Material Interactions with Molten LiCl-Li2O-Li

The electrolytic reduction of oxide nuclear fuel in a molten lithium chloride electrolyte containing 1-2wt% lithium oxide is a fuel cycle process that has been established on an engineering scale. The electrochemical window of lithium oxide must be exceeded in order to conduct this process at an appreciable rate, as a result of which Li+ ions are reduced to elemental lithium during the process. The generation of elemental lithium (Li) during the reduction of actinide oxides leads to the formation of a ternary molten solution consisting of lithium chloride, 1-2wt% lithium oxide, and elemental lithium. The resulting ternary melt of LiCl-Li2O-Li is a complex fluid that exhibits an array of peculiar physical properties. This dissertation attempts to investigate the molten ternary LiCl-Li2O-Li system, both in terms of its physical chemistry and the manner in which it interacts with materials. The first part of this dissertation research focused on development of an experimental system specifically for the ternary LiCl-Li2O-Li system. The development of analytical methodologies for characterizing material interactions with molten LiCl-Li2O-Li required extensive high temperature engineering and the development of first-of- a-kind in situ techniques. Experimental methods were developed that facilitated the characterization of unperturbed surface films formed in the molten environment.The physical chemistry of molten solutions of LiCl and Li in the presence as well as the absence of Li2O was investigated using in situ Raman spectroscopy. The observed Raman spectrum is the first reported evidence that a salt soluble, molecular, Li-rich phase exists in molten solutions of LiCl and Li. The Raman spectra obtained from these solutions provides the first evidence for the presence of the lithium cluster Li8 in a fluid phase. This observation is indicative of a nanofluid-type colloidal suspension of Li8 in a molten LiCl salt matrix. The presence of Li clusters in molten solutions of LiCl-Li has significant implications in that a well-defined solubility limit may not exist due to the dispersion mechanism of colloidal suspension in addition to physical dissolution. This discovery may explain numerous previously unattributed physical properties exhibited by these molten solutions. The corrosion behavior of three categories of alloys (Fe-Cr-Ni, Ni-Cr-Fe, and Ni-Cr-Mo) in molten LiCl-Li2O-Li was studied and forms the crux of this dissertation. It was observed that while the presence of a low concentration of Li (0.6wt%). The effect of the presence of trace quantities of moisture on the corrosion of materials in molten LiCl-Li2O-Li was investigated, and the efficacy of methods used to dry the salt such that these effects do not occur was demonstrated. It was determined that material interactions with melts containing low Li concentrations are governed by electrochemical oxidation phenomena in accordance with the basicity (pO2-) of the melt. However, molten solutions containing an excess of Li leads to corrosion of materials in a manner more typical of liquid metal environments. While these regimes appear separate with regard to corrosion, evidence is presented that both Li and Li2O behave independently over a broader range of melt compositions.The electroless deposition of Ti compounds on materials exposed to molten LiCl-Li2O-Li was observed during the course of characterizing material interactions with these molten solutions. Characterization of these effects yielded important information demonstrating the ternary nature of the LiCl-Li2O-Li system. It was found that the activity (based on concentration) of O2- affects the electrochemistry of material interactions with molten LiCl-Li2O-Li in a manner that is in agreement with the Lux-Flood model of molten salt basicity. Furthermore, corrosion products were observed to form in melts containing physically dissolved Li that suggest that chemical reactions previously observed in liquid metal environments may occur in molten LiCl-Li2O-Li. Thus, LiCl-Li2O-Li exhibits both molten salt and liquid metal effects. In conclusion, the research conducted for this dissertation has led to several novel findings that are summarized in Table below. Importantly, this study has also identified knowledge gaps in our existing understanding of molten LiCl-Li2O-Li system and interactions with materials, especially with respect to the combined electrochemical and liquid metal type behavior of the system

Alternative Anodes for the Electrolytic Reduction of Uranium Dioxide

Reprocessing of spent nuclear fuel is an essential step in closing the nuclear fuel cycle. In order to consume current stockpiles, ceramic uranium dioxide spent nuclear fuel will be subjected to an electrolytic reduction process. The current reduction process employs a platinum anode and a stainless steel alloy 316 cathode in a molten salt bath consisting of LiCl-2wt% Li2O and occurs at 700⁰C. A major shortcoming of the existing process is the degradation of the platinum anode under the severely oxidizing conditions encountered during electrolytic reduction. This work investigates alternative anode materials for the electrolytic reduction of uranium oxide.The high temperature and extreme oxidizing conditions encountered in these studies necessitated a unique set of design constraints on the system. Thus, a customized experimental apparatus was designed and constructed. The electrochemical experiments were performed in an electrochemical reactor placed inside a furnace. This entire setup was housed inside a glove box, in order to maintain an inert atmosphere.This study investigates alternative anode materials through accelerated corrosion testing. Surface morphology was studied using scanning electron microscopy. Surface chemistry was characterized using energy dispersive spectroscopy and Raman spectroscopy. Electrochemical behavior of candidate materials was evaluated using potentiodynamic polarization characteristics. After narrowing the number of candidate electrode materials, ferrous stainless steel alloy 316, nickel based Inconel 718 and elemental tungsten were chosen for further investigation. Of these materials only tungsten was found to be sufficiently stable at the anodic potential required for electrolysis of uranium dioxide in molten salt. The tungsten anode and stainless steel alloy 316 cathode electrode system was studied at the required reduction potential for UO2 with varying lithium oxide concentrations. Electrochemical impedance spectroscopy showed mixed (kinetic and diffusion) control and an overall low impedance due to extreme corrosion. It was observed that tungsten is sufficiently stable in LiCl - 2wt% Li2O at 700⁰C at the required anodic potential for the reduction of uranium oxide. This study identifies tungsten to be a superior anode material to platinum for the electrolytic reduction of uranium oxide, both in terms of superior corrosion behavior and reduced cost, and thus recommends that tungsten be further investigated as an alternative anode for the electrolytic reduction of uranium dioxide

Review-Metallic Lithium and the Reduction of Actinide Oxides

Extensive research and process development has been conducted on the electrolytic reduction of actinide oxides inmolten LiCl-Li2O. It is now accepted that the reduction of these metal oxides occurs via two separate reduction mechanisms: direct electro-chemical reduction and mediated chemical reduction by metallic lithium. The deposition of metallic lithium at the cathode (mediated chemical reduction mechanism) during the process is known to be essential in order to achieve high process throughputs and reduction yields, and yet a knowledge gap exists regarding the nature of metallic lithium in this system. This review summarizes the formation of lithium during the process and its dispersion into the molten salt electrolyte. Previously reported aspects of the physical chemistry of the LiCl-Li2O-Li system are presented with a specific focus on the dispersion of Li in the solution. Finally, issues regarding the effect of the presence of lithium on the electrolytic reduction process are discussed. Evidence shows that electrochemically generated metallic lithium is likely a significant source of experimental uncertainty, low current efficiency and Li2O consumption in the oxide reduction process. (C) The Author(s) 2017. Published by ECS. All rights reserved

Presence of Li Clusters in Molten LiCl-Li

Molten mixtures of lithium chloride and metallic lithium are of significant interest in various metal oxide reduction processes. These solutions have been reported to exhibit seemingly anomalous physical characteristics that lack a comprehensive explanation. In the current work, the physical chemistry of molten solutions of lithium chloride and metallic lithium, with and without lithium oxide, was investigated using in situ Raman spectroscopy. The Raman spectra obtained from these solutions were in agreement with the previously reported spectrum of the lithium cluster, Li-8. This observation is indicative of a nanofluid type colloidal suspension of Li-8 in a molten salt matrix. It is suggested that the formation and suspension of lithium clusters in lithium chloride is the cause of various phenomena exhibited by these solutions that were previously unexplainable