Calculation of oxygen potential change of irradiated UO₂ and UO₂-PuO₂ mixed oxide fuels using the intra-cation complex model

The oxygen potential change of irradiated uranium dioxide and urania-plutonia mixed oxide fuels, which is caused by the formation of solid solutions of some fission product metals with the fuel oxides, was calculated using the intra-cation complex model. It assumes weak-bound complexes between the cations of relatively positive and negative charges with respect to U⁴⁺. Partial molar entropy of oxygen, Δ S ̄_O2, was derived by differentiating the logarithm of the number of ways of arranging the cationic defects and complexes over the cation sublattices. The fission product metals which dissolve into the fuel matrix were classified into +2, +3 and +4 valence state ion groups, and their effect on Δ G ̄_O2 was clarified in the equations of Δ G ̄_O2 representation. Comparison of the calculated results with the Δ G ̄_O2 values for irradiated fuels as well as for the fuels containing some fission product simulant cations revealed that agreement was good, showing that the Δ G ̄_O2 change on irradiation can be expressed as a function of M³⁺ fraction.