A thermodynamic study of fluorite/bixbyite equilibria in the UO2–PuO1.5–PuO2 system

• Five-endmember (UO 2 , PuO 2 , PuO 1.5 , Pu 0.25 Pu 0.75 O 1.625 and U 0.25 Pu 0.75 O 1.625 ) thermodynamic models of the fluorite and bixbyite solid solutions are developed and applied to simulate phase equilibria in the UO 2 –PuO 1.5 –PuO 2 system at different Pu/(Pu+U) ratios. • The cubic lattice parameters of the fluorite and bixbyite phases are modelled with an ion-packing approach, consistently with the thermodynamically constrained fractions of the five endmembers. • The cubic lattice parameters are plotted as functions of the temperature or O/M ratio, allowing a direct fitting of the model to relevant experimental data. • The results of the modelling suggest that Pu/(Pu+U) ratios in fluorite, bixbyite, and in the rhombohedral phase remain the same at temperatures below ∼1000 K, i.e., no inter-phase U/Pu- partitioning occurs despite complex exsolution phenomena within fluorite and bixbyite. Thermodynamic properties of U 1− z Pu z O 2+ δ fluorite (FCC) and U 1− z Pu z O 2+ δ bixbyite (BCC) in the UO 2 −PuO 1.5 −PuO 2 system are assessed by considering phase equilibrium constraints and data on the variation of oxygen to metal ratio ( O/M ) as a function of the chemical potential of O 2 . Thermodynamically, both BCC and FCC are described as ordered solid solutions allowing for a decrease in the configurational entropy of their oxygen/vacancy distributions at the specific values of δ = –0.5 and δ = –0.375 ( O/M = 1.5 and O/M = 1.625). With this approach, fluorite/bixbyite equilibria in PuO 1.5 –PuO 2 and in UO 2 –PuO 1.5 –PuO 2 are reproduced well with little effort. Moreover, we show that a large manifold of experimental data on the UO 2 –PuO 1.5 –PuO 2 system is consistent with the assumption that Pu/(Pu+U) ratios in individual phases remain equal to the total Pu/(Pu+U) ratio in the system, i.e., phase equilibration does not involve inter-phase U/Pu-partitioning.