[en] The calculations of the phase diagram and thermodynamic parameters of mixed carbide and nitride fuels were carried out on the basis of free energy of formation of the various binary compounds. All the solutions were assumed to behave ideally. For the given temperature and composition, the program calculates the stable phases field among the assumed phase fields. The program also indicated whether the assumed phase fields are stable for a particular composition. The calculations are carried out using Newton-Raphson iteration technique, based on the mass balance equations and independent chemical equilibria. In literature only a schematic phase diagram is available. This report presents the calculated phase diagrams at different temperatures for different oxygen impurity levels and plutonium contents. Based on these calculations it is suggested that for the nitride fuel it will be advantageous to have a fuel in MCNO+MO₂ phase field, as in this phase field does not have (i)high nitrogen pressure due to sesquinitride phase or (ii) the compatibility problems due to the presence of free carbon/free metal phase. (author). 34 refs., 14 figs., 9 tabs