Kori unit 1, the first PWR (Pressurized Water Reactor) in Korea, was permanent shut down in 2017. In Korea, according to the Nuclear Safety Act, the FDP (Final Decommissioning Plan) must be submitted within 5 years of permanent shutdown. According to NSSC Notice, the types, volumes, and radioactivity of solid radioactive wastes should be included in FDP chapter 9, Radioactive Waste Management, Therefore, in this study, the types depending on generation characteristics and radiological characterization methods and process of solid radioactive waste were analyzed. Solid radioactive waste depending on the characteristics of the generation was classified into reactor vessel and reactor vessel internal, large components, small metals, spent nuclear fuel storage racks, insulation, wires, concrete debris, scattering concrete, asbestos, mixed waste, soil, spent resins and filters, and dry active waste. Radiological characterization of solid radioactive waste is performed to determine the characteristics of radioactive contamination, including the type and concentration of radionuclides. It is necessary to ensure the representativeness of the sample for the structures, systems and components to be evaluated and to apply appropriate evaluation methods and procedures according to the structure, material and type of contamination. Therefore, the radiological characterization is divided into concrete and structures, systems and components, and reactor vessel, reactor vessel internal and bioshield concrete. In this study, the types depending on generation characteristics and radiological characterization methods and process of solid radioactive waste were analyzed. The results of this study can be used as a basis for the preparation of the FDP for the Kori unit 1.

Radiological characterization, one of the key factors for any successful decommissioning project for a nuclear facility, is defined as a systematic identification of the types, quantities, forms, and locations of radioactive contamination within a facility. This characterization is an essential early step in the development of a decommissioning plan, in particular during transition period after permanent shutdown of the facility, and also to be used for classification of decommissioned radioactive wastes so that their disposal criteria can be met. Therefore, the characterization should be well planned and performed. In the transition period, the characterization information developed during the operational phase is usually reexamined with respect to the applied assumptions, the actual status of the facility after shutdown, the accuracy of the required measurements and changes in its radiological properties to support the development of the final decommissioning plan. Based on some national (Korean, USA’s and Japanese) laws including the related regulations, and some related documents published by OECD/NEA, IAEA, and ASTM, key elements of radiological characterization, which should be developed in the transition period, could be proposed as the followings. The key elements might be an operational history including facility operation history and contamination by events and/or accidents, radiological inventory of the facility and site area, characterization survey including in-situ survey and/or sampling and analyses, radiological mapping (which is able to identify radiological contamination levels of SSCs, and the facility area and, if contamination may be suspected, the surroundings) with tabulating, residual radioactivity (or derived concentration guideline levels) of selected major radionuclides for remediation of the site, (retainable and retrievable) recording, and quality control and quality assurance. In review process of the operational history, interviews of current or former long-tenured knowledgeable employees of the facility should be conducted to identify conditions that may have been missing from the records.

Prior to dismantling a nuclear facility, full site characterization should be carried out to identify basic data for various stages of decommissioning, such as deregulation of sites and structures, selection of decontamination technology, decommissioning methods, and waste management and disposal. Radiological characterization is implemented through information collection, on-site measurement, sampling and analysis, and theoretical calculations and proven codes for radioactive material at the time of decommissioning of the nuclear facility. There are issues in that it takes a lot of time and money to collect and analyze samples for characterization of contaminated sites and radioactive structures. Therefore, in the entire process of decommissioning a nuclear facility, a technology that can quickly measure the radiological characteristics of various decommissioning objects and wastes on site is required. In this project, the utilization of gamma cameras that can be analyzed in the field for quick and accurate characteristic evaluation at the dismantling site was studied. A gamma camera, iPIX from Canberra (Now it became Mirion Technology), was tested in this study. It is a unique gamma imager, which have a CdTe sensor with TIMEPIX chip and a coded aperture collimator, quickly locates and identifies low to high level radioactive sources from a distance while estimating the dose rate at the measurement point in real time. It also can be combined with CZT sensor which called iPIX-NID (nuclide Identification) provides users with clear understanding of radionuclides presence with no need of any spectroscopic knowledge. iPIX with iPIX-NID convert the gamma camera into a hot-spot detector with radionuclide information. To verify the applicability of a gamma camera in Nuclear power plant, it was implemented to Kori unit-1 which was permanently shut down from 2017. Various Systems were observed at restricted area including reactor cooling system, boron recovery system, residual heat removal system, containment spray system, and etc. The locations of hot spots were clearly revealed by iPIX and these results can be used for selecting the locations of destructive samples and help to decide the conservative decision making. Condensate water systems in turbine building were also observed by a gamma camera and showed no nuclide. Based on this preliminary gamma camera applications, further investigation and tests will be carried out to Kori Unit-1.

An application of the final decommissioning plan for unit 1 of Kori NPP was submitted to NSSC on 14 May 2021. We have been implementing the project related to the radiological characterization for the plan since 2019. However, the project was not running smoothly due to the regulatory environment. The destructive sampling from the objects was not allowed, so only smear (swipe) samples are available. In this study, the sampling way and the analytical results of radionuclides are presented. In addition, we propose in-situ measurement using gamma camera and in-situ gamma spectroscopy to obtain more comprehensive radiological information on the object.