Theoretical Studies of Structure and Dynamics of Molten Salts: The LiF–ThF₄ System

Abstract

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.

Polarizable force field parameterization and theoretical simulations of ThCl4–LiCl molten salts

“…The Shannon's ionic crystal radii sum of Th 4+ (0.94 Å for coordination number CN = 6) and Cl − (1.81 Å for coordination number CN = 6) is 2.75 Å, which is 0.09 Å longer than that in the melts. This kind of distance changes has also been observed in other MSs …”

“…With the increase of concentration, there appears a pronounced Th 4+ Th 4+ peak at 4.3 Å for x ThCl4 = 0.67. Interestingly, the distance of Th 4+ Th 4+ in ThCl 4 LiCl MS is exactly the same with the value in ThF 4 LiF MS at the concentration of thorium x = 0.67 . It is noteworthy that the value is smaller than twice the Th 4+ Cl − bond distance, indicating the formation of network with nonlinear bridging chlorides.…”

“…The detailed particle numbers at different molar percentages of ThCl 4 are given in Table , which are identical to the numbers of ThF 4 LiF system in Ref. . The systems were first equilibrated in a cube for 500 ps in the isothermal‐isobaric ( NPT ) ensemble at a target pressure of 1 atm.…”

“…From the simulated results, the interplay between the microscopic structures and the dynamical properties is elucidated. Previously, Salanne and we separately studied the thermodynamic and transport properties of the ThF 4 LiF MS . The current study allows us to investigate the anion effects in the thorium‐based MS.…”

“…Therefore, knowledge and understanding the physical properties of relevant molten salt (MS) mixtures at a fundamental level are required. Up to now, there have been numerous studies involving trivalent metal halides, however, structural studies of similar systems involving tetravalent systems are relatively limited . Particularly, considering the chemical similarities between Th and U/Pu, investigation of the structure/property of thorium‐based MS will be helpful to understand the behaviors of other actinides.…”