Bentonite Buffer Material Research Articles

High-Temperature Corrosion Characteristics of Copper for Deep Geological Disposal of Spent Nuclear Fuel

In this study, we evaluated the high-temperature corrosion characteristics of copper(Cu), which is considered a candidate material for disposal containers in deep geological repositories for spent nuclear fuel, through electrochemical methods. Deep geological repositories serve to isolate spent nuclear fuel from humans and the environment, employing a multi-barrier concept consisting of engineered and natural barriers. Among these, engineered barriers consist of canisters, bentonite buffer materials, and others. Various countries are progressing with conceptual designs, with Cu, known for its corrosion resistance, being considered a key material for canister. While Cu is predicted to experience minimal corrosion in groundwater environments where deep geological repositories are typically situated, recent studies suggest that exposure to temperatures exceeding 100oC may lead to some corrosion. In previous studies by the authors, Cu corrosion evaluations were conducted in environments with various compositions at temperatures ranging from 60 to 70oC. The results confirmed that corrosion of copper could occur in the presence of corrosive species such as Cl-. Expanding upon these findings, the current study aims to assess various base metal and weld of Cu under high-temperature and high-pressure conditions using electrochemical methods.

Assessment of thermal-hydraulic-mechanical-chemical (THMC) performances of Ca-type bentonite-graphite mixtures as buffer materials for a high-level radioactive waste repository

The bentonite buffer material is the most important element of the engineered barrier system (EBS) used to dispose of high-level radioactive waste. Therefore, there are functional criteria or performance targets for the bentonite buffer material to maintain the long-term performance of the EBS for the entire disposal system. Thermal conductivity is a key parameter in the design of an EBS as it has the greatest influence on the buffer temperature. As the peak temperature of the buffer should not exceed the thermal design criterion, it is necessary to increase the thermal conductivity of the bentonite buffer material. Therefore, in this study, a small portion of graphite was added to Ca-type bentonite and thermal, hydraulic, and mechanical properties were measured separately, along with overall THMC (thermal-hydraulic-mechanical-chemical) properties. Additionally, while thermal conductivity was examined for varying graphite contents, other properties such as hydraulic conductivity, swelling pressure, and unconfined compressive strength were tested with a 3 % graphite addition. In particular, the thermal conductivity was measured for 3 % graphite addition to the pure bentonite under the different dry densities and saturation conditions and a regression analysis was performed to predict thermal conductivity based on the dry density and degree of saturation values. Furthermore, the saturated hydraulic conductivity, swelling pressure, unconfined compressive strength, and sorption efficiency of cesium (Cs) ions in bentonite and bentonite-graphite mixtures were measured. Moreover, prototype buffer blocks for bentonite-graphite mixtures were manufactured to evaluate viability of production. The addition of 3 % graphite to pure bentonite satisfied the functional criteria of saturated hydraulic conductivity and swelling pressure when the dry density was 1.63 g/cm3 or higher, and the thermal conductivity of the bentonite-3 % graphite mixtures was 10–30 % higher than that of pure bentonite.

Effects of specimen size and loading rate on the mechanical property of Bentonil-WRK bentonite for engineered barrier system

This study aims to investigate the effects of specimen size and loading rate on the mechanical property of Bentonil-WRK bentonite buffer material for engineered barrier system. Highly instrumented unconfined compressive strength tests were performed on cylindrical specimens prepared using cold isostatic pressing (CIP) technique at varying testing conditions: (a) CIP pressures of 29.43 MPa, 39.24 MPa, and 58.86 MPa, (b) specimen sizes of 30 × 60mm, 40 × 80mm, and 50 × 100mm, and (c) loading rates of 0.5 %/min, 1.0 %/min, and 1.5 %/min. As CIP pressure increases, the unconfined compressive strength (qu), failure strain (εf), and elastic modulus (E) of Bentonil-WRK increase similar to Gyeongju compacted bentonite (KJ-II). Increase in diameter results in a decrease in qu and εf, and increase in E. Minimal effect of loading rate was observed on qu and εf. However, increasing loading rate tends to increase E. In addition, decrease in diameter increases the elastic threshold axial strain (εe,th). Increase in diameter and loading rate slightly increases Poisson's ratio. Prediction models are provided for assessing the effects of specimen diameter on qu and εf. Furthermore, correlation models of qu to ρd and E are proposed for Bentonil-WRK bentonite buffer material.

Evaluation and prediction of swelling properties of bentonite under the action of salt solution

Bentonite demonstrates exceptional water absorption and expansion characteristics, along with a high degree of impermeability. The expansion characteristics of bentonite buffer materials are influenced by the underground aqueous solutions containing various base ions. In this paper, the swelling characteristics of bentonite with different dry densities under the action of different base ions and different concentrations of aqueous solution were studied by indoor limited expansion rate and swelling force tests. And the swelling characteristics of bentonite under the action of salt solution were predicted by Back Propagation Neural Network (BPNN). The findings demonstrated that: The swelling process of bentonite can be categorized into four distinct stages: initial swelling, rapid expansion, gradual enlargement, and stable volumetric increase. The expansion rate of bentonite is influenced by the dry density, as well as the type and concentration of the solution. The influence of various solutions on the expansion rate of bentonite follows the order: NaCl < CaCl2 ≈ MgCl2 < KCl. The type and concentration of the solution, as well as the initial dry density of the sample, exert significant influence on the swelling force of bentonite. The swelling characteristics of bentonite are accurately predicted by BPNN. The findings of this study offer valuable insights for the scientific utilization and rational disposal of bentonite in engineering applications.

Experimental study on heat and moisture coupling migration behavior of unsaturated bentonite buffer materials for high-level radioactive waste repository

This paper independently developed a set of experimental equipment that can approximately simulate the service working environment of unsaturated bentonite buffer. By conducting heat and moisture transfer experiments on unsaturated bentonite under the combined action of heating-cooling path and water erosion conditions, the evolution laws of temperature, humidity, electrical conductivity at different locations away from the heat source, as well as the heat and moisture coupling migration behavior were studied. The results show that during the heating-cooling process, the temperature evolution laws is similar at different positions away from the heat source. Although the temperature of the heat source reaches 100 °C, its impact range is relatively small. The evolution law of humidity varies greatly at different positions, and is significantly influenced by the temperature field, as well as the coupling effect of hydraulic field. The change in electrical conductivity is basically consistent with the change in humidity, and is also subject to the combined coupling effect of temperature field and hydraulic field. Under the condition of no water erosion and increasing heating/cooling rate, the amplitude of temperature change decreases, and humidity and electrical conductivity show a trend of migration away from the heat source throughout the heating cooling stage.

Modelling Hydraulic and Swelling Pressure Characteristics of Bentonite Buffer Material for High-Level Nuclear Waste Containment Conditions

Modelling Hydraulic and Swelling Pressure Characteristics of Bentonite Buffer Material for High-Level Nuclear Waste Containment Conditions

Apparatus for swelling deformation of compacted bentonite utilizing multi-ring moulds for evaluation of dry density and water content distribution

In the geological disposal of high-level radioactive waste, bentonite buffer materials are required to have self-sealing property, which is the ability to fill gaps by swelling deformation between the buffer material and the waste containment vessel or surrounding bedrock. Self-sealing property was evaluated based on the pressure generated after the gap filling under the assumption of constant dry density and water content in the specimen and buffer material. On the other hand, it takes tens to hundreds of years for the transitional period until the buffer contacts with groundwater, fills the gap, and the internal state converges to a uniform state. To assess changes more accurately in the condition of the buffer material, it is necessary to evaluate time courses in the density and water content of the buffer material because of groundwater infiltration and swelling deformation. From these, the authors developed an experimental apparatus for swelling deformation of bentonite, which consists of a mould with 2 mm-thick stacked rings. It is aimed to evaluate the distribution of dry density, water content, and degree of saturation inside the specimen by cutting the specimen with a 2 mm thick ring after the end of experiment. In this paper, results of preliminary experiments to confirm the applicability of this apparatus to swelling deformation test of bentonite was firstly described. Then, experiments were conducted on compacted bentonite specimens using this apparatus, and changes in the dry density and water content distribution inside the specimens were evaluated in terms of swelling deformation behaviour.

Strain-dependent-deformation property of Gyeongju compacted bentonite buffer material for engineered barrier system

This study aims to investigate the strain-dependent-deformation property of Gyeongju bentonite buffer material. A series of unconfined compressive tests were performed with cylindrical specimens prepared at varying dry densities (ρd = 1.58 g/cm3 to 1.74 g/cm3) using cold isostatic pressing technique. It is found that as ρd increase, the unconfined compressive strength (qu), failure strain, and elastic modulus (E) of Gyeongju compacted bentonite (GCB) increases. Normalized elastic modulus (Esec/Emax) degradation curves of GCB specimens are fitted using Ramberg-Osgood model and the elastic threshold strain (εe,th) is determined through the fitted curves. The strain-dependency of E and Poisson's ratio (v) of GCB were observed. E and v were measured constant below εe,th of 0.14 %. Then, E decreases while v increases after exceeding the strain threshold. The Esec/Emax degradation curves of GCB in this study suggests wider linear range and higher linearity than those of sedimentary clay in previous study. On top of that, the influence of ρd is observed on Esec/Emax degradation curves of GCB, showing a slight increase in εe,th with increase in ρd. Furthermore, an empirical model of qu with ρd and a correlation model between qu and E are proposed for Gyeongju bentonite buffer materials.

Pressure evaluation of a gap space in the engineered barrier system in a high-level radioactive waste repository

Disposal canisters and buffer materials are main components in an engineered barrier system designed for high-level radioactive waste disposal. The design temperature of the buffer material is below 100 °C in most countries, and many studies have been conducted to increase the design temperature of the buffer material to increase the disposal density of the repository. A 1-cm air gap exists between the canister and buffer material that can cause several variations in the thermal–hydraulic–mechanical performance of the compacted bentonite buffer material. Therefore, considering the air-gap effect under high-temperature conditions is necessary because high temperatures can cause pressure variations in the air gap and compacted bentonites. Therefore, an experimental system was developed to measure the pressure variations in a confined cell, which contained air and compacted bentonites from room temperature to 150 °C. The pressure of the confined cell increased with increasing water content of the compacted bentonite, and the pressure changes were similar up to 100 °C irrespective of the presence of the 1-cm air gaps. Moreover, the pressure values at 150 °C were 10–15% higher in samples without gaps.

Hydraulic properties of bentonite buffer material beyond 100 °C

Bentonite buffer materials are important components of engineered barrier systems for the disposal of high-level radioactive waste produced during nuclear power generation. The design temperature of the buffer material is < 100 °C, and increasing the design temperature can reduce the required disposal area. This characteristic necessitates the evaluation of the thermal–hydraulic–mechanical properties of the buffer at temperatures above 100 °C to increase its target temperature. Therefore, the hydraulic properties of Gyeongju (KJ) bentonite buffer material were evaluated in this study, including the soil–water characteristic curve (SWCC) and hydraulic conductivity. An experimental system was manufactured to measure the suction and saturated hydraulic conductivity of KJ bentonite buffer material above 100 °C; the relative humidity of KJ bentonite buffer material was measured at 25–149 °C with an initial water content of 0, 0.06, and 0.12 under constant saturation conditions. The suction decreased as the temperature increased (10%–25% reduction at 99 °C-149 °C). The Van-Genuchten SWCC fitting parameters were also derived at 25 °C-149 °C using previously reported and newly generated experimental results, and the applicability of the modified Van-Genuchten SWCC model in this temperature range was verified. The hydraulic conductivity was proportional to temperature up to 100 °C, in agreement with the theoretical model results. Between 100 °C and 150 °C, the hydraulic conductivity increased nonlinearly because of molecular motion and structural changes inside the sample.

Characterization of Compacted Ca- and Na-Bentonite with Copper Corrosion Products in the KAERI Underground Research Tunnel

In a deep geological disposal system, bentonite buffer material is an important barrier used to protect the disposal canister from the inflow of groundwater and prevent the outflow of radionuclides. This study aimed to characterize the mineralogical and chemical reactions of bentonite caused by copper corrosion of the canister in a radioactive waste repository. We investigated the d-spacings of montmorillonite in Gyeongju bentonite (Ca-type, KJ-I) under groundwater-saturated conditions over 10 years and compared their characteristics with those of Wyoming bentonite (Na-type, MX-80) in the Korea Atomic Energy Research Institute Underground Research Tunnel. Mineralogical investigations using X-ray diffraction and focused ion beam energy-dispersive spectroscopy indicated that no transformation of smectite or neo-formed clay phases occurred. In the Ca-type bentonite (KJ-I), the swelling was observed when it was in contact with rolled plate (RP) and cold-spray-coated (CSC) copper, with d-spacing expansions of 2.9% and 3.8%, respectively. In contrast, the Na-type bentonite (MX-80) showed d-spacing expansions of 17.6% and 19.6% when it was in contact with the RP and CSC Cu, respectively. The Cu concentration and distribution indicated that the corrosion products dissolved and then diffused into the surrounding bentonite, with maximum penetration depths of 2.0 and 0.5 mm over 10 years, respectively.

Prediction Model for Saturated Hydraulic Conductivity of Bentonite Buffer Materials for an Engineered-Barrier System in a High-Level Radioactive Waste Repository

Prediction Model for Saturated Hydraulic Conductivity of Bentonite Buffer Materials for an Engineered-Barrier System in a High-Level Radioactive Waste Repository

Long-Term Chemical Alteration of 238Pu-Doped Borosilicate Glass in a Simulated Geological Environment with Bentonite Buffer

Chemical degradation of borosilicate glass doped with 238Pu was modelled in conditions of a failed underground radwaste repository in granite host rock with bentonite buffer material after penetration of aqueous solutions at temperature of 90 °C. The total duration of the experiment exceeded two years. It is shown that wet bentonite preserved its barrier function and prevents migration of plutonium to the solution. The total amount of plutonium adsorbed on bentonite clay during the experiment did not exceed 0.02% of the initial amount of plutonium in the glass sample. Estimated accumulated dose of self-irradiation of the glass sample after the experiment varies from 3.16 × 1015 to 3.39 × 1015 α-decays per gram, which is equivalent to more than 1000 years storage of 239Pu doped sample with the same Pu content. Beishan granite remained intact, with no evidence of Pu penetration into the granite matrix along mineral grain boundaries.

Investigation of the various properties of several candidate additives as buffer materials

Bentonite buffer material is a critical component in an engineered barrier system (EBS) for disposing high-level radioactive waste (HLW). The bentonite buffer material protects the disposal canister from groundwater penetration and releases decay heat to the surrounding rock mass; thus, it should possess high thermal conductivity, low hydraulic conductivity, and moderate swelling pressure to safely dispose the HLWs. Bentonite clay is a suitable buffer material because it satisfies the safety criteria. Several additives have been suggested as mixtures with bentonite to increase the thermal-hydraulic-mechanical-chemical (THMC) properties of bentonite buffer materials. Therefore, this study investigated the geotechnical, mineralogical, and THMC properties of several candidate additives such as sand, graphite, granite, and SiC powders. Datasets obtained in this study can be used to select adequate additives to improve the THMC properties of the buffer material.

Effect of thermal ageing on physical properties of MX80 bentonite under high-temperature conditions

In a deep geological repository (DGR) system, the buffer layer is indispensable to ensuring the safe disposal of high-level radioactive nuclear waste (HLW). Because the heat generated by the decay of the spent nuclear fuel in a canister is released to the surrounding buffer layers, the bentonite buffer material experiences long-term high-temperature conditions. Therefore, the variations in physical properties of bentonite buffer material under high-temperature conditions are one of the important parameters in the DGRs for HLW. A series of tests on the specific gravity, specific surface area (SSA), Atterberg's limits, and free swelling ratio of MX80 bentonite after heating for different times at a high temperature of 200 °C were conducted, to investigate the influence of thermal ageing time on its physical properties. Then, the microscopic investigations, including X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), were conducted to explain the aforementioned variations from the microscopic point of view. Based on the crystal layer structure characteristics of montmorillonite, the quantitative relationships between the physical properties of bentonite and its mineral composition or bound water content were established to further explain the mechanism by which thermal ageing affects the physical properties of MX80 bentonite. The results indicate that with increasing heating time, the specific gravity, SSA, liquid limit, plastic limit, plastic index and free swelling ratio decrease sharply by 2.5%, 4.5%, 3.1%, 5.5%, 2.8% and 30.2%, respectively, within 15–30 days of heating. After 30 days of heating, their variations are negligible. Under high-temperature conditions, the transformation of mineral composition, desorption of bound water, and changes in the micro-morphology are the fundamental reasons for the variations in the physical properties of bentonite, and they influence and interact with each other.

Constitutive model for swelling properties of unsaturated bentonite buffer materials during saturation

Bentonite-based materials are candidate buffer materials for the geological disposal of radioactive waste in many countries. Once the disposal facility is closed and the groundwater level recovers, the buffer will gradually become saturated by the infiltration of groundwater. The swelling properties of saturated bentonite affect the distribution of dry density and the stress state within the buffer. Therefore, to evaluate the long-term safety of the buffer, a model must be developed that can continuously express the mechanical behavior of bentonite from the construction and operation of the repository for disposal (the unsaturated state) to the resaturation phase (saturated state). In this study, the existing elastoplastic constitutive model for saturated expansive soils was extended to predict the mechanical response in the unsaturated state. In formulating this model, the effective degree of saturation and the plastic volumetric strain were employed as the hardening parameters to express the changes in stiffness and swelling properties due to desaturation or saturation. In addition, a method was proposed for estimating the material parameters added to the expanded constitutive model. This study demonstrated the validity of the proposed constitutive model and the parameter estimation method by comparing the results of simulations with those of laboratory tests.

Linear thermal expansion behavior of compacted bentonite buffer materials

In a geological repository system, buffer is indispensable to ensure the safe disposal of high-level radioactive waste (HLW). Because heat generated from spent nuclear fuel in a canister is released to the surrounding buffers, thermal properties of such materials are fundamental in determining the overall disposal safety. Specifically, given that thermal expansion causes thermal stress to canisters and intact rock masses in the near-field location, it is imperative to evaluate the thermal expansion characteristics of the buffer, particularly when bentonite is used. This study investigates the linear thermal expansion properties of Kyeongju bentonite buffer, a type of Ca-bentonite produced in South Korea. The linear thermal expansion coefficient of dried bentonite was measured considering the heating rate, dry density, and temperature variation using dilatometer equipment. The linear thermal expansion coefficient values of the KJ bentonite buffers were found to be 4.0–6.2 × 10⁻⁶/°C. Based on test results, a numerical analysis was conducted, and the thermal strain values were similar between the test and numerical analysis. The overall linear thermal expansion coefficient of the KJ bentonite, considering radially confined or unconfined conditions and dried or saturated states, was predicted to be between 3.2 × 10⁻6/°C and 1.0 × 10⁻5/°C.

Thermal conductivity evaluation for bentonite buffer materials under elevated temperature conditions

In most countries, the upper limit of the buffer temperature in a geological repository for nuclear waste is set below 100 °C because of possible illitization. This smectite-to-illite transformation likely causes the swelling function of the buffer to deteriorate. However, stipulation of a higher temperature for the buffer would significantly increase the disposal density and cost effectiveness of the repository. Hence, understanding the characteristics and providing a database of information pertaining to the buffer at elevated temperatures is crucial. This study involved an evaluation of the thermal conductivity of bentonite buffer materials, as this property is the most important parameter for determining the design temperature. An experimental system suitable for operation at high temperatures was established, and was used to measure the thermal conductivity of bentonite buffer materials at different saturation values in the range 0–0.22 from room temperature to 150 °C. The dry density of the bentonite buffer materials is approximately 1.75 g/cm³, and the higher the initial saturation, the higher the measured thermal conductivity. The specimen and probe were placed in an oven and completely sealed with heat resistance tape to prevent water from evaporating from the pores of the specimen. The thermal conductivity of the bentonite buffer materials, which initially increased with temperature, decreased slightly between 100 and 150 °C. In general, the thermal conductivity values were similar between room temperature and 150 °C.

Thermal Conductivity Evaluation for Bentonite Buffer Materials Under Elevated Temperature Conditions

In most countries, the upper limit of the buffer temperature in the repository is set below 100 °C because of possible illitization. This smectite-to-illite transformation will likely deteriorate the swelling function of the buffer. However, if a higher temperature is stipulated for the buffer, then the disposal density and cost effectiveness of the repository will increase significantly. Hence, understanding the characteristics and providing a database associated with the buffer under elevated temperature conditions is crucial. In this study, the thermal conductivity of bentonite buffer materials is evaluated, as thermal conductivity is the most important parameter in determining the design temperature. An experimental system that can accommodate high temperatures is established, and the thermal conductivity of bentonite buffer materials under different saturation values ranging between 0 and 0.22 from room temperature to 150 °C is measured. The dry density of the bentonite buffer materials is approximately 1.75 g/cm3, and the higher the initial saturation, the higher is the measured thermal conductivity. The specimen and probe placed in an oven are completely sealed with heat resistance tape to prevent water from evaporating from the specimen. The thermal conductivity of specimens increases with temperature but decreases slightly between 100 °C and 150 °C.

Contemplation of relative hydraulic conductivity for compacted bentonite in a high-level radioactive waste repository

A deep geological repository has been suggested for the disposal of high-level radioactive waste (HLW) produced from nuclear power plants. The deep geological repository system can be divided into engineered barrier system (EBS) and natural barrier system (NBS). The engineered barriers are feature of the disposal system by humans, composed of a disposal canister, a backfill material, a gap-filling material, and a buffer material. Compacted bentonite buffer materials are one of the most important components as it protects the disposal canister from external impacts and the penetration of groundwater, and it retard radionuclide transport.Since groundwater flows into the compacted bentonite buffer materials, it is important to investigate the unsaturated hydraulic properties of compacted bentonite buffer materials such as water retention curve (WRC) and relative hydraulic conductivity. The power law is usually used to describe the relative hydraulic conductivity, and it can be defined as unsaturated hydraulic conductivity divided by saturated hydraulic conductivity. The power coefficient (L) in the power law is mainly applied to values near 3 regardless of unsaturated soil properties. Therefore, this paper evaluated the relative hydraulic conductivity function of compacted bentonite buffer materials using the van-Genuchten model, which was converted to the potential law equation. Based on the process suggested in this research, the L values for MX-80 and FEBEX bentonites were derived. The L values obtained using the proposed process exhibited <5 ~ 10% difference from previously reported values. Furthermore, the L values for Korean compacted bentonite were calculated as 2.64 and 3.23 with dry densities of 1.59 g/cm3 and 1.73 g/cm3, respectively. This shows the importance of the proper estimation of the L value.