Existence of a miscibility gap in uranium neodymium oxide materials used as nuclear fuels simulants

• Structural evolution in temperature of a (U0.72Nd0.28)O2-x sample was determined.
• A miscibility gap was detected for temperatures up to 745 K.
• A simple ionic model was used to estimate the tie-line corresponding to this system.

During the irradiation of oxide nuclear fuels used in pressurized water reactors, many microstructural modifications occur which affect the fuel performance. This work focuses on the thermodynamic evolution of the fuel under these conditions, with a special regard to the influence of lanthanides content. For this purpose, simulant materials have been synthesized to model irradiated fuels. They consist of (U,Nd)O2 doped ceramics, neodymium being added since it is the most abundant lanthanide fission product. A previous study carried out on these systems highlighted, for the first time, the possible existence of a miscibility gap in the U–Nd–O ternary system, with an experimental evidence of a biphasic equilibrium at room temperature, for a Nd content larger than 6 wt%. In the current work, high temperature X-ray diffraction experiments up to 1973 K on (U0.72Nd0.28)O2-x samples are reported. The results confirm the existence of a miscibility gap. In fact, the system is monophasic at elevated temperatures and a phase separation occurs below 745 K, with two face-centered cubic phases in equilibrium. A preliminary evaluation of the tie-line associated to the biphasic system at room temperature is proposed.