Oxygen Interactions Between Sodium and Uranium-Plutonium Oxide Fuel

The oxygen concentration of sodium in equilibrium with uranium-25% plutonium oxide fuel has been measured at temperatures of 650 to 900°C for fuels with oxygen-to-metal ratios of 1.90, 1.94, and 1.97. The oxygen concentration in sodium at the three-phase Na-MO_2-x-Na₃MO₄ (M = uranium plus plutonium) equilibrium has also been measured for the same temperature range. This three-phase equilibrium was established by reacting mixed-oxide fuel with sodium to which Na₂O had been added. The oxygen concentrations in sodium were determined by the vanadium-wire equilibration method. The oxygen concentration in sodium at the three-phase equilibrium varied from ∼0.1 to 0.4 ppm in the temperature range investigated. Oxygen concentrations in EBR-II primary sodium, which have been measured by the same method, vary from 0.4 to 0.9 ppm oxygen depending on the cold-trap operation. These values indicate that the sodium -fuel reaction product, i.e., Na₃MO₄, is stable in sodium at the temperatures (<1000°C) and oxygen levels present in EBR -II.