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Corrosion behavior of Monel 400 and electrochemical performance of Li-Bi reference electrode in molten LiCl-Li2O-Li
AuthorPhillips, William C.
Chemical and Materials Engineering
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Closing the nuclear fuel cycle is an important step to improve the safety, economics, and environmental impact of nuclear power. To integrate the oxide fuels that are used in the current reactor fleet into a metallic fuel cycle applicable to some proposed advanced reactors, an electrolytic oxide reduction process will need to be performed. Currently, this process is conducted in a LiCl-Li2O electrolyte at 650°C. As the process proceeds, metallic Li is produced and dissolves into the electrolyte, resulting in a tertiary system composed of LiCl, Li2O, and Li. The materials used to construct the containers and equipment in direct contact with the electrolyte have been shown to suffer degradation due to exposure to the resulting LiCl-Li2O-Li electrolyte. This thesis investigates Monel 400 as an alternative material for construction of equipment used for the electrolytic reduction of used nuclear fuel due to this alloy’s high corrosion resistance that is not dependent on the development of protective chromium oxide surface films. Exposure testing was performed by submerging Monel 400 samples in LiCl-2wt%Li2O with varying concentrations of Li at 650°C for 20 hours. Post exposure analysis of the samples was performed using X-ray diffraction, field emission scanning electron microscopy, optical microscopy, micro-Vickers hardness testing, Raman spectroscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma – optical emission spectroscopy. Due to the unique electrochemical properties of molten LiCl-Li2O-Li, conventional reference electrodes used in other molten salt electrolytes are not applicable in this system. Therefore, a reference electrode based on the Li|Li+ couple was designed and constructed. The reference electrode designed for this study was verified to provide a stable reference voltage comparable to reference electrodes commonly used for rigorous aqueous electrochemistry. This reference electrode was used to observe the relative electrochemical potentials of Ni and Cu in the LiCl-Li2O-Li system at 650°C.The observed stability of Monel 400 over the range of molten salt compositions studied was superior to that of stainless steel 316, Inconel 615, and Inconel 718 that had been previously studied by this research group. The high corrosion resistance of Monel 400 in LiCl-Li2O-Li at 650°C observed in this study indicates that this material would be superior to other materials more commonly used for the construction of equipment necessary for the electrolytic reduction of used nuclear fuel. Results from analytical studies show that the interaction of M400 with LiCl-Li2O-Li system is complex and is likely to involve the formation of a protective Li1-xNi1+xO2 film on the surface that is subsequently degraded prior to analyses. It is suggested that the corrosion resistance and mechanical properties of Monel 400 exposed to LiCl-Li2O-Li at 650°C should be investigated further to verify the performance of this material in the variable environments encountered during the electrolytic reduction of used nuclear fuel.