MgCl2-Mediated Mechanisms of Oxygen Solubility in Molten Chlorides

In molten salt reactors, oxygen impurities accelerate structural material corrosion. Moreover, the ingress of oxide species may promote the formation of insoluble actinide oxychlorides or oxides, potentially raising safety concerns. To limit the undesired precipitation of such phases, MgCl2 has been proposed as a complementary fuel component to sequester oxygen by enhancing the solubility of actinide oxide species. In this study, we provide direct, molecular-level evidence of an MgCl2-mediated oxygen solubility mechanism. This was achieved through high-temperature 17O nuclear magnetic resonance spectroscopy on NaCl-MgCl2-LaCl3 melts with various amounts of dissolved 17O-enriched LaOCl, corroborated by classical molecular dynamics with the polarizable ion model and first-principles NMR calculations. Experiment and simulation both show that adding MgCl2 promotes the sharing of O2- coordination between Mg2+ and La3+ ions, disrupting La-O clustering that facilitates oxide precipitation. Altogether, these results establish a mechanistic framework for tuning oxygen speciation and solubility through compositional design in high-temperature molten salt systems.