• Energy Research

Abstract In many countries which adopted a radiological criterion for decommissioning or site release, a Multi-Agencies Radiation Survey and Site Investigation Manual methodology has been extensively used as a guide for designing decommissioning surveys, especially a final status survey, in order to demonstrate a decommissioned site’s compliance with the radiological criteria. The methodology recommends the use of nonparametric hypothesis testing in a final status survey as a statistical tool in making a decision on whether or not the mean contamination level in a survey unit meets a concentration limit which is derived from the radiological criterion. When designing a final status survey, it is of a great importance to ensure that the hypothesis test of the survey has a sufficient power to release a survey unit in question when it is clean by determining the sample size necessary for achieving the power. For this purpose, the MARSSIM methodology recommends using sample sizes which are 20% greater than statistically necessary for the test. However, this study shows that, the MARSSIM-recommended sample size generally provides a conservatism much larger than necessary. Therefore, it is recommended to determine a reasonable sample size of the final status survey for a survey unit not by just following the recommendation of the MARSSIM but by performing a detailed investigation of the power of the test. This kind of effort is valuable as it could result in saving lots of costs when the determined reasonable sample size is smaller than recommended by the MARSSIM methodology.

The decommissioning project of a nuclear facility is a large-scale process that is expected to take about 15 years or longer. The range of risks to be considered is large and complex, then, it is expected that various risks will arise in decision-making by area during the project. Therefore, in this study, the risk family derived from the Decommissioning Risk Management (DRiMa) project was reconstructed into a decommissioning project risk profile suitable for the Kori Unit 1. Two criteria of uncertainty and importance are considered in order to prioritize the selected 26 risks of decommissioning project. The uncertainty is scored according to the relevant laws and decommissioning plan preparation guidelines, and the project importance is scored according to the degree to which it primarily affects the triple constraints of the project. The results of risks are divided into high, medium, and low. Among them, 10 risks are identified as medium level and 16 risks are identified as low level. 10 risks, which are medium levels, are classified in five categories: End state of decommissioning project, Management of waste and materials, Decommissioning strategy and technology, Legal and regulatory framework, and Safety. This study is a preliminary assessment of the risk of the decommissioning project that could be considered in the preparation stage. Therefore, we expect that the project risks considered in this study can be used as an initial data for reevaluation by reflecting the detail project progress in future studies.

Abstract The oldest commercial reactor in South Korea, Kori Unit 1 Nuclear Power Plant (NPP), will be permanently shut down starting in 2017. Since decommissioning of NPPs will generate a huge amount of radioactive metallic waste, proper treatment for decommissioning wastes is one of the key factors to decommission a plant successfully. Many studies have been conducted and experience accumulated for waste minimization and the reduction of disposal volume. Melting technology is one of the proven technologies for effectively managing the metallic waste and has been adopted by many countries. However, a licensed melting facility to dispose of metallic wastes from the NPP decommissioning has not been implemented in Korea. The purpose of this study is to establish a management plan for metallic waste and to evaluate the preliminary doses according to the acceptance criteria and operations at the facility. Based on the available research papers and empirical data from domestic and international experience, the operational concept and the melting technology principle were established and its dose and volume reduction effects were evaluated. The concept of operating a melting plant is to finally free release the ingots after melting or free release after a planned storage period. Based on this concept, the radioactive concentration of the scrap metal which can be treated was derived, demonstrating that melting can be applied to low level and very low level wastes above the clearance level. The RESRAD-RECYCLE computer code was used to assess worker doses and was rated within the regulatory limits in most scenarios, except for slag workers. Factors affecting the dose, such as methods of radiation protection, operating method and working time, will be important factors in establishing the melting facility and the radiological limits should also be determined accordingly.

Korea's first commercial nuclear power plant at Kori site was permanently shut down in 2017 and is currently in transition stage. Preparatory activities for decommissioning such as historical site assessment, characterization, and dismantling design are being actively carried out for successful D&D (Dismantling and Decontamination) at Kori site. The ultimate goal of decommissioning will be to ensure the safety of workers and residents that may arise during the decommissioning of nuclear facilities and, thereby finally returning the site to its original status in accordance with the release criteria. Upon completion of decommissioning, the resident's safety at a site released will be assessed from the evaluation of dose caused by radionuclides expected to be present or detected at the site. Although the U.S. commercial nuclear power plants with decommissioning experience use different site release criteria, most of them are 0.25 mSv/y. In Korea, both the unrestricted and restricted release criteria have been set to 0.1 mSv/y by the Nuclear Safety and Security Commission. However, since the dose is difficult to measure, measurable concentration guideline levels for residual radionuclides that result in dose equivalent to the site release criteria should be derived. For this derivation, site reuse scenario, selection of potential radionuclides, and systematic methodology should be developed in planning stage of Kori site decommissioning.In this paper, for calculation of a preliminary site-specific Derived Concentration Guideline Levels (DCGLs) for the Nuclear Power Plant site, a novel approach has been developed which can fully reflect practical reuse plans of the Kori site by taking into account multiple site reuse scenarios sequentially, thereby striking a remarkable distinction with conventional approaches which considers only a single site scenario. Keywords: Decommissioning, RESRAD, Site release, DCGL, Residual activity, Farmer scenario, Industrial worker scenario