In Situ Experimental Approach of the Speciation in Molten Lanthanide and Actinide Fluorides Combining NMR, EXAFS and Molecular Dynamics

Abstract

We study the nature and the distribution of the different anionic species existing in fluoride melts of nuclear interest. Our approach is based on the combination of NMR and EXAFS spectroscopy, both sensitive to the microstructure of the melt, with Molecular Dynamics calculations. Because of its analogy with actinide fluorides, we have studied zirconium fluoride and its mixtures with alkali fluorides. In MF-ZrF4 systems, MD simulations, in agreement with 91Zr and 19F NMR and EXAFS experiments, put in evidence the coexistence of 3 different Zr-based complexes [ZrF6]2-, [ZrF7]3-, and [ZrF8]4- in the melt. Their proportions depend on the amount of ZrF4 and on the nature of the alkali (Li+, Na+ and K+). In the case of lanthanide fluorides, a similar approach performed in the system MF-LaF3 evidences some structural similarity, mainly with the existence of different complexes and upon free fluorine content evolution.