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Local structure and dissolution mechanism of ThF4 in FLiBe molten salts
Jianchao Sun, Weijie Hua, Junheng Yingsu, Miao Shena Xiaobin Fu*, Yuan Qian*
https://doi.org/10.1016/j.cjsc.2026.101046
Molten salts; ThF4 dissolution; Ionic coordination; High-temperature NMR
ABSTRACT
TMSR represents a promising pathway for sustainable nuclear energy, where ThF4 serves as a fuel dissolved in fluoride carrier salts such as FLiBe. However, the solubility and structural behavior of ThF4 in FLiBe remain poorly understood. This study integrates HT-NMR, ss-NMR, and AIMD simulations to systematically investigate the coordination structure and dissolution mechanisms of ThF4 in FLiBe melts. The results show that at low ThF4 concentration (1.2 mol%), Th4+ predominantly exists as isolated coordinated ThF84- complexes with an average Th-F coordination number of approximately 8.0. As the ThF4 concentration increases, the average Th-Th coordination number rises from nearly 0 to approximately 2.0, indicating the progressive formation of fluoride-bridged Th-F-Th oligomers. An independent 19F resonance at +43 ppm is directly observed in the molten state at 600 °C, providing experimental evidence for poorly soluble Th-F-Th oligomeric species that do not participate in rapid ionic exchange. Structural analysis indicates that the formation of these oligomers originates from the depletion of free F- ions and discrete BeF42- units, while the weakened spatial separation effect of Li+ ions further promotes Th-F-Th bridge formation. Concentration-dependent ss-NMR measurements reveal that the characteristic signals of these bridged species emerge above approximately 5.5 mol% ThF4, accompanied by new resonances at -3, 20, and 50 ppm. These oligomeric species exhibit poor solubility, which limits the dissolution of ThF4 in FLiBe. These findings elucidate a molecular-scale mechanism underlying the limited solubility of ThF4 in FLiBe, which offers a valuable method for investigating the structural behavior of other actinide fuels and fission products in molten salts.
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