Phase diagram calculations of the UPuN system with carbon and oxygen impurities

Abstract

The most common method for the preparation of mixed nitride fuels is the carbothermic reduction of a UO 2 + PuO 2 + C mixture in a nitrogen atmosphere. A mixed nitride fuel thus formed has carbon and oxygen impurities which are kept well below 5000 ppm. For a given overall composition of the nitride fuel and the temperature, the present work calculates the stable phases in equilibrium and the amounts of those phases. These calculations are based on the principle of mass balance and minimization of the Gibbs free energy for the system. The Gibbs free energy of formation of the binary compounds UN, PuN, UO, PUO, UC, PUC, UO 2, PuO 2 and UN 1.5 are used for the calculations. In general, the binary compounds are assumed to form ideal solid solutions, but in certain cases, available or estimated interaction parameters were used to see the effect of deviation from the ideal solution assumption on the phase diagram. Partial phase diagrams of (U, Pu)CN were drawn for different oxygen impurity concentrations at various temperatures. Comparison with the experimental data available in the literature is carried out. Based on these calculations it is suggested that a separate oxide phase has to be left deliberately in the mixed nitride fuel matrix during preparation to ensure fuel clad chemical compatibility.