Electrochemical separation of uranium and cerium in molten LiCl–KCl

The electrochemical separation of uranium from cerium in LiCl–KCl eutectic and the electrochemical behavior of Ce(III) were studied. According to the cyclic voltammogram of Ce(III) and the former result of U(III), electrodeposition potential was determined at ?1.65 V (vs Ag/AgCl). The uranium metal was successfully deposited and separated from cerium. The morphology of deposit and cross section of electrode were investigated by SEM, firstly uranium deposit alloys with stainless steel and forms a thin transition layer, and secondly the uranium metal layer grows from the transition layer. The separation factors of uranium/cerium on different recovery ratios were determined through a series of steps. It was found that the content of cerium in the deposit and separation factors declined with increasing the initial concentration of U³⁺ in molten salts; the separation factors remained stable at around 20 in different uranium recovery ratios.     

Development of an integrated sieve-crucible assembly for sequential operation of liquid salt separation and vacuum distillation

The solid cathode processing is necessary to separate salt from the cathode product for the preparation of a uranium ingot with a high purity in the pyroprocess. The capacity of a salt distiller should be sufficiently large to reach the throughput of uranium electro-refining process. In this study, an assembly composing a liquid separation sieve and a distillation crucible was developed for the sequential operation of a liquid salt separation and a vacuum distillation in the same tower. The feasibility of the sequential salt separation was examined by the rotation test of the sieve-crucible assembly and sequential operation of a liquid salt separation and a vacuum distillation. The adhered salt in the uranium deposits was removed successfully. The salt content in the deposits was below 0.1 wt% after the sequential operation of the liquid salt separation—salt distillation. From the results of this study, it could be concluded that efficient salt separation can be realized by the sequential operation of liquid salt separation and vacuum distillation in one distillation tower since the operation procedures are simplified and no extra operation of cooling and reheating is necessary.     

Producing metallic titanium through electro-refining of titanium nitride anode

Titanium nitride (TiN) was used as consumable anode to produce metallic titanium in molten salts. The electrochemical dissolution of TiN was investigated. It was found that nitrogen (N₂) was monitored at the anode during electrolysis. The titanium ion species was changed between Ti^2 + and Ti^3 + depending on the electrochemically dissolving potentials of TiN. Furthermore, the product on the cathode was analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that pure titanium powders can be prepared by the TiN electro-refining in a molten salt bath.     

Determination of the thickness of an electrodeposited thorium film with SiC alpha detectors

Alpha spectroscopy can be used to quantify actinide (e.g., U, Pu) concentration in a molten salt electrorefining environment. One could electroplate actinide samples directly onto a semiconductor alpha particle detector to obtain representative isotopic concentrations from a measured alpha particle energy spectrum. In this work, we fabricated a SiC Schottky device that can be partially biased to a depletion depth of 8.8 µm that is able to measure the energy spectrum of 4.012 MeV alpha particles emitted from a thorium film with a thickness. We also present a method in calculating the thickness of a medium thick alpha source with a dE/dx detector.     

Electrochemical study on various types of anode materials in LiCl–KCl eutectic salt used in the electro-winning process

The electrochemical behaviors of the molybdenum, graphite, and glassy carbon anode have been observed during the electrolysis of uranium chloride in an UCl₃–LiCl–KCl molten salt. The cyclic voltammogram data of the molybdenum anode showed that Mo anode could be corroded by the formation of Mo compounds or its dissolution. However, the experimental data of the graphite and the glassy carbon anode showed that the chlorine evolution started from 1.2 V and the anode potential remained stable during uranium electrolysis. Therefore, the graphite and glassy carbon may be used as an inert anode in the electro-winning process of pyroprocessing.     

Investigation of the evaporation of rare earth chlorides in a LiCl–KCl molten salt

Uranium dendrites which were deposited at a solid cathode of an electrorefiner contained a certain amount of salts. These salts should be removed for the recovery of pure metal using a cathode processor. In the uranium deposits from the electrorefining process, there are actinide chlorides and rare earth chlorides in addition to uranium chloride in the LiCl–KCl eutectic salt. The evaporation behaviors of the actinides and rare earth chlorides in the salts should be investigated for the removal of salts in the deposits. Experiments on the salt evaporation of rare earth chlorides in a LiCl–KCl eutectic salt were carried out. Though the vapor pressures of the rare earth chlorides were lower than those of the LiCl and KCl, the rare earth chlorides were co-evaporized with the LiCl–KCl eutectic salt. The Hertz–Langmuir relation was applied for this evaporation, and also the evaporation rates of the salt were obtained. The co-evaporation of the rare earth chlorides and LiCl–KCl eutectic were also discussed.     

Quantitative analysis of trivalent uranium and lanthanides in a molten chloride by absorption spectrophotometry

As an analytical application for pyrochemical reprocessing using molten salts, quantitative analysis of uranium and lanthanides by UV/Vis/NIR absorption spectrophotometry was performed. Electronic absorption spectra of LiCl–KCl eutectic at 773 K including trivalent uranium and eight rare earth elements (Y, La, Ce, Pr, Nd, Sm, Eu, and Gd as fission product elements) were measured in the wavenumber region of 4,500–33,000 cm^?1. The composition of the solutes was simulated for a reductive extraction condition in a pyroreprocessing process for spent nuclear fuels, that is, about 2 wt% U and 0.1–2 wt% rare earth elements. Since U(III) possesses strong absorption bands due to f–d transitions, an optical quartz cell with short light path length of 1 mm was adopted in the analysis. The quantitative analysis of trivalent U, Nd, Pr, and Sm was possible with their f–f transition intensities in the NIR region. The analytical results agree with the prepared concentrations within 2σ experimental uncertainties.     

NaCl-KCl-CeCl3熔盐体系电解精炼铈的研究

用Ce作为模拟金属,开展了NaCl-KCl-CeCl3熔盐体系电解精炼提纯金属Ce的研究,为乏燃料干法后处理提纯U, Pu等金属提供了研究基础。研究了CeCl3浓度、电解温度和阴极电流密度对电解精炼Ce的收率和电流效率的影响,得到优化的工艺条件为:CeCl3浓度为20%~25%,电解精炼温度为900~950℃,阴极电流密度为0.35~0.45 A·cm^-2,得到了成型致密的金属铈锭,产品的收率大于70%。杂质元素Al, Fe, Cu, Cr,Mn的含量均有大幅度的下降,尤其是Fe的去除效果明显从1300×10^-6降低为200×10^-6。     

The solid–liquid separation characteristics of pure LiCl–KCl eutectic salt using different types of crucibles

Uranium deposits were recovered at the solid cathode of an electrorefining system, and deposited uranium dendrite normally contains about 30–40 wt% LiCl–KCl eutectic salts. Therefore, a separation of the eutectic salts from deposited uranium is essential for reusing these salts and uranium. A process such as distillation was employed for cathode processing due to the advantages of a minimal generation of secondary waste, a compact unit process, and simple and low-cost equipment. However, the realization of a wide evaporation area or high distillation temperature is limited by various factors such as the material or structure of a distiller. Also, the electrical energy flow from outside has a lot of consumption to maintain the high temperature. Hence, in this study, solid–liquid separation experiments are proposed to increase the throughput of the salt removal process by the separation of the liquid salt prior to the distillation of the LiCl–KCl eutectic salt. The solid–liquid separation of salt was carried out in a vertical type distiller. The behavior of the solid–liquid separation of pure eutectic salt was investigated as a function of temperature, pressure, sieve size, and crucible shape. From the experimental results using pure eutectic salts, the amount of salt separation was achieved at more than 94 wt%. The rate of solid–liquid separation of salt using 600 °C is higher than that of 500 °C under the same condition. The influence of a vacuum for solid–liquid separation can be disregarded, and the separation rate of a 100 mesh was higher than that of a 150 mesh. In addition, the rate of separation for salts using a porous crucible is higher than that in a non-porous crucible.     

锕系元素在熔盐体系中的电化学及萃取特性研究进展

熔融盐是一类非水溶剂,分为高温熔盐、室温熔盐和低温熔盐。 作为反应介质和电解介质,因其优良性能,可以溶解很多难溶于水的活泼金属。 近年来,该领域的研究热点是将熔盐作为干法后处理的电解质分离和回收锕系元素。 本文综述了锕系元素在高温熔盐中的电化学行为、热力学等物理特性,介绍了近几年室温离子液体(RTILs)的研究进展及锕系元素在RTILs中萃取特性的最新研究成果,展望了熔盐体系未来的研究方向。     

Determination of uranium in salt solutions containing high levels of lanthanides via epithermal neutron activation analysis

Epithermal Instrumental Neutron Activation Analysis has been used to measure the concentration of uranium in eutectic salt solutions in support of a research program in which the actinide elements are separated from rare earths and other fission products using high-temperature electo-deposition. The uranium response over three decades in concentration follows a negative power function; and high concentrations of samarium interfere with the determination of uranium but can be accurately corrected. The EINAA method was successfully used to analyze NIST SRM 278 Obsidian and NIST SRM 1566a Fly Ash.     

Characteristics of an integrated cathode assembly for the electrolytic reduction of uranium oxide in a LiCl-Li2O molten salt

Summary Electrochemical behavior of the reduction of uranium oxide was studied in a LiCl-Li₂O molten salt system with an integrated cathode assembly. The mechanism for the electrolytic reduction of uranium oxide was studied through cyclic voltammetry. By means of a chronopotentiometry, the effects of the thickness of the uranium oxide, the thickness of the MgO membrane and the material of the conductor of an integrated cathode assembly on the overpotential of the cathode were investigated. From the voltamograms, the reduction potential of the uranium oxide and Li₂O was obtained and the two mechanisms of the electrolytic reduction were considered with regard to the applied cathode potential. From the chronopotentiograms, the exchange current, the transfer coefficient and the maximum allowable current based on the Tafel behavior were obtained with regard to the thickness of the uranium oxide, and of the MgO membrane and the material of the conductor of an integrated cathode assembly.     

Quantitative analysis of barium and strontium in simulated oxide fuel during electrolytic reduction and salt distillation

Barium and strontium contents of alkaline-earth fission products in simulated oxide fuel were quantitatively analyzed during electrolytic reduction and salt distillation. Electrolytic reduction of the simulated oxide fuel was conducted at 650 °C in molten Li₂O–LiCl salt. The residual salt was then separated by salt distillation at 900 °C. Ba and Sr were partially separated from the simfuel and accumulated in the salt during oxide reduction. Ba and Sr levels of 0.21–0.56 wt% of their initial simfuel masses were found in the distilled salt; 2.28–3.34 wt% were found in the fuel remaining after the salt distillation.     

Studies of leaching of copper ores and flotation wastes

In Poland, there are significant deposits of copper ores. During the copper extraction, large amounts of flotation wastes are produced. In the ores and flotation wastes many other important elements are present. The main goal of this work was analysis of uranium content and to elaborate procedures for recovery of U from these materials. Two types of ores and four types of waste were examined. It has been found that uranium content varies from 4.5 to 25 ppm. The other elements have also been determined in these materials: Cu (4–5 % in ores and 0.3–1.7 % in waste), Ag, Re, Mo, La, Ni, V, etc. For leaching, sulfuric acid and sodium carbonates of various concentrations (temperature, time) were used. The optimum conditions for leaching have been found. The concentration of uranium in the final solution was generally less than 25 μg/mL. The other elements are also present in the leaching solutions. Simultaneous liquid–liquid extraction of uranium with these elements from leaching solution is under study. In our opinion, only such combined procedure for the recovery of uranium together with the accompanying elements could be cost-effective.     

Development of the anode-liquid cathode module (ALCM) for a high-throughput electrowinner

The employment of multiple electrode pairs is one of the ways to achieve a high-throughput electrowinner. In order to improve the U/TRU recovery rate and also to obtain economic advantages for a molten salt electrowinning process, which is a major step in pyroprocessing, the development of an electrode module having a compact structure is required. Here, we designed an anode-liquid cathode module (ALCM), where two graphite anodes are symmetrically arranged centering on a liquid Cd cathode with a mesh stirrer to prevent the growth of U dendrites, to be applied to the electrowinning process. As a preliminary study, basic electrochemical characterization of the designed electrode module was conducted and its capability for U recovery was evaluated in the ALCM-employed lab-scale electrowinner including LiCl–KCl-UCl₃ at 500 °C. Also, the morphology and component of the recovered U residue by distillation of the cathode product was examined by FE-SEM and EDS analyses.     

Biosorption of uranium(VI) by free and entrapped Chlamydomonas reinhardtii: kinetic,equilibrium and thermodynamic studies

Biosorption of uranium from aqueous solution onto the free and entrapped algae, “Chlamydomonas reinhardtii” in carboxymethyl cellulose (CMC) beads was investigated in a batch system using bare CMC beads as a control system. CMC can be a potential natural biosorbent for radionuclide removal as it contains carboxyl groups. However, limited information is available with the biosorption of uranium by CMC, when adsorption isotherm, kinetics and thermodynamics parameters are concerned. The biosorbent preparations were characterized by swelling tests, FTIR, and surface area studies. The effects of pH, temperature, ionic strength, biosorbent dosage, and initial uranium concentrations on uranium biosorption were investigated. Freely suspended algae exhibited the highest uranium uptake capacity with an initial uranium ion concentration of 1,000 mg/L at pH of 4.5 and at 25 °C. The removal of U(VI) ion from the aqueous solution with all the tested biosorbents increased as the initial concentration of U(VI) ion increased in the medium. Maximum biosorption capacities for free algal cells, entrapped algal cells, and bare CMC beads were found to be 337.2, 196.8, and 153.4 mg U(VI)/g, respectively. The kinetic studies indicated that the biosorption of U(VI) ion was well described by the pseudo-second order kinetic model. The variations in enthalpy and entropy for the tested biosorbent were calculated from the experimental data. The algal cells entrapped beads were regenerated using 10 mM HNO₃, with up to 94 % recovery. Algal cells entrapped CMC beads is a low cost and a potential composite biosorbent with high biosorption capacity for the removal of U(VI) from waters.     

Sorption of uranium on magnetite nanoparticles

Magnetite (Fe₃O₄) nanoparticle was synthesized using a solid state mechanochemical method and used for studying the sorption of uranium(VI) from aqueous solution onto the nanomaterial. The synthesized product is characterized using SEM, XRD and XPS. The particles were found to be largely agglomerated. XPS analysis showed that Fe(II)/Fe(III) ratio of the product is 0.58. Sorption of uranium on the synthesized nanomaterials was studied as a function of various operational parameters such as pH, initial metal ion concentration, ionic strength and contact time. pH studies showed that uranium sorption on magnetite is maximum in neutral solution. Uranium sorption onto magnetite showed two step kinetics, an initial fast sorption completing in 4–6 h followed by a slow uptake extending to several days. XPS analysis of the nanoparticle after sorption of uranium showed presence of the reduced species U(IV) on the nanoparticle surface. Fe(II)/Fe(III) ratio of the nanoparticle after uranium sorption was found to be 0.48, lower than the initial value indicating that some of the ferrous ion might be oxidized in the presence of uranium(VI). Uranium sorption studies were also conducted with effluent from ammonium diuranate precipitation process having a uranium concentration of about 4 ppm. 42% removal was observed during 6 h of equilibration.     

Consecutive recovery of uranium oxides from uranyl-containing molten alkali metal sulfate electrolytes

The oxygen coefficient (atomic ratio O/U) of a cathode product and the cathode current yield in consecutive recovery of uranium oxides from molten salt mixtures (nLi₂SO₄ + (1 ? n)Cs₂SO₄) + UO₂SO₄ and (Li,Na,K)₂SO₄ + UO₂SO₄ in air were analyzed as dependent on the electrolyte and the mean solvent-salt cation radius. The increasing mean solvent-salt cation radius and decreasing UO₂SO₄ concentration in the electrolyte during long-term electrolysis suppress the solvolysis of uranyl ions and decrease the oxygen coefficient in the cathode product. The cathode current yield decreases systematically during electrolysis and drops to zero long before the uranium is completely recovered from the melt. The ultimate uranium recovery increases as the mean solvent-salt cation radius increases.     

Study on the time-dependent simulation and input parameter sensitivity of spent nuclear fuel electrorefining

A computer program for the simulation of spent nuclear fuel electrorefining has been developed based on general Butler–Volmer kinetics and mass transport within a diffusion layer model. The effect of a solid electrode area change due to the dissolution and deposition, the solubility of the element in the liquid Cd, and a volume change due to the development of metallic compounds were considered in this program. The program has been verified through Tomczuk et al.’s experiment, and the results show a good agreement of within 4 and 2 % for U amount in the anode and cathode, respectively. The sensitivity of several input parameters such as the standard electrode potential, activity coefficient, diffusion coefficient, diffusion layer thickness, and transfer coefficient has been tested. It was found that the standard electrode potential is very sensitive, as a 3 % variation leads to a U amount change of more than 30 and 50 % in the anode and molten salt, respectively. The sensitivity of the other input parameters is much less than the standard electrode potential, but significant within the data difference with the other references, except for the transfer coefficient. This result is useful for understanding the effectiveness of each input parameters on the simulation results.     

Current pulse measurement of capacitance during molten salt electrochemical experiments

In earlier work in our laboratories, a current pulse method was developed that allows in situ (dynamic) measurements of electrode capacitance. The present work describes the successful application of the technique to the study of electrode properties in molten salt electrolytes. As expected, the electrode capacitance increases as the electrode surface area exposed to a molten salt bath increases. Furthermore, creep of the bath along the surface of a conductive ceramic anode and subsequent ingress into the anode pores is observed as an increase in capacitance. The pulse technique also gives an indication of phase changes that occur during the reduction of a solid titanium dioxide cathode and a highly sensitive measure of the temperature at which initial freezing of the calcium chloride electrolyte begins. These observations provide useful in situ information about changes in electrode properties in molten salt electrolytes that are difficult to obtain from other techniques. For consideration in the Special Edition: Oldham Festchrift Dedicated to our dear friend Keith B. Oldham on the occasion of his 80th birthday.