Theoretical studies of structure and dynamics of molten salts: the LiF-ThF4 system

LiF-ThF4 molten salt (MS) is the fuel for advanced MS reactors. Knowledge of the microscopic MS structure and dynamics is required for an understanding of the macroscopic physical and chemical properties of the MS phases. We have performed molecular dynamics simulations on LiF-ThF4 MS at different molar percentages (LiF/ThF4 = 20.0 to 0.5) and temperatures (1100 to 1400 K). Experimental deductions and recent theoretical results on the coordination structures and transport properties of the MS are well reproduced. The density of states of the [ThF8](4-) species and the character of the Th-F bonding are investigated. The interplay between the microscopic structures and the dynamical properties is elucidated. Corresponding to the smaller effective radius of Zr, the activation barrier of the M(4+)-F(-) dissociation and the lifetime of the first coordination shell of M(4+) are both smaller for M = Th than for M = Zr in the respective LiF-MF4 systems. The shorter Zr-F bond is stronger than the longer Th-F bond, while the coordination number of the predominant [ZrF7](3-) species is smaller than that of the dominant [ThF8](4-) species. An approximate formula is proposed for the lifetime of F(-) ions in the first solvation shell of molten MFn (M = Y, Zr, Th) in terms of the radial distribution function.