The UO2–ZrO2 system at high temperature (T > 2000 K): importance of the meta-stable phases under severe accident conditions

In the framework of the severe accident R&D studies led by CEA, a better knowledge of ‘corium’, mainly a UO2–ZrO2 mixture, which would result from the melting of a nuclear reactor core, is fundamental to mastering this kind of hypothetical accident. Among the available corium physical characteristics, the knowledge of the final crystalline compounds and their quantities gives basic information about the physico-chemistry mechanisms and the solidification path (thermodynamic calculation). Meta-stable phases in the UO2–ZrO2 system (tetragonal solid solution), which appear during transient phases of severe accidents, have been observed in a recent real material experiment. A specific XRD (X-ray diffraction) approach has been developed for corium analysis coupling identification of the solid solutions and Rietveld method for quantitative data. Similar transient phases had been characterized on samples coming from TMI-2 nuclear reactor corium samples.