Electrochemical Measurement of Li2O in Molten LiCl Salt
Authors: Burak Adam J , Simpson Michael Forrest 
Published: February 2020

Nuclear fuel reprocessing is an important technology designed to increase the sustainability of nuclear power generation, as well as minimize long-lived radioactive waste production. There are multiple reprocessing schemes, most notably PUREX and pyroprocessing. While PUREX and related aqueous separation processes technically have superior maturity due to large scale implementation in France, Japan, Great Britain, and Russia, pyroprocessing has attracted substantial interest in recent years largely based on the perception that it represents a much lower proliferation threat and is ideally suited for metallic fast reactor fuel recycle. The Republic of Korea in particular has invested a tremendous amount of research funding over the last decade to develop pyroprocessing for spent fuel from commercial pressurized water reactors. Pyroprocessing utilizes a high temperature electrochemical cell called an electrorefiner to separate actinides from fission products. To apply this technology to oxide fuel, as in the case of the Korean spent fuel, an oxide reduction step must precede electrorefining. The current leading technology for oxide reduction is a high temperature electrolytic process carried out in molten lithium chloride salt. Electrolytic reduction relies on a lithium cycle, which consists of oxidation of lithium metal to lithium oxide at the cathode and reduction of the lithium oxide back to lithium metal at the anode. The net result is that uranium oxide is reduced to uranium metal, and oxygen gas is evolved. Lithium oxide concentration in the molten salt electrolyte is an important process variable. If it decreases, it can be a sign of lithium oxide accumulation in the fuel basket and will result in increased anode potential that ultimately leads to anode degradation. Currently, the only proven way of measuring lithium oxide concentration in lithium chloride involves sampling followed by acid-base titration. Measuring lithium oxide concentration in real time is an important objective to maintain optimal process control. The technique being developed for this purpose involves measuring the change in the open circuit potential (OCP) between a molten pool of a lead-lithium alloy and a Ni/NiO reference electrode. Because OCP measures equilibrium potential, this method can be combined with the Nernst Equation to continuously measure lithium oxide concentration. Mixtures of lithium chloride and lithium oxide were prepared with lithium oxide concentrations ranging from 0.00 to 2.75 wt%. A lead-lithium pool containing 2.0 wt% lithium was placed below the molten salt pool. After the salt melted it was allowed an hour to equilibrate, then an OCP measurement was obtained. As expected, the OCP increased with increasing concentration of Li2O. The measured sensitivity was about 200 mV for a Li2O concentration change from 0 to 2.5 wt%. Achieving repeatable and stable reference electrode potential is key to making accurate measurements from this method. The reference electrodes were made of high-density MgO tubes with low density, porous MgO plugs. Nickel wire and NiO powder were inserted into these tubes. The reference potential varied significantly between different sets of reference electrodes, which is hypothesized to be due to variations in the thickness of the porous MgO plugs. Methods for standardizing the reference electrodes have been attempted, and the results will be discussed. Another factor that may effect this measurement is the solubility of lithium metal in the zero oxidation state in the molten LiCl. Lithium solubility measurements were made in LiCl-Li2O with varying concentrations of Li2O. The results varied from 0.7 to 1.5 mole%. This low level of solubility is expected to cause minimal loss of lithium from the lead-lithium sensor. And experiments with different volumes of salt over a fixed volume of lead-lithium confirmed that the measured OCP is relatively insensitive to any decrease in the lithium concentration in the lithium-lead pool.