Activity Measurements of Gadolinium(III) Chloride in Molten LiCl-KCl Eutectic Salt Using Saturated Gd/GdCl₃Reference Electrode

Abstract

The activity coefficients of rare earth chlorides in molten salt systems are of great interest for the research, development and commercialization of pyrochemical processing of spent nuclear fuels. In this paper, the activity coefficient of GdCl 3 in molten LiCl-KCl eutectic salt has been studied using a two electrode electrochemical cell with a gadolinium metal rod as working electrode and a saturated Gd|GdCl 3 -LiCl-KCl reference electrode. Use of this analyte reference electrode allows for the elimination of hereto unavoidable errors in the experiments. It is determined that the activity coefficient of GdCl 3 is a function of concentration; decreasing from 3.9 × 10 −4 at low concentrations to 4.9 × 10 −5 at about 0.62 mol % GdCl 3 , remaining unchanged up to about 2 mol % GdCl 3 after which it begins to increase with increasing concentration of GdCl 3 . Further it is also determined experimentally that presence of high levels of CsCl tends to decrease the activity coefficient of The pyrochemical processing (pyroprocessing) of spent nuclear fuel and the development of high temperature separations process technology is one of the flowsheet options for closing the spent fuel cycle. During pyrochemical separations, it is proposed that the uranium and transuranic species are selectively separated from the spent fuel in a high temperature electrorefining process. LiCl-KCl eutectic salt serves as the electrolyte for the electrorefining cell. A stainless steel basket, in which spent fuel is placed, serves as the anode. The cathode can be an inert metal (steel), a reactive metal (aluminum) or liquid metal cathode (cadmium). Inert solid cathodes are excellent at recovering pure uranium deposits, 1 reactive solid metal cathodes are good at collecting uranium and transuranic elements with less rare earth impurities 2 whereas liquid metal cathodes allow for the co-deposition of uranium and transuranic elements during the electrorefining process but can result in high levels of rare earth impurities in the cathode deposit.3 Actinide drawdown process can be used to achieve high levels of actinide recovery from the salt prior to disposal. In theory, actinides are electrochemically more noble (more positive standard reduction potentials) than the rare earths and other fission products present in the molten salt hence can be selectively reduced on the cathode. Standard reduction potentials (E 0 ) does not account for the deviation from ideal behavior both the actinides and rare earths are reported to exhibit in the molten salt systems. This deviation can be quantified using the activity coefficients of the species.The motivation for future research and development of the nuclear fuel cycle is both efficient utilization of the fuel as well as waste minimization. After an actinide recovery step, the actinide-free, fission product-rich salt will need to be disposed safely after immobilizing the salt in waste forms suitable for long term storage in a deep geological repository. Recently, flowsheets were proposed for the further re...

Study of Potentiometry for Monitoring Activity of GdCl3 in Molten LiCl-KCl Salt

“…Bagri and Simpson, in a recent study, summarized the reported activity coefficient of GdCl 3 salt in LiCl-KCl salts, and compared their results using a Gd metal electrode and a reference electrode with Gd metal in a GdCl 3 -saturated LiCl-KCl salt. 9 To calculate the formal potential and activity coefficient of GdCl 3 , its Gibbs free energy of formation must be known, and it is needed to convert the Gd metal electrode potential vs the Ag/AgCl RE to a potential vs the Cl 2 /Cl − RE. For convenience, the potential difference of Ag/AgCl RE with respect to a Cl 2 /Cl − electrode from a recent study by Yoon et al 15 was used here.…”

“…Potentiometry is a well-established method for analyzing both aqueous and molten salt electrolytes. [7][8][9][10] It is based on the measurement of open circuit potentials (OCPs), also known as electromotive force (EMF) potentials, of an electrochemical cell. The OCP is dependent on temperature and the chemical activities of the species involved in the two half-cell reactions that comprise the overall cell reaction.…”

“…11,12 A potentiometry approach is seldom used for chemical analysis in molten salt systems, probably due to the fact that molten salt systems are complicated and a good working electrode that selectively responds to the component to be tested is difficult to find. Recently, Bagri and Simpson 9 studied the activity of GdCl 3 in LiCl-KCl salts and the effect of CsCl on the activity of GdCl 3 , using a GdCl 3 solute reference electrode and reported the activity coefficient of GdCl 3 compared to literature data. Their results showed that at the GdCl 3 concentration range of 0.4 to 1.5 mol%, the activity coefficient of GdCl 3 appeared reasonably constant; at a higher GdCl 3 concentration range, the activity coefficient increased with GdCl 3 concentration.…”

Rare Earth Electrochemical Property Measurements and Phase Diagram Development in a Complex Molten Salt Mixture for Molten Salt Recycle

“…As presented elsewhere in this report, equilibrium potentials were measured for LaCl3/La, CeCl3/Ce, NdCl3/Nd, and GdCl3/Gd in eutectic LiCl-KCl. The measured equilibrium potentials of the metals relative to a reference electrode were used to calculate apparent potentials (E * ) versus a standard Cl -/Cl2 potential, activity, and activity coefficient at concentrations ranging up to 4 mol % in eutectic LiCl-KCl [79,80]. The data indicated that the equilibrium potentials were more negative than those of the actinide chlorides over a wide range of rare earth chloride concentrations.…”

Effect of Metal Chloride Impurities on Equilibrium Potential of Fe/FeCl2 in Eutectic LiCl-KCl

“…12 Recently, Bagri et al published a series of papers on the measurement of equilibrium potentials for a select set of rare Earth metals (La, Ce, and Gd) in molten LiCl-KCl as a function of the rare Earth chloride concentration. [13][14][15] They used the EMF method with each rare Earth metal used as the working electrode. In the cases of CeCl 3 and GdCl 3 activity measurement, special reference electrodes were fabricated consisting of Ce/CeCl 3 and Gd/GdCl 3 .…”