MgCl ₂ -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, MgCl 2 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 MgCl 2 -mediated oxygen solubility mechanism. This was achieved through high-temperature 17 O nuclear magnetic resonance spectroscopy on NaCl–MgCl 2 –LaCl 3 melts with various amounts of dissolved 17 O-enriched LaOCl, corroborated by classical molecular dynamics with the polarizable ion model and first-principles NMR calculations. Experiment and simulation both show that adding MgCl 2 promotes the sharing of O 2– coordination between Mg 2+ and La 3+ 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.