Advanced Characterization of Molten Salts

Wednesday, 4 October 2017: 15:40
Camellia 3 (Gaylord National Resort and Convention Center)
A. Merwin, C. Benmore, and M. A. Williamson (Argonne National Laboratory)
Standard methods of analysis are frequently incapable of characterizing molten salts due to the high temperatures and chemically reactive nature of these systems. As a result, fundamental properties of molten salt solutions, and material interactions with them, remain unknown despite their industrial significance. The molten LiCl, LiCl-Li2O, and LiCl-Li2O-Li systems are of specific interest to the electrolytic reduction process used to treat oxide nuclear fuels prior to pyrochemical reprocessing. In this study, we employ two advanced techniques, impedance sensitive electrochemistry and synchrotron based high energy X-ray diffraction (HEXRD), to characterize molten LiCl and to demonstrate the benefits of these methods of analysis.

Impedance dependent electroanalytical chemistry was used to study the electrochemical behavior of molten LiCl-Li2O, and to investigate the processes associated with Li+ reduction from the melt. During this study, a unique behavior of the AC signal was observed to coincide with Li+ reduction. This effect has been investigated in terms of the electrochemical reversibility of the Li|Li+ redox couple, and has yielded information that could not be obtained using standard DC voltammetry techniques. The advantages of employing impendence sensitive methods of electroanalysis for the study of molten salts over typical DC voltammetry methods will be discussed.

The nature of elemental Li suspended in molten LiCl has been the subject of research for more than 50 years without yielding conclusive results. The study of molten solutions containing Li is exceptionally challenging due to the reactivity of molten Li, and as a result requires advanced methods of analysis. In attempt to circumvent these challenges, a system has been developed for containing molten salts and liquid metals during synchrotron characterization at the Advanced Photon Source (APS). By employing the APS, the system is capable of in situ characterization of molten LiCl while electrochemically forming Li in the melt. Standard methods of synchrotron analysis are incapable of characterizing molten LiCl due to the low energy state of the electrons of interest (55 eV in the case of Li). HEXRD offers a distinct advantage in this regard, as it is capable of yielding the structure factor and the corresponding pair distribution function of low energy electron states using high energy X-rays. In the current experiment, 100 KeV X-rays were propagated through an atmospheric chamber, the melt containing crucible, and a molten LiCl electrochemical cell, in order to characterize the molten solution during electrochemical analysis. This apparatus enables the study of the physical chemistry of the melt, as well as in situ characterization of material interactions with the high temperature fluid. The ability of this technique to characterize molten LiCl-Li2O-Li at 650°C will be presented along with an analysis of the chemistry of the melt itself.