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류강우,이세종,김혁재,김광표 (사)한국방사선산업학회 2021 방사선산업학회지 Vol.15 No.2
In Korea, in order to complete the decommissioning of the nuclear facility, radiationdose due to residual radioactivity in the site or remaining buildings following (dose based)decommissioning satisfy site reuse criteria. When safety assessment of site reuse after decommissioning,site specific parameter should be used for reliable assessment. In this study, the sensitivityanalysis of the distribution coefficient, transfer factors, and bioaccumulation factor, which are themajor site specific parameter for the safety assessment were performed. The nuclides were selectedbased on decommissioning cases of overseas NPP, and 12 of the total 13 nuclides were selected asthe target nuclides. Resident farmer scenario was applied as an exposure scenario. Distributioncoefficient, transfer factors, and bioaccumulation factor were selected for sensitivity analysis. NDD(Normalized Dose Difference) was used as a sensitivity analysis method. For the application ofNDD, the statistical distribution presented in NUREG/CR-6697 was used. The 5th percentile wasapplied as the low value, the mean value was applied as the base value, and the 95th percentile wasapplied as the high value. As a result of sensitivity analysis, it was found that the contaminatedlayer distribution coefficient was sensitive to 99Tc and 90Sr. The distribution coefficients of theunsaturated and saturated layers were found to be insensitive to all nuclides. When the site has onesoil texture, the distribution coefficient was sensitive to 14C, 90Sr, 99Tc, and 241Am. In the case of planttransfer factor, it was sensitive to 63Ni, 90Sr, 99Tc, 238Pu, 241Am, and 241Pu. Meat transfer factor wassensitive to 55Fe, and milk transfer factor was sensitive to 63Ni. The bioaccumulation factor for fishwas insensitive to all nuclides. As a result of the sensitivity analysis, it was found that the degree ofsensitivity of the distribution coefficient and the transfer factor varies depending on the nuclide. Anddistribution coefficient and the transfer factor have a large influence on the change in the maximumdose overall. Therefore, in order to evaluate the safety of site reuse after decommissioning of nuclearfacilities, it is considered that reliable results can be derived only when the site-specific parametersare obtained and reflected in the evaluation.
이세종,류강우,김민성,김기훈,김광표 한국방사선산업학회 2021 방사선산업학회지 Vol.15 No.3
Hundreds of residual radionuclides exist on the site after decommissioning nuclearfacilities, which spread around the site through routes such as groundwater leaching, surfaceleakage, and atmospheric diffusion. Exposure of individuals living offsite of nuclear facilitiesafter decommissioning is affected by the diffusion characteristics of radionuclides. Therefore,it is necessary to analyze the radiation dose due to changes in the diffusion characteristics ofradionuclides. In this study, an analysis of radiation due to changes in groundwater characteristicswas performed. Radionuclides of 3H, 14C, 99Tc, and 129I were selected as the source terms for theevaluation. Exposure scenario was selected as resident farmer scenario considering all intakepathways. Radiation dose assessment was conducted using RESRAD-OFFSITE, and groundwatercharacteristic input parameters were selected for groundwater flow direction, groundwater andsurface water usage ratio. For other input parameters, literature review was performed. The flowdirection of groundwater was divided into eight directions, groundwater and surface water usageratio was divided into 0.1 units. Based on the selected conditions, the changes in the radiation dosedue to changes in the flow direction of groundwater, groundwater and surface water usage ratiowere analyzed. According to the analysis of the exposure radiation dose about the direction ofgroundwater flow, in case of the flow direction increases from 45° to 90°, the effect of groundwateris eliminated and the radiation dose decreased sharply by 10~10,000 times. From 270° to 315°,the radiation dose increased dramatically, increasing to the same level as the radiation dose in theflow direction of 0°. As a result of the radiation dose analysis according to the groundwater andsurface water usage ratio, 3H decreased by about 1,000 times, 99Tc, and 129I by about 10,000 times,as the ratio of groundwater decreased. However, for 14C, there was no change in the radiation dosewhen the groundwater usage ratio reached zero. The results of this study can be used to selectinput parameters related to groundwater characteristics in the safety assessment around the siteafter nuclear facility decommissioning in the future.