During the decommissioning of a nuclear power plant, the structures must be dismantled to a disposal size. Thermal cutting methods are used to reduce metal structures to a disposal size. When metal is cut using thermal cutting methods, aerosols of 1 μm or less are generated. To protect workers from aerosols in the work environment during cutting, it is necessary to understand the characteristics of the aerosols generated during the cutting process. In this study, changes in aerosol characteristics in the working environment were observed during metal thermal cutting. The cutting was done using the plasma arc cutting method. To simulate the aerosols generated during metal cutting in the decommissioning of a nuclear power plant, a non-radioactive stainless steel plate with a thickness of 20 mm was cut. The cutting condition was set to plasma current: 80 A cutting speed: 100 mm/min. The aerosols generated during cutting were measured using a highresolution aerosol measurement device called HR-ELPI+ (Dekati®). The HR-ELPI+ is an instrument that can measure the range of aerodynamic diameter from 0.006 μm to 10 μm divided into 500 channels. Using the HR-ELPI+, the number concentration of aerosols generated during the cutting process was measured in real-time. We measured the aerosols generated during cutting at regular intervals from the beginning of cutting. The analyzed aerosol concentration increased almost 10 times, from 5.22×106 [1/cm3] at the start of cutting to 6.03×107 [1/cm3] at the end. To investigate the characteristics of the distribution, we calculated the Count Median Aerodynamic Diameter (CMAD), which showed that the overall diameter of the aerosol increased from 0.0848 μm at the start of cutting to 0.1247 μm at the end of the cutting. The calculation results were compared with the concentration by diameter over time. During the cutting process, particles with a diameter of 0.06 μm or smaller were continuously measured. In comparison, particles with a diameter of 0.2 μm or larger were found to increase in concentration after a certain time following the start of cutting. In addition, when the aerosol was measured after the cutting process had ended, particles with a diameter of 0.06 μm or less, which were measured during cutting, were hardly detected. These results show that the nucleation-sized aerosols are generated during the cutting process, which can explain the measurement of small particles at the beginning of cutting. In addition, it can be speculated that the generated aerosols undergo a process of growth by contact with the atmosphere. This study presents the results of real-time aerosol analysis during the plasma arc cutting of stainless steel. This study shows the generation of nucleation-sized particles at the beginning of the cutting process and the subsequent increase in the aerosol particle size over time at the worksite. The analysis results can characterize the size of aerosol particles that workers may inhale during the dismantling of nuclear power plants.

Bentonite is a promising buffer material for high-level radioactive waste (HLW) disposal due to the high nuclides sorption capacity and swelling property. However, bentonite has the potential to generate colloid particles, with small particle sizes less than 1,000 nm when in contact with groundwater. The bentonite colloids easily form pseudo-colloid with the released nuclides and migrate through the water-conducting rock to the biosphere. Therefore, understanding the generation and migration of bentonite colloids is crucial in assessing the safety of the HLW repository. In this study, an artificial fracture system was prepared to investigate colloid release from compacted bentonite. A 250 mm diameter acrylic artificial fracture system was used, with 30 mm of compacted calcium bentonite installed. Artificial groundwater flow was injected into the system at a flow rate of 250 μL/h, and every 6 mL of leachate was collected by a fraction collector. A film-type pressure sensor was equipped to monitor the swelling pressure, and the swelling was observed using a digital microscope. The results indicate that the compacted bentonite formed a mineral ring originating from the swelling of the bentonite, and the end of the ring generated colloid particles due to chemical erosion. Although the release rate of colloids increased with increasing flow rate, the colloid ratio depended on the low ionic strength of the injected artificial groundwater. This work contributes to the understanding of the chemical erosion and colloid release mechanism of compacted bentonite.

The design of a radioactive waste disposal system should include both natural and engineered barriers to prevent radionuclide leakage and groundwater contamination. Colloids and gases can accelerate the movement of radionuclides and affect their behavior. It is important to consider these factors in the long-term stability evaluation of a deep geological repository. An experimental setup was designed to observe the acceleration of nuclide behavior caused by gas-mediated transport in a simulated high temperature and pressure environment, similar to a deep disposal repository. The study used specimens to simulate gas flow in engineered barriers, based on conditions 1000 years after repository closure. In the experiment, bentonite WRK with a dry density of 1.61 g/cm3 was used after compaction. Measurements were taken of the saturation time and gas permeability of compacted bentonite. In this study, gas was injected into saturated buffer materials at various pressures to evaluate the penetration phenomenon of the buffer material according to the gas pressure. It was observed that gas penetrated the buffer material and moved upward in the form of gas bubbles at a specific pressure. Furthermore, when a flow was continuously induced to penetrate the buffer material, erosion occurred, and the eroded particles were found to be able to float upward or be transported by gas bubbles. In future studies, analysis will be conducted on the transport rate of fine particles according to the size of gas bubbles and the characteristics of the nuclides adsorbed on the fine particles.

Background: After the coronavirus disease 2019 (COVID-19) pandemic, many changes in education were observed, particularly non-face-to-face classes. Many studies have focused on the suitability of non-face-to-face classes and the satisfaction of both students and teachers with this teaching method. However, no study on the suitability of numerous teaching methods in teaching physical therapy subjects has been conducted. Objectives: To investigated the suitability of three teaching methods, namely, face-to-face, non-face-to-face, and mixed classes, in teaching physical therapy subjects. Design: Survey research. Methods: 336 students studying physical therapy participated. A survey was conducted on the suitability of face-to-face, non-face-to-face, and mixed classes for 16 subjects. The survey was conducted using a Google questionnaire. Results: Face-to-face classes were found to be suitable for teaching the basic fields of physical therapy, and face-to-face and mixed classes were suitable for teaching the principle of diagnostic evaluation. Furthermore, faceto- face classes were found to be suitable for teaching physical therapy interventions. In other subjects, face-to-face classes were suitable for teaching rehabilitation medicine, non-face-to-face classes were suitable for teaching medical terms, and mixed classes were suitable for teaching daily life activities. Conclusion: The results of this study can be used as a basis for choosing the appropriate methods for learning and teaching physical therapy subjects.

Colloid Formation and Migration (CFM) project is being carried out within the Grimsel Test Site (GTS) Phase Ⅵ. Since 2008, the Korea Atomic Energy Research Institute (KAERI) has joined CFM to investigate the behavior of colloid-facilitated radionuclide transport in a generic Underground Research Laboratory (URL). The CFM project includes a long-term in-situ test (LIT) and an in-rock bentonite erosion test (i-BET) to assess the in-situ colloid-facilitated radionuclide transport through the bentonite erosion in the natural flow field. In the LIT experiment, radionuclide-containing compacted bentonite was equipped with a triple-packer system and then positioned at the borehole in the shear zone. It was observed that colloid transport was limited owing to the low swelling pressure and low hydraulic conductivity. Therefore, a postmortem analysis is being conducted to estimate the partial migration and diffusion of radionuclides. The i-BET experiment, that focuses more on bentonite erosion, was newly designed to assess colloid formation in another flow field. The i-BET experiment started with the placement of compacted bentonite rings in the double-packer system, and the hydraulic parameters and bentonite erosion have been monitored since December 2018.

When decommissioning a nuclear power plant, the structure must be made to a disposable size. In general, the cutting process is essential when dismantling a nuclear power plant. Mainly, thermal cutting method is used to cutting metal structures. The aerosols generated during thermal cutting have a size distribution of less than 1 μm. The contaminated structures are able to generate radioactive aerosols in the decommissioning. Radioactive aerosols of 1 μm or less are deposited in the respiratory tract by workers’ breathing, causing the possibility of internal exposure. Therefore, workers must be protected from the risk of exposure to radioactive aerosols. Prior knowledge of aerosols generated during metal cutting is important to ensure worker safety. In this study, the physical and chemical properties of the aerosol were evaluated by measuring the number and mass concentrations of aerosols generated when cutting SUS304 and SA508 using the laser cutting method. High-resolution aerosol measuring equipment (HR-ELPI+, DEKATI) was used to measure the concentration of aerosols. The HR-ELPI+ is an impactor-type aerosol measuring equipment that measures the aerosol number concentration distribution in the aerodynamic diameter range of 6 nm to 10 um in real-time. And analyze the mass concentration of the aerosol according to the diameter range through the impactor. ICP-MS was used for elemental mass concentration analysis in the aerosol. Analytical elements were Fe, Cr, Ni and Mn. For the evaluation of physical and chemical properties, the MMAD of each element and CMAD were calculated in the aerosol distribution. Under the same cutting conditions, it was confirmed that the number concentration of aerosols generated from both materials had a uni-modal distribution with a peak around 0.1 um. CMAD was calculated to be 0.072 um for both SUS304 and SA508. The trend of the CMAD calculation results is the same even when the cutting conditions are changed. In the case of MMAD, it was confirmed that SUS304 had an MMAD of around 0.1 μm in size for only Fe, Cr and Mn. And SA508, Fe, Cr, Ni and Mn were all confirmed to have MMAD around 0.1 μm in size. The results of this study show that a lot of aerosols in the range of less than 1 μm, especially around 0.1 μm in size, are generated when metal is cut using laser cutting. Therefore, in order to protect the internal exposure of workers to laser metal cutting when decommissioning NPPs, it is necessary to protect from nano-sized aerosols beyond micron size.

Safe storage of spent nuclear fuel in deep underground repositories needs an understanding of the long-term alteration (corrosion) of metal canisters and buffer materials. We conducted a small-scale laboratory alteration tests on some metal (Cu and Fe) chips by embedding them into the compacted bentonite blocks, which were placed in anaerobic water for 1 year. Some additives like lactate, sulfate, and bacteria were separately loaded into the water to promote biochemical reactions. The bentonite blocks immersed in the water were finally dismantled after 1 year, and they showed that their alteration was insignificant. However, the Cu chip exhibited some microscopic etch pits on its surface, wherein sulfur component was slightly detected. Overall, the Fe chip was more corroded than the Cu chip under the same condition. The secondary phase of the Fe chip was locally found as carbonate materials, such as siderite (FeCO3) and calcite ((Ca, Fe)CO3). These secondary products could imply that the local carbonate production around the Fe chip may be initiated by an evolution (alteration) of bentonite and a diffusive provision of biogenic CO2 gas. These laboratory scale test results suggest that the long-term alteration (corrosion) of metal canister/bentonite blocks in the engineered barrier could be possible and may be promoted by microbial activities.

Colloid-facilitated migration has been significantly concerned with the acceleration of the radionuclide mobility in the HLW repository. In the repository system, the compacted bentonite, which is the buffer material, could be the major source for colloid generation; hence, the understanding of colloid generation from the bentonite is the essential to expect the colloid-facilitated radionuclide migration. This study aimed to investigate the colloid generation using a bentonite-based micro-scale flow path system, which called microfluidics. In order to fabricate the microfluidics, direct milling method was applied to make a mold by computer numerical control. The fabricated mold applied to prepare the microfluidic chip by Polydimethylsiloxane (PDMS), in which the size of microchannel was designed to be one micrometer. Initially, sylgard 184 and curing agent mixed and stirred for 10 min, afterwards the bubbles in the paste was removed in the vacuum desiccator for 30 min. Then the paste was poured into the mold, and finally dried for 4 hours at 80°C in a dry oven. The compacted Ca-bentonite chip was prepared by the cold isostatic pressing (CIP) method with the dry density of 1.6 g·cm−3. The microfluidic chip and compacted bentonite chip were assembled by an acryl jig, the flow rate was adjusted by 20 mL syringe equipped syringe pump. The degree of colloid generation accompanied with the erosion of bentonite was gravimetrically examined after the experiment. The effect of the pH and ionic strength on the colloid formation was investigated through the particle size, stability and aggregation. To the best of our knowledge, this is the first examination for the colloid generation using microfluidics; these results would give information to understand the colloid formation from the compacted Ca-bentonite in the HLW repository system.

A method to effectively scavenge highly mobile radioiodide into a solid material was developed. Under an anaerobic condition, as copper(II) was strongly associated with bicarbonate (HCO3 −) in solution, malachite quickly formed, and then it was gradually transformed to a compact crystal of CuI (marshite) attracting iodide. The formation of CuI crystal was principally led by the spontaneous Cu-I redox reaction centering around the copper phase over the presence of sulfate (SO4 2−). The transformed CuI crystal was poorly soluble in water. Interestingly, this redox-induced iodide crystallization was rather promoted over the existence of anionic competitors (e.g., HCO3 − and SO4 2−). Unlike the conventional methods, these competing anions positively behaved in our system by supporting that the initial malachite was more apt to be reactive to largely attract highly mobile I−. Under practical environments, such a selective I− uptake and fixation into a crystalline form will be a promising way to effectively remove I− in a great capacity.

The radioactive waste disposal systems should consist of engineering and natural barriers that limit the leakage of radionuclide from spent nuclear fuel and fundamentally block groundwater from contact with radioactive waste. These considerations and criteria for designing a disposal system are important factors for the long-term stability evaluation of deep geological repository. Colloids and gases that may occur in the near-field and groundwater infiltrated from outside can be means to accelerate the behavior of radionuclide. The gas produced and infiltrated in the disposal system is highly mobile in the porous medium, and reactive gases in particular can affect the phase and behavior of radionuclide. A free gas phase (bubble) can be formed inside the canister if the partial pressure of the generated gas exceeds the hydrostatic pressure. If the gas pressure exceeds the critical endurance pressure of canister and buffer, then a gas bubble may push through the canister perforation and the buffer. It is also known that when gas bubbles are formed, radionuclide or colloids are adsorbed on the surface of the bubbles to enable accelerated movement. An experimental setup was designed to study the acceleration of nuclide behavior induced by gas-mediated transport. A high temperature and pressure reaction system that can simulate the deep disposal environment (500 m underground) was designed. It is also designed to install specimens to simulate gas flow in engineered barriers and natural barriers. The experimental scenario was set based on 1,000 years after the closure of the repository. According to the previous modeling results, the surface temperature of the canister is about 30 to 40 degrees and the gas pressure can be generated between the canister and the buffer is 5 MPa or more. In the experimental conditions, the saturation time of compacted bentonite was measured and the gas permeability of the compacted bentonite according to the dry density was also measured. Further studies are needed on the diffusion of dissolved gas into the compacted bentonite and the permeation phenomenon due to gas overpressure.