Electrorefining of Uranium Alloys Containing Palladium and Neodymium in 3LiCl–2KCl–UCl3 Melts D. I. Nikitin, I. B. Polovov, O. I. Rebrin Russian Metallurgy Metally, 2023 Abstract—The technology of pyrochemical processing of mixed nitride uranium–plutonium spent fuel that is applied at the experimental and demonstration energy complex of the Siberian Chemical Plant includes several certain procedures finally aimed at extraction of the target fission products. The penultimate stage of processing is planned to be the electrorefining of the products of the previous stage, namely, metallized spent nuclear fuel (SNF). To implement electrolytic refining, it is necessary to determine the processes and technological regimes of electrolytic refining of the alloys modeling the product of this stage at the processing module. The results of electrorefining of the model alloys (modeling raw materials of the electrorefining stage) on an enlarged laboratory electrolysis cell are presented. The initial parameters of uranium refining in the melts based on 3LiCl–2KCl–UCl3 have been determined earlier. The basic parameters of refining are the use of the 3LiCl–2KCl–UCl3 (10.1 wt % UCl3) electrolyte and conducting experiments at 550°C. The uranium alloys containing palladium and neodymium are prepared by direct melting of uranium metal, PdAP-1 palladium metallic powder, and neodymium metal (99.99%) in a medium of high-purity argon (99.998%). At 550°C, the cathodic deposits are typical dendritic forms of orthorhombic α-uranium tending to needle formation with an increase in the cathode current density. An increase in the process time and the cathode current density leads to a decrease in the current efficiency because of electrode short circuit caused by cathodic deposit needles or metal fell from the cathode. The conditions of the cathodic process are clarified as a result of electrorefining experiments. For the electrorefining of the alloys U–Pd (1.59 wt %), U–Pd (1.62 wt %), U–Pd (1.54 wt %), U–Pd (1.58 wt %)–Nd (5.64 wt %), U–Pd (1.84 wt %)–Nd (6.49 wt %), and U–Pd (1.79 wt %)–Nd (6.54 wt %), uranium cathodic deposits are produced. They are subjected to chemical analysis, which shows a high purity of the resulting metallic uranium and the absence of metallic palladium and molybdenum. The palladium and uranium separation factor exceeds 5000, and that of neodymium and uranium is higher than 1000, which corresponds to the requirements imposed on purification from fission products at this stage of pyrochemical processing of SNF. Palladium is accumulated in anodic tailings, while the major mass of neodymium dissolves in molten electrolyte.
Anodic Processes of Uranium Alloys Containing Palladium and Neodymium in 3LiCl–2KCl–UCl3 Melts D. I. Nikitin, I. B. Polovov, O. I. Rebrin, A. V. Shchetinskii, A. S. Dedyukhin Russian Metallurgy Metally, 2023 Abstract A combined technological scheme for the reprocessing of a mixed nitride uranium–plutonium spent fuel, which consists of pyrochemical operations and hydrometallurgical refining of uranium, plutonium, and neptunium, is currently being developed at the reprocessing unit of the pilot demonstration power complex of the Siberian Chemical Plant. According to this scheme, the target pyrochemical reprocessing products (the actinide content not lower than 99.9%) purified from the main mass of fission products are directed to hydrometallurgical reprocessing. Pyrochemical reprocessing requires a technology of electrorefining of the metallized spent nuclear fuel. To carry out electrorefining, it is necessary to determine processes and conditions of anodic dissolution of the alloys simulating the product of primary “metallization” operation. The results of studying the anodic dissolution of model U–Pd and U–Pd–Nd alloys with different concentrations of palladium and neodymium in the melts based on 3LiCl–2KCl–UCl3 (10.1 wt % UCl3) at 550°C using different methods are presented. The uranium alloys containing palladium and neodymium are prepared by direct melting of metallic uranium, PdAP-1 metallic palladium powder, and metallic neodymium (99.99%) in a high-purity argon medium (99.998%). Electrochemical measurements are carried out on an Autolab 302N potentiostat/galvanostat equipped with a Booster 20A high-current device. The anodic polarization curves consist of only one oxidation wave attributed to the dissolution of uranium metal. An increasing in the palladium content in the alloy from 1.5 to 10.0 wt % does not affect the shape of the polarization curves. An increase in the neodymium content in the alloy from 1.0 to 10.0 wt % does not either influence on the shape of the polarization curves. The parameters of electrorefining of uranium alloys containing palladium and neodymium are determined. The limiting current density of uranium dissolution from the uranium alloys containing palladium and neodymium in the 3LiCl–2KCl–UCl3 (10.1 wt % UCl3) electrolyte at 550°C is 0.4 A/cm2. Palladium does not transfer into the melt due to anodic dissolution, and neodymium is accumulated in the electrolyte only for refining the alloy with 10.0 wt % neodymium, which is much higher than the possible real concentrations of components of the electrorefined uranium alloy in the technological flowsheet of processing the spent nuclear fuel.
POSSIBILIITY OF URANIUM EXTRACTION FROM SPENT NUCLEAR FUEL IN FUSED ELECTROLITES CONTAINING RARE ELEMENTS Dmitry I. Nikitin, Ilya B. Polovov, Oleg I. Rebrin Bulletin of the Tomsk Polytechnic University Geo Assets Engineering, 2023 Link for citation: Nikitin D.I., Polovov I.B., Rebrin O.I. Possibility of uranium extraction from spent nuclear fuel in fused electrolytes containing rare elements . Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 10, рр. 210-218. In Rus. The relevance of the research is caused by the plans of using electrolytic separation of metalized spent nuclear fuel as a stage in pyrochemical reprocessing of mixed nitride uranium-plutonium fuel. The main aim is to determine the parameters of uranium electrolytic separation from an alloy with simulators of fission products (precious metals and rare earth elements) simulating spent nuclear fuel in salt mixtures based on 3LiCl–2KCl with additives of rare earth element chlorides. Objects: model spent nuclear fuel – uranium alloy with simulators of fission product (precious metals and rare earth elements) with a mass compound of Pd:Ru:Ag:Rh=25:1:3:3, Nd:Ce:La:Pr:Sm:Y=15:10:5:5:5:1). Methods: electrorefining, X-ray fluorescence and X-ray diffraction method of analysis, inductively coupled plasma mass spectrometry, assessment of distribution of components in the system. Results. The data obtained showed that uranium deposits have dendrite formations of alpha-uranium at 550 °C in orthorhombic crystal system toward needle cathode current density. The resulting cathode deposits are free from impurities of ruthenium, rhodium, molybdenum, praseodymium and yttrium. The purification coefficient for palladium reaches 3000, and for silver 1700. Noble metals accumulate in the anode sludge, and even with the complete depletion of the anode material. The concentration of noble metals in the cathode deposit does not exceed 0,0015 wt %. Despite the high concentration of rare-earth chlorides in the electrolyte, which simulates the accumulation of rare-earth ions during repeated reprocessing of spent nuclear fuel, the concentration of rare-earth metals in the cathode product did not exceed 0,007 wt %. During uranium electrowinning from a model spent nuclear fuel in the 3LiCl–2KCl–UCl3 electrolyte (10,1 wt %) with REE chlorides, which imitate their accumulation in the electrolyte during repeated processing, at 550 °C, as well as the initial cathode current density of 0,2 A/cm2, the specific amount of electricity is 1,0 A∙h/cm2. A cathode uranium deposit is released with a current efficiency exceeding 90 % with all anode mass depletion, and purification coefficient 1800 for the sum of precious metals and for the sum of rare earth metals.
Uranium electrorefining in 3LiCl-2KCl based melts Dmitry I. Nikitin, Daniil A. Zolotarev, Artur D. Mukhametdyanov, Vladimir A. Volkovich, Ilya B. Polovov Ecs Transactions, 2020 The process of uranium electrorefining in 3LiCl–2KCl based melts was investigated. Dense and compact uranium deposits were obtained at low current densities, while application of relatively high current densities led to the formation of uranium dendrites. Cathodic current efficiency in the majority of experiments confirmed the three-electron scheme of uranium reduction. Cathodic current efficiency under certain conditions exceeded 95%. The amount of salt retained in the cathodic deposit varied from 7 to 26%, these values typical for dendrite electrolytic metals.
Electrode processes in 3LiCl-2KCl-UCl3 melts: Investigation of temperature and uranium concentration influence D. A. Zolotarev, D. I. Nikitin, I. B. Polovov Aip Conference Proceedings, 2019 Electrode processes in 3LiCl–2KCl–UCl3 melts were studied at different temperatures and uranium concentrations using a variety of electrochemical techniques. It was found that the anodic process leads to the formation of U(IV) species while uranium metal is the only product of uranium(III) ions electroreduction. Characteristic potentials and limiting current densities were determined both for potentiostatic and galvanostatic regimes of uranium electrodeposition. Varying uranium concentration and temperature did not influence the mechanism of the cathodic reaction.
