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Degradation of Structural Alloys in Molten LiCl-Li2O-Li
Materials Science and Engineering
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The necessity to curtail carbon emissions to limit anthropogenic climate change is at odds with the continued increase in energy demand caused by economic and population growth in the developing world. In order to simultaneously increase the standard of living for billions of people while limiting greenhouse gas emissions, it is necessary to develop and implement carbon free sources of energy on a large scale. Such energy sources should cover increases in capacity and replace legacy coal and nuclear power plants. Nuclear fission provides a powerful, compact, and reliable source of electricity and process heat. However, the long-term storage of nuclear waste poses a number of environmental and nuclear weapons proliferation concerns. Reprocessing of used nuclear fuel (UNF) offers the promise of reducing both the longevity and volume of waste, while simultaneously reducing nuclear security concerns and increasing the energy generated by a given quantity of mined uranium. Pyroprocessing technology has been developed to separate the fission products from the remaining actinide elements using electrochemical separations in a molten salt electrolyte. The incorporation of oxide based nuclear fuels from light water reactors requires the reduction of this material to a metallic form. Currently, the state-of-the-art process for this reaction is electroreduction in a molten LiCl-Li2O electrolyte maintained at 650°C. In order to achieve a high reduction efficiency, the applied potential necessarily surpasses the reduction potential of Li2O, causing the formation of metallic Li at the cathode. As metallic Li is soluble in molten LiCl, the electrolyte becomes a ternary mixture of LiCl, Li2O, and Li. While the increasing concentration of Li during the electrolytic reduction of UNF has been widely observed, little work has been performed thus far to investigate the effect of metallic Li on the corrosion of the structural materials used to contain the electrolyte. This is the first study to explore the long-term effect of metallic Li dissolved in LiCl-Li2O on alloys. Knowledge of the corrosion rate of a material in a given environment is necessary in order to accurately predict the expected lifetime of a component, thus preventing expensive and potentially hazardous failures. As such, the work presented in this dissertation was performed to elucidate the corrosion mechanisms and the rate of material degradation of structural materials during extended exposure to molten LiCl containing varying concentrations of Li2O and Li. Stainless Steel 316L (SS316L) was studied since the current containers for engineering scale oxide reduction processing are constructed of this material. Inconel 625 (I625) was also studied as an alternative to SS316L due to its high temperature corrosion performance in other systems, and previous short-term studies that have shown good performance in the LiCl-Li2O-Li system. Duplicate samples of each material were exposed to molten LiCl containing 1 or 2wt% Li2O and 0, 0.3, 0.6, or 1wt%Li for periods of 500 and 1000 hours, at a temperature of 650°C inside an Ar glovebox. Post exposure analysis of the sample surfaces was conducted via scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDS) and focused ion beam (FIB) milling, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. SEM-EDS analysis was also performed on the sample cross section after both FIB milling and mechanical cross sectioning. Both SS316L and I625 formed a protective oxide layer composed primarily of LiCrO¬2 in the absence of metallic Li, which limited the degradation of both alloys. However, the presence of metallic Li in the molten salt solution prevented the formation of this protective oxide layer, resulting in extensive attack of the base materials. In both cases, selective dissolution of minor alloying elements was observed. In SS316L, intergranular attack of the base alloy resulted from the selective dissolution of Cr, Mo, and Mn, while I625 showed the selective dissolution of Cr, Mo and Nb, and a porous, Ni foam like microstructure was observed. Corrosion rates were approximately an order of magnitude greater in the tertiary LiCl-Li2O-Li electrolyte than observed in LiCl-Li2O in the absence of Li. Thus, these results should be taken into consideration when designing systems for the oxide reduction of UNF.