Studies in the PuO2 – ZrO2 Pseudo-binary Phase Diagram

Abstract

The phase relationship between ZrO2 and PuO2 was examined in the ZrO2-rich region by high temperature X-ray diffractometry. At 1273 K, for the studied range of compositions, the samples exhibited two stable phases: monoclinic and cubic. At higher temperatures, several configurations were observed depending on the temperature and the composition. It was concluded that there should be a eutectoid line at 1463 K and that the composition of the eutectoid point was in between 2.3 and 3.1 mol% PuO2.Then, the pseudo-binary phase diagram was calculated by minimisation of Gibbs energies, on the whole range of concentrations and up to 3200 K. A good fitting of experimental data was obtained. The resulting diagram shows three solid solution domains. A large fee domain allows complete solubility at high temperature. On the ZrO2 rich side, small amounts of PuO2 can be in solution either under monoclinic or tetragonal form depending on the temperature. These solid solution domains are separated by two-phase ones.This experimental and computational study provides a re-evaluation of the pseudo-binary diagram PuO2 – ZrO2. It gives a clearer view of the previously uncertain domains and it provides a good starting point for studying more complex phase relationships both in the frame of rock-like fuels and minor actinide transmutation.