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

Abstract

Abstract This article focuses on the speciation of molten fluoride mixtures based on ThF4 and UF4 actinides used in molten salt reactors. The local structure of molten AF-MF4 systems (A = Li+, Na+, K+; M = Th4+, U4+) was studied in situ by combining measurements by high temperature X-ray absorption spectroscopy and molecular dynamics simulations. In the molten state, 20 higher than the melting temperature, these mixtures are composed of free fluorine and anionic species [MF7]3−, [MF8]4− and [MF9]5− whose distribution varies with the amount of MF4 (M = Th4+, U4+). Regardless of the cation, these complexes consist of an average of 8 F− neighbors for MF4 content is lower than 35 mol%. This value decreases to 7 as the size of the alkaline ion A (A = Li+, Na+, K+) increases. The [MFx]4−x species are linked together by bridging fluorine ions to form long chains [MxFY]4x−y. The addition of a few percent UF4 to the eutectic LiF-ThF4 composition (77.5 mol% - 22.5 mol%) at leads to a slight increase in the population of free fluorine ions due to the breakdown of the Th-F links to form complexes [UFx]4−x isolated or connected to the (ThxFy)4x−y chains. This change in the structure of the liquid results in a slight decrease in viscosity when a few mol. % of UF4 is added.