Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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Catherine Bessada, Didier Zanghi, Mathieu Salanne, Ana Gil-Martin, Mathieu Gibilaro, Pierre Chamelot, Laurent Massot, Atsushi Nezu, Haruaki Matsuura, 'Investigation of ionic local structure in molten salt fast reactor LiF-ThF4-UF4 fuel by EXAFS experiments and molecular dynamics simulations', J. Mol. Liq. 307 112927 (2020) doi:10.1016/j.molliq.2020.112927

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.