Potentiometric Sensor for Multivalent Ions in High Temperature Molten Salt
Authors: Gese Natalie 
Published: February 2020

Natalie Gese*, Brenda Serrano, Jan-Fong Jue, Guy Fredrickson
Idaho National Laboratory, Idaho Falls, ID, USA
A solid-state potentiometric sensor [1] is being developed for ion selective monitoring of multivalent ion concentrations in LiCl-KCl molten salt. Based on the present chemical and mechanical stability studies it has been demonstrated that a solid electrolyte based sensor composed of polycrystalline ß"- alumina is well suited to aggressive molten salt environments where the sensor would be directly placed.
A natural consequence of electrorefining spent nuclear fuel is that during electrochemical oxidation of uranium, active metals and fission products also report to the ternary chloride molten salt comprising LiCl-KCl-UCl3. The UCl3 in the electrolyte is necessary for efficient electrotransport of uranium from the anode to the cathode and for obtaining a high purity cathode product; however, uranium favors the metal phase relative to other actinides [2]. As many batches of spent fuel are processed the UCl3 concentration decreases in the electrolyte. Elements in the spent fuel which are more stable in the molten salt as chlorides than uranium, participate in redox displacement reactions with UCl3. One of the main elements of concern is plutonium which forms PuCl3 at the expense of UCl3 and accumulates in the salt phase. Monitoring PuCl3 in the salt phase is necessary for safeguarding, process efficiency and control, and maintaining regulatory limits. Currently, spectroscopic methods are used to determine the composition of the salt between batches of fuel processed but there is no method in place to monitor actinide concentrations in the salt phase during electrorefining. There are significant time delays which are associated with the transfer, handling and analysis required. A real-time in situ ion selective sensor for plutonium and other actinides would provide many advantages as method for determining plutonium concentration so that electrorefining operations can be further expedited, the salt chemistry can be maintained efficiently, and a method for safeguarding can be employed for monitoring the salt phase.
Gadolinium was used as a multivalent ion surrogate for plutonium and other actinides in this study. Ion exchange of mobile sodium ions for gadolinium ions was achieved in polycrystalline Na-ß"-alumina ion conducting ceramics to prepare the sensor material [3]. Annealing was also performed to enhance the mechanical integrity of the Gd-ß"-alumina prepared through the ion exchange process. Conductivity as a function of temperature and the activation energy for ionic conduction was determined with Electrochemical Impedance Spectroscopy (EIS). Ion exchange, chemical stability and mechanical integrity were verified with SEM/EDS analysis. X-ray diffraction (XRD) was used to characterize the phase stability and phase changes in the Gd-ß"-alumina as a result of ion exchange and annealing. The potential between the Pt/Gd-ß"-alumina/Pt cell was measured as function of change in gadolinium concentration in LiCl-KCl. The approach and results obtained in development of this sensor will be presented.
This work was supported by the Department of Energy's Office of Nuclear Energy (DOE-NE), Fuel Cycle Research and Development (FCR&D), Materials Protection, Accounting and Control for Transmutation (MPACT). The authors would like to thank Dr. Shelly Lee for creating this research incentive.
[1] Fray, D., Metall and Materi Trans B2003, 34 (5), 589-594.
[2] Tomczuk, Z.; Ackerman, J. P.; Wolson, R. D.; Miller, W. E., J. Electrochem. Soc.1992, 139 (12), 3523-3528.
[3] Dunn, B.; Farrington, G. C., Solid State Ionics 1983, 9-10 (Part 1), 223-225.