[Korea"s nuclear technology (9)] Korea bets on pyroprocessing technology < KAERI News < News & Information (2024)

This is the ninth in a series of articles that highlight the challenges and opportunities facing Korea's nuclear power industry. -Ed.

By Song Ki-chan

Pyroprocessing is a technology that recovers valuable resources such as uranium from the spent fuel of nuclear power plants. The recovered resources are recycled into fuel for next-generation sodium fast reactors (SFRs), resulting in increased uranium usage efficiency and a marked decrease in radiotoxicity and the amount of radioactive waste generated. In this context, pyroprocessing technology is crucial to advanced nuclear systems due to increased safety and economic efficiency.

The basic concept of pyroprocessing is group recovery, which enhances the proliferation resistance significantly, as separation of sole plutonium is impossible. The plutonium with miner actinides (MA) that account for 1.4 percent of the spent fuel is recovered as a group and fabricated as SFR fuel. The mixture is burnt out as SFR, resulting in transmutation of the long half-life elements. This reduces the 300,000 year waste management period by a factor of 1,000. A part of the uranium from recovered spent fuel is blended with this plutonium and MA mixture.

Caesium and strontium, which are major components regarding heat load in spent fuel, are recovered, stored for a while, and consequently disposed of. The extraction of transuranics (TRU), Cs, and Sr from spent fuel allows the repository burden to be reduced by a factor of 100, compared to the instances without extraction. The fission products are recovered and transferred to the repository. As a result of pyroprocessing, both repository efficiency and uranium usage are increased by up to 100 times.

The feature of pyroprocessing is an electro-chemical process technique for recycling spent nuclear fuel for reuse in an SFR. The core reactions take place in an electrorefining and an electrowinning system. In these systems, solid cathode is used to recover pure uranium, and liquid cadmium cathode (LCC) is used for the recovery of TRU elements and residual uranium.

The metal ingot transferred from the electrolytic reduction process acts as an anode in an electrorefining reactor. Due to differences in the reduction potentials of metal ions at the solid cathode, uranium can be selectively deposited on the surface of solid cathode. After deposition, the TRU elements will start to co-deposit with uranium on the solid cathode. Beyond this condition, the salt is transferred to the electro-winning process.

In the cadmium cathode, uranium and TRU are recovered together as the reduction potentials of these elements are similar. The LCC collects most of the actinides and some of the rare earth elements. This explains why pyroprocessing has a strong nuclear proliferation resistance.

Therefore, unlike reprocessing technology which enables the separation of weapon grade plutonium, pyroprocessing recovers plutonium, neptunium, americium and curium together. This aspect makes pyroprocessing a promising, non-proliferative technology for recycling spent fuel in the future.

Pyroprocessing technology in the world

Due to the above-mentioned benefits, many countries have shown interest in pyroprocessing technology. In the United States, the pyroprocess was developed around 40 years ago, and has been used to treat the spent fuel from EBR-II in laboratory scales.

Japan has also developed the technology as a second option for spent fuel treatment, with the Rokashomura plant also used for reprocessing spent fuel.

In 2008, the European Commission started the Actinide Recycling by Separation and Transmutation (ACSEPT) program in which the dry process is studied along with the wet process. Russia, India and China also have paid attention to the development of pyroprocessing technology for spent fuel treatment.

There are two challenges that need to be solved in order to fully develop pyroprocessing technology: improving the capacity and proving the technology by using real spent fuel. With regard to the treatment capacity, the United States and other major countries have conducted experiments under laboratory conditions using 1 to 10 kilogram batches. However, many technological barriers remain that must be overcome in order to achieve commercial scale pyroprocessing. High capacity and high throughput are crucial factors when considering technological barriers. Radiation shied, remote operation and criticality are also among the key factors in process design.

KAERI's pyroprocessing technology

The pyroprocess includes the decladding of spent fuel, voloxidation to remove the volatile fission products, electroreduction in order to produce uranium and TRU metal ingot, an electrorefining process to recover pure uranium, an electrowinning process to separate the TRU mixture and a waste treatment process. These pyroprocessing technologies have been significantly improved recently in terms of high capacity and high throughput.

The Korea Atomic Energy Research Institute is currently developing an electrolytic reduction process, the beginning stage of the pyroprocess, to demonstrate the laboratory scale operation of electrolytic reduction, and at the same time produce the engineering data to be incorporated into the design of the equipment, instruments and facilities for the Pyroprocess Integrated Inactive Demonstration (PRIDE) and Engineering Scale Pyroprocessing Facility (ESPF).

PRIDE is a testing facility that uses surrogate fuel produced from naturally occurring uranium rather than reprocessed fuel.

The inactive tests (around 10 kgU/batch) of the electrolytic reduction process based on a ceramic cathode basket have been completed with a more than 99 percent reduction yield. Design and construction of a new electrolytic reduction system (20 kgU/batch) equipped with a metal cathode basket which can be linked to an electrorefining process have been completed. The technology to suppress the vaporization of molten salts and to enable a reuse of the molten salts was verified. Current density on the anode was increased from 100 mA/cm2 to 500 mA/cm2 enabling high speed electrolytic reduction.

The continuous electrorefining system (CERS), the purpose of which is to separate pure uranium from the impure uranium mixture, is composed of an electrorefiner, cathode processor, and melting furnace. The UCl3-chlorinator and transportation are also needed to operate the CERS. In the electrorefiner, the uranium deposition is initiated in molten lithium chloride-potassium chloride salt with about 9 percent by weight of uranium chloride, and then the uranium dendrites are deposited and separated from the electrode spontaneously, and finally collected at the bottom of the reactor. The collected uranium dendrites are conveyed to the cathode processor continuously. The cathode processor distills the salt from the conveyed metal, and purified metal is transferred to the melting furnace. The metal is melted and consequently reformed to ingot for storage or for future use. The CERS was designed to continuously operate with a capacity of 20 kg/batch. An electrorefiner with 50 kgU/batch has been designed for PRIDE.

Uranium deposits from an electrorefiner contain about 30-40 percent of weight in salts. In order to recover pure uranium and convert it into a metal ingot, the salts have to be removed from the uranium deposits. Above 99 percent of salt weight removal of uranium deposits has to be achieved for the following uranium metal ingot casting process. The characteristics of salt evaporation depend largely on the vapor pressures of the components and the temperature. A vacuum evaporation was applied to the salt removal system. The vacuum pressure and the hold temperature are the key factors of the evaporation system.

The casting process is to consolidate the refined uranium deposits into a solid cylindrical metallic form, which will be used as raw material for SFR fuel or will be stored for future use. This process is followed by the cathode process, where almost 99 percent of salts mixed with uranium deposits are removed.

The ingot casting equipment consists of a vacuum chamber, charger, crucible and mold. The distilled uranium dendrite is fed from the distiller to the charger and the uranium dendrite goes inside the crucible made of graphite coated with zirconia. The throughput is 20 kgU/24 hours.

The first step of the electrowinning study is to establish lab-scale electrowinning equipment for TRU recovery and to produce a scale-up data from the results of performance tests. The final target of this study can be accomplished in the second step by establishing the electrowinning system of engineering scale (PRIDE) which consists of a liquid cadmium cathode (LCC) electrowinning, a cadmium distillation, and evaluating the performance including an efficiency of TRU recovery.

The purpose of this study is to develop electrowinning technology which is able to recover TRU from a molten salt system as a major process in the pyroprocessing technology with proliferation resistance. The scope of this study includes an electrowinning technology of LCC to recover group actinides such as uranium and TRU (neptunium, plutonium, americium, curium) in the molten salt (LiCl-KCl) transferred from an electrorefining process which collects uranium of high purity, a cadmium distillation technology to separate the cadmium and actinides from recovered actinide/cadmium products by LCC, and a computer analysis technology to simulate the electrolytic process of molten salt system for TRU recovery.

During the pyroprocessing of LWR spent oxide fuels, two different types of waste salts are expected to be generated: One is LiCl waste salt containing alkali and alkaline-earth fission products fission products from an electrolytic reduction process and the other is LiCl-KCl eutectic waste salt containing rare-earth FPs from an electrorefining process. Since these waste salts are radioactive, heat-generative and highly soluble in water, they must be fabricated into durable waste forms that are compatible with the environment inside of a geologic repository for long periods of time. Current technology for disposing of waste salts from the pyroprocess is non-selective, total incorporation of waste salts in the zeolite matrix to form a ceramic waste form, which results in the significant increase in the volume of the disposed waste.

KAERI has two key R&D concepts in developing innovative waste salt treatment technologies. The first is minimization of waste salt generation by the removal of fission products, and then recycling of the cleaned salt to the main pyroprocess. The second technology improves safety during interim storage or final disposal by fabricating high-integrity final waste forms.

To achieve these purposes, KAERI has developed various FPs removal and waste solidification technologies such as melt crystallization, oxidative precipitation, and SAP solidification. The performance was found to be successful with small scale equipment. Scale-up of the salt regeneration and solidification units and verification of the performance in the scaled-up processes will be carried out in a stepwise manner according to the KAERI's long-term R&D plan which is financially supported by the national long-term nuclear R&D program.

Construction and performance evaluation of the lab-scale system was completed by the end of 2009 and an engineering-scale mock-up system will be designed and constructed by 2011. The demonstration of an engineering-scale (10t-HM/yr) pyroprocessing system is scheduled by 2016.

Summary

The main purposes of pyroprocess R&D in KAERI are both to increase equipment throughput, and to reduce the final volume of waste that has to be disposed of. The adoption of graphite cathode in the electrorefiner and waste salt regeneration by crystallization method are applied successfully to achieve these goals. Based on the results of bench and laboratory-scale tests, an inactive engineering-scale integrated pyroprocess (PRIDE) with a capacity of 10 ton-U per year is planned to be constructed and tested by the end of year 2016. This is to be followed by an active test in ESPF (Engineering Scale Pyroprocessing Facility).

2010.03.09

The Korea Herald