Development of a Li₂O Sensor Based on a Yttria Stabilized Zirconia Membrane for Oxide Reduction in a Molten LiCl-Li₂O Electrolyte at 650°C

LiCl-Li₂O salt is a widely used electrolyte for the electrochemical reduction of spent oxide (mainly uranium oxide) fuels, and the Li₂O concentration is usually controlled at about 1.41 mol % for optimum operation and corrosion mitigation of the platinum anode material. Due to the small difference in reduction potential between UO₂ and Li₂O, some Li₂O will be reduced during the oxide reduction process, leading to a Li₂O deviation from the desired 1.41 mol %. Monitoring of the Li₂O concentration is desired for proper control of the electrochemical oxide reduction process. In this paper, a Li₂O sensor based on a yttria stabilized zirconia (YSZ) membrane was developed and tested to evaluate the feasibility of using the Li₂O sensor to monitor the Li₂O concentrations in the range of 0.57 to 1.69 mol % Li₂O in LiCl-Li₂O salt systems—one without dissolved Li metal and another with 0.24 mol % dissolved Li metal. The experimental results show that the open circuit potential of the Li₂O sensor logarithmically responded to the concentration of Li₂O in LiCl-Li₂O salts with or without the presence of Li metal, suggesting that YSZ appears promising for real-time monitoring of the Li₂O concentration in LiCl-Li₂O salt for an oxide reduction process. The Li₂O sensor developed herein is based on a potentiometry measurement that requires a stable, reliable reference electrode (RE), particularly for long-term Li₂O concentration monitoring. To this end, a novel Ag/AgCl RE that is contained in a high-density MgO tube with a closed end was developed and exhibited stable electrode potential, chemical compatibility with LiCl-Li₂O, and good mechanical strength. The performance of the newly developed Ag/AgCl RE was demonstrated in Li₂O monitoring by comparing it with a traditional Ni/NiO RE.