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Department of Nuclear EngineeringStarting with Kori unit 1 on 2016, decontamination and dismantling projects (D&D project) of Hanbit 1, 2, Haneul 1, 2 and Wolsung 1, 2, 3, 4 are planned at the Republic of Korea. Specifically, since Kori unit 1 would be the first commercial nuclear power plant for the D&D project, it is more important to make adequate preparation in advance. In addition, currently, KHNP (Korea Hydro & Nuclear Power) aims for instant decommissioning (deferment with 5 years) instead of deferred decommissioning (deferment with 15 ~ 30 years). According to the raw timeline that has been announced, the preparation stage should be managed until 2024, which is the deadline for complication on SNF (spent nuclear fuel). These preparations are focused on minimization of radioactive waste during the main on-site decontamination and dismantling process, minimization of radiation exposure to workers in the facility, and radiation leakage minimization to the environment. Through this, the reduction in waste management budget can be achieved. Especially, radioactive wastes need to be tactfully managed to attain efficient management of budget requirements. Among the radioactive wastes that would be generated, concrete, which originates from the bioshield at the primary circuit that activates due to high neutron absorption, has the maximum contribution. Therefore, the exact estimation and minimization of management wastes can only be attained by assessing the radioactive inventory in the bioshield. Therefore, the exact estimation and minimization of the management waste could only be achieved from the assessment of radioactive inventory in bioshield. Looking at the case of Connecticut Yankee nuclear power plant that had been decommissioned in 1968, the United States of America which is where a large percentage of an error on pre-radioactive inventory analysis, the error caused the generation of additional 163,954 of 200 L of LLW (Low-Level Waste) that lead to increasing at 228 % of waste management budget and eventually concluded to delaying and change on whole D&D project. In order to prevent such failure, specific modeling on radioactivity inventory of Kori unit 1 has been done. Before the initiation of modeling on Kori unit 1, literature review and case study on radioactive inventory assessment in similar foreign nuclear power plants were made for identification of variables and information which should be identified on the modeling. Based on the case study, the research is objective to investigate the radioactivity inventory of the bioshield on Kori unit 1 and estimated the potential amount and cost of radioactive waste managing and give guidance to workers form external dose analysis. In addition, compared to studies on other foreign nuclear power plants, three-dimensional neutron flux distribution and nuclides behavior after the shutdown of the facility with time passes were also considered. Trojan nuclear power plant was used as benchmark for validation of the computation model for ensuring the reliability of it. On-site monitoring on radioactivity that had been initiated by IAEA (International Atomic Energy Agency) and result with MCNP 6 was compared and verified for its reliability. After the verification had been completed, specific Kori unit 1 bioshield assessment with MCNP 6 based on Monte Carlo probability theory was adopted with Boltzmann neutron transport scheme and activity of 60Co, 152Eu, 154Eu, and 134Cs which hold high sensitivity on regulation for clearance on Republic of Korea was assessed with MS-EXCEL adopting Bateman balance theory. Based on radioactivity inventory analysis, the regulation for clearance level was adopted and the amount of the potential LLW has been estimated with an additional change of them from 5 to 30 years after the shutdown of Kori unit 1. Finally, the potential external dose to workers on bioshield was classified using VISIPLAN 4.0 ALARA adopting governing balance scheme. As a result, the bioshield in Kori unit 1 showed average an 812 Bq/g of contamination with major radioactive nuclide as 60Co. The clearance boundary was estimated as 425 cm from the reactor core with potential 3689 of LLW drums generation and 44 M USD would be required on managing. The average permittable working time was an average 14 hours. The research provides results within a reasonable amount of error and can be utilized as a basic tool to assess other domestic PWR nuclear power plants.clos