On the melting behaviour of uranium/plutonium mixed dioxides with high-Pu content: A laser heating study

The melting behaviour of mixed uranium–plutonium dioxides (MOX) has been investigated by laser heating under controlled atmosphere in the PuO2-rich composition range (with amount-of-substance fraction x(PuO2) ⩾75%). The observed solidus/liquidus points are in agreement with the newly measured melting point of pure plutonium dioxide (3017 K). They suggest the existence of a minimum freezing temperature at a composition x(PuO2) between 50% and 80%, in contrast with earlier research carried out with traditional furnace heating methods. The current results have been obtained under optimised experimental conditions aimed at maintaining the integrity and original composition of the samples throughout the laser heating cycles. With this goal in mind, experiments have been carried out under controlled gas pressure (pressurised air or argon) and short time duration (<0.25 s). A critical discussion of the present results highlights the fact that the formation of the gas phase has to be taken into account in the study of the high-Pu MOX behaviour at high temperature. The experimental results obtained thus correspond to slightly hypo-stoichiometric (U, Pu)O2−x compositions in equilibrium with the gas phase.