Leonid Burakovsky ^* Dean L Preston Andrew A Green

• Los Alamos National Laboratory (DOE), Los Alamos, New Mexico, United States

Mixed oxides of uranium and plutonium (MOX) are currently considered as reference fuel for the new generation of fast breeder reactors such as ASTRID. The key factor determining the performance and safety of fuel such as MOX is its operational limits in the application environment which are closely related to material's structure and thermodynamic stability. They are in turn closely related to the ambient (zero pressure) melting point (T m ); thus, T m is an important engineering parameter. However, the current knowledge of T m of MOX is limited and controversial as several studies available in the literature do not converge on the unique behavior of T m as a function of x. In this study we present a theoretical model for the melting curve (liquidus) of a mixture, and apply it to MOX being considered as a mixture of pure UO 2 and PuO 2 . The model uses the known melting curves of pure constituents as an input and predicts the melting curve of their mixture. It has only one free parameter which must be determined independently. In the case of MOX, T m of MOX as a function of x as given by the model has a local minimum at x ≈ 0.64, in small disagreement with our previous ab initio molecular dynamics studies which place this minimum at x = 0.7.