Electrolytic Reduction Process Research Articles

Li2O concentration influenced local structure and properties of molten LiCl salt by machine learning driven molecular dynamics simulation

Pyroprocessing is one of the promising technologies for the reprocessing of spent nuclear fuel. In the electrolytic reduction process of pyroprocessing, LiCl serves as the electrolyte with a minor addition of Li2O as the initial oxygen ion input. Investigating the impact of Li2O concentration on the molten salt system is essential for enhancing the efficiency of electrolytic reduction. In the present study, the LiCl-Li2O system is simulated using deep potential molecular dynamics simulation. The properties including self-diffusion coefficient, shear viscosity, ionic conductivity, heat capacity, radial distribution functions, coordination numbers, short-range order, medium-range order, Voronoi structure, and angle distribution functions are explored at different Li2O concentrations. This study reveals that the variations of physical properties at different Li2O concentrations in LiCl molten salt can be attributed to the changes in local structure. For example, unnormal high proportions of two Voronoi structures are found in the molten salt when the Li2O concentration increases, which may explain the changes of properties. These findings offer valuable insights for refining the parameters of electrolytic reduction and elucidating the correlation between microscopic and macroscopic phenomena.

Effects of electrode configuration on the current density distribution of the electrolytic reducer

Pyroprocessing for the recycling of light water reactor spent fuel into metal nuclear fuel involves the electrolytic reduction process to convert oxides into metals. Optimization of the electrolytic cell is essential in the current engineering-scale development stage of electrolytic reduction research. This study investigates the effects of the conventional impermeable anode shroud, typically used in the development of laboratory-scale electrolytic reducers, and the effects of parallel multiple electrode configurations, aimed at the scale-up of the electrolytic reducer, on the current density distribution of the electrolytic reducer. The current density distribution was analyzed using COMSOL modeling. The exclusion of the impermeable anode shroud resulted in increased total currents under constant cell voltage conditions and contributed to the narrowing of the current density distribution on the electrodes. In addition, parallel multiple electrode configurations proved effective in increasing total currents and narrowing the current density distribution on the electrodes. These results emphasize the need to use a permeable anode shroud and a parallel multiple electrode configuration to scale-up the electrolytic reducer.

Electrolytic Reduction of UO2 Microspheres Synthesized Via Internal Gelation Method

Pyroprocessing is the combination of process steps to extract and recycle actinides from fission products in spent nuclear fuels in high-temperature molten salt media. Pyroprocessing has been studied extensively over the past decades. One of the key sub-processes is the electrolytic reduction of uranium oxides. The electrolytic reduction process originated from the FFC Cambridge process, an approach for electrochemically reducing titanium dioxide into titanium metal in molten CaCl2. In the electrolytic reduction of uranium oxides, the uranium oxide feed is applied as the cathode in LiCl-Li2O molten salt electrolyte at 650°C. LiCl appears more promising than conventional CaCl2 owing to the lower melting point (878 K), and higher decomposition voltage (3.46 V at 923 K). Additionally, Li2O is expected to speed up the reduction via an additional chemical reduction pathway with the participation of Li metal, and prevents the direct dissolution of the Pt anode.This work aims to study the morphology effect of the oxide feed on the reduction kinetics and the incorporation of impurities during the electrolytic reduction process. The electrolytic reduction of UO2 is carried out on sintered UO2 pellets, as well as various UO2 microspheres synthesized via internal gelation. The introduction of uranium oxide microspheres holds several advantages. The spheres provide a higher surface area, which is expected to improve the overall reduction efficiency of uranium oxide during electrolysis. In addition, it is easier to handle the feed during loading and unloading of the cathode basket compared to powder feeds.The potentiostatic and galvanostatic electrolysis of UO2 is performed, and the comparison between the electrolytic reduction of UO2 pellets and microspheres is investigated. The SEM micrographs in figure 1show the morphology change of a UO2 pellet to uranium metals particles. A significant morphology change between the flat surface of the sintered UO2 feed and the reduced U particles is observed at the surface. Further characterization is performed using XRD and TGA to identify the products of the electrolytic reduction process qualitatively and quantitatively. Results show that galvanostatic electrolysis shows a better reduction efficiency compared to potentiostatic electrolysis, applied both below and above the Li formation onset potential. Overall, there is barely un-reacted UO2 visible in the XRD patterns of the reduced feed after galvanostatic electrolysis. U metal and UO are identified in the XRD patterns depending on electrolysis parameters. The presence of UO can be as an intermediate of UO2 reduction during electrolysis, which is a rare compound to be reported in literature. Overall, the reduction of microspheres does improve the overall reduction efficiency compared to the conventional pellet feeds. Figure 1

Synthesis of Carbon – TiC/TiB2 Composites at the Electrolytic Reduction of Fused Salts

This article presents the results of laboratory testing of the technology for the synthesis of composite carbon—titanium carbide/titanium diboride (C—TiC/TiB 2) material directly in course of electrolytic reduction of cryolite-alumina melts. After the preliminary standard treatment purposing the mixing of initial components—petroleum coke of different fractions, rutile titanium oxide TiO 2, boron oxide B 2 O 3 and binding agent—the cathode specimens were pressed to the specified sizes and thermally treated at the temperature of 1050 °С. Experiments with holding time of 5, 15 and 24 h during the electrolytic reduction process have demonstrated the acceptable wettability of cathode specimen surface. X-ray phase analysis and SEM-EDS-analysis have detected the presence of TiC/TiB 2 and titanium oxycarbide TiC x O 1-x within the volume of cathodes and on their surface. After 15 and 24 h experiments the aluminium layer with good adhesion to the surface was noticed on the cathode surface; which is indicative of wettability of the electrode with metal. This tested technology can be extended to the creation of wide range of composite products in the С—МеС/МеВ 2 system.

(Invited) Consumable Fe Anode for Use in Direct Electrolytic Reduction of Spent Oxide Fuel

The direct electrolytic reduction (DER) process can be used to reduce UO2 to U metal in a molten LiCl-Li2O electrolyte. In the baseline process, UO2 is loaded in a steel basket that serves as the cathode, while O2 gas is generated at a Pt anode. The use of Pt for anodes is non-optimal because of its cost and tendency to corrode via lithium platinate formation during the DER process. Other inert metals and cermets for DER anodes have been extensively researched to reduce the cost. Alternatively, reactive anodes may be considered that form disposable metal oxides rather than O2 gas. Thus, we investigated the behavior of pure iron rods as anodes for this process. Experiments were performed using a LiCl-2wt%Li2O electrolyte at 650oC in an Ar atmosphere glove box with <10 ppm O2 and H2O. UO2 powder was loaded into a stainless-steel cathode basket. A 7.56-mm diameter iron rod was used as the anode. Anode and cathode potentials were measured relative to a Ni/NiO reference electrode encased in a MgO tube with a porous MgO plug. The cell was run via controlled anode potential with increments from 0.2–1.0 V versus Ni/NiO. The headspace gas was sampled during experiments and analyzed for O2. Throughout the experiment, the cathode potential was consistent with either direct reduction of UO2 or reduction of Li2O, which forms Li that will also reduce UO2 to U metal. LECO oxygen analysis indicated 32% reduction of UO2 with a cell efficiency of 37%. Li2O concentration remained constant in the salt; thus, net oxidation must have occurred at the anode. The Fe anode was pulled out of the salt and photographed after stages of holding the potential at 0.2–1.0 V versus Ni/NiO in 0.1 V increments for about 11-12 min at each stage. Yet unidentified metal oxides were observed on the anode surface after holding at potentials of 0.4 V and 0.6–1.0 V. O2 was detected in the headspace gas when the anode potential was increased to 1.0 V. Thus, iron oxides form at lower potentials, while O2 gas formation occurs at a threshold potential. O2 formation may be enabled by the oxide layer passivating the iron against further oxidation. Results of ICP-MS of salt samples taken throughout the experiment showed no increase in Fe concentration in the salt. A metallic deposit formed on the outside of the cathode basket, implying some electrotransport occurred from the anode to the cathode. The results of this study show that cheap, disposable iron anodes are promising for the DER process. After an initial stage of formation of an iron oxide phase on the surface of the anode, O2 gas forms at a sufficiently high potential (1.0 V versus Ni/NiO). Further work is needed to achieve a more detailed mechanistic understanding of the effect of anode potential on specific reactions at the anode. Optimization of the anode potential may serve to promote O2 formation and minimize the rate of consumption of iron anodes.DISCLAIMERThis work of authorship and those incorporated herein were prepared by Consolidated Nuclear Security, LLC (CNS) as accounts of work sponsored by an agency of the United States Government under Contract DE-NA-0001942. Neither the United States Government nor any agency thereof, nor CNS, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility to any non-governmental recipient hereof for the accuracy, completeness, use made, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency or contractor thereof, or by CNS. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency or contractor (other than the authors) thereof.COPYRIGHT NOTICEThis document has been authored by Consolidated Nuclear Security, LLC, under Contract DE-NA-0001942 with the U.S. Department of Energy/National Nuclear Security Administration, or a subcontractor thereof. The United States Government retains and the publisher, by accepting the document for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this document, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, or allow others to do so, for United States Government purposes.

New Oxygen-Evolving Inert Anode Made of Nickel Metal Applicable to Electrolytic Reduction of UO2 in LiCl–Li2O Melt

The development of an O2-evolving inert anode is of crucial importance for the electrolytic reduction process of oxide nuclear fuels using LiCl–Li2O melts at 923 K. As scaled-up anodes for practical use, metallic anodes are preferable. In this study, Fe, Ni, and Fe–Ni metals were electrochemically examined and the results indicate that Ni metal coated with NiO is a promising anode material. Ni metal is easily dissolved in LiCl in the form of Ni2+ ions over the potential range >2.3 V (vs Li+/Li). However, in LiCl–Li2O, after NiO was formed at the surface of Ni metal, the dissolution of Ni2+ ions was inhibited and O2 evolution occurred over the potential range >2.6 V at a high current density. Oxygen gas was stably evolved during current-controlled electrolysis at currents up to 0.6 A (0.94 A cm−2) performed using a Ni rod anode of 3 mm diameter, which was heat-treated in air and covered with a MgO protective tube at around the interface between the melt and the cover gas. Moreover, it was demonstrated that about 100 g of UO2 was completely reduced to the metallic form in 8.7 h using a Ni plate anode.

(Invited) Electrochemical Recycling of Used Nuclear Fuel

Current nuclear energy systems produce approximately twenty percent of the electricity generated in the U.S. and account for approximately fifty-five percent of the carbon-free electricity. As the U.S. strives for even lower carbon emissions such as net-zero carbon emission by 2050, nuclear energy will play an increased role in providing baseload electricity to the nation. An opportunity to implement advanced reactor and fuel cycle technologies that offer enhanced resource utilization, waste management and non-proliferation features will be available as the role of nuclear energy expands to meet the growing demand of carbon-free electricity. For example, the U.S. can transition from the current once-through fuel cycle to a closed fuel cycle with used fuel recycling that provides actinides for energy production and encapsulates fission product waste in durable, engineered waste forms.Argonne National Laboratory has a rich-history in developing new methods to recycle used nuclear fuels that dates back to the late 1950s. This early work focused on developing methods to recycle the actinides extracted from used nuclear fuel and closing the nuclear fuel cycle. Early techniques consisted of melting the used metallic fuel discharged from the Experimental Breeder Reactor II in lime-stabilized zirconia crucibles to produce a liquid uranium alloy that could be separated from the resulting oxide dross containing the fission product elements. The resulting uranium alloy was recycled to produce fresh nuclear fuel. Subsequently, more elegant techniques such as molten salt – liquid metal extraction processes were developed to recover and recycle the actinides present in the oxide dross, and to separate the actinides from the fission product elements present in used metallic fuel. In both cases, the actinides were recycled in fresh nuclear fuel and the fission products were sequestered in waste forms.Argonne’s research and development efforts in used fuel recycling expanded to include areas such as oxide to metal conversion via chemical reduction in a molten salt flux and developing new processes for recycling the actinides extracted from nitride and carbide fuels. Work in this area evolved to include the design and development of electrochemical technologies for recovering actinide metals from used nuclear fuels and an electrolytic reduction process for the conversion of metal oxides to metallic forms.The transition of used nuclear fuel processing from chemical to electrochemical methods established the need for fundamental electrochemical data for the actinide and fission product elements in molten salt media, provided opportunities for the design and demonstration of purpose-built electrochemical processing equipment, and allowed for the use of process monitoring and control technologies to enable efficient electrochemical cell operations. Actinide metal nucleation and growth in a molten LiCl - KCl eutectic salt is an example of the type of fundamental data that has been collected to elucidate the behavior of actinides during the electrodeposition process. Results from those experiments revealed that actinide nucleation transitions from progressive to instantaneous growth with increasing actinide chloride concentration in the molten salt. This behavior influenced the design of electrochemical processing equipment. Other data such as actinide ion diffusion coefficients were also determined from the experiments.Electroanalytical process monitoring and control techniques are being developed for several molten salt applications. These techniques are ideally suited for molten salt applications due to the specificity of chemistry information that is produced, simplicity of equipment design, and functionality at elevated temperature and in radiation environments. Argonne developed a multi-function sensor and methodology that yields reproducible electrochemical data such as the redox potential of the salt and the identities of species present in the system for applications in used fuel processing and monitoring heat transfer fluids.Argonne’s molten salt activities span the range from fundamental property measurement to pilot-scale process demonstrations. The presentation will provide a survey of Argonne’s electrochemical fuel recycling activities including actinide electrochemistry, process design and development, and electroanalytical sensor development.ACKNOWLEDGMENTS: The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Li2O Entrainment during UO2 Reduction in Molten LiCl-Li2O: Part II. Effect of Cathodic Reduction Mechanism

A study was performed to assess the effect of controlling the reduction mechanism on Li2O entrainment in an electrolytic UO2 reduction process. The reduction mechanism was controlled by isolating the UO2 particles from the lead, eliminating the direct reduction mechanism. Cathode design made it possible to eliminate the direct UO2 reduction mechanism, as evidenced by the cathode potential, the result being indirect reduction via electrolytically produced Li metal. Characterization of the reduced product was achieved via acid-base titration to measure Li2O entrainment and thermogravimetry to measure reduction extent. Significantly increased entrainment was observed when the electrolytic UO2 reduction portion was eliminated compared to normal operation. In addition to the entrainment the representativeness of dip sampling, compared to cup sampling, was investigated. A statistically significant difference between Li2O concentration measurements from dip and cup samples was observed, dip samples measured 8 ± 3% less Li2O than cup samples.

Viscosity of molten MgF2-LiF-MgO system and structure investigation using classical molecular dynamics simulations

The effect of temperature and fluoride structure on the viscosity of a molten 32.8MgF2-67.2LiF system with varying MgO concentration was investigated for the environmentally sound electrolytic reduction process of magnesium extraction. Whereas the temperature dependence of the viscosity was not significant, increasing viscosity was observed with the increase in the MgO concentration. Using classical molecular dynamics simulation, the molten fluoride structure in the 32.8MgF2-67.2LiF with varying MgO concentration was investigated at 1123 K. From the result of the radial distribution function and coordination numbers, the formation of a characteristic bonding structure between the Mg-F and O-Mg was verified. The F-Mg-F bond-angle distribution results indicated the formation of a marginally distorted octahedral structure of MgF6 in the present MgF2-LiF-MgO system. Moreover, the Mg-F-Mg bond angle distribution revealed the formation of a three-dimensional MgF6 network structure by sharing one or two fluorine atoms. Owing to the increasing edge-sharing MgF6 structure unit and formation of a larger three-dimensional network associated by the oxygen atoms, the viscosity increased by the addition of MgO in the 32.8MgF2-67.2LiF system.

Thermodynamic Calculations on the Chemical Behavior of SrO During Electrolytic Oxide Reduction

Strontium is known as a salt-soluble element during the electrolytic oxide reduction (EOR) process. The chemical behavior of SrO during EOR was investigated via thermodynamic calculations to provide quantitative data on the chemical status of Sr. To achieve this, thermodynamic calculations were conducted using HSC chemistry software for various EOR conditions. It was revealed that SrO reacts with LiCl salt to produce SrCl2, even in the presence of Li2O, and that the ratio of SrCl2 depends on the initial concentration of Li2O dissolved in LiCl. It was found that SrO reacts with Li to produce Sr during EOR and that the reduced Sr reacts with LiCl salt to produce SrCl2. As a result, the proportions of metallic forms were lower in Sr than in La and Nd under various EOR conditions. The thermodynamic calculations indicated that the three chemical forms of SrO, SrCl2, and Sr co-exist in the EOR system under an equilibrium with Li, Li2O, and LiCl.

Quantitative Description of the Equilibrium Potentials and Atomic Radius of the Co–Ln Alloy by a Mathematical Equation

The intermetallic compound formations of Co–Ln (Ce, Pr, Gd, Dy and Er) in the molten LiCl-KCl eutectic at 873 K for an electrolytic reduction process was investigated with various electrochemical techniques of cyclic voltammetry, square wave voltammetry and open-circuit chronopotentiometry. The deposited Co–Ln alloy samples were prepared by potentiostatic electrolysis and further characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS). A mathematical model about the equilibrium potentials of Co–Ln intermetallic compounds and atomic radius was proposed in this work. The fitted equation is Y = 0.1402X − 0.6457 (R2 = 0.992). Because of the close similarity of chemical properties, the equilibrium potentials of Co–An intermetallic compounds can be simulated and predicted through the mathematical equation. In addition, the relationship between the equilibrium potentials and atomic radius of Ni–Ln and Cu–Ln alloy compounds were also in accordance with the mathematical model, while show a good linear relationship. The work described here can broaden the applications of mathematical model and highlight its potentials in future molten salt electrolysis based spent fuel pyroprocessing.

Corrosion Behavior of Fe-Cr-Al Alloy for Anode Current Collector in Oxygen Atmosphere

Solid Oxide membranes (SOMs) have been applied to metal production through oxide reduction in order to accomplish low-cost production of primary products through simple pre-processing, low capital costs, small manufacturing plants, and low energy costs. Especially, the SOM process is eco-friendly, which makes it even more attractive. The SOM process is a direct electrolytic reduction process using a solid-oxide oxygen ion conducting membrane with high-temperature molten salts as a reaction medium. The transported oxygen ions from the cathode during the direct electroreduction process create a chemically harsh environment that can be corrosive for the alloy. Therefore, there is a need for the high-temperature corrosion behavior of alloy in high-temperature oxygen atmosphere. In this study, the hot corrosion behavior of FeCrAl alloy was investigated at 1150 ℃ in oxygen atmosphere with alumina shielding and without alumina shielding. The surface and cross-sectional morphologies and the characteristics of alloy were analyzed by SEM, EDS. As a result, FeCrAl alloy with alumina shielding was less oxidation than that without alumina shielding.

Eco-Friendly Electroreduction of Indium Tin Oxide Using SOM Anode in Molten Salts

Indium Tin Oxide(ITO) has been widely used at transparent electrode of Light Emitting Diode(LED) or the material of Liquid Crystal Display(LCD). However, waste ITO is generated after process and wet recycling technology has been used. In the case of wet recycling technology, a large amount of acid required and process time is so long. Then, if waste ITO could be recycled by dry process, electroreduction process, it is expected to solve the economic and environmental problems. However, when graphite is used as anode, it generates greenhouse gases such as CO or CO2 gas and causes formation of dirty C dusts. In this study, electroreduction of Indium Tin Oxide using Solid Oxide Membrane anode was investigated in a molten salt of CaF2-NaF-CaO at 1150 °C under the inert atmosphere to replacing previous anode materials. The SOM process is a direct electrolytic reduction process using a solid-oxide oxygen ion conducting membrane, namely, yttria-stabilized zirconia (YSZ), with high-temperature molten salts as a reaction medium. To verify the possibility of replacing, waste ITO was used because of the large use in industry. Electrochemical behavior was investigated by CV(Cyclic Voltammetry) in molten salts using waste ITO and inert working electrodes. As a result, InSn metal was reduced by SOM process. After electroreduction process, the optimum ITO particle size is considered to be the range of 600 μm to 1000 μm due to considering the maximum reduction efficiency dependent on the sedimentation rate, specific melt droplet growth, and reduced InSn film formation. The reduced InSn alloy was analyzed by XRD and EDS and they were confirmed as InSn alloy, which is almost the same as the initial ITO composition.

Characterization of Ce-Bi intermetallic compound formations by electrolytic reduction in molten LiCl-KCl eutectic

The intermetallic compound formations of Ce-Bi on an inert W electrode in the molten LiCl-KCl eutectic with CeCl3 and BiCl3 for an electrolytic reduction process was investigated with various electrochemical techniques of cyclic voltammetry, square wave voltammetry, and open circuit chronopotentiometry. The electrochemical reduction of Ce3+ on the inert electrode at 773 K was observed at more positive potential with Bi3+ than that without Bi3+ due to lowering activity of Ce by intermetallic compound formations. Cyclic voltammogram and square wave voltammogram exhibited four reduction peaks with overlapping signals. Application of peak separation technique allow five intermetallic compounds of BiCe1/2, BiCe, BiCe4/3, BiCe5/3, BiCe2 to be determined. Intermetallic compound formations of Ce-Bi were confirmed with open circuit chronopotentiogram which exhibited five plateau corresponding to a state in which two phases of intermetallic compounds coexist on the electrode surface. Analysis of cyclic voltammograms according to molar ratio of Bi to Ce showed that the amount of CeBi2 and CeBi at the reduction potential above −1.5 V were increased at the higher molar ratio of Bi to Ce. The experimental results indicated that intermetallic compounds of Ce-Bi could be formed on the inert electrode in LiCl-KCl-CeCl3-BiCl3 melt by electrolytic reduction and the type and amount of the resulting intermetallic compound on the inert electrode by electrolytic reduction could be varied depending on the applied potential and molar ratio of Bi to Ce, respectively.

Application of Phase-Field Theory to Model Uranium Oxide Reduction Behavior in Electrolytic Reduction Process

Application of Phase-Field Theory to Model Uranium Oxide Reduction Behavior in Electrolytic Reduction Process

Thermodynamic investigation on the behavior of rare earth oxides during electrolytic reduction process

A quantitative analysis on the behavior of rare earth (RE) oxides during the electrolytic reduction process was conducted through thermodynamic calculations. The calculation results can identify the oxide form of RE elements as RE2O3 including Ce and Pr, and provide a quantitative explanation on the slow reduction of RE oxides, compared to UO2, as observed in previous experiments. The concept of the effective salt amount was also introduced to verify the impact of metallic lithium on nearby salt.

Estimation on Feeding Portions of Slitting Decladded Fuel Fragments to Electrolytic Reduction Process

Performance tests of mechanical decladding technology for estimating the feeding portions of the recovered fuel fragments to an electrolytic reduction process were conducted in terms of the fuel rod burnups of 27.3 to 65.7 GWd/tonne uranium (tU) for the used pressurized water reactor nuclear fuel. The decladding efficiencies with fuel burnups were quantitatively obtained from slitting decladding tests. Based on the average fuel rod burnups, fuel rods with an average burnup of up to 52.3 GWd/tU showed above 99%, but higher burnup fuels of above 54.9 GWd/tU were below 97.52% in the decladding efficiency. It was interpreted that variations in decladding efficiency with fuel burnups were closely linked to the opening characteristics of the gap between the pellets and cladding. However, the fuel fragment size distribution after slitting decladding has little difference in fuel burnup changes between 34.8 and 55.4 GWd/tU. Hence, feeding portions of the fuel fragments from an assembly basis by using the decladding efficiency and recovered fragment size distribution data were estimated with burnup variations of 35 to 52.5 GWd/tU.

Electrochemical behavior of chalcogen and halogen fission products in pyro-electrochemical reduction process

An electrochemical process using a molten LiCl has been developed to reduce metal oxides from spent fuel (SF) from nuclear power plants, under the name of an electrolytic reduction process as a part of pyroprocessing. The researches on the electrolytic reduction have been investigated to determine the process conditions such as the shape of cathode, the form of feed material, and cell configuration. SF contains various kinds of oxides and chalcogen and halogen (group VIB and VIIB) compounds are expected to be dissolved into LiCl which is adopted as an electrolyte of the electrolytic reduction process. However, the behaviors of such compounds have not been experimentally clarified yet. In this work, the chemical and electrochemical behaviors of chalcogen and halogen compounds during the electrolytic reduction process were thermodynamically analyzed to understand their stability and final forms. LPP diagrams were used to determine the probable compounds on cathode and anode, respectively. Chemical and electrochemical calculations were carried out to find that it is required to suppress the reactions associated with chalcogens to protect anode and increase current efficiency.

High-temperature corrosion characteristics of yttria-stabilized zirconia material in molten salts of LiCl-Li2O and LiCl-Li2O-Li

ABSTRACT The isothermal and cyclic corrosion behavior of yttria (Y2O3)-stabilized zirconia (ZrO2) in a LiCl-Li2O molten salt were investigated at 650 °C in an argon atmosphere. During isothermal and cyclic corrosion tests in the molten salt of LiCl-Li2O for 168 h and 7 thermal cycles, the corrosion rate was very low, whereas under the molten salt of LiCl-Li2O-Li for 168 h, the corrosion rate was almost 10 times higher than that in the molten salt of LiCl-Li2O. No corrosion product was detected until 168 h for the isothermal corrosion test, however, after 7 thermal cycles, a very-low-intensity Li2ZrO3 peak was detected at the beginning stage of the chemical reaction between ZrO2 and Li2O. Additionally, in the molten salt of LiCl-Li2O-Li for 168 h, a large amount of Li2ZrO3 was formed, with evidence of marked cracks, pores, and spallations on the corroded surface. The introduction of Y2O3-stabilized ZrO2 was beneficial in increasing the hot corrosion resistance of the structural materials used to handle molten salts containing Li2O at elevated temperature without forming a lithium at the cathode during the electrolytic reduction process.

Advanced Characterization of Molten Salts

Standard methods of analysis are frequently incapable of characterizing molten salts due to the high temperatures and chemically reactive nature of these systems. As a result, fundamental properties of molten salt solutions, and material interactions with them, remain unknown despite their industrial significance. The molten LiCl, LiCl-Li2O, and LiCl-Li2O-Li systems are of specific interest to the electrolytic reduction process used to treat oxide nuclear fuels prior to pyrochemical reprocessing. In this study, we employ two advanced techniques, impedance sensitive electrochemistry and synchrotron based high energy X-ray diffraction (HEXRD), to characterize molten LiCl and to demonstrate the benefits of these methods of analysis. Impedance dependent electroanalytical chemistry was used to study the electrochemical behavior of molten LiCl-Li2O, and to investigate the processes associated with Li+ reduction from the melt. During this study, a unique behavior of the AC signal was observed to coincide with Li+ reduction. This effect has been investigated in terms of the electrochemical reversibility of the Li|Li+ redox couple, and has yielded information that could not be obtained using standard DC voltammetry techniques. The advantages of employing impendence sensitive methods of electroanalysis for the study of molten salts over typical DC voltammetry methods will be discussed. The nature of elemental Li suspended in molten LiCl has been the subject of research for more than 50 years without yielding conclusive results. The study of molten solutions containing Li is exceptionally challenging due to the reactivity of molten Li, and as a result requires advanced methods of analysis. In attempt to circumvent these challenges, a system has been developed for containing molten salts and liquid metals during synchrotron characterization at the Advanced Photon Source (APS). By employing the APS, the system is capable of in situ characterization of molten LiCl while electrochemically forming Li in the melt. Standard methods of synchrotron analysis are incapable of characterizing molten LiCl due to the low energy state of the electrons of interest (55 eV in the case of Li). HEXRD offers a distinct advantage in this regard, as it is capable of yielding the structure factor and the corresponding pair distribution function of low energy electron states using high energy X-rays. In the current experiment, 100 KeV X-rays were propagated through an atmospheric chamber, the melt containing crucible, and a molten LiCl electrochemical cell, in order to characterize the molten solution during electrochemical analysis. This apparatus enables the study of the physical chemistry of the melt, as well as in situ characterization of material interactions with the high temperature fluid. The ability of this technique to characterize molten LiCl-Li2O-Li at 650°C will be presented along with an analysis of the chemistry of the melt itself.