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 quasichemical 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.