Investigation of ionic local structure in molten salt fast reactor LiF-ThF4-UF4 fuel by EXAFS experiments and molecular dynamics simulations

Abstract

This article focuses on the speciation of molten fluoride mixtures based on ThF 4 and UF 4 actinides used in molten salt reactors. The local structure of molten AF-MF 4 systems (A=Li + , Na + , K + ; M= Th 4+ , U 4+ ) was studied in situ by combining measurements by high temperature X-ray absorption spectroscopy and molecular dynamics simulations. In the molten state, 20°C higher than the melting temperature, these mixtures are composed of free fluorine and anionic species [MF 7 ] 3-, [MF 8 ] 4and [MF 9 ] 5whose distribution varies with the amount of MF 4 (M= Th 4+ , U 4+ ). Regardless of the cation, these complexes consist of an average of 8 Fneighbors for MF 4 content is less than 35 mol. %. This value decreases to 7 as the size of the alkaline ion A (A=Li + , Na + , K + ) increases. The [MF x ] 4-x species are linked together by bridging fluorine ions to form long chains [M x F Y ] 4x-y . The addition of a few percent UF 4 to the eutectic LiF-ThF 4 composition (77.5 mol% -22.5 mol. %) at 700°C leads to a slight increase in the population of free fluorine ions due to the breakdown of the Th-F links to form complexes [UF x ] 4-x isolated or connected to the (Th x F y ) 4x-y chains. This change in the structure of the liquid results in a slight decrease in viscosity when a few mol. % of UF 4 is added.

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

“…13. 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.…”

“…The spread between R max,Th-F = 2.26 Å and R cutoff,Th-F = 3.17 Å is 0.91 Å, hence more than 6 times greater than in the Bessada et al 10 calculated average bond lengths in the LiF-ThF 4 melt as a function of coordination number, and the former are seen to consistently decrease as the latter increases. This can be explained mainly by an increased repulsion between the F À ligands with increasing coordination.…”

“…The composition LiF : ThF 4 = (0.75 : 0.25) (Fig. 6), which can be singled out because of its importance as Molten Salt Fast Reactor (MSFR) fuel, is compared with data by Bessada et al 10 Although recorded at different temperatures, the spectra are comparable to each other. The experimental data for all compositions can be found in the ESI.…”

“…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. This element-specific, non-destructive, oxidation state-sensitive technique requires very small masses of sample, making it very powerful and ideally suited for radioactive, corrosive salts at high temperature.…”

Probing the ionic structure of FLiNaK–ZrF4 salt mixtures by solid-state NMR

“…The formation of different [Zr x F y ] 4xÀy ionic structures and crystal phases during the solidication of the salt melts could be discussed. Aer heating into melts, Zr 4+ ions tended to form a coordinated structure with F À ions as [ZrF 7 ] 3À or [ZrF 6 ] 2À , which is similar to the results that [AF 7 ] 3À , [AF 8 ] 4À and [AF 9 ] 5Àcoexisted in LiF-AF 4 (A ¼ Th 4+ or U 4+ ) molten salt 25,26. When cooling down, K 3 ZrF 7 crystal phase would be formed rst as impacted by the weak bonding energy of K + and F À ions.…”