Corrosion-related challenges remain a significant research topic in developing next-generation Molten Salt Reactors (MSRs). To gain a deeper understanding of preventing corrosion in MSRs, previous studies have attempted to improve the corrosion resistance of structural alloys by coating surfaces such as alumina coating. To conduct a corrosion test of coating alloys fully immersed in molten salt, it’s important to ensure that the coating application process is carefully carried out. Ideally, coating all sides of the alloy is necessary to avoid gaps like corners of the alloy, while only applying a one-sided coating alloy can lead to galvanic corrosion with the base metals. Using the droplet shape of eutectic salt applied to only one side of the coating alloy would avoid these problems in conventional corrosion immersion tests, as corrosion would occur solely on the coating surface. Although the droplet method for corrosion tests cannot fully replicate corrosion in the MSRs environment, it offers a valuable tool for comparing and evaluating the corrosion resistance of different coating surfaces of alloys. However, the surface area is important due to the effect of diffusion in the corrosion of alloy in molten salt environments, but it is difficult to unify in the case of droplet tests. Therefore, understanding the droplet-alloy properties and corrosion mechanism is needed to accurately predict and analyze these test systems’ behavior highlighting unity for corrosion tests of different coating surfaces of alloys. To analyze the molten salt droplet behavior on various samples, pelletized eutectic NaCl-MgCl2 was prepared as salt and W-, Mo-coating, and base SS316 as samples. At room temperature, the same mass of pelletized eutectic NaCl-MgCl2 was placed on different samples under an argon atmosphere and heated to a eutectic point of 500°C in a furnace. After every hour, the molten droplets were hardened by rapid cooling at room temperature outside the furnace. The mass loss of salts and the contact area of the samples were measured by mass balance and SEM. The shape, surface area to volume ratio, and evaporation of the droplets of NaCl-MgCl2 per each coating sample and hour were analyzed to identify the optimal mass to equalize the contact coating surface of alloys with salts. Furthermore, We also analyzed whether their results reached saturation of corrosion products through ICP-MS. This will be significant research for the uniformity of the liquid-drop shape corrosion test of the coating sample in molten eutectic salts.

LiCl-KCl eutectic possesses unique properties such as a low melting point, high thermal conductivity, and good electrical conductivity. These properties make it suitable for various applications, including nuclear power generation, pyroprocessing in nuclear waste management, and thermal energy storage systems. In most experiments using LiCl-KCl, the molten salt composition is an important factor; therefore, periodic analysis through sampling is necessary for monitoring compositional changes. Although manual sampling is typically used, it is time-consuming and can introduce errors due to low reproducibility. To address this issue, we have developed an automatic molten salt sampling device using the cold-finger method. This method involves immersing the tip of a tungsten rod in hightemperature LiCl-KCl, removing it after a few seconds, and allowing the adhered molten salt to solidify instantly. A collector then scratches and drops the solidified sample. These processes are carried out automatically using servo motors, enabling the sampling device to move around the molten salt system. We have optimized the sampling conditions, such as insertion and withdrawal rate, immersion time, and the interval between continuous sampling, based on the molten salt temperature. The temperature was set between 500°C and 850°C, considering the operating temperatures of the applications. In addition to sampling speed, the sampling depth is a key condition for determining the sampling mass. Therefore, we examined the amount of sample depending on the sampling depth and, particularly, considered the change in salt height when sampling is performed continuously. As a result, we determined the number of sampling iterations required to reach the target sample mass. Furthermore, to minimize the initial salt loss, we noted that sampling from the salt surface resulted in less representative samples. To determine the reliability, we compared the results of surface sampling with those obtained when sampling at the middle of the salt. This study will enable highly reproducible and reliable sampling by providing a prototype for an automatic sampling device for molten salt along with guidelines.

Molten salts based on magnesium chloride can be used in the nuclear power reactor because they have a high heat capacity and heat stability, and allow for a faster neutron spectrum. However, magnesium chloride is highly hygroscopic, leading to the formation of moisture-related impurities, which result in the corrosion of structural materials and negatively affect the operation of the reactor. The dehydration of magnesium chloride is studied using both thermal and electrochemical treatments. According to previous studies, water impurities in magnesium chloride molten salt transform into magnesium oxide over 650 degrees Celsius. The temperature profile of the molten salt is suggested to separate magnesium chloride and magnesium oxide, focusing on cooling rate near the freezing point of magnesium chloride. Two layers separated by a phase boundary on the salt surface appear due to the density difference between magnesium oxide and magnesium chloride. Further, the removal of oxide ions remaining in the molten salt is carried out by electrochemical treatment. Two different cells, each consisting of two electrodes, are used. One cell is composed of graphite anode and nickel cathode. The other is composed of tin oxide anode and nickel cathode. As the reaction proceeds, carbon dioxide and oxygen are generated in graphite and tin oxide, respectively, and magnesium electrodeposition occurs at the cathode. The amount of purified magnesium oxide is measured to the endpoint, which is notified by the reduced current. The efficiency of each method is compared by measuring the weight ratio of the purified part to the unpurified part. Thermogravimetric analyzer (TGA) and UV-vis spectroscopy are used to check the quality of the purified part. Only magnesium oxide remains at a temperature above the boiling point of magnesium chloride. Therefore, the amount of magnesium oxide in the purified part can be measured by the mass change of the salt through the TGA method. For UV-vis spectroscopy, the transmittance is measured which depends on the weight percent of the impurities in the purified part. The suggested purification method using both thermal and electrochemical treatment is assessed quantitatively and qualitatively. It is expected that hygroscopic molten salts other than magnesium chloride will be able to be dehydrated through the above process.

Molten chloride salts are promising candidates as a coolant for Molten Salt Reactors (MSRs) because of their low cost, high specific heat transfer, and thermal energy storage capacity. The NaCl- MgCl2 eutectic salts have enormous latent heat (430 kJ/kg) and financial advantage over other types of molten chloride salt. Despite the promise of the NaCl-MgCl2 eutectic salt, problems associated with structural material corrosion in the MSR system remain. The hygroscopicity of NaCl-MgCl2 and high MSRs operating temperature accelerate corrosion within structural alloys. Especially, MgCl2 reacts with H2O in the eutectic salt to produce HCl and Cl2, which are known to further exacerbate corrosion by the chlorination of structural materials. Therefore, several studies have worked to purify impurities associated with MgCl2, such as H2O. Thermal salt purification of NaCl-MgCl2 eutectic salt is one method that reduces HCl and Cl2 gas generation. However, MgO and MgOHCl are generated as the byproduct of thermal purification through a reaction between MgCl2 and H2O. The corrosion behavior of MgO within structural alloys after thermal treatment is not well known. This paper demonstrates corrosion behavior within structural alloy after thermal treatment at various temperature profiles of the NaCl-MgCl2 eutectic salt. According to the temperature range, MgCl2·H2O are separated at 100~200°C, and MgOHCl and HCl begin to occur at 240°C or higher. Finally, MgOHCl produces MgO and HCl at 500°C or higher temperatures. After thermal treatments, the H2O, MgOHCl, and MgO content were measured by Thermo Gravimetric Analyzer (TGA) to evaluate significant products causing corrosion. The structural materials were analyzed by the Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS) and using the mass change method to observe the type of localized corrosion, the corrosion rate, and the corrosion layer thickness. This study is possible in that it can reduce economic costs by reducing the essential use of expensive, high-purity molten salts because it is related to a substantial financial cost problem considering the amount of molten salt used in industrial sites.

During electrorefining, fission products, such as Sr and Cs, accumulate in a eutectic LiCl-KCl molten salt and degrade the efficiency of the separation process by generating high heat and decreasing uranium capture. Thus, the removal of the fission products from the molten salt bath is essential for reusing the bath, thereby reducing the additional nuclear waste. While many studies focus on techniques for selective separation of fission products, there are few studies on processing monitoring of those techniques. In-situ monitoring can be used to evaluate separation techniques and determine the integrity of the bath. In this study, laser-induced breakdown spectroscopy (LIBS) was selected as the monitoring technique to measure concentrations of Sr and Cs in 550°C LiCl-KCl molten salt. A laser spectroscopic setup for analyzing high-temperature molten salts in an inert atmosphere was established by coupling an optical path with a glove box. An air blower was installed between the sample and lenses to avoid liquid splashes on surrounding optical products caused by laser-liquid interaction. Before LIBS measurements, experimental parameters such as laser pulse energy, delay time, and gate width were optimized for each element to get the highest signal-to-noise ratio of characteristic elemental peaks. LIBS spectra were recorded with the optimized conditions from LiCl-KCl samples, including individual elements in a wide concentration range. Then, the limit of detections (LODs) for Sr and Cs were calculated using calibration curves, which have high linearity with low errors. In addition to the univariate analysis, partial least-squares regression (PLSR) was employed on the data plots to obtain calibration models for better quantitative analysis. The developed models show high performances with the regression coefficient R2 close to one and root-mean-square error close to zero. After the individual element analysis, the same process was performed on samples where Sr and Cs were dissolved in molten salt simultaneously. The results also show low-ppm LODs and an excellent fitted regression model. This study illustrates the feasibility of applying LIBS to process monitoring in pyroprocessing to minimize nuclear waste. Furthermore, this high-sensitive spectroscopic system is expected to be used for coolant monitoring in advanced reactors such as molten salt reactors.