An Experimental Study of Molten Salt Electrorefining of Uranium Using Solid Iron Cathode and Liquid Cadmium Cathode for Development of Pyrometallurgical Reprocessing

This paper describes how electrochemical techniques have been used to develop a method for high-precision, real-time quantitative measurements of the concentration of actinides, present in in molten salts as actinide chlorides, for pyrochemical process monitoring applications. Possible reasons for discrepancies between reported measurements obtained with electrochemical techniques have been investigated and a combination of methods to improve their precision has been established. The combination of methods consists of selecting a suitable electroanalytical measurement technique, experimentally verifying assumptions used in its theoretical analysis, ensuring reproducible conditions at the electrode/electrolyte interface, and using an improved method to eliminate the need to know the electrode surface area. By following the developed procedures and refining both experimental techniques and data analysis methods, precise and reproducible measurements were obtained for U and Pu in LiCl/KCl eutectic at 773 K. Preliminary results showed that cyclic voltammetry, along with a method of standard area addition, are very promising tools for in situ quantitative measurements with a degree of precision comparable to destructive analysis techniques. Electrorefining is the main step in pyrochemical reprocessing. During this process, actinides are separated from the bulk of the fission product elements by electrochemical dissolution and electrotransport onto a solid or liquid cathode. The goal of the process is to maximize the separation and recovery of actinides from chlorinated fission products so that minimal amounts of actinides are lost to the process waste stream. [1][2][3][4][5] In addition to efficient actinide recovery, there is a need to address the safeguarding the recycle system. 5 Highprecision, real-time concentration measurements of actinides, present as actinide chlorides, in molten salts are required for process monitoring and control and could be used to support traditional material control and accountability techniques. Development of these measurements is essential to implement and operate a commercial fuel reprocessing facility. [6][7][8][9][10] Electrochemical techniques are well suited for in situ monitoring because they allow rapid, real-time measurements, are compatible with remote handling operations, and do not require the use of standards. Also, the equipment is not affected by the high radiation field present in a fuel processing operation.9,10 Unlike offline destructive analysis techniques, in situ methods do not require representative sample collection and preparation, and therefore avoid problems with sample contamination and degradation. In addition, analytical results can be received in a relatively short period (for example, less than 2 minutes).In an electrochemical cell, the measured potential (E), current (i), or charge (Q) is related to the quantity of the analyte in the solution; therefore, it can serve as an analytical signal for concentration measurements. 11,12 Depending on the applied...

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confidence: 99%

Method Development for Quantitative Analysis of Actinides in Molten Salts

Tylka

Willit

Prakash

et al. 2015

“…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. 4 Many studies on the thermochemical properties of uranium have been done in LiCl-KCl molten eutectic salt in different temperature ranges.…”

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confidence: 99%

Electrochemical Properties and Analyses of CeCl₃in LiCl-KCl Eutectic Salt

Yoon

Phongikaroon

2015

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...

“…In contrast, most fission products (FPs) still remain in the electrolyte. 8 Actually, Lns are the major and most awkward FPs and are difficult to be separated from Ans due to their similar chemical properties. On the other hand, Lns are well known to possess large neutron capture cross sections, which can spoil the neutron economy of reactor cores in the transmutation operation.…”

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confidence: 99%

“…A typical process in pyrochemical reprocessing is the electrorefining, [5][6][7][8][9][10] in which lanthanides (Lns) and Ans in spent fuels are dissolved into molten chloride salt by anodic dissolution. Subsequently, Ans (e.g.…”

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confidence: 99%

“…[1][2][3][4] The aim of the traditional pyrochemical processing technology is to achieve a group-selective separation of all actinides (Ans) for recycling or transmutation, generating the minimum of ultimate waste flows with an acceptable decontamination of fission products. A typical process in pyrochemical reprocessing is the electrorefining, [5][6][7][8][9][10] in which lanthanides (Lns) and Ans in spent fuels are dissolved into molten chloride salt by anodic dissolution. Subsequently, Ans (e.g.…”

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confidence: 99%

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Electrochemical Extraction of Cerium by Forming Ce-Zn Alloys in LiCl-KCl Eutectic on W and Liquid Zn Electrodes

Wang

Liu

Tang

et al. 2015

The electrochemical formation of Ce-Zn alloys in LiCl-KCl eutectic was systematically investigated by cyclic voltammetry (CV), square wave voltammetry (SWV) and open circuit chronopotentiometry (OCP). Due to the decrease of Ce activity in the metal phase of formed Ce-Zn alloys, the deposition potential of Ce(III) at a liquid Zn electrode was found to be 0.64 V more positive than that on an inert W electrode. The co-reduction behaviors of Ce(III) and Zn(II) ions were also studied in LiCl-KCl eutectic. Signals associated with six and nine Ce x Zn y intermetallic compounds were detected by CV and OCP techniques, respectively. In addition, potentiostatic electrolyses were carried out to extract Ce at a liquid Zn electrode in LiCl-KCl-CeCl 3 melt and on a W electrode in LiCl-KCl-CeCl 3 -ZnCl 2 melt, respectively. Deposited samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The results showed that intermetallic compound CeZn 11 was prepared on both the inert W electrode and liquid Zn electrode. Pyrochemical reprocessing of spent nuclear fuels is now considered to be one of the most promising options in nuclear fuel cycles due to their outstanding properties such as high thermal and radiation stability which allows a shorter cooling time of fuels before reprocessing, compactness, high actinide content and inherent proliferation resistance.1-4 The aim of the traditional pyrochemical processing technology is to achieve a group-selective separation of all actinides (Ans) for recycling or transmutation, generating the minimum of ultimate waste flows with an acceptable decontamination of fission products. A typical process in pyrochemical reprocessing is the electrorefining, [5][6][7][8][9][10] in which lanthanides (Lns) and Ans in spent fuels are dissolved into molten chloride salt by anodic dissolution. Subsequently, Ans (e.g. U) are selectively deposited on the stainless steel cathode or liquid cadmium cathode with respect to their different redox potentials in melt. In contrast, most fission products (FPs) still remain in the electrolyte.8 Actually, Lns are the major and most awkward FPs and are difficult to be separated from Ans due to their similar chemical properties. On the other hand, Lns are well known to possess large neutron capture cross sections, which can spoil the neutron economy of reactor cores in the transmutation operation. Therefore, though quite challenging, it is still imperative to efficiently separate Ans from Lns.In recent years, liquid-liquid reductive extraction has attracted considerable interest as a promising technique for the separation of minor actinides (MAs) from Lns.11-15 During a typical separation process, the solute cations in molten salt are recovered into the liquid metal phase via direct electrolysis or reductive extraction, due to the low activity coefficients of their corresponding metal phases in liquid metals. Since the operation of liquid metals plays an important role in the pyrochem...