Melting behaviour of mixed plutonium and iron oxides: An experimental study

The present work describes original experimental results obtained on the melting and solidification behaviour of mixed PuO₂ – Fe₃O₄ samples. The samples were laser-heated into the liquid phase in a pressurised autoclave under a controlled atmosphere of argon or air, whereby the sample temperature was measured in-situ with the help of fast pyrometers. In addition, the oxygen release in the argon experiments was measured by an O₂ probe. By these means, solidus and liquidus temperatures in the Fe₃O₄-PuO₂ system were derived as a function of the PuO₂ mole fraction at two oxygen partial pressures (pO₂ = 1e-6 and pO₂ = 0.2 bar). It was shown that the system exhibits a eutectic reaction, and that the observed liquidus and eutectic temperatures are higher for higher oxygen contents. This is due to the stabilisation of stoichiometric oxides in an oxidising atmosphere, as opposed to the appearance of oxygen-defective, lower-melting phases in an inert/reducing environment. The occurrence of the eutectic in the system was confirmed by electron microscopy analysis. It occurs at a composition between 22 mol % and 23 mol % of PuO₂ in the Fe₃O₄-PuO₂ pseudo-binary system. Outside the Fe₃O₄ – PuO₂ plane, the eutectic temperature was observed to depend upon the oxygen potential, varying from approximately 1800 K in air to approximately 1700 K in inert (Ar) gas. The experimental solidus/liquidus data were used to derive a thermodynamic model on the system. Thermodynamic calculations supported the oxygen release of the samples in both argon and air and the interpretation of the quenched microstructure of the samples.