Integrated Experiments of Electrometallurgical Pyroprocessing Using Plutonium Oxide

Electrometallurgical pyroprocessing is a promising technology to realize actinide fuel cycle. Integrated experiments to demonstrate electrometallurgical pyroprocessing of PuO₂ in continuous operation were carried out. In each test, 10–20 g of PuO₂ was reacted with Li reductant to form metal product. The reduction products were charged in an anode basket of the electrorefiner with LiCl-KCl-UCl₃ electrolyte. Using the anode, deposition of uranium on the solid cathode was carried out when PuCl₃/UCl₃ concentration ratio was low. After the Pu/U ratio in the salt electrolyte was increased enough, Pu and U were recovered simultaneously on a liquid cadmium cathode. By heating up the deposits for distillation of the salt and the cadmium, U metal or Pu-U alloyed metal was obtained as residues in the crucible. It was the first result to demonstrate the recovery of metal actinides in the continuous operation of pyroprocessing of oxide fuels.     

Electrochemical Reduction of (U,Pu)O2 in Molten LiCl and CaCl2 Electrolytes

The electrochemical reduction of UO₂-PuO₂ mixed oxides (MOX) was performed in molten LiCl at 923 K and CaCl₂ at 1,123 K to evaluate the behavior of the plutonium quantitatively and to define the optimum conditions for the electrochemical reduction of those materials.

In LiCl, excess deposition of lithium metal can be avoided and the MOX was smoothly reduced at ?0.65 V vs. Bi-35 mol% Li reference electrode. The reduction ratio calculated from the mass change of the samples taken during the electrochemical reduction and the ratio evaluated by gas-burette method were in good agreement. The cathodic current efficiency remained 30–50% mainly due to the deoxidation of tantalum cathode basket. Although dissolution of plutonium and americium into the electrolyte was found by the chemical analysis, the dissolved amount was negligible and had no immediate influence on the feasibility of the electrochemical reduction process.

In CaCl₂, reduction of the MOX occurred in whole range of the tested cathode potential (?0.15 V to ?0.40 V vs. Ca-Pb reference electrode). The cathodic current efficiency was around 30%. Although the MOX was completely reduced at ?0.25 V, the reduction was interrupted by formation of the surface barrier made of the reduced material and the vacancy between the reduced and the non-reduced areas at ?0:30 V. Plutonium and americium dissolved also into the CaCl₂ electrolyte to slightly higher concentrations than those observed in LiCl electrolyte. The analyses for the reduction products showed that the amount of those actinides lost from the cathode was much larger than that found in the electrolyte, probably due to the formation of mixed oxide precipitate.     

Observation of Lithium Isotope Effect Accompanying Electrochemical Insertion of Lithium into Tin

Electrolysis of an organic electrolyte solution containing lithium ions was conducted to observe lithium isotope fractionation accompanying electrochemical insertion of lithium from the electrolyte to tin metal. The experimental setup consisted of a three-electrode electrolysis cell with a tin wire as cathode, lithium foils as anode and reference electrode and 1 M LiPF₆ dissolved in 1:2 volume ratio of ethylene carbonate and methylethyl carbonate as electrolyte and a power supply. The supplied electric energy was mostly consumed for the lithium insertion from the electrolyte to the tin cathode within the range of the cathode potential, relative to the reference electrode potential, from 0.05 V to 0.30 V. The single-stage separation factor increased with increasing cathode potential and seemed to asymptotically approach to the limiting value of 1.015 at 25®C, with ⁶Li being preferentially fractionated into tin metal.     

Electrochemical Study on the Reduction Mechanism of Uranium Oxide in a LiCl-Li2O Molten Salt

By means of a linear sweep voltammetry, a cyclic voltammetry and a chronopotentiometry, the electrolytic reduction of uranium oxide has been studied to establish the reduction mechanisms, which are based on a simultaneous uranium oxide reduction and a Li₂O electrowinning, and the formation and electrolysis of lithium uranate. From the voltammograms, the reduction potentials of the uranium oxide and Li₂O were obtained. From the chronopotentiometries based on the results of the voltammograms, the uranium oxide was reduced to uranium metal through the reduction mechanisms showing a more than 99% conversion. For a verification of the reduction mechanisms feasibility, basic data on the electrolytic reduction of the uranium oxide was obtained from the experiments and the characteristics of the closed recycle of Li₂O were discussed.     

Application of a boron doped diamond (BDD) electrode as an anode for the electrolytic reduction of UO2 in Li2O–LiCl–KCl molten salt

A boron doped diamond thin film electrode was employed as an inert anode to replace a platinum electrode in a conventional electrolytic reduction process for UO₂ reduction in Li₂O–LiCl molten salt at 650 °C. The molten salt was changed into Li₂O–LiCl–KCl to decrease the operation temperature to 550 °C at which the boron doped diamond was chemically stable. The potential for oxygen evolution on the boron doped diamond electrode was determined to be approximately 2.2 V vs. a Li–Pb reference electrode whereas that for Li deposition was around ?0.58 V. The density of the anodic current was low compared to that of the cathodic current. Thus the potential of the cathode might not reach the potential for Li deposition if the surface area of the cathode is too wide compared to that of the anode. Therefore, the ratio of the surface areas of the cathode and anode should be precisely controlled. Because the reduction of UO₂ is dependent on the reaction with Li, the deposition of Li is a prerequisite in the reduction process. In a consecutive reduction run, it was proved that the boron doped diamond could be employed as an inert anode.     

氯化锂熔盐中八氧化三铀电解还原基础研究

将氧化物乏燃料直接电解还原为粗金属的过程是目前以电解还原-电解精炼为特征的主流干法后处理流程的重要步骤。二氧化铀（UO₂）是乏燃料的最主要成分,将致密的UO₂芯块转化为八氧化三铀（U₃O₈）粉末后,再进行电化学还原能有效提高还原速率。因此,以U₃O₈为研究对象,开展其在氯化锂（LiCl）熔盐中的电解还原机理研究,对后处理干法流程的开发具有重要的现实意义。本文在650 ℃的LiCl熔盐中,采用循环伏安法和恒电位电解法,研究U₃O₈的电解还原行为;对电解后的样品,运用XRD、SEM等手段分析其组成和形貌,并推测相应的还原机理。     

Nitration by a simulated fuel technique for nitride fuel re-fabrication

Nitration reaction of a spent nuclear oxide fuel through a carbothermic reduction and the change in thermal conductivity of the resultant nitride specimens were investigated by a simulated fuel technique for use in nitride fuel re-fabrication from spent oxide fuel. The simulated spent oxide fuel was formed by compacting and sintering a powder mixture of UO₂ and stable oxide fission product impurities. It was pulverized by a 3-cycle successive oxidation-reduction treatment and converted into nitride pellet specimens through the carbothermic reduction. The rate of the nitration reaction of the simulated spent oxide fuel was decreased due to the fission product impurities when compared with pure uranium dioxide. The amount of Ba and Sr in the simulated spent oxide fuel was considerably reduced after the nitride fuel re-fabrication. The thermal conductivity of the nitride pellet specimen in the range 295-373 K was lower than that of the pure uranium nitride but higher than the simulated spent oxide fuel containing fission product impurities.     

Preliminary investigation of possible low-temperature fusion

Preliminary tests have been made with electrolytic cells utilizing 0.2N LiOD in D₂O as the electrolyte and a palladium cathode surrounded by a wire-wound platinum anode operating at cathode current densities of 100–400 mA/cm². The cathodes were swaged to diameters of 2.8 or 5.5 mm with 8.5 cm of active length. The electrolyte temperature was controlled, heat was removed by flowing water in a cooling jacket, and the cell was insulated. Cooling water and electrolyte temperatures were measured by thermocouples, and neutron and gamma-ray spectra were recorded. The electrolyte level was periodically monitored and replenished with D₂O. Tests up to 2 weeks in duration were made with no sustained release of energy in excess of the electrical power input, although there was one period of 12 h when an unaccountable heat excess was observed. In another test, an anomalous neutron flux was measured during the first few hours that was 3.5 standard deviations above the background.     

俄罗斯氧化物乏燃料电沉积流程研究进展

高温熔盐干法后处理以熔盐作为电解质，通过电解精炼和电沉积回收核燃料中的铀和钚。目前，俄罗斯、美国、日本、韩国和欧盟等国均在积极发展乏燃料高温熔盐干法后处理技术的研究，其中俄罗斯的金属氧化物核燃料电沉积流程是经典的流程之一。本文对俄罗斯原子反应堆研究所（Research Institute of Atomic Reactors,RIAR）发展的氧化物乏燃料高温熔盐电沉积干法后处理的发展现状、流程及特点进行了综述。     

Development of an environmentally benign reprocessing technology—pyrometallurgical reprocessing technology

Present status of research and development on pyrometallurgical reprocessing technology in Central Research Institute of Electric Power Industry is described with emphasis on electrorefining and waste salt immobilization. As for the electrorefining, three different electrodes — anode basket, solid cathode, and liquid-cadmium cathode — have been being investigated; prismatic anode basket with faster rotation was found to accelerate metal-fuel dissolution. Morphology and collection efficiency of electrodeposited uranium on solid cathode were found to vary with ratio of cathode to anode surface area. Liquid-cadmium cathode with a paddle-shape stirrer was developed and determined maximum uranium concentration into the cathode without dendrite formation. As for waste salt immobilization, sodalite is proposed as waste form and synthesized by dry reaction without gas generation. Measured leachability of the synthesized sodalite is as low as those of vitrified waste form.     

Electrolysis of Burnup-Simulated Uranium Nitride Fuels in LiCl-KCl Eutectic Melts

The electrochemical behavior of burnup-simulated uranium nitride fuels containing representative solid fission product elements, UN+Mo (Mo = 2.84 wt%), UN+Pd (Pd = 4.6 wt%) and (U, Nd)N (NdN = 8.0 wt%), was investigated in the molten LiCl-KCl eutectic salt with 0.54 wt% UCl₃ in order to clarify the effects of fission products on the dissolution of actinide nitrides and the behavior of FPs in the electrorefining of spent nitride fuel. The rest potentials of burnup-simulated UN pellets were similar to that of pure UN. The electrochemical dissolution of UN began at about _0:75V vs Ag/AgCl reference electrode in all samples as well as that of pure UN. After the electrolyses at the constant anodic potential of ?0:65––0:60V vs Ag/AgCl, most of UN was dissolved into LiCl-KCl as UCl₃ at the anode, and U was recovered in the liquid Cd cathode in all samples. Furthermore, Nd was dissolved at the anode and accumulated into LiCl-KCl as NdCl₃, while Mo and Pd were not dissolved but remained at the anode.     

Corrosion behavior of ceramic structural materials in an electrolytic reduction process

The electrolytic reduction of spent oxide fuel involves the liberation of oxygen in a molten LiCl electrolyte, which results in a chemically aggressive environment that is too corrosive for typical alloying structural materials. Therefore, the choice of the optimum material for the processing equipment that handles molten salt is critical. We investigated the corrosion behaviors of CaO-stabilized ZrO₂ (CSZ) and mullite (Al₆Si₂O₁₃) at 650°C for 168 h in molten (1, 3) wt% Li₂O–LiCl. The as-received and tested specimens were examined by scanning electron microscopy/X-ray energy dispersive spectrometry and X-ray diffraction. CSZ showed a much better hot-corrosion resistance in the presence of Li₂O–LiCl molten salt than mullite. The surface corrosion layers of mullite consisted of LiAlSiO₄ in 1 wt% Li₂O–LiCl, and a LiAlO₂ phase appeared as the Li₂O concentration increased to 3 wt%. Furthermore, Li₂SiO₃ was the only corrosion product observed at 3 wt% Li₂O–LiCl. The surface corrosion layers of CSZ were composed mainly of tetragonal-ZrO₂ with partial monoclinic-ZrO₂ in 1 wt% Li₂O–LiCl, and a Li₂ZrO₃ phase appeared at 3 wt% Li₂O–LiCl. There was no corrosion product detached from the surface for those specimens. CSZ was beneficial for increasing the hot-corrosion resistance of the structural materials that handle high-temperature molten salts containing Li₂O.     

Separation Factor of Hydrogen and Deuterium by Suspension Electrolysis Using Mercury Cathode

The electrolytic separation factor between hydrogen and deuterium was examined using mercury or else platinum cathode immersed in IF NH₄Cl in 10v/oD₂O water containing cobalt sulfide powder in suspension. Several other kinds of powdered materials in suspension were also studied. In the case of mercury pool electrode, the materials added in suspension were effective in enhancing the hydrogen/deuterium separation factor, but powder suspension was ineffective on platinum plate electrode. The powdered material added in suspension served as catalyst on the hydrogen evolution reaction at the mercury cathode. The influence of the applied potential on the separation factor was studied over the temperature range of 15°~80°C. The results provided an indication of the rate-determining steps governing the electrolytic hydrogen evolution.

The experimental values obtained for the separation factor and activation energy gave an insight into the mechanism of the rate-determining step of the hydrogen evolution at the working electrode.     

高温熔盐中氧化物乏燃料的电化学还原研究进展

干法后处理流程可应用于快堆乏燃料后处理。由美国开发的熔盐电解精炼流程是目前最具应用前景的干法后处理流程之一。为了将电解精炼流程应用于氧化物乏燃料后处理,需要将氧化物乏燃料转化为金属。目前电化学还原是应用最广的氧化物乏燃料还原方法,但是该过程仍然存在亟待解决的关键科学与技术问题。本文针对氧化物乏燃料电化学还原研究进展进行综合阐述,主要包括过程简介、研究现状及电化学还原机理等几个方面。     

乏燃料干法后处理中铀电沉积行为模拟研究

基于多物理场耦合软件COMSOL,在LiCl-KCl熔盐体系中建立以电解槽为阳极、双石墨棒为阴极的氯化铀电沉积行为三维数值模型。通过研究熔盐中铀离子的运动情况以及对阴极几何形状的实时计算,得到了铀电沉积的沉积层厚度随时间的变化情况,得到了铀离子阴极沉积行为与阴极表面的位置、熔盐铀离子浓度、反应温度、平均电流密度之间的关系。研究中,还将模拟结果与氯化铀熔盐电解实验数据进行对比,计算与实验结果拟合良好,证明铀电沉积行为模拟的可靠性,模拟结果可为乏燃料干法后处理中铀的提取提供设计参考。      

Studies on disintegrating spherical fuel elements of high temperature gas-cooled reactor by a electrochemical method

Spherical fuel elements of a high temperature gas-cooled reactor were disintegrated through a electrochemical method with NaNO₃ as electrolyte. The X-ray diffraction spectra and total carbon contents of the graphite fragments were determined, and the results agreed with those from simulated fuel elements. After conducting the characterization analysis and the leaching experiment of coated fuel particles, the uranium concentrations of leaching solutions and spent electrolyte were found to be at background levels. The results demonstrate the effectiveness of the improved electrochemical method with NaNO₃ as electrolyte in disintegrating the unirradiated fuel elements without any damage to the coated fuel particles. Moreover, the method avoided unexpected radioactivity contamination to the graphite matrix and spent electrolyte.     

On the effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel

Radiation induced oxidative dissolution of UO₂ is a key process for the safety assessment of future geological repositories for spent nuclear fuel. This process is expected to govern the rate of radionuclide release to the biosphere. In this work, we have studied the catalytic effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel. The experimental studies were performed using UO₂ pellets containing 0%, 0.1%, 1% and 3% Pd as a model for spent nuclear fuel. H₂O₂ was used as a model for radiolytical oxidants (previous studies have shown that H₂O₂ is the most important oxidant in such systems). The pellets were immersed in aqueous solution containing H₂O₂ and and the consumption of H₂O₂ and the dissolution of uranium were analyzed as a function of H₂ pressure (0–40 bar). The noble metal inclusions were found to catalyze oxidation of UO₂ as well as reduction of surface bound oxidized UO₂ by H₂. In both cases the rate of the process increases with increasing Pd content. The reduction process was found to be close to diffusion controlled. This process can fully account for the inhibiting effect of H₂ observed in several studies on spent nuclear fuel dissolution.     

Recovery of Neptunium by Electrolysis of NpN in LiCl-KCl Eutectic Melts

The electrochemical behavior of neptunium nitride, NpN, in the LiCl-KCl eutectic melt containing NpCl₃ at 450, 500 and 550°C was investigated from the viewpoint of the application of electrochemical refining in a fused salt to nitride fuel cycle. The electrochemical dissolution of NpN began nearly at the potential theoretically evaluated, though this reaction was irreversible owing to small partial pressure of N₂ in the salt and the reaction rate was slow. Under the electrolysis in the NpCl₃-LiCl-KCl eutectic melt, NpN was dissolved into the salt as Np³⁺ at the anode, and Np metal was deposited at the cathode. About 0.5 g of Np metal was obtained by heating the deposit containing the salt at 800°C for 3.6 ks.     

Direct electrochemical reduction of solid uranium oxide in molten fluoride salts

The direct electrochemical reduction of UO₂ solid pellets was carried out in LiF-CaF₂ (+2 mass.% Li₂O) at 850 °C. An inert gold anode was used instead of the usual reactive sacrificial carbon anode. In this case, oxidation of oxide ions present in the melt yields O₂ gas evolution on the anode. Electrochemical characterisations of UO₂ pellets were performed by linear sweep voltammetry at 10 mV/s and reduction waves associated to oxide direct reduction were observed at a potential 150 mV more positive in comparison to the solvent reduction. Subsequent, galvanostatic electrolyses runs were carried out and products were characterised by SEM-EDX, EPMA/WDS, XRD and microhardness measurements. In one of the runs, uranium oxide was partially reduced and three phases were observed: nonreduced UO₂ in the centre, pure metallic uranium on the external layer and an intermediate phase representing the initial stage of reduction taking place at the grain boundaries. In another run, the UO₂ sample was fully reduced. Due to oxygen removal, the U matrix had a typical coral-like structure which is characteristic of the pattern observed after the electroreduction of solid oxides.     

电化学调价制备2AF Ⅰ.二甲基羟胺和甲基肼的电解氧化

为将电化学应用于先进后处理流程中的1BP调价制备2AF,采用循环伏安法研究了甲基肼和二甲基羟胺的电化学行为。使用无隔膜的电解池,以钛基镀铂电极为阳极,以钛电极为阴极,在恒电流的条件下进行了电解实验。研究表明,甲基肼在阳极被直接电解氧化;甲基肼被完全破坏后,二甲基羟胺在阴极发生间接的氧化反应。