Calculation of the thermodynamic functions of urania-gadolinia fuel

Recently, increasing interest has been shown in the application of mixed urania-gadolinia nuclear fuel in reactors of the PWR type. It has been found that the addition of gadolinia, a burnable poison for neutrons, to urania permits one to increase the charge of fuel in a nuclear reactor, to level off the power produced along the cross section of the fissile core, and to increase the safety of the reactor. The addition of gadolinia changes many properties of the nuclear fuel, in particular, its thermodynamic properties, which are of great importance in predicting the behavior of the fuel during burning. Existing experimental results on the thermodynamic features of mixed urania-gadolinia fuels correspond to relatively narrow ranges of temperature and Gd2O3 content. Therefore, it is of particular importance to develop a thermodynamic model that will permit one to calculate various thermodynamic parameters of the urania-gadolinia system over wide temperature and composition ranges. Thermodynamic models, based on statistical and chemical thermodynamic descriptions of the defect structure of solids, have been developed to date for UO2±x and (U,Pu)O2±x fuels. There is no information on thermodynamic models for urania-gadolinia mixed oxides