A model for the solidus and liquidus of a ternary mixture: ThO ₂ -UO ₂ -PuO ₂ mixed-oxide (MOX) system as an example

Self-consistent thermodynamic description of a mixture (a compound, an alloy, etc.) including both its equation of state (EOS) and solidus and liquidus boundaries is a challenging problem. We review several standard approaches with the associated mixing rules and discuss their shortcomings. We propose a new thermodynamics-based analytic model for the EOS of a N-component mixture, N≥2. The model uses N−1 free parameters the values of which can be easily determined from the available experimental data. The calculation of the liquidus surface of a N-component mixture, which the model also suggests, requires the knowledge of the melting curves of each of the constituents of the mixture, but no other information such as their EOSs is really needed. We apply the new model to a ternary mixture in general, and to the ThO2-UO2-PuO2 mixed-oxide (MOX) system as an example in particular.