Thermodynamic Assessment of CrCl₂ with NaCl-KCl-MgCl₂-UCl₃-UCl₄ for Molten Chloride Reactor Corrosion Modeling

The formation of corrosion species in molten salt reactor systems is driven by the salt redox condition, which can be indicated by the uranium oxidation state ratio (U⁴⁺/U³⁺). In chloride salts, chromium is well known to have the highest tendency to deplete from alloy surfaces; however, no available thermodynamic database suitably represents the effect of trivalent and tetravalent uranium chloride on the corrosion potential. In this work, we extend the Molten Salt Thermal Properties Database-Thermochemical with Gibbs energy models suitable for application to alloy corrosion in chloride molten salt-fueled nuclear reactors. The work required the development of fully constrained thermodynamic models, utilizing the modified quasi-chemical model in the quadruplet approximation for the melt and a single sublattice model for the solid solution phases allowing accurate estimations of thermodynamic properties even in systems for which few thermodynamic data are available. The effort included differential scanning calorimetry (DSC) measurements to find the previously unreported phase equilibria of the CrCl₂-UCl₃ system and to confirm the equilibria of the MgCl₂-UCl₃ system. Elemental period correlations allowed the estimation of thermodynamic values for compounds in the NaCl-CrCl₂ and KCl-CrCl₂ systems allowing a well-informed description of CrCl₂ behavior in the compositions of technical interest for the Na-K-Mg-U³⁺-U⁴⁺ chloride salt.