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1. Introduction
The Radiation and Decommissioning Laboratory of CRI of KHNP, its initial name was the Radiation Technology Office, established in 1997, under its vision as the “The world’s best leader for radiation and decommissioning technology”, is dedicated to becoming a total solution provider for operating and decommissioning NPPs by solving plant operational issues as well as by developing new technologies for generating future value. The laboratory contributes to providing effective guidelines on the back-end cycle for NPPs concerning decommissioning, spent fuel storage, radiation safety, water chemistry, and radioactive waste treatment and disposal for protecting humans and the environment. In addition, the laboratory as a KOLAS provides technical support related to the calibration of the radiometric instruments, performance tests of impregnated activated carbon, radiation safety, water chemistry management, and spent fuel safety management.
2. R&D Fields
2.1 Decommissioning and SF Technology Fields
Decommissioning and SF Technology Group develops key technologies to decommission permanently shutdown NPPs and remediate them to a harmless state in economical and safe methods. As a big achievement, the development of the final decommissioning plan (FDP) for Kori Unit 1 was completed in 2020. The group is conducting not only the development of the FDP but also various activities such as the development of the initial decommissioning plan (IDP), spent fuel management program, dry storage facilities, and its approval from the regulatory body. Furthermore, for pressurized heavy water reactor (PHWR), it plans to contribute to the decommissioning projects in order to ensure they get underway without any setbacks by evaluating the characteristics of the radiologically contaminated structures, optimizing the technology for the decommissioning process, evaluating the site of radiation contamination, evaluating the safety of clearance and developing a model for the dry storage of spent nuclear fuel.
2.2 Radiation and Chemistry Technology Fields
Radiation and Chemistry Group develops the preventive water-chemistry management technology for primary and secondary systems to ensure the safety of NPPs in operation and provides various technical supports, such as solving water- chemistry issues, analyzing radioactive waste, and developing the aging management program (AMP). The group has developed system decontamination technology to minimize the radiation exposure of workers during the dismantling of permanently shutdown NPPs. It has been accredited by the KOLAS for testing 14 items, including personal exposure dosimeters, performance evaluation of impregnated activated carbon, and radiochemical analysis for radioactive waste disposal, as shown in Fig. 1. As a new business, the group develops the technology for selling and utilizing tritium extracted from PHWR NPPs in Korea and other countries and provides technical support for its commercialization to other companies and exports Tritium Removal Facilities (TRF).
2.3 Radwaste and Environment Technology Fields
Radwaste and Environment Group develops technologies for treating and disposal of waste generated from NPPs, radiation protection and emergency response, and radiation environmental impact assessment. The group has actively responded to the problem of radioactive waste treatment and disposal by conducting an integrity evaluation of the high-integrity container (HIC) developed for safe disposal and by developing a mega-watt class plasma torch melting technology. Moreover, the group is developing technologies for evaluating the environmental behavior of radioactive materials discharged from NPPs, assessing the off-site radiological dose, protecting residents in the event of an accident, and evaluating the internal and external radiation exposure dose. The group actively manages the optimization and field application of underground water monitoring in operational NPPs, supports the operation of vitrification facility, and provides technical support for periodic safety evaluations. In particular, it has conducted empirical research on the vitrification of Fukushima radioactive waste that was outsourced from Japan with great success.
3. Major R&D Achievements
3.1 Segmentation of Reactor Vessel and Internals for Decommissioning of Kori Unit 1
Since the permanent shutdown of Kori Unit 1 in June 2017, the customization and advancement of dismantling technology for Korean NPPs have been ongoing. The decommissioning of a commercial NPP consists of various processes, including decommissioning design, decontamination, cutting, waste treatment and disposal, and site remediation, of which the cutting of radioactive structures is classified as the key process. The technical level of activated component segmentation is the same as that of advanced countries. Commercial NPPs are estimated to have a greater gap between the actual project stages due to the absence of an established track. According to the analysis, a demonstration test under systematic and various conditions was carried out to verify the cutting technology and to accumulate references. Cutting the pressure vessel and internal structure is classified as a major process of significant technical difficulty. The pressure vessel and internal structure of Kori Unit 1, which have been continuously exposed to neutrons for over 40 years, are representative radioactive structures. Considering the heavy weight and radioactive characteristics, the cutting process of radioactive structures requires a high degree of attention in terms of industrial and radiological safety. Decommissioning experts must exclusively carry out the cutting process of radioactive structures due to their technical difficulty. The laboratory has developed a cutting process and equipment for the pressure vessel and internal structures jointly with domestic specialized companies and has conducted validation studies on the effectiveness and detailed procedures through demonstration tests.
Although the technical difficulties presented by remote operation and handling are considerable, the laboratory has demonstrated an underwater cutting technology that can safely perform the cutting process from a radiological standpoint and verified the cutting process and the detailed procedures for cutting into about 800 pieces. The laboratory has also conducted an intensive demonstration test on the thermal cutting technology based on the oxy-propane cutting system, as shown in Fig. 2, a leading cutting technology capable of reducing the work period due to its rapid cutting ability. The nuclear reactor remote-cutting technology, developed as part of the core nuclear technology development project, was selected as a next-generation world-class technology by the Ministry of Trade, Industry, and Energy in 2021 in recognition of its contribution to increasing exports and boosting national economic development based on its advanced technology and competitiveness. The government has selected the remote autonomous decommissioning automation technology as a representative technology development project for accelerating Korea’s autonomous, self-reliant technological capability and commercializing it, with the goal of completing it by 2030 [1].
3.2 Development of a Cutting/Dismantling Technology for CANDU-type Radioactive Structures to Secure a Foundation for the Decommissioning of PHWR
Wolsong Unit 1 is currently being prepared to become the world’s first NPP with a PHWR to be decommissioned. The laboratory is developing a technology to ensure the safe and economical dismantling and decommissioning of radioactively contaminated nuclear reactors and their surrounding structures and is conducting joint research with the CANDU Owners Group Inc. (COG) to dismantle the Calandria nuclear reactor. The dismantled radioactive wastes were classified for the complex shape of the reactor structure by reflecting the power plant’s operation history, and the amount of waste was calculated.
The laboratory has completed the conceptual design of twenty types of remote dismantling equipment based on the established dismantling strategy and process, prioritizing the radiation safety of the workers responsible for dismantling, as shown in Fig. 3. The joint COG research benchmarked the decommissioning strategy of the Gentilly-2 in Canada, the same type of reactor as Wolsong Unit 1, drawing heavily on similar decommissioning experiences, such as Canada’s refurbishment projects [2].
3.3 Development of System Decontamination Fundamental Technology for PHWR Decommissioning
In December 2019, Wolsong Unit 1 was permanently shutdown and needs to perform system decontamination to remove the corrosion oxide film on the inner surfaces of the system before performing major decommissioning activities. The characteristics of the corrosion oxide film are fundamental in system decontamination. Unlike the pressurized water reactor (PWR), the PHWR is mainly composed of carbon steel, and thus there is a limit to applying that of PWR. Therefore, the laboratory conducted research from April 2019 to September 2021 in order to develop a conceptual design for system decontamination to maximize the effect of system decontamination of PHWR. For PHWR, the coolant system and moderator system are physically separated, therefore, it was originally classified into Range 1 (Coolant System) and Range 2 (Moderator System). Range 1 was selected as a coolant system, shutdown cooling system, coolant purification system, and pressure and inventory control system. Range 2 was selected as a moderator system and moderator purification system. Range 1 (Reactor Coolant System) of system decontamination mainly uses organic acid since the reactor coolant system is made of carbon steel, and if necessary, an oxidation process will be additionally performed. Range 2 (Moderator System) of system decontamination mainly uses the oxidation/reduction process because the system is made of stainless steel [3].
3.4 Development of Korean Dry Storage Module for PWR Spent Nuclear Fuel
In Korea, all the spent nuclear fuels were stored in NPP spent fuel pool. However, starting with Hanbit site in 2030, the spent fuel pool of the storage capacity of Hanul and Kori sites is expected to reach saturation point in order. Therefore, the laboratory completed a feasibility evaluation as the first step of developing the Korean dry storage module for PWR spent nuclear fuel and began the detailed design as a second step in 2023. The detailed design includes a safety analysis including criticality safety, radiation shielding, thermal analysis, structural and seismic design, operation procedures, and International Atomic Energy Agency (IAEA) safeguards.
The laboratory plans to apply for licensing in 2027 to install the Korean dry storage module in domestic nuclear power plant sites. The main advantage of this model is that it is about 50% less expensive than overseas commercial dry storage cask and has excellent seismic performance. Moreover, it can fundamentally prevent the CISCC (Chloride- Induced Stress Corrosion Cracking) and intentional aircraft crash, and the adoption of a replaceable cylinder in the dry storage module has great benefits in terms of maintenance. The configuration of the Korean dry storage module for PWR spent nuclear fuel is shown in Fig. 4. The laboratory has completed domestic patent registration (No. 10-2036458) for the Korean dry storage module and is promoting PCT registration for oversea export [4].
3.5 Development of a Gamma Ray Spectroscopy Method for Whole Body Counters Using an AI Technology
NPPs use whole-body counters to evaluate the internal dose when radiation workers inhaled radioactive isotopes. Among them, sodium iodide (NaI) scintillation detectors have a superior sensitivity than that of high-purity germanium (HPGe) detectors and can thus measure the amount of nuclides incorporated in a radiation worker within 3 minutes. However, it has a disadvantage in the accuracy of analyzing gramma-ray emitting nuclides with similar energy due to its poor spectral resolution. As the Koran regulatory body is planning to enact a regulation requiring mandatory compliance with the performance standard of ANSI N 13.30 (2011) when evaluating the internal exposure dose, there are concerns that if it is applied to NaI detectors, the performance for some nuclides emitting gamma rays with similar energies may not meet the test criteria. As such, the laboratory is developing an AI technology to minimize the effects of interfering nuclides to meet the ANSI criteria. In July 2021, the laboratory developed Korea’s first deep learning technology-based radionuclide analysis algorithm, which was confirmed to be superior in terms of discrimination to the conventional method, as shown in Fig. 5.
3.6 Development of Plasma Torch Melting Technology (PTM) for Treating Radioactive Waste
The laboratory developed a PTM, starting with the first-generation 150 kW facility in 1996, followed by the second-generation 500 kW in 2012 and the third-generation 1.5 MW facility with a drum (200 L) direct processing method in 2018, respectively. The laboratory has been conducting research projects to advance the third-generation PTM facility, as shown in Fig. 6, since 2020. The primary objectives of the research are to develop the treatment process for each type of waste, optimize the operation parameters, and set up the standardization and reliability of the facilities. The view of plasma melting, melt discharging, and final products are shown in Fig. 7, respectively.
