Ni/NiO Reference Electrode Potential Measurements in Molten CaCl₂–CaO

Abstract

A direct oxide reduction (DOR) process is used to reduce PuO2 to Pu metal via Ca metal. Molten CaCl2 is commonly used as the electrolyte for this reaction. As the DOR reaction proceeds, CaO accumulates in the salt and must be measured in real-time to know when the reaction has gone to completion. Open circuit potential (OCP) was measured in this study between a reference electrode consisting of Ni/NiO contained in an MgO tube and a tungsten working electrode. Two methods of CaO additions were performed over several experiments, and both methods correlated to a 0.11 to 0.23V decrease in OCP up to 14.4 wt% CaO. CaO concentrations were separately measured via sampling and acid-base titration. A plot of OCP versus concentration of CaO fits the Nernst Equation with the CaO powder additions up to 5.2wt% and also at the higher end of the CaO pellet concentration range. While the MgO tubes were observed to be structurally stable in the molten salt, scanning electron microscopy and energy dispersive X-ray spectroscopy showed that Mg in the tubes had been partially replaced with Ca.

Application of Cyclic Voltammetry with W Electrodes for Measurement of High CaO Concentration in Molten CaCl2

“…11 For detecting oxide ions in a calcium chloride salt, open circuit potentiometry (OCP) measurements are especially notable, as Dale et al found a correlation between OCP measurements to oxide ion concentrations up to 14.4 wt% and showed that correlation matched the Nernst equation for concentrations up to 5.2 wt%. 13 Alternatively, Zhang et al proposed an in situ real-time electrochemical sensor using three tungsten electrodes to detect oxide ion concentrations in molten salt via cyclic voltammetry (CV). 14 Oxide ions sensing with tungsten and cyclic voltammetry is possible because of an oxidation reaction between the tungsten electrodes and the dissolved CaO and oxide ions solution.…”