 + 22.46 T kcal/ mole), and about the carbon monoxide pressures associated with various oxycarbide compositions. Where it was possible to check the calculated values they were in good agreement with measured data.