Jin-Mok Hur

PYROPROCESSING TECHNOLOGY DEVELOPMENT AT KAERI

Pyroprocessing technology was developed in the beginning for metal fuel treatment in the US in the 1960s. The conventional aqueous process, such as PUREX, is not appropriate for treating metal fuel. Pyroprocessing technology has advantages over the aqueous process: less proliferation risk, treatment of spent fuel with relatively high heat and radioactivity, compact equipment, etc. The addition of an oxide reduction process to the pyroprocessing metal fuel treatment enables handling of oxide spent fuel, which draws a potential option for the management of spent fuel from the PWR. In this context, KAERI has been developing pyroprocessing technology to handle the oxide spent fuel since the 1990s. This paper describes the current status of pyroprocessing technology development at KAERI from the head-end process to the waste treatment. A unit process with various scales has been tested to produce the design data associated with the scale up. A performance test of unit processes integration will be conducted at the PRIDE facility, which will be constructed by early 2012. The PRIDE facility incorporates the unit processes all together in a cell with an Ar environment. The purpose of PRIDE is to test the processes for unit process performance, operability by remote equipment, the integrity of the unit processes, process monitoring, Ar environment system operation, and safeguards related activities. The test of PRIDE will be promising for further pyroprocessing technology development.

STATUS OF PYROPROCESSING TECHNOLOGY DEVELOPMENT IN KOREA

The Korea Atomic Energy Research Institute (KAERI) has been developing pyroprocessing technology for recycling useful resources from spent fuel since 1997. The process includes pretreatment, electroreduction, electrorefining, electrowinning, and a waste salt treatment system. This paper briefly addresses unit processes and related innovative technologies. As for the electroreduction step, a stainless steel mesh basket was applied for adaption of granules of uranium oxide. This basket was designed for ready handling and transfer of feed material. A graphite cathode was used for the continuous collection of uranium dendrite in the electrorefining system. This enhances the throughput of the electrorefiner. A particular mesh type stirrer was designed to inhibit uranium spill-over at the liquid Cd crucible. A residual actinide recovery system was also tested to recover TRU tracer. In order to reduce the waste volume, a crystallization method is employed for Cs and Sr removal. Experiments on the unit processes were tested successfully, and based on the results, engineering-scale equipment has been designed for the PRIDE (PyRoprocess Integrated inactive DEmonstration facility).

Current Status of Pyroprocessing Development at KAERI

Pyroprocessing technology has been actively developed at Korea Atomic Energy Research Institute (KAERI) to meet the necessity of addressing spent fuel management issue. This technology has advantages over aqueous process such as less proliferation risk, treatment of spent fuel with relatively high heat and radioactivity, and compact equipments. This paper describes the pyroprocessing technology development at KAERI from head-end process to waste treatment. The unit process with various scales has been tested to produce the design data associated with scale-up. Pyroprocess integrated inactive demonstration facility (PRIDE) was constructed at KAERI and it began test operation in 2012. The purpose of PRIDE is to test the process regarding unit process performance, remote operation of equipments, integration of unit processes, scale-up of process, process monitoring, argon environment system operation, and safeguards-related activities. The test of PRIDE will be promising for further pyroprocessing technology development.

An Electrochemical Reduction of Uranium Oxide in the Advanced Spent-Fuel Conditioning Process

Abstract The Korea Atomic Energy Research Institute is currently developing the Advanced Spent-Fuel Conditioning Process (ACP) based on a pyrochemical process. An electrochemical reduction process has been developed as a key unit of the ACP. In this work, an electrochemical reduction of U3O8 powder in a LiCl-Li2O molten salt has been investigated in an electrochemical cell with a unique cathode assembly, which consists of a porous magnesia membrane, oxide powder, and a solid electricity conductor. The experimental results suggest successful demonstration of this process, exhibiting a reduction conversion of U3O8 of >99% for a batch.

AN EXPERIMENTAL STUDY ON AN ELECTROCHEMICAL REDUCTION OF AN OXIDE MIXTURE IN THE ADVANCED SPENT-FUEL CONDITIONING PROCESS

An electrochemical reduction of a mixture of metal oxides was conducted in a LiCl molten salt containing 3 wt% Li₂O at 650℃. The oxide reduction was carried out by applying a current to an electrolysis cell, and the Li₂O concentration was analyzed during each run. The concentration of Li₂O in the electrolyte bulk phase gradually decreases according to Faraday’s law due to a slow diffusion of the O₂- ions. A hindrance effect of the unreduced metal oxides was observed for the reduction of the uranium oxide. Cs, Sr, and Ba of high heat-load fission products were diffused into and accumulated in the salt phase as predicted with thermodynamic consideration.

CHEMICAL BEHAVIOR OF FISSION PRODUCTS IN THE PYROCHEMICAL PROCESS

Abstract The chemical behavior of lanthanide oxides has been studied both for the electrolytic reduction process and the electrorefining process. At high concentration of Li2O in LiCl, lanthanide oxides reacted with Li2O to form mixed oxides, LiLnO2 (Ln = lanthanides), which decomposed to the starting materials at relatively low Li2O concentration. The chemical behavior of lanthanide oxides under the condition of electrorefining process was investigated by optical fiber spectrophotometry and X-ray diffraction. Lanthanide oxides reacted with U3+ to produce Ln3+ and UO2. The solubility of lanthanide oxides was measured under the electrolytic reduction and the electrorefining condition. All of the lanthanide oxides except Eu2O3 had relatively low solubility values in LiCl-KCl eutectic mixture at 450°C. Electrochemical behavior of Br-, I-, and Se2- in LiCl was also investigated by cyclic voltammetry and by X-ray diffraction. All of the anions reacted with platinum anode and gave platinum compounds.

Electrochemical reduction of UO2 to U in a LiCl–KCl-Li2O molten salt

An electrochemical reduction of UO2 to U in a LiCl–KCl-Li2O molten salt has been investigated in this study. A diagram showing equilibrium potentials (relative to Cl2/Cl−) plotted versus the negative logarithms of oxide-ion activity (pO2−) was constructed. The crushed UO2 pellets in the cathode basket of an electrolytic reducer were successfully reduced to U. The reduction of UO2 is proved to proceed mainly through chemical reaction with in situ generated Li and K at the cathode. The control of cathode potential is essential to prevent the deposition and subsequent vaporization of K metal at the cathode for the applications of a LiCl–KCl-Li2O molten salt as an electrolyte for the metal production from its oxide sources.

KINETIC MODELING STUDY OF A VOLOXIDATION FOR THE PRODUCTION OF U₃O₈ POWDER FROM A UO₂ PELLET

A kinetic model for the oxidation of a UO₂ pellet to U₃O? powder has been suggested by considering the mass transfer and the diffusion of oxygen molecules. The kinetic parameters were estimated by a fitting of the experimental data. The activation energies for the chemical reaction and the product layer diffusion were calculated from the kinetic model. The oxidation conversion of a UO₂ pellet was simulated at various operating conditions. The suggested model explains the oxidation behavior of UO₂ well.

MOLTEN SALT VAPORIZATION DURING ELECTROLYTIC REDUCTION

The suppression of molten salt vaporization is one of the key technical issues in the electrolytic reduction process developed for recycling spent nuclear fuel from light-water reactors Since the Hertz-Langmuir relation previously applied to molten salt vaporization is valid only for vaporization into a vacuum, a diffusion model was derived to quantitatively assess the vaporization of LiCl, Li₂O and Li from an electrolytic reducer operating under atmospheric pressure. Vaporization rates as a function of operation variables were calculated and shown to be in reasonable agreement with the experimental data obtained from thermogravimetry.

High temperature corrosion behavior of Ni-based alloys

The high temperature corrosion behavior of N07263, N06600, and N06625 in LiCl-Li2O molten salt was investigated at temperatures ranging from 650 to 850 °C in a glove box. The high temperature corrosion behavior was observed using measurements of the oxide morphology and thickness, the extent of internal corrosion, and the compositional changes in the scale and in the substrate. Corrosion tests were performed, and these demonstrated that the main corrosion products were Fe(Ni,Co)3, FeNi3, and LiCrO2. The internal corrosion of N07263 was localized, while that of N06600 maintained intergranular corrosion throughout the test temperature range. N06625 exhibited uniform intergranular corrosion behaviors at low and high temperatures. N07263 exhibited superior corrosion resistance, as evidenced by its corrosion layer which was more continuous, dense, and adherent when compared with those of N06600 and N06625.