In Situ Experimental Approach of Speciation in Molten Fluorides: A Combination of NMR, EXAFS, and Molecular Dynamics

Examination of the short-range structure of molten salts: ThF4, UF4, and related alkali actinide fluoride systems

“…In agreement with previous studies, 9,10,16,27,35,37 the distribution ranges between 6 and 10. From the works related to LiF-ThF 4 and LiF-UF 4 melts, 8,9,16,23,27,31,35 it is clear that the dominant contributions are from CN = 7, 8, and 9 at all temperatures and compositions. Guo et al, 27 who performed MD calculations of dilute solutions of ThF 4 in AF (A = Li, Na, K) systems, observed a bimodal distribution with virtually only [ThF 7 ] 3À and [ThF 8 ] 4À complexes present, and very small concentrations of [ThF 9 ] 5À (mean values at 1373 K indicated in cyan in Fig.…”

“…In the KF-ThF 4 system it was found previously, 37 also with a combined EXAFS-MD approach, that the dominant complexes at X(ThF 4 ) = 0.25, 1073 K, were [ThF 6 ] 2À (14.5%), [ThF 7 ] 3À (68.7%), and [ThF 7 ] 3À (16.8%), for an expected CN of 7.03. 37 In a later work 10 the authors reported a mean coordination of 7.5 in the KF-ThF 4 system at X(ThF 4 ) = 0.45, T = 1173 K, and slightly higher at T = 1073 K. These values are in excellent agreement with the data collected in this work, i.e. E[CN] = 7.12 at X(ThF 4 ) = 0.167, T = 1060 K, E[CN] = 6.99 at X(ThF 4 ) = 0.33, T = 1123 K, and E[CN] = 7.53 at X(ThF 4 ) = 0.50, T = 1209 K (Table 6).…”

“…5,6 Understanding the speciation and structure of the molten salts, which are closely related to their thermodynamic and transport properties, is crucial for the design and operation of any intended application. Salt systems which have been subject of this type of analysis in the literature include AF-ZrF 4 (A = Li, Na, K), 7 KF-ThF 4 , 8 LiF-ThF 4 9 and LiF-ThF 4 -UF 4 systems. 10 In these studies, the experimental technique used is in situ high temperature Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy.…”

“…Another approach, known as cumulant expansion, is also inadequate because of a strong correlation between the fitting parameters. 8,12,13 Hence, this work relied on configurations generated from Molecular Dynamics (MD) simulations as the underlying structural models. MD has demonstrated already in numerous studies to be a strikingly valuable tool in its own right for calculating the thermo-physical and thermo-chemical properties of molten salts.…”

Chemical Interactions between Thorium(IV) and O2- in the FLiBe and FLiNaK Molten Salt Systems

“…Besides, they also found that the complex ThOF 2 can be converted to ThO 2 and ThO in molten fluorides. Bessada et al [30] confirmed the existence of ThOF 2 compound in fluoride media containing oxide through nuclear magnetic resonance (NMR) measurements. However, these studies mainly focused on the formation of Th(IV) oxide/oxyfluoride species rather than their speciation processes.…”