Local structure and early-stage aggregation behavior of Mo in LiF-BeF2-UF4-ZrF4 molten salts

Redox-dependent speciation and transport of molybdenum (Mo) in actinide-bearing fluoride melts strongly influence fission-product behavior in molten-salt reactors, yet their atomistic origins remain unclear. Here, on-the-fly machine-learning molecular dynamics simulations were performed to investigate the local structure and early-stage aggregation behavior of Mo in LiF-BeF 2 -UF 4 -ZrF 4 molten salt at 923 K under systematically varied fluorine stoichiometries. For isolated Mo, increasing fluorination shortens the Mo-F distance, increases both the coordination number and the positive effective Bader charge of Mo, and significantly suppresses diffusivity. In contrast, U- and especially Zr-centered fluoride coordination environments remain relatively stable and mainly act as the multicomponent host matrix. In two-Mo systems, direct Mo-Mo bonding persists under fluorine-deficient and moderately fluorinated conditions, but disappears at high fluorination; the most compact dimer is observed under the neutral condition. In four-Mo systems, aggregation evolves from a compact U/Zr-associated cluster to a Mo-dominated compact cluster, partial pairing, and finally complete dispersion with increasing fluorination. Separate auxiliary simulations involving Nb and Te as representative noble-metal fission products were further performed to compare heteroatomic association. These results provide an atomistic basis for understanding redox-controlled fission-product speciation, early-stage aggregation, and mobility in reactor-relevant fluoride salts.