The swelling capacity of bentonite buffers is vital in high-level radioactive waste (HLW) repositories, as it minimizes groundwater infiltration, prevents nuclides from reaching the biosphere, and stabilizes the HLW canisters. As swelling capacity is a function of temperature, understanding bentonite’s behavior at approximately 100°C (its presumed upper limit) is essential. However, research on this subject has been scarce. Hence, this study explored the effects of thermal treatment of Ca-bentonite at 105°C under injected water pressures. The results suggest a 19% reduction in “swell index” and a 35%–36% decrease in the total pressure in thermally treated bentonite. The heated samples demonstrated higher hydraulic conductivity than the non-heated ones, indicating potential performance deterioration in controlling the fluid movement. Furthermore, the injected water pressure (base pressure) was not fully transmitted to the sample owing to the difference between the base and back pressures, leading to variations in the total pressure despite maintaining a constant differential pressure. Thus, the results demonstrated a degradation in bentonite’s swelling capacity and its compromised role in safe HLW disposal, when subjected to treatment at 105°C. The insights from this research can assist in HLW repository design, while highlighting the need for further research into bentonite’s performance.

A multi-barrier can be applied for the deep geological disposal of high-level radioactive waste. The multi-barrier comprises an engineered barrier and the natural barrier of the host rock. In the engineered barrier, the bentonite buffer is the key component for the disposal container, and the bentonite buffer thickness is given important consideration when designing the engineered barrier. This study reviewed the safety functions of bentonite buffers. Subsequently, the requirements and factors necessary to determine the thickness of the bentonite buffer, including criteria for radiological safety and the thermal stability of the disposal system, were identified. Additionally, the bentonite buffer thicknesses required for the top, bottom, and side of the disposal container were calculated. A double-layered emplacement method is also proposed for the bentonite buffer to improve disposal efficiency in terms of thermal management. Based on radiological safety and thermal stability analyses, an optimal thickness of 0.36 m was found to be appropriate for the bentonite buffer surrounding the disposal container. The thickness of the bentonite buffer above the disposal container can be determined based on the excavation damaged zone depth. The study findings can be used as a reference when designing deep geological disposal systems.

We analyzed the mineral composition of compacted calcium bentonite (GJ-I) and uncompressed sodium bentonite (MX80), both of which were exposed for two years in the YS03 borehole. The YS03 borehole is characterized by a high concentration of anaerobic microorganisms, including sulfate-reducing bacteria, elevated levels of hydrogen sulfide, and high pH conditions. The compacted Ca bentonite showed minimal alteration, with a small amount of new magnetite formation. However, an X-ray diffraction (XRD) analysis revealed that the uncompressed Na bentonite underwent a complete transformation from montmorillonite to muscovite, goethite, and magnetite. Therefore, it is suspected that the compactness of the bentonite significantly impacts the rate of alteration. Furthermore, an X-ray fluorescence (XRF) analysis demonstrated a marked increase in iron oxide in the Na bentonite, whereas key elements of montmorillonite such as alumina (Al2O3), silica (SiO2), and magnesium oxide (MgO) showed substantial decreases. The presumed cause of the alteration in the uncompressed MX80 bentonite is the presence of Fe cations coupled with a high pH environment. We believe that Fe cations, which were likely released from the corrosion of cast iron, played a significant role in altering the montmorillonite lattice.

This review explores the potential of pillared bentonite materials as solid acid catalysts for synthesizing diethyl ether, a promising renewable energy source. Diethyl ether offers numerous environmental benefits over fossil fuels, such as lower emissions of nitrogen oxides (NOx) and carbon oxides (COx) gases and enhanced fuel properties, like high volatility and low flash point. Generally, the synthesis of diethyl ether employs homogeneous acid catalysts, which pose environmental impacts and operational challenges. This review discusses bentonite, a naturally occurring alumina silicate, as a heterogeneous acid catalyst due to its significant cation exchange capacity, porosity, and ability to undergo modifications such as pillarization. Pillarization involves intercalating polyhydroxy cations into the bentonite structure, enhancing surface area, acidity, and thermal stability. Despite the potential advantages, challenges remain in optimizing the yield and selectivity of diethyl ether production using pillared bentonite. The review highlights the need for further research using various metal oxides in the pillarization process to enhance surface properties and acidity characteristics, thereby improving the catalytic performance of bentonite for the synthesis of diethyl ether. This development could lead to more efficient, environmentally friendly synthesis processes, aligning with sustainable energy goals.

Hydraulic conductivity is a critical design parameter for buffers in high-level radioactive waste repositories. Most employed prediction models for hydraulic conductivity are limited to various types of bentonites, the main material of the buffer, and the associated temperature conditions. This study proposes the utilization of a novel integrated prediction model. The model is derived through theoretical and regression analyses and is applied to all types of compacted bentonites when the relationship between hydraulic conductivity and dry density for each compacted bentonite is known. The proposed model incorporates parameters such as permeability ratio, dynamic viscosity, and temperature coefficient to enable accurate prediction of hydraulic conductivity with temperature. Based on the results obtained, the values are in good agreement with the measured values for the selected bentonites, demonstrating the effectiveness of the proposed model. These results contribute to the analysis of the hydraulic behavior of the buffer with temperature during periods of high-level radioactive waste deposition.

The bentonite buffer material is a crucial component in an engineered barrier system used for the disposal of high-level radioactive waste. Because a large amount of heat from the disposal canister is released into the bentonite buffer material, the thermal conductivity of the bentonite buffer is a crucial parameter that determines the design temperature. At the Korea Atomic Energy Research Institute (KAERI), a new standard bentonite (Bentonil-WRK) has been used since 2022 because Gyeongju (KJ) bentonite is no longer produced. However, the currently available data are insufficient, making it essential to investigate both the basic and complex properties of Bentonil-WRK. Thus, this study evaluated its geotechnical and thermal properties and developed a thermal conductivity empirical model that considers its dry density, water content, and temperature variations from room temperature to 90°C. The coefficient of determination (R2) for the model was found to be 0.986. The thermal conductivity values of Bentonil-WRK were 1–10% lower than those of KJ bentonite and 10–40% higher than those of MX-80 bentonites, which were attributable to mineral-composition differences. The thermal conductivity of Bentonil-WRK ranged between 0.504 and 1.149 W·(m−1·K−1), while the specific heat capacity varied from 0.826 to 1.138 (kJ·(kg−1·K−1)).

A semi-natural composite of κ-carrageenan and bentonite, two natural biopolymers, was synthesized through free radical polymerization. This synthesis aimed to obtain a biodegradable, biocompatible, and swellable composite that is environmentally friendly. The components used in this synthesis are readily available, making it economically feasible and promising for potential biomedical applications. The composite is pH-responsive and intended for oral delivery of metformin hydrochloride and aminophylline, which have low bioavailability and undesirable side effects, respectively. The organic composite exhibits the advantage of reducing drug release in the acidic gastric medium. This composite is a stimuli-responsive polymeric material that has garnered significant attention in recent years for its application in oral drug delivery systems. These materials enable site-specific and controlled drug release while minimizing toxicity. The carrageenan-g-poly(acrylamide-co-acrylic acid)/bentonite composite was characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FE-SEM), which confirmed the successful synthesis of the composite. The swelling behaviour and point of zero charge of the composite were studied at different pH values, which showed a strong influence on the swelling properties of the composite. The drug loading capacity of the composite was measured at pH 5.3, and it was 70.60 mg/g for metformin and 95.66 mg/g for aminophylline at pH(3). The in vitro release profile of both drugs from the composite was also affected by the ionic strength, and it exhibited a lower release rate with higher salt concentration. The maximum release percentage of the drugs from carrageenan-g-poly(acrylic acid-acrylamide)/bentonite in simulated gastric, intestinal, and colon fluids was achieved within 40 h. The maximum release was 80% for metformin in simulated intestinal fluid (SIF) and 75% for aminophylline after 40 h.

Tc-99 is considered as one of the major fission products in the context of disposal of spent nuclear fuel, due to the long half-life and chemical stability. In the atmospheric aqueous solutions, Tc is expected to exist in the form of TcO4 ‒ and thus is considered as an environmental concern according to its high solubility and mobility. Therefore, the development of an effective and economically viable adsorbent for aqueous Tc(VII) is imperative from the perspective of decontamination and remediation of contaminated environments. In this work, the adsorption behaviors of Re(VII), as a chemical surrogate of Tc(VII), onto the bentonites modified with two different organic cations such as hexadecyl pyridinium (HDPy) and hexadecyl trimethylammonium (HDTMA) were quantitatively analyzed and compared with each other. For the sorption experiment, adsorbents were prepared by surface modification of bentonite. Before the modification, the initial bentonite was pre-treated with 1 M NaClO4 and then reacted with HDPy or HDTMA. The modification process was performed at room temperature for 24 hours with various concentrations of organic cations, which were set to a range of 50-400% compared to the cation exchange capacity (CEC) of bentonite. After the reaction, the dried and crushed modified bentonites were filtered with the sieve with a mesh size of 63 μm. Aqueous Re(VII) solutions were prepared by dissolution of NH4ReO4 (Sigma-Aldrich) in deionized water with three different Re(VII) concentrations of 10-4M, 10-5M, and 10-6M. After that, the modified bentonite and the aqueous Re(VII) solutions were mixed at a liquid-to-solid ratio of 1 g/L. Aliquots of the samples were extracted for quantification analysis with ICP-MS after syringe filtration (pore size: 45 μm) at reaction times of 10, 50, 100, and 500 minutes. According to the results, a considerably fast adsorption reaction of Re(VII) onto all modified bentonites was observed, revealing exceptional sorption affinity of HDPy- and HDTMA-modified bentonites. For both organic cations, bentonites modified with the concentrations of organic cations ranging from 200 to 400% relative to the CEC of bentonite showed almost complete removal of aqueous Re(VII). For bentonites modified with lower concentrations of organic cations, the HDTMA presented a relatively larger sorption capacity than the HDPy. The result obtained through this study is expected to be referred to as a case study for the synthesis of cost-efficient and highly effective adsorbent material for highly mobile anionic radionuclides such as I‒ and TcO4 ‒.

After the Fukushima accident in 2011, relevant concerns regarding the contamination of the natural environment rose abruptly. For example, water contaminated by radionuclides such as Cs and Sr may directly flow into the ocean and threaten the marine ecosystem. In this respect, costeffective and efficient decontamination techniques need to be developed and verified to remediate the contaminated water. Prussian blue (PB) is known as a representative material that can adsorb Cs by ion-trapping and is widely used for medical purposes. However, there is a limitation that PB itself is non-separable and highly mobile in aqueous system, so it needs a fixture, such as bentonite, to be collected after the adsorption. Furthermore, while the performance of PB toward Cs is relatively well known, its behavior toward Sr has rarely been reported. The object of this study is to investigate the sorption characteristics of Cs and Sr onto PB-functionalized bentonite at various conditions. The adsorbent employed in the present work was prepared by mixing bentonite, FeCl3, and K4[Fe(CN)6] at room temperature for 24 hours in the aqueous solution. The concentrations of FeCl3 and K4[Fe(CN)6] were set to a range of 5-200 % compared to the cation exchange capacity of bentonite. After that, the PB-functionalized bentonite was sieved with a mesh size of 63 μm and then reacted with the Cs and Sr solution at various liquid-to-solid (L/S) ratios of 2-10 g/L for up to 500 minutes. Moreover, synthetic seawater containing additional Cs and Sr was reacted with PBfunctionalized bentonite to characterize the ion selectivity of PB. After the completion of the adsorption experiment, a part of the adsorbent was separated and desorption of Cs and Sr with 2 M of nitric acid was performed. For the quantification of aqueous Cs and Sr concentrations, ICP-MS was employed after the filtration with a pore size of 0.45 μm. The result obtained in this study revealed a high sorption affinity of Cs and Sr onto PBfunctionalized bentonite. The analysis results also presented that the sorption reactions of Cs and Sr reached their steady state within 10 minutes of reaction time. Furthermore, the ion selectivity toward Cs and Sr was verified through sorption test with synthetic seawater. According to the high sorption affinity and selectivity, the PB-functionalized bentonite synthesized through this study is expected to be widely used for remediating the Cs- and Sr-contaminated groundwater and seawater, particularly in nuclear waste-relevant industries.

The bentonite buffer material is a crucial component for disposing of high-level radioactive waste (HLW). Several additives have been proposed to enhance the performance of bentonite buffer materials. In this study, unconfined compression tests were conducted on bentonite mixtures as well as pure bentonite buffer material. Joomunjin and silica sands were added at a 30% ratio, and graphite was added at 3% along with bentonite. The unconfined compression strength (UCS) and elastic modulus of pure bentonite were found to be 20% to 50% higher than those of bentonite mixtures under similar dry density and water content conditions. This decrease in strength can be attributed to the reduced cross-sectional area available for bearing the applied load in the bentonitemixture. Furthermore, the 3% graphite-bentonite mixture exhibited a 10% to 30% higher UCS and elastic modulus compared to the 30% sand-bentonite mixtures. However, since the strength properties of additive-bentonite mixtures are lower than those of pure bentonite, it is essential to evaluate thermohydraulic-mechanical functional criteria when considering the use of bentonite mixtures as buffer materials.

Effective containment and disposal of high-level radioactive waste is critical to ensure long-term environmental and human safety. Especially bentonite, which is widely used as a buffer material due to its favorable characteristics such as swelling ability and low permeability, plays an important role in preventing the migration of radioactive waste into the surrounding environment. However, the long-term performance of bentonite buffer remains an area of ongoing investigation, with particular attention focused on erosion mechanisms induced by swelling and groundwater flow. The erosion of the bentonite buffer can significantly impact the integrity of buffer and lead to the formation of colloids, which could potentially facilitate the transport of radionuclides through groundwater. Therefore, quantification of bentonite buffer erosion based on an understanding of the underlying mechanisms and factors that influence bentonite buffer erosion, is essential for the safety assessment of high-level radioactive waste repositories. In this study, we aimed to develop a bentonite buffer erosion model using the Adaptive Processbased total system performance assessment framework for a geological disposal system (APro) proposed by the Korea Atomic Energy Research Institute (KAERI). The impact of bentonite erosion on performance assessment can be broadly divided into bentonite property degradation by the penetration of the bentonite buffer into rock fractures and the formation of pseudocolloids. To simulate this phenomenon, Two-region model based on a dynamic bentonite diffusion model is adopted, which can quantify the extent of bentonite intrusion and loss by erosion. Using this Tworegion model, a numerical model was developed to simulate the degradation of bentonite properties based on the amount of bentonite intrusion, as well as to simulate the migration of pseudocolloids in the near-field by deriving the amount of pseudocolloid production based on the loss of bentonite and the sorption rate of radionuclides. To check the applicability of the developed numerical model, preliminary analysis was performed for the effect of bentonite erosion in terms of process-based performance assessment. It is anticipated that this comprehensive model developed in this study will contribute to the accurate and reliable assessment of the long-term performance and safety of high-level radioactive waste repositories.

In the high-level waste disposal systems, colloids generated through the chemical erosion of bentonite buffers can serve as critical mediators for the transport of radionuclides from the disposal environment to the biosphere. The stability of these colloids is influenced by the chemical composition of the groundwater. According to DLVO theory, the Critical Coagulation Concentration (CCC) is the ionic strength at which the total repulsive force between colloids is either less than or equal to the total attractive force. At ionic strengths lower than the CCC, electrostatic double-layer repulsion outweighs van der Waals attraction, forming a repulsive barrier between particles. Conversely, at ionic strengths higher than the CCC, attractive forces dominate, leading to particle aggregation. To investigate the CCC of bentonite colloids, this study focused on Ca-type WRK bentonite. Colloids separated from a ten g/L bentonite suspension underwent centrifugation (1,200 g for 30 minutes) and dialysis (3,500 MWCO) to produce colloid samples. After adjusting the ionic strength from 0.1 mM to 10 mM, the particle size distribution was monitored as a function of aggregation time for approximately 20 days. Rate constants, calculated based on variations in ionic strength, were used to interpret the observed results. The experimental outcomes revealed that the CCC value for WRK bentonite colloids was an order of magnitude lower with CaCl2 than with NaCl. This suggests that Ca ions have a more significant impact on colloid stability, which has implications for the longterm safety of high-level waste disposal systems.

In environments where buffer materials are exposed to increased temperature due to the decay heat emitted by radioactive waste, it is crucial to assess the performance of the buffer material in relation to temperature effects. In this study, we conducted experiments using Bentonil-WRK, a calcium-type bentonite, compacted to a dry density of 1.65 g/cm3 and an initial water content of 15%. The experimental temperature conditions were set to 30, 60, 90, 110, and 130°C. We observed that the swelling pressure of the compacted bentonite buffer decreased as the temperature increased. The findings from this study can provide valuable guidance for the design of high-level waste repository in Korea.