Characterization of the Corrosion Layer onto Stoichiometric Uranium Oxide Created by Liquid Sodium toward a Kinetic Modeling

We studied two scenarios of the interaction between uranium dioxide and liquid sodium in conditions similar to in-reactor conditions for defective fuel pins in contact with sodium: scenario 1 with a constant oxygen potential and scenario 2 with a given starting amount of oxygen. The samples after interaction were characterized by X-ray diffraction, grazing incidence X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The experimental results show that the reaction between liquid sodium containing dissolved oxygen and stoichiometric UO₂ leads to the production of a corrosion layer, which develops uniformly on the surface without any dependence upon UO₂ grain orientation or grain size. Depending on the oxygen feeding, the crystallographic phases of the corrosion layer change. If trisodium uranate Na₃UO₄ is the only phase obtained in scenario 2, for the first time, it is observed that both Na₃UO₄ and NaUO₃ phases are formed within the corrosion layer in scenario 1.