Electrochemical impedance spectroscopy (EIS), linear polarization (LP), Tafel and cyclic voltammetry (CV) were performed to investigate the exchange current density (i 0) of U/U 3+ reaction in LiCl-KCl eutectic salt under different UCl 3 concentrations (0.5 wt%-4 wt%) and temperature conditions (723 K-798 K). The EIS spectra were measured by applying minimum overpotential from equilibrium potential and fitted to the proposed equivalent circuit. From the EIS experiments, the calculated values of i 0 were ranging from 0.0054 A cm-2 to 0.102 A cm-2. For the LP and Tafel methods, i 0 values were determined from the linear fitting at small and large overpotential ranges of the current-potential curves. The measurement for i 0 via CV was done analogously to the LP method. All i 0 values have the linear trend with the change of concentration and temperature; however, these values measured by LP, Tafel, and CV methods are greatly influenced by the change in electrode surface area. Overall, i 0 agreed within 33% relative error range with the EIS method being the most consistent and accurate in comparison to reported literature values. Dimensionless analysis was done on the EIS data sets providing insightful correlations for prediction of i 0 under various experimental conditions.

Thermodynamic and electrochemical properties of cerium in LiCl-KCl eutectic salt have been measured and studied at different concentrations (0.5 -4 wt%) and temperatures (698 K -798 K) via both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques as a part of developing a fundamental understanding and methodology in materials detection and accountability for pyroprocessing technology. CV experiments were performed to determine the diffusion coefficient and apparent standard potential of CeCl 3 on the tungsten working electrode. The diffusion coefficient was calculated by using Delahay equation, and raging from 0.48 × 10 −5 to 1.01 × 10 −5 cm 2 s −1 . Results reveal that the calculated diffusion coefficient of CeCl 3 in the salt follows the Arrhenius temperature relationship and it is weakly affected by the changes in concentration of CeCl 3 . The apparent standard potentials were calculated from peak potentials showing linear relationship with temperature. Exchange current density values of Ce 3+ /Ce couple in the salt were obtained from EIS experiments, ranging from 0. Pyroprocessing technology has been proposed as another promising method for the recovery and recycle of uranium and actinide elements from the used nuclear fuel. An essential step in this technology is the electrorefining process in which uranium is selectively recovered by using solid cathodes in chloride-based molten salt at high temperature.1 Then, co-recovery of uranium and transuranic elements can be accomplished by replacing the solid cathodes with a liquid cadmium cathode because the reduction potentials of the elements become close when the liquid cadmium is used as a cathode electrode.2,3 Since uranium is the major element in most nuclear fuel cycle paths as well as pyroprocessing technology, the assessment of accurate thermochemical data for the element in the molten salt is extremely important. 4Many studies on the thermochemical properties of uranium have been done in LiCl-KCl molten eutectic salt in different temperature ranges. Masset et al. 5,6 investigated diffusion coefficients of actinides and lanthanides in LiCl-KCl via cyclic voltammetry (CV) and chronopotentiometry (CP). Kuznetsov et al. 7,8 studied the electrochemical behaviors of actinides and rare-earth metals in LiCl-KCl salt. They performed CP and chronoamperometry (CA), and linear sweep voltammetry to determine the diffusion coefficients. Hoover et al. in 2014 9 extended the uranium concentration in LiCl-KCl molten salt up to 10 wt% and observed the electrochemical and thermodynamic behaviors of uranium using CV, CP, and anodic stripping voltammetry. These data are valuable to a development of kinetic models, which can be useful for understanding the main features of actinide deposition at the electrode surface, and also for prediction of material distribution in an electrorefiner of a safeguarding aspect. Zhang 10 developed a kinetic model for electrorefining system showing that the model is capable of predicting the kinetic features and...

No abstract

Electrochemical and thermodynamic properties of Ce 3+ /Ce on liquid cadmium cathode (LCC) were investigated in LiCl-KCl salt at various temperatures. Cyclic voltammograms only attributing to Ce 3+ /Ce reaction were obtained by background subtraction method and diffusion coefficients of Ce on the LCC were determined at temperatures ranging from 723 K to 798 K. Furthermore, thermodynamic properties of the Ce-Cd intermetallic compound formations were measured via an open circuit chronopotentiometry technique at temperature from 698 K to 823 K. The linear dependence of the Gibbs free energy of the formation was observed as a function of temperature, from which the enthalpy and entropy of the formation were calculated. Here, activity and partial molar Gibbs free energies of Ce in two phase coexisting states were calculated through the measurement of electromotive force. Tafel and linear polarization were conducted with varying Ce concentration in Cd. The exchange current densities (i 0 ) of Ce on the LCC were calculated from both techniques in LiCl-KCl-1 wt% CeCl 3 at 773 K. The resulting data sets of i 0 from both methods show a near linear trend of i 0 within the solubility limit of Ce in Cd. However, i 0 of Ce remains unchanged with increasing Ce concentration beyond the solubility limit. Treatment of the long term radioactive wastes from nuclear fuel cycle have become an essential matter in nuclear industries and public concern.1 One of the most promising option is known as Pyrochemical or Pyroprocessing technology, whose main method is electrorefining in chloride form of salt. This technology has a potential to treat used oxide/metal fuels and produce plutonium (Pu) in company with uranium (U), and minor actinides (MAs), resulting strong resistance for nuclear proliferation.1-3 The main step within this system is an electrorefining process, in which most of U is selectively recovered using a solid cathode in molten lithium-potassium chloride (LiCl-KCl) salt at high temperature (typically at 773 K). Then, the residual U, Pu and MAs are simultaneously collected with small amount of lanthanides into a liquid cadmium cathode (LCC) due to their small activity in liquid metals. 4 Since the operation of the LCC plays an important role in the feasibility of electrochemical processes toward material detections and accountability, 5 extensive studies have been done to understand electrochemical and thermodynamic features of U, Pu, and MAs on the LCC. Shirai et al. 6,7 investigated the reaction of U 3+ /U and Pu 3+ /Pu couples on the LCC. The redox reaction for both couples were almost reversible, and the reduction potential on the LCC showed more positive values than that at the molybdenum electrode. Murakami and co-workers 8 measured diffusion coefficients of actinides and rare earth elements with LCC by performing chronopotentiometry (CP) in LiCl-KCl at temperature ranging from 723 K to 823 K. Castrillejo et al. 5,9,10 investigated the activities and Gibbs energy of rare earth materials in both Cd and Bi liquid electro...

Experimental studies were performed to provide measurement and analysis of zirconium (Zr) electrochemistry in LiCl-KCl eutectic salt at different temperatures and concentrations using cyclic voltammetry (CV). An additional experimental set with uranium chloride added into the system forming UCl 3-ZrCl 4-LiCl-KCl was performed to explore the general behavior of these two species together. Results of CV experiments with ZrCl 4 show complicated cathodic and anodic peaks, which were identified along with the Zr reactions. The CV results reveal that diffusion coefficients (D) of ZrCl 4 and ZrCl 2 as the function of temperature can be expressed as D Zr(IV) = 0.00046exp(-3716/T) and D Zr(II) = 0.027exp(-5619/T), respectively. The standard rate constants and apparent standard potentials of ZrCl 4 at different temperatures were calculated. Furthermore, the results from mixture of UCl 3 and ZrCl 4 indicate that high concentrations of UCl 3 hide the features of the smaller concentration of ZrCl 4 while Zr peaks become prominent as the concentration of ZrCl 4 increases.