乏燃料熔盐电解后处理动力学模型研究

电解精炼是乏燃料干法后处理工艺中的关键环节。针对氯化锂-氯化钾（LiCl-KCl）熔盐环境中的电解精炼行为,基于电极表面处的反应过程建立了动力学模型。通过与现有实验数据的比较验证了模型的有效性。通过该模型,可以预测电解精炼过程中液镉阳极中乏燃料的溶解和阴极处金属沉积的动力学特征,以及所涉及元素的分电流、电极电位和熔盐中离子浓度的演变。除此之外,该模型还能模拟多元素复杂体系电解精炼的过程。当使用锆、铀、钚及稀土作为阳极时,模拟结果显示在阳极处锕系元素和稀土元素随时间逐渐溶解,而惰性金属几乎不溶解。在熔盐中,钚的浓度逐渐增加而铀的浓度逐渐减少,当钚开始在固体阴极发生沉积后,铀的沉积速率减小,而稀土元素和锆在阴极的沉积量极少。     

Electrorefiner Liquid Cadmium Cathode Crucible Thermal Shock

Abstract

A liquid cadmium cathode is used in an electrorefiner to remove plutonium and minor actinides from spent nuclear fuel by pyroprocessing. Liquid cadmium in a beryllia crucible, originally at 35°C, is lowered into 500°C salt electrolyte to begin reprocessing. Crucible cracking from thermal stress would release cadmium into the liquid salt causing electrorefiner failure. This study's purpose was to predict if the ceramic crucible would fail. A handbook method showed it would. An analytical model eliminating two large conservatisms predicted no failure. A beryllia crucible preheated to 321°C was successfully immersed in electrorefiner salt without failure. The conclusion is that handbook methods can be severely conservative in predicting thermal stress failures for immersion in low thermal conductivity fluids.     

Modeling the Molten Salt Electrorefining Process for Spent Metal Fuel Using COMSOL

Molten salt electrorefining process is one of the key steps of the pyrochemical reprocessing flow sheet for the spent metallic fuel from fast reactors. The electrorefining process is simulated using COMSOL Multiphysics. This involves solving multiple equations corresponding to electrochemical reactions at the electrode surfaces, mass transfer of metal ions in the electrolyte, potential distribution in the electrolyte, and overall material balance of metal ions in a coupled manner. The model is validated using the data of laboratory scale electrorefining experiments from literature. The results show a good agreement with the present experimental data, the variation being less than 10% for the U and Pu concentration changes in liquid Cd anode and molten salt, and U deposit on solid cathode.     

A study on the recovery of actinide elements from molten LiCl–KCl eutectic salt by an electrochemical separation

Pyroprocessing is a prominent way for the recovery of the long-lived elements from the spent nuclear fuel. Electrorefining is a key technology for pyroprocessing and generally composed of two recovery steps—deposition of uranium onto a solid cathode and the recovery of TRU (TRansUranic) elements. In this study, it was investigated on electrochemical separation of actinides to develop an actinide recovery system in a molten LiCl–KCl eutectic salt. In the electrorefining experiment, uranium was successfully separated from cerium. The effects of the anode material and the surface area were investigated during the electrolysis experiments for a more efficient electrorefining system. Anode potential decreased with an increasing anode surface area, however, an anode effect was observed in case of a complicated anode structure for high surface area. Glassy carbon was found to be the best anode material among the molybdenum, graphite, glassy carbon, and oxide materials. It was found that the solid cathode with a perforated ceramic container could be one of the candidates for a salt clean-up process to remove residual actinide elements in the salt after the recovery step.     

Recovery of chlorine from dilute hydrochloric acid by electrolysis using a chlorine resistant anion exchange membrane

A new chlorine resistant anion exchange membrane enables innovative possibilities for hydrochloric acid electrolysis for recovery of chlorine. This is of interest for hydrochloric acid that is neutralized in the chemical industry because purity and concentration are not sufficiently high for recycling. In the common electrolysis process hydrochloric acid is fed into the anode compartment and needs a satisfactory HCl concentration for supplying the anode with chloride ions. Using an anion exchange membrane as a cell separator the feed flows into the cathode chamber where a low HCl concentration is acceptable because Cl⁻ ions at the anode can be supplied by addition of a salt which is not consumed. Experimental data of the membrane and the process are presented: membrane permselectivity improved up to above 97% using CaCl₂ as added salt, chlorine current efficiency up to 98% and oxygen content as low as 0.5 vol%, cell voltage at 4 kA m⁻² 2.3 V, equivalent to 1740 kWh per t produced chlorine, even at low HCl concentrations. Thus, the power consumption is comparable with the common process. A problem of the new process is the high water transport through the membrane. Therefore, experiments for two process alternatives were carried out. Disadvantages of water transport can be avoided by using a high concentrated CaCl₂ solution as anolyte and catholyte and as absorption medium for diluted HCl gas streams. Additionally, a cell design was investigated where the anode is directly connected to the membrane in an empty (gas filled) anode compartment.     

Application of Extraction Chromatography to Nuclear Fuel Reprocessing

Abstract

The potentialities of applying extraction chromatography to the reprocessing of reactor fuels on an industrial scale have been investigated. The stationary phase was undiluted (100%) tri-n-butyl phosphate (TBP) and the mobile phases were nitric acid or nitrate salt solutions with or without reducing agents for plutonium.

Several extraction chromatographic processes for the recovery of nuclear grade uranium and plutonium are described. The flowsheets are based on a systematic determination of the distribution coefficients of relevant metal species (particularly those of uranium, neptunium, plutonium, americium, ruthenium, zirconium and niobium) in the chromatographic systems employed.

The Purochromex process developed for the recovery of uranium and plutonium from light-water reactor fuels and the Eurochromex process developed for the separation of highly enriched uranium from irradiated U/A1 alloy, U/Zr alloy and uranyl sulfate fuels have successfully been hot-tested on a laboratory scale and cold-tested on an “industrial scale.”

Some complementary studies related to the separation processes. such as radiation degradation of the stationary phase and the removal of tributyl phosphate from product and waste streams, are also described.     

Purification of Plutonium Chloride Solutions via Precipitation and Washing

Abstract

Pyrochemical operations at Los Alamos Plutonium Facility use high temperature melts of calcium, sodium, and potassium salts in the plutonium metal purification process. Aqueous chloride operations recover the remaining plutonium from the residue salts, generating copious quantities of corrosive aqueous waste while occupying a large area of the facility. To minimize these problems, an alternative flow sheet was tested in which dissolved salts were precipitated and washed to remove interstitial chloride before further processing in aqueous nitrate operations. Results of the tests will be discussed from the perspective of chloride removal, plutonium recovery, filterability, and waste minimization.     

Anodic dissolution of tantalum and niobium in acetone solvent with halogen additives for electrochemical synthesis of Ta2O5 and Nb2O5 thin films

Tantalum(V) and niobium(V) oxide films, which are typically difficult to prepare by electrochemical methods using aqueous solutions, are easily fabricated in an acetone bath using Ta and Nb anodes as the metal sources and a metal-free solvent containing halide ions as the supporting electrolyte. At the initial stage of electrolysis, anodic oxidation of the metal anode proceeds in the presence of water as an impurity in the acetone solvent. Subsequently, pitting corrosion of the oxide film on the metal anode occurs as a result of the action of halide ions. In this stage, anodic corrosion proceeds only in the presence of Br₂, and not in acetone containing I₂. Finally, Ta or Nb species are deposited directly on the cathode surface via the reactions with cathodically generated hydroxide ions, and the films need to be annealed at high temperature to effect crystallization. In these processes, the metal plate acts as a soluble anode with respect to Br⁻ and as a metal source for electrodeposition. The coating on a stainless steel substrate prepared by the present technique acts as an effective barrier against electrolytic corrosion.     

The Analysis of Valence State of Np and Pu in Chinese High‐Level Liquid Waste

Abstract

The valence states of neptunium and plutonium in simulated and genuine Chinese high‐level liquid waste (HLLW) have been studied by solvent extraction. A TRPO process has been developed in our laboratory in recent years in which 30% TRPO‐kerosene was used as extractant. This process demonstrates good extraction efficiency for neptunium as well as uranium, plutonium, and americium. A countercurrent solvent extraction experiment with genuine HLLW has been carried out with 16 stages of centrifugal extractors. The results of valence state experiments showed that the neptunium and plutonium were mainly in the tetravalent state respectively. It is not necessary to regulate the valence state of neptunium before HLLW treatment by the Chinese TRPO process.     

Separation of Calcium and Cadmium by Electrodialysis in the Presence of Ethylenediaminetetraacetic Acid

ABSTRACT

The separation of calcium and cadmium ions in a system containing ethylenedi-aminetetraacetic (EDTA) as the complexing agent has been studied using a laboratory-scale batch electrodialyzer. The theoretical distribution diagram constructed for the Ca—Cd—EDTA system suggested that cadmium preferably formed negatively charged complexes while calcium remained in the positively charged uncomplexed form. The experimental results confirmed that under the influence of an electric field, calcium was exclusively transported to the cathode while more than 90% of cadmium totally removed from the middle compartment of the batch electrodialyzer migrated toward the anode. The separation effect resulting from EDTA complexation was studied within the 1·l5–4·0 pH range.     

Compatibility Issues of Yttria‐Stabilized Zirconia Solid Oxide Membrane in the Direct Electro‐Deoxidation of Metal Oxides

Direct electro‐deoxidation of metal oxides has become quite popular in the production of metals and alloys. In this process, metal oxide cathode is directly reduced to a metal in a molten CaCl₂ salt bath. The anode material used is graphite. Over the years, graphite is reported to cause numerous process difficulties. Recently, based on the solid oxide membrane technology, yttria‐stabilized zirconia (YSZ) has been tested as oxygen ion conducting membrane for the anode. The success of using a membrane implies its long‐term stability in the bath. In this paper, it is seen that YSZ chemically degrades in a static melt of CaCl₂ or CaCl₂–CaO. The degradation occurs by leaching of yttria into solution leading to the formation of monoclinic zirconia which, being porous, reacts with the molten electrolyte to form calcium zirconate. However, on application of voltage, YSZ degrades via a different mechanism. The metallic calcium produced during electrolysis increases the electronic conductivity of the salt, apparently leading to the electrochemical reduction of zirconia to ZrO_2?x. As a result, localized pores are formed which allow the infiltration of salts. Addition of yttria to the salt is seen to prevent both the chemical and electrochemical degradation of the membrane.     

The development of photo-electrochemical cells from systems with photocatalytic reactions

Photo-electrochemical cells were constructed using an illuminated rutile crystal as the anode and several another counter electrodes as the cathode. Their principle is based on a separation of the local cell processes which occur in heterogeneous photocatalytic reactions on an illuminated rutile catalyst. During the cell operation, methanol or iso-propanol, which was used as solvent of the electrolyte, was oxidized respectively to formaldehyde and acetone. Dissolved methylene blue, oxygen gas or proton was reduced respectively to leuco-methylene blue, hydrogen peroxide and hydrogen. When the metal cathode such as Pt was replaced by an illuminated p-GaP electrode utilizing proton discharge as the cathode reaction, the cell performance was improved, as expected from the results of water photolysis. The quantum yield and the energy efficiency for the cells were obtained.     

Studies on the Separation of Cadmium from Solutions by Foam Separation. II. Precipitate Flotation of Cadmium Hydroxide

Abstract

Foam separation of cadmium in relation to pH from solutions of different metal concentrations was carried out by means of lauryl sulfate. The effect of inert salt on the removal of cadmium hydroxide and cadmium cations by adsorbing colloid floation was also studied. The precipitate flotation results reflect the precipitation of the metal in the form of a hydroxide. The precipitation pH values calculated are approximately those at which cadmium removal over 50% is obtained. The presence of electrolyte has a negative effect on the results of precipitate flotation of cadmium hydroxide and adsorbing colloid flotation of cadmium cations with lauryl sulfate.     

A high-capacity lithium–air battery with Pd modified carbon nanotube sponge cathode working in regular air

We report a lithium–air battery with a free-standing, highly porous Pd-modified carbon nanotube (Pd–CNT) sponge cathode. The Pd-CNT sponge was synthesized through a chemical vapor deposition growth followed with an electrochemical deposition process. To build a whole lithium–air battery, the air cathode is integrated with a ceramic electrolyte-protected lithium metal anode and non-volatile ionic liquid electrolyte. The lithium anode is stable during the operation and long-time storage and the ionic liquid is chemically inert. By controlling the amount of ionic liquid electrolyte, the sponge is wet but not fulfilled by the electrolyte. Such configuration offers a tricontinuous passage for lithium ions, oxygen and electrons, which is propitious to the discharge reaction. In addition, the existence of Pd nanoparticles improves the catalytic reactivity of the oxygen reduction reaction. The battery is durable to any humidity level and delivers a capacity as high as 9092 mA h g⁻¹.     

On the use of vinylene carbonate (VC) as an additive to electrolyte solutions for Li-ion batteries

Vinylene carbonate (VC) was tested as an additive to electrolyte solutions for Li-ion batteries. For the model electrodes, synthetic graphite was chosen as the anode material, while LiMn₂O₄ spinel and LiNiO₂ were chosen as the cathode materials. The test solution was 1 M LiAsF₆ in a 1:1 mixture of ethylene and dimethyl carbonates (EC-DMC). Cyclic voltammetry (CV), chronopotentiometry, impedance spectroscopy, electrochemical quartz crystal microbalance (EQCM), FTIR and X-ray photoelectron spectroscopies have been used in this study. It was found that VC is a reactive additive that reacts on both the anode and the cathode surfaces. The influence of this additive on the behavior of Li-graphite anodes is very positive, since it improves their cyclability, especially at elevated temperatures, and reduces the irreversible capacity. The spectroscopic studies indicate that VC polymerizes on the lithiated graphite surfaces, thus forming poly alkyl Li-carbonate species that suppress both solvent and salt anion reduction. The presence of VC in solutions reduces the impedance of the LiMn₂O₄ and LiNiO₂ cathodes at room temperature. However, we have not yet found any pronounced impact of VC on the cycling behavior of the cathodes, either at room temperature or at elevated temperatures. Thus, VC can be considered as a desirable additive for the anode side in Li-ion batteries, one which has no adverse effect on the cathode side.     

Room temperature molten salts as lithium battery electrolyte

In the present study, are reported investigations obtained with the room temperature molten salt (RTMS) ethyl-methyl-imidazolium bis-(trifluoromethanesulfonyl)-imide (EMI-TFSI) in order to use it as solvent in lithium battery. The thermal stability, viscosity, conductivity and electrochemical properties are presented. A solution of 1m lithium bis-(trifluoromethanesulfonyl)-imide (LiTFSI) in EMI-TFSI has been used to test the electrolyte in a battery with LiCoO₂ and Li₄Ti₅O₁₂ as respectively cathode and anode materials. Cycling and power measurements have been obtained. The results have been compared with those obtained with a molten salt formulated with a different anion, BF₄⁻ and with a conventional liquid organic solvent EC/DMC containing LiTFSI. The 1m LiTFSI/EMI-TFSI electrolyte provides the best cycling performance: a capacity up to 106 mAh g⁻¹ is still delivered after 200 cycles, with 1C rate at 25 °C.     

Formation of metal fog during molten salt electrolysis observed in a see-through cell

Visual observations of several molten salt electrolysis processes were made in a two-compartment, see-through quartz cell. The electrolyses of aluminium, magnesium, lead, zinc, sodium and potassium were studied. The colour of the melt in the anode compartment was pale yellow for fluoride-chloride melts and red for chloride melts, caused by the presence of dispersed anode gases during electrolysis. In the cathode compartment, streamers of metal fog were formed. The colours of the metal fog were purple for aluminium, grey for magnesium, lead and zinc, blue for sodium and green for potassium.The metal fog tended to sink to the bottom of the cell, which indicated that it had a higher density than that of the melt. The metal fog also penetrated into the anode compartment, probable due to convection and diffusion in the melt. The most probable explanation of the nature of the metal fog is that it consisted ofdispersed metal particles. This chemically unstable phase dissolved easily in the melt and was oxidized quickly by the anode gases.     

Electrodeposition of titanium(IV) oxide film from sacrificial titanium anode in I2-added acetone bath

A TiO₂ film was fabricated by a simple electrochemical method using a sacrificial titanium anode as a cationic source in an I₂-dissolved acetone bath, where the solvent contains iodide ions as a supporting electrolyte but no Ti salt as an electrolyte. At the initial stage of electrolysis, anodic oxidation of Ti anode occurred under the presence of water as an impurity to acetone. Subsequently, TiO²⁺ was produced as a result of the dissolution of oxide films under the influence of iodide ions, and was then electrodeposited on the cathode surface. The morphologies of as-deposited films were found to be dependent on the film thickness, which in turn is determined by the voltage applied during the electrolysis. Moreover, the obtained films show photocatalytic activity for decomposition of gaseous acetaldehyde without annealing. In this paper, the electrodeposition mechanism is discussed in detail.     

Computerized scaled cells to study the effect of additive ratios and concentrations on nodulation during copper electrorefining

Scaled copper electrorefining cells were designed, built and computerized to simulate as closely as possible industrial conditions at three Canadian copper refineries. The industrial dimensions of Falconbridge, Kidd Metallurgical Division, were considered while designing scaled cells. Anode width to cell width ratio, anode width to cathode width ratio, anodic surface to cathodic surface ratio, as well as electrolyte volume to cathodic surface ratio, which was about 60 L m^–2, were consistent with Kidd's industrial ratios. However, the cell design also allowed simulation of INCO's Copper Cliff Copper Refinery (CCCR) or Noranda's Canadian Copper Refinery (CCR). Electrorefining cells were 135.0 cm deep by 14.7 cm wide. Electrolyte flow rate was parallel to the electrodes. Electrolyte was circulated from the lower part of the electrorefining cells to the top where there was an overflow going to the electrowinning circuit. The equipment was computer controlled using Labview software. Experiments were conducted using this scaled electrorefining set-up to evaluate the effect of various ratios and concentrations of additives on nodulation during copper electrorefining under high current densities. Cathodic polarization curves, SEM micrographs, porosity analyses and copper grain analyses were used to characterize the cathodes produced.     

Beneficiation of Pu Residues by Ultrafine Grinding and Aqueous Biphasic Extraction

Abstract

Aqueous biphase systems are heterogeneous liquid/liquid systems that result from appropriate combinations of inorganic salts and water-soluble polymers such as polyethylene glycol. Colloid-size particles that are suspended in an aqueous biphase system will partition to one of the phases, depending on a complex balancing of particle interactions with the surrounding solvent. With regard to waste treatment applications, aqueous biphase systems are similar to conventional solvent extraction but do not utilize an organic diluent, which may itself become a source of pollution. In addition, the water-soluble polymers that have been used in biphase formation are inexpensive, nontoxic, and biodegradable. The application of aqueous biphasic extraction to the beneficiation of plutonium residues will be discussed.