Thermochemical modelling of the oxygen potential of uranium oxide fuel pellets under irradiation

The elemental and chemical composition of nuclear fuel pellets are key factors influencing the material properties of the pellets, along with physical properties and irradiation history. These are connected through the chemical behavior of the various fission products present in irradiated fuel. In nuclear fuel performance codes, the chemical composition of nuclear fuel is almost always taken into account only through parameterized correlations of various material properties, most of which are affected by the chemical composition. The oxidation state of the fuel is one of the most important chemical properties influencing the material properties of the fuel, and it can only be determined with the knowledge of the chemical composition. The oxidation state of a complex mixture of substances such as nuclear fuel can be represented by the oxygen potential. The oxygen potential is a measure of the tendency of a chemical substance to oxidize adjacent materials, and it is related to the oxygen partial pressure over the substance at chemical equilibrium. The oxygen potential of fuel can be calculated with the principles of Gibbs energy minimization, provided the elemental composition, temperature, pressure and suitable thermochemical data are available. The computing power necessary to apply Gibbs energy minimization to such a chemically complex system as nuclear fuel has been unavailable until recently. In this work, the elemental composition of the fuel in a PWR rod is obtained from a burnup calculation and the temperature and pressure on the fuel surface calculated with a fuel performance code. These are combined with a Gibbs energy minimization routine developed at VTT and thermochemical data from the open literature to calculate the oxygen potential of the fuel surface over the lifetime of the fuel rod.