Thermodynamic assessment of the Na-O and Na-U-O systems: Margin to the safe operation of SFRs

A thermodynamic model for the Na-O system was developed for the first time using the CALPHAD method after review of the structural, thermodynamic, and phase diagram data available on this system. Differential Scanning Calorimetry measurements were moreover performed to assess the phase equilibria and liquidus temperature in the Na₂O-Na₂O₂ composition range. A CALPHAD model for the Na-U-O system was furthermore developed on the basis of both reviewed experimental data, and thermodynamic functions of the sodium uranates derived by combining ab initio calculations and a quasi-harmonic statistical model. The phase equilibria in this ternary system are particularly relevant for the safety assessment of the nuclear fuel-sodium coolant interaction in Sodium-cooled Fast reactors (SFRs). The model predicts the stability of the ternary phase field UO₂-Na₃UO₄-Na₄UO₅, which is consistent with the most recent literature data. Further optimization was moreover performed to fit the sodium partial pressures measured experimentally in the NaUO₃-Na₂U₂O₇-UO₂ and NaUO₃-Na₂UO₄-Na₂U₂O₇ phase fields, yielding an overall consistent description. Finally, the oxygen content required to form pentavalent Na₃UO₄ and hexavalent Na₄UO₅ in liquid sodium at 900 K were calculated to be 0.7 and 1.5 wppm, respectively, which are levels typically encountered in SFRs.