Masatoshi Iizuka

Behavior of plutonium and americium at liquid cadmium cathode in molten LiCl–KCl electrolyte

The effects of the electrochemical conditions on the behavior of plutonium and proper conditions for plutonium recovery at the liquid cadmium cathode (LCC) used in the pyrometallurgical reprocessing were studied by the electrotransport experiments with small, not stirred electrodes. Plutonium was successfully collected up to 7.75 wt% without disturbance by solid phase formation at the surface. Plutonium collected beyond saturation formed intermetallic compound PuCd6 and accumulated at the bottom of the LCC. The cathodic current density adequate for plutonium collection was proportional to the concentration of plutonium in the molten salt. The behavior of americium was reasonably explained by a local equilibrium model between plutonium and americium at the surface of the LCC, although thermodynamic study and quantitative analysis are needed for further understanding. The plutonium collection rate in practical electrorefiner was estimated to be 324 g/h for one LCC by extrapolation of the experimental results.

Electrode reaction of plutonium at liquid cadmium in LiCl–KCl eutectic melts

The electrode reaction of the Pu3+/Pu couple at the surface of a liquid Cd electrode was investigated by cyclic voltammetry at 723, 773 and 823 K in LiClKCl eutectic melt. It was found that the diffusion of Pu3+ in the salt phase was a rate-determining step in the cathodic reaction, but the dissolution of Pu from the liquid Cd phase was so slow that the anodic wave broadened slightly. The redox potentials of the Pu3+/Pu couple at the liquid Cd electrode at 723, 773 and 823 K were observed at more positive potentials, 0.299, 0.269 and 0.239 V, respectively, than those at a solid Mo electrode. This potential shift was thermodynamically analyzed by a lowering of activity of Pu in the Cd phase due to the formation of the alloy PuCd6 at the interface. As well as the Gibbs energy of formation of PuCd6, the Gibbs energies of formation of the other intermetallic compounds PuCd2 and PuCd4 could also be estimated by the analysis of the anodic peaks in cyclic voltammograms of the Pu3+/Pu redox couple at a Cd-coated Mo electrode.

Actinides recovery from molten salt/liquid metal system by electrochemical methods

Abstract Electrochemical methods were examined for the recovery of actinides from the electrorefiner which is used in pyrometallurgical reprocessing of spent metal fuel for fast reactors. Uranium was successfully collected at the solid steel cathode from both liquid cadmium and molten salt solvents. In electrotransport from liquid cadmium, the behavior of uranium and rare earths was as expected by a computer simulation code based on the diffusion layer model at the interface between the electrolyte and the electrodes. In electroreduction from the molten salt electrolyte, a considerable amount of uranium was reduced at the CdLi anode by direct chemical reduction with lithium, especially at a lower anodic current density. The decrease in collection efficiency of uranium due to the direct chemical reduction would be avoided by maintaining the anode potential higher than the deposition potential of uranium.

Development of an environmentally benign reprocessing technology—pyrometallurgical reprocessing technology

Abstract 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.

Electrodeposition of uranium in stirred liquid cadmium cathode

Abstract The electrodeposition of U in a liquid Cd cathode was known to be hampered by the formation of dendritic U on the Cd surface. Electrotransports of uranium to the stirred liquid Cd cathode were carried out at 773 K for different cathode current densities and different Reynolds number of stirring. The maximum amount of U taken in the liquid Cd cathode without forming dendrites was found to increase with an increasing Reynolds number of stirring and decrease with increasing cathode current density.

Application of normal pulse voltammetry to on-line monitoring of actinide concentrations in molten salt electrolyte

Abstract The applicability of electroanalytical methods – square wave voltammetry (SWV) and normal pulse voltammetry (NPV) – to on-line monitoring of actinide concentrations in molten chlorides was investigated for use in pyrometallurgical processes. For SWV curves for plutonium, the concentration dependence of the peak current was not linear at high concentration. However, the limiting currents for uranium and plutonium determined by NPV were proportional to their concentrations up to about 1.7 wt%. The fission product elements do not seem to affect the accuracy in determination of actinide concentrations because the limiting currents for uranium and plutonium were not influenced by the addition of gadolinium. It was found to be difficult to determine neptunium concentration in the presence of plutonium due to the overlap of the output current waves. From those results, NPV was considered more promising although the optimization of applied potential waveform is necessary for use at higher concentrations.

Development of an Innovative Electrorefiner for High Uranium Recovery Rate from Metal Fast Reactor Fuels

To increase the uranium recovery rate of molten salt electrorefining step in pyrometallurgical reprocessing of metallic fast reactor fuels, tests were carried out using electrorefiners equipped with mechanisms for scraping cathode deposits. After the modifications in the design of the anode basket and scraper mechanism, no stalling of the anode and scraper rotation due to interference by cathode deposits occurred. Under the condition that codissolution of zirconium and uranium was allowed in order to obtain maximum throughput, a current of 400–450A was maintained until 82% of the initially loaded uranium was recovered. The uranium recovery rate for the same duration reached 789 g U/h (32.9 g U/h_L per electrode volume). On the assumption that an electrorefiner operates for 20 h/d and 200 d/y in an actual pyrometallugical reprocessing facility, this result corresponds to a uranium recovery rate of 3.16 t U/y using only one electrode assembly of about 30 cm diameter, which should be a sufficiently high performance for practical use. From these results, the engineering feasibility of uranium recovery using an electrorefiner with cathode deposit scraper mechanism has been demonstrated.

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.

Behavior of U-Zr Alloy Containing Simulated Fission Products during Anodic Dissolution in Molten Chloride Electrolyte

To investigate the distribution of fission product elements in the electrorefining of spent metallic fuel, electrorefining tests were carried out using U-Zr alloy containing simulated fission products. An intermediate region was found between undissolved alloy and Zr-rich regions formed by the anodic dissolution of uranium. The composition of this region determined by wavelength-dispersive X-ray spectroscopy (WDS) analysis corresponded to that of δ-phase (63–78 at% Zr) in the U-Zr binary phase diagram. This region is expected to form in the course of the anodic dissolution sequence that begins from an undissolved U-Zr alloy and ends with a Zr-rich region. From the results of chemical and WDS analyses of anode residue samples, it was predicted that noble metals would stay in the anode residue as long as zirconium remains even after most of the uranium in the alloy was dissolved. Analytical results indicated no large difference betweenthe noble metal behavior in simulated material and in irradiated EBR-II driver fuels. Decontamination factors of uraniumfrom molybdenum and palladium were defined as ratios between their concentrations in the initial alloy loaded into an anode and those in cathode products, and evaluated from the analytical results. Although there was some scattering of data, decontamination factors of around 70 for molybdenum and 10 for palladium would be expected.

Electrode reaction of the Np3+/Np couple in LiCl–KCl eutectic melts

The electrochemical behaviour of the Np3+/Np couple in the LiCl–KCl eutectic salt was investigated by electromotive force measurements, cyclic voltammetry and chronopotentiometry in the temperature region between 723 and 823 K. The standard redox potential of the Np3+/Np couple vs Ag/AgCl (1.00 wt %) was measured and given by the equation, ENp3+/Np° = −2.0298 + 0.000706 T, where E is in V and T in K. The electrode reaction of the Np3+/Np couple was almost reversible under the conditions studied. The diffusion coefficient of Np3+, DNp3+, in the LiCl–KCl eutectic melts between 723 and 823 K was represented by the equation, DNp3+ = 2.22 × 10−6 − 6.88 × 10−9T + 5.60 × 10−12T2 cm2 s−1. The adsorption and desorption peaks of Np at the Mo working electrode caused by underpotential deposition were also observed in the cyclic voltammograms, and the work function of Np was evaluated as 3.04 eV by peak analysis of the cyclic voltammograms.