Kensuke Kinoshita

Measurement of standard potentials of actinides (U,Np,Pu,Am) in LiCl–KCl eutectic salt and separation of actinides from rare earths by electrorefining

Abstract Pyrochemical separation of actinides from rare earths in LiCl–KCl eutectic–liquid metal systems has been studied. The electromotive forces of galvanic cells of the form, Ag|Ag(I), LiCl–KCl‖actinide(III), LiCl–KCl|actinide, were measured and standard potentials were determined for uranium, neptunium and plutonium to be −1.283 V, −1.484 V and −1.593 V (at 450°C vs. Ag/AgCl (1wt%–AgCl)), respectively. A typical cyclic voltammogram of americium chloride has two cathodic peaks, which suggests reduction Am(III)→Am(II) occurs followed by reduction of Am(II) to americium metal. Standard potential of Am(II)/Am(0) was estimated to be −1.642 V. Electrorefining experiments to separate actinides (U, Np, Pu and Am) from rare earths (Y, La, Ce, Nd and Gd) in LiCl–KCl eutectic salt were carried out. It was shown that the actinide metals were recovered on the cathodes and that americium was the most difficult to separate from rare earths. The actinide separation will be achieved by means of the combination of electrorefining with multistage extraction.

Distribution behavior of uranium, neptunium, rare-earth elements ( Y, La, Ce, Nd, Sm, Eu, Gd) and alkaline-earth metals (Sr,Ba) between molten LiClKCI eutectic salt and liquid cadmium or bismuth

Abstract Distribution coefficients of uranium neptunium, eight rare-earth elements (Y, La, Ce, Pr, Nd, Sm, Eu and Gd) and two alkaline-earth metals (Sr and Ba) between molten LiCl-KCI eutectic salt and either liquid cadmium or bismuth were measured at 773 K. Separation factors of trivalent rare-earth elements to uranium or neptunium in the LiCl-KCl/Bi system were by one or two orders of magnitude larger than those in the LiCl-KCl/Cd system. On the contrary, the separation factors of alkaline-earth metals and divalent rare-earth elements to trivalent rare-earth elements were by one or two orders of magnitude smaller in the LiCl-KCl/Bi system.

Separation of Uranium and Transuranic Elements from Rare Earth Elements by Means of Multistage Extraction in LiCl-KCl/Bi System

A pyrometallurgical partitioning process is being developed for recovering transuranic elements (TRUs) from high-level liquid waste. For estimating the separation efficiencies of TRUs from rare earth elements (REs), preliminary measurements were made of the distribution coefficients of TRUs in an LiCl-KCl/Bi system at 773 K. Actual separation of TRUs from REs was experimented by multiple-batch extraction in LiCl-KCl/Bi system. The experimental results agreed well with estimates obtained from measured separation factors for single-step reductive extraction. A simulation study was performed in counter-current extraction, for which the TRU recovery yields and separation efficiencies were derived from the measured distribution coefficients. The results indicated that counter-current extraction should permit recovering” more than 99% of every actinide contained in the high-level liquid waste, while ensuring a mass ratio of TRUs to REs high enough for deriving metallic fuels for FBR.

Pyroprocessing of Light Water Reactor Spent Fuels Based on an Electrochemical Reduction Technology

A concept of pyroprocessing light water reactor (LWR) spent fuels based on an electrochemical reduction technology is proposed, and the material balance of the processing of mixed oxide (MOX) or high-burnup uranium oxide (UO2) spent fuel is evaluated. Furthermore, a burnup analysis for metal fuel fast breeder reactors (FBRs) is conducted on low-decontamination materials recovered by pyroprocessing. In the case of processing MOX spent fuel (40 GWd/t), UO2 is separately collected for ~60 wt% of the spent fuel in advance of the electrochemical reduction step, and the product recovered through the rare earth (RE) removal step, which has the composition uranium:plutonium:minor actinides:fission products (FPs) = 76.4:18.4:1.7:3.5, can be applied as an ingredient of FBR metal fuel without a further decontamination process. On the other hand, the electroreduced alloy of high-burnup UO2 spent fuel (48 GWd/t) requires further decontamination of residual FPs by an additional process such as electrorefining even if RE FPs are removed from the alloy because the recovered plutonium (Pu) is accompanied by almost the same amount of FPs in addition to RE. However, the amount of treated materials in the electrorefining step is reduced to ~10 wt% of the total spent fuel owing to the prior UO2 recovery step. These results reveal that the application of electrochemical reduction technology to LWR spent oxide fuel is a promising concept for providing FBR metal fuel by a rationalized process.

Estimation of Material Balance in Pyrometallurgical Partitioning Process of Transuranic Elements from High-level Liquid Waste

A pyrometallurgical partitioning process for transuranic elements (TRUs) from high-level liquid waste (HLLW) has been studied to develop an effective and safety method of nuclear waste disposal. A salt and metal as solvents, Li as reductant and chlorine gas can be recycled in the process. The process is expected to generate less secondary radioactive waste because of no irradiation damage of the solvents, and to require a relatively compact installation because of larger critical mass in comparison with a conventional aqueous method. In this study, the material balance of the solutes and the volume of the solvents were estimated in the pyrometallurgical partitioning process that was constructed on the basis of our previous study. The results show that the volume of the salt as solvent is about 200 l and that of Cd, Bi and Pb as metal solvent is 54, 71 and 140 l, respectively, in the case that HLLW from PUREX reprocessing of 1t LWR spent fuel is treated in the process. The results also show that the amount of Li in the waste salt disposed from the process is kept less than the amount of Li2O needed for glass matrix.

Electrodeposition of Uranium and Transuranic Elements onto Solid Cathode in LiCl-KCl/Cd System for Pyrometallurgical Partitioning

A pyrometallurgical partitioning process is being developed for recovering transuranic elements (TRUs) from high-level liquid waste. In the process, actinides are separated from fission product, especially rare earth elements (REs), by means of an electrorefining technique or a reductive-extraction technique. In this study, electrorefining experiments were carried out in LiClKCl/Cd system to recover actinides from salt bath containing actinides and REs. Uranium and neptunium could be depleted from the salt bath and recovered onto a solid cathode with high collection efficiency and high selectivity. Plutonium and americium, however, were difficult to be recovered at high current efficiency because reduction of Nd3+ to Nd2+ at about—1.7V consumed cathodic current prior to the deposition of Pu or Am. The rotation of the cathode had rather negative effect against deposition of Am and Pu in case of coexistence of much amount of Nd because Nd2+ was removed from the cathode surface quickly and the reaction of Nd3+ to Nd2+ was promoted. At higher current density, Pu and Am could be recovered onto solid cathode but current efficiency became too low. The result indicated that electrorefining technique in the pyro-partitioning was effective for U and Np but not for Pu and Am.

Pyrometallurgical Partitioning of Uranium and Transuranic Elements from Rare Earth Elements by Electrorefining and Reductive Extraction

High-level liquid waste generated from PUREX reprocessing contains a small amount of transuranic elements, such as Np, Pu, Am, and Cm, with long-lived radioactivities. A pyrometallurgical partitioning process is being developed to recover transuranic elements from such waste. Small amounts of U contained in the high-level liquid waste are also recovered in the process. A key issue for developing the process is effective separation of U and the transuranic elements from the rare-earth elements, because the two elemental groups are chemically analogous. A series of process tests were carried out in the present study to demonstrate that a combination of electrorefining and reductive extraction is useful for separating U and transuranic elements from the rare-earth elements. The results indicate that 99% of U and each transuranic element is recovered by the combination process as a product, and that the quantity of rare-earth elements contained in the product is smaller than the transuranic elements by weight. The overall mass balance of U and transuranic elements in the system ranged within the experimental errors assigned to sampling and analysis.

Equilibrium Distribution of Actinides Including Cm Between Molten LiCl-KCl Eutectic and Liquid Cadmium

Equilibrium distribution of actinides both in molten LiCl-KCl eutectic and liquid cadmium were measured from the concentration data obtained in electrorefining tests and reductive extraction tests. Separation factors for U, Np, Am, Cm against Pu were derived in the practical temperature range of 700 K to 783 K. The derived separation factors are consistent with the reported values measured at 773 K and 723 K. The temperature dependence for Cm is different compared to the other actinides (U, Np and Am). This behavior remains unclear and additional experimental measurements of distribution coefficient of Cm are required before ruling on the real behavior.

Countercurrent Extraction Test with Continuous Flow of Molten LiCl-KCl Salt and Liquid Cd for Pyro-Reprocessing of Metal FBR Fuel

The three-stage countercurrent extraction test with molten LiCl-KCl and liquid Cd was carried out for the development of spent salt treatment process in pyrometallurgical reprocessing of metal FBR fuel. Both salt and Cd solvent were supplied successively from each feed tank into the extractor at a constant flow rate and recovered separately into individual recovery tanks. Uranium, transuranic elements, and rareearth elements were substituted by Ce, Gd, and Y, respectively. More than 98% of Ce and 93% of Gd were recovered, and 27% of Y was corecovered in the Cd solvent. The separation factors of Gd and Y against Ce were evaluated to be 0.28–0.29 and 0.0057–0.0073, respectively. The former is almost equal to and the latter is better than that in the equilibrium. As the connecting part between each stage had a simple structure, a small amount of Cd in each stage mixed with that in the neighboring stage, which made the separation efficiency lower than expected. A calculation study showed the relationship between the Cd mixing rate and recovery yield or separation factor, and it showed that the recovery yield and separation efficiency were expected to be much improved by the modification of the connecting part to prevent Cd from mixing.

Performance of Pyroprocess Equipment of Semi-Industrial Design and Material Balance in Repeated Engineering-Scale Fuel Cycle Tests Using Simulated Oxide/Metal Fuels

Fuel cycle tests using uranium and simulants with process equipment of 1 ton HM/yr throughput were conducted to develop an equipment design for long-term and hot-cell operation with stable performance, and to investigate the influence of impurities on the behavior of sensitive materials such as molten chlorides and active metals on material mass balance during repeated engineering-scale operations. These cycle tests were performed in two phases. The first phase simulated the introduction of spent oxide fuel into the metallic fuel cycle by the sequential operations of the UO2 electroreduction, electrorefining of the reduction product, salt distillation using the electrorefining product, and injection casting of U-Zr alloy using the recovered uranium metal. The second phase, consisting of electrorefining, salt distillation, and injection casting, simulated the repeated metallic fuel cycle. The major achievements and results in these cycle tests are summarized as follows: 1. Simulated metallic fuel (U-Zr alloy rods) was successfully fabricated using UO2 as the starting material. 2. The electrorefining, product transfer, salt distillation, and injection casting equipment operated satisfactorily, and their performance was sufficiently high, taking the target processing rate of 5 kg/day into account. 3. Regarding electroreduction, the reduction rate was approximately half the target value, and the cathodic current efficiency was also low. The reasons for the unsatisfactory result are considered to be Li2O stagnancy at the cathode, the parasitic generation of lithium and the subsequent oxidation out of the cathode, and possibly the reaction between the reduced uranium and the oxygen gas evolved at the anode. Improvement of equipment design should be continued to moderate the influence of these factors on the electroreduction performance. 4. Favorable material mass balance of uranium, zirconium, and ruthenium (simulated fission products) was kept during the cycle tests, including the electrorefining, product transfer, salt distillation, and injection casting steps. No influence of three-time repetition of the fuel cycle tests was found from this viewpoint. The representativity of the anode residue and cathode product samples from the electrorefining step, which strongly influences the material mass balance evaluation, would be improved by performing anode residue treatment including metal waste consolidation and cathode processing for all the cathode products.