Material Interactions with Molten LiCl-Li2O-Li
Chemical and Materials Engineering
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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%) promotes the formation of protective Cr surface films, Cr and Mo are preferentially leached by melts containing high concentrations 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.