Experimental results on the U-Zr-Fe-O liquid miscibility gap and its sub-systems

Alan Brunel

A. Quaini, C. Bonnet, L.Cormier.

Université Paris-Saclay, CEA, Service de la Corrosion et du Comportement des Matériaux dans leur Environnement, 91191, Gif-sur-Yvette, France and Sorbonne Université, CNRS UMR7590, MNHN, IRD, Institut de minéralogie, de physique des matériaux et de la cosmochimie (IMPMC), 4 place Jussieu, F-75005 Paris, France.


During a severe accident involving the core meltdown in a pressurised water reactor, the nuclear fuel can react with the zircalloy cladding and structural materials to form a complex mixture of solid and liquid phases called corium. This corium can present a miscibility gap involving a metallic and an oxide liquid at equilibrium. To get information on this corium and the miscibility gap, the study of the U-Zr-Fe-O system and its sub-systems is essential.
We have studied the Fe-Zr-O sub-system at around 2000°C and 2700°C respectively in the iron rich region and a zirconium richer region to observe the liquid miscibility gap. The investigated samples were obtained by arc fusion and underwent a pyrometer-assisted heat treatment in an arc furnace, followed by a quenching like cooling in Ar. The presence and the composition of the different phases were determined by SEM-EDS to confront the experimental results to thermodynamic calculations performed using the TAF-ID database. Results show a good agreement between the experiments and the calculations for the iron rich region. However, significant discrepancies concerning the composition of the liquids were observed for the zirconium richer region at 2700°C. A good agreement was also observed between the experiments and the calculations for the U-Fe-O liquid miscibility gap around 2700°C.
The same kind of study has been realised in the UO2-Zr-Fe pseudo-ternary system to observe the influence of adding iron in a U-Zr-O based liquid miscibility gap for a set of different U/Zr ratio. The metallic liquid becomes richer in iron when iron is added, resulting in the decrease of the uranium and zirconium molar fractions by dilution, while the oxide liquid composition exhibits small changes, with no iron solubility in the oxide liquid. The amount of zirconium in the metallic liquid is underestimated by the calculation for low amounts of iron but is better described for higher iron amounts. This can be explained by the need of improvement of the thermodynamic model of the Fe-Zr-O ternary system.

Event Timeslots (1)

Wednesday – 15th September 2021
Alan Brunel