Derived Concentration Guideline Levels (DCGLs), which represent the residual radioactivity concentration limits, serve as the pivotal criteria for decontamination during decommissioning of nuclear power plants and are essential for license termination. The analysis of radionuclides in various media to check site-specific and radionuclide-specific DCGLs is a resource-intensive and time-consuming processes, and there are some radionuclides that are hard to analyze. In the decommissioning of the Rancho Seco nuclear power plant in the United States, a conservative approach was adopted. Potentially highly contaminated areas on the site were identified by collecting and analyzing soil samples, and radionuclides exceeding the Minimum Detectable Concentration (MDC) were selected as the potential Radionuclide of Concern (ROC), and surrogate DCGLs for hard-to-detect radionuclides were applied to soil samples. For soil samples in the Rancho Seco nuclear power plant, Cs-137 contributed more than 90% of the total radioactivity. DCGLs of the ROC were obtained using the scaling factors through analysis of Cs- 137 for a large amount of soil samples. In Korea, the scaling factor methodology has not been applied to the decommissioning of commercial nuclear power plants. An initial investigation was undertaken to assess the viability of implementing Surrogate Derived Concentration Guideline Levels (DCGLs) in the dismantling of Kori Unit 1, drawing insights from the U.S. nuclear power plant decommissioning experiences. To do this approach, the concentration ratio of radionuclides of interest to key radionuclide in contaminated soil should be known and consistent. But related information is not available at this time. So Surrogate DCGL for representative C-14, Fe-55, Ni-59, Ni-63, and Sr-90 was obtained using the scaling factors applied to radioactive waste data, specifically Decontaminated Aqueous Waste (DAW) and Spent Resin. In order to develop a reliable surrogate DCGLs the Kori Unit 1 site, it is important to analyze the radionuclides in the soil for the Kori Unit 1 decommissioning site to obtain consistent concentration ratio of the radionuclides of concern to the key radionuclides. When a the suitable DCGL is developed, it can be used for FSS planning and prior decision-making ensuring the safe and effective decommissioning of Kori Unit 1 and similar nuclear power plants.

As the decommissioning of nuclear power plants progresses, interest in the inevitably generated radioactive waste is also increasing. Especially, because the containers of ILW packages are significantly more expensive than the containers of LLW packages, the special attention should be focused on minimizing the number of the containers of ILW packages. The radiation dose limit for packaging of ILW shall not exceed 2 mSv/h and 0.1 mSv/h on contact and at 2 m, respectively in South Korea. Meanwhile, The DEMplus provides various environmental geometry and all properties such as materials, absorptions, and reflections and the estimation of the radiation dose rates is based on the radiation interactions of the designed 3D geometry model. With the consideration of the radiation dose rate by using DEMplus and its strategy of packaging and cutting plan, the number of containers for ILW packages generated from decommissioning of Reactor Vessel Internal (RVI) of a nuclear power plant that has been in operation for decades was optimized in this paper. The modular shielded containers (MSC) with shielding inserted were used for radioactive wastes that require shielded packaging. In order to verify the accuracy of the estimated radiation dose rate by using DEMplus, the estimated results were compared with those obtained using MicroShield. The trends of the estimated radiation dose rates using DEMplus and the estimation of MicroShield were similar to each other. The results of this study demonstrated the feasibility of using DEMplus as a means of estimating the radiation dose limit in packaging plan of the radioactive waste.

In thermal cutting process, gas flow injected from the nozzle has a significant effect on the cutting materials. The gas flow is difficult to observe gases are transparent, therefore, in this study, Schlieren method was adopted to visualize the gas flow inside the kerf. The kerf shape was inserted between two slices of transparent glass in order to imitate the real cutting environment. In order to get the flow characteristics, a high speed camera was equipped and the image processing was applied to compare the before and after injection images. As a result, the method for visualizing gas flow was successfully developed and also expected to be applied to the analysis of purging gas in various welding environments.

A safety assessment of radioactive waste repositories is a mandatory requirement process because there are possible radiological hazards owing to radionuclide migration from radioactive waste to the biosphere. For a reliable safety assessment, it is important to establish a parameter database that reflects the site-specific characteristics of the disposal facility and repository site. From this perspective, solubility, a major geochemical parameter, has been chosen as an important parameter for modeling the migration behavior of radionuclides. The solubilities were derived for Am, Ni, Tc, and U, which were major radionuclides in this study, and on-site groundwater data reflecting the operational conditions of the Gyeongju low and intermediate level radioactive waste (LILW) repository were applied to reflect the site-specific characteristics. The radiation dose was derived by applying the solubility and radionuclide inventory data to the RESRAD-OFFSITE code, and sensitivity analysis of the dose according to the solubility variation was performed. As a result, owing to the low amount of radionuclide inventory, the dose variation was insignificant. The derived solubility can be used as the main input data for the safety assessment of the Gyeongju LILW repository in the future.

Site characterization for decommissioning Kori Unit 1 is ongoing in South Korea after 40 years of successful operation. Kori Unit 1’s containment building is assumed to be mostly radioactively contaminated, and therefore radiation exposure management and detailed contamination investigation are required for decommissioning and dismantling it safely. In this study, site-specific Derived Concentration Guideline Levels (DCGLs) were derived using the residual radioactivity risk evaluation tool, RESRAD-BUILD code. A conceptual model of containment building for Kori Unit 1 was set up and limited occupational worker building inspection scenario was applied. Depending on the source location, the maximum contribution source and exposure pathway of each radionuclide were analyzed. The contribution of radionuclides to dose and exposure pathways, by source location, is expected to serve as basic data in the assessment criteria of survey areas and classification of impact areas during further decommissioning and decontamination of sites.

대한민국 첫 상업원전인 고리1호기는 40년간의 성공적인 운전을 끝내고 2017년 6월 18일 영구정지 되었다. 고리1호기는 본격적인 해체에 앞서 터빈건물에 폐기물처리시설 건설을 계획하고 있다. 각종 방사성폐기물은 폐기물처리시설에서 제염, 해체, 절단, 용융되어 자체처분 되거나 방사성폐기물 처분장으로 보내 진다. 해체폐기물 중 대형금속방사성폐기물은 주로 1차 계통측 기기들로 높은 방사능을 띄고 있어 해체활동 중 작업자의 피폭관리가 필요하다. 본 논문에서는 대형금속방사성폐기물 중 크기가 가장 크고 형상이 복잡한 증기발생기를 선정하여 RESRAD-RECYCLE 코드를 이용하여 작업자 피폭선량을 평가하고 저감화 방안을 수립 하고자 한다.

노후 원전 해체의 경우 부지 특성 및 최종 상태 조사 보고서에 해당 부지내 잔존가능성이 있는 방사성 핵종 정보에 대한 내용 을 포함하여야 한다. 미국 NRC의 경우 이에 해당하는 해체기술관련문서(DTBD)를 부지 특성 조사시에 부지이력조사(HSA) 와 같이 사업자 측이 제출하도록 규제하고 있다. 또한 해체기술관련문서는 방사선학적 부지 조사와 해체완료계획서에 포함 되어야 하는 내용으로써 부지 규제 해제와 재이용에 관해서 중요한 자료를 제공한다. 이 논문은 부지 별 잠재적 핵종에 대해 미국 원전의 해체 사례중 부지 특성 및 최종 상태조사 과정에서 결정하는 방법론을 분석하고 2017년 고리 1호기의 영구 운 전정지 후 이루어질 해체 과정에 필수적인 규제 지침과 기술적 근거 수립에 도움이 되고자 한다.

미국의 원전해체 지침서인 MARSSIM과 MARLAP에서는 의사결정 전 부지조사를 계획하고, 계획한 조사를 시행하는데 Data Life Cycle의 사용을 권장하고 있다. 부지조사 계획단계에서 설정되는 데이터품질목표(DQO)는 부지조사 수행 및 설계의 모든 측면에서 얻을 수 있는 데이터를 최선으로 활용하는 방법을 제시하고, 부지조사 계획을 체계화하여 공식적으로 중요한 의사결정을 내리는데 필요한 정보를 얻을 수 있도록 한다. DQO의 일곱 과정 중 5~7단계에서는 앞선 단계에서 수집한 정보를 이용하여 합리적이고 신뢰성있는 의사결정을 내릴 수 있도록 부지조사를 설계하는 과정이다. 이 과정 중 설정되는 회색영역은 관심핵종에 대한 조사단위의 평균농도가 실제로 DCGL을 초과하지 않음에도 불구하고, 초과한다고 판단하여 제염활동과 같은 추가적인 활동을 하도록 결정내리는 Type II 의사결정 오류로 인한 결과가 미약할 것으로 생각되는 농도범위로 정의된다. 회색영역은 부지에서 수집한 Sample에 속한 특정 핵종의 평균농도와, 규제한도로서 설정될 수 있는 잔류방사능 유도농도(DCGL)를 이용하여 정할 수 있다. 회색영역을 설정함으로써 가장 자원효율적으로 부지조사계획을 설정할 수 있으며 의사결정 오류에 대한 영향을 최소화 할 수 있다. 선행연구로 도출된 고리 1호기의 DCGL을 이용하여 회색영역을 설정하고, 이를 이용해 올바른 의사결정을 내릴 수 있도록 하는 Sample의 농도평가 방법을 제시하였다.