Buffer Material Research Articles

Study on surface erosion threshold and erosion mass of bentonite applied in nuclear waste repository based on fractal structure characteristics

Bentonite exhibits good self-sealing performance and low permeability after hydration, and therefore is selected as a buffer material for high-level radioactive waste repositories, isolating the canisters containing nuclear waste from the external environment. The bentonite cushion directly contacts the surrounding rock and generates gels at the contact interface with fracture water. Surface erosion occurs once the shear stress exerted by the flowing water exceeds the erosion threshold of the bentonite gel, resulting in gel particle detachment from the soil–water interface and transport through fissure. The mass loss of the buffer by erosion will weaken its functionality as designed and directly endanger the long-term safety of the repository. The erosion behavior of bentonite is related to the structure of the gel formed by bentonite hydration. In this paper, the X-ray small-angle scattering experiments were used to prove that the gel structure has fractal characteristics and to obtain the mass fractal dimension. Based on the fractal structure of the bentonite gel, the erosion threshold that determines whether erosion will occur was quantified as a function of the solid volume fraction of the gel by taking the inter-particle attractive bond as the primary source of resistance. Furtherly, the cumulative erosion mass model that evaluates the extent of erosion was constructed to reveal the relationship between the cumulative erosion mass and cumulative water flow after the erosion has achieved stability. The proposed relations are verified by the experimental results from published literature and are of great significance for judging whether erosion will occur and evaluating the degree of erosion.

Analysis of natural organic matter chemistry in bentonite clay under compaction using different dry densities and duration

The Nuclear Waste Management Organization of Canada has adopted an adaptive phased management plan for the long-term storage of used nuclear fuel in a Deep Geological Repository (DGR). The DGR barrier system employs copper-coated used fuel containers (UFCs) surrounded by buffer material which is composed of highly compacted bentonite clay. The natural organic matter (NOM) composition of the compacted bentonite is of practical importance for the safety assessment as it regulates the biogeochemical processes at the interface between the UFCs and the buffer layer. However, insufficient investigations on NOM constituents limits the understanding of biogeochemical dynamics in bentonites compacted under different dry densities. This study analyzed the carbon content and NOM composition using targeted compound analysis and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy with bentonites compacted at 1.1, 1.4 or 1.6 g/cm3 dry density for 1, 3, 6, 12 or 18 months. Organic carbon contents were similar across bentonites with different compaction densities at various durations as compared to the reference clay (powdered bentonite sample before compaction). The overall NOM composition measured by solid-state 13C NMR suggested a predominance of alkyl and aromatic carbon in bentonite samples. No marked variations were observed for the NOM components (i.e., alkyl, O-alkyl and aromatic carbon) between the compacted bentonites and the reference clay. Targeted compound analysis revealed that the extractable lipid concentrations in the compacted clays did not significantly vary from the reference bentonite with only some compounds exhibiting nano-gram level differences. Bentonites with relatively lower dry densities (1.1 and 1.4 g/cm3) demonstrated slightly higher extractable compound concentrations, but these differences were not statistically significant. In contrast, the compound concentrations of bentonites compacted to 1.6 g/cm3 were similar or slightly lower compared with the reference bentonite, likely associated with limited microbial growth under high dry compaction density. Taken together, these results suggested that NOM composition and quantity did not significantly alter in bentonites under various pressures nor with longer experimental durations. These findings highlight that compacted bentonite exhibited geochemical stability under the simulated repository environments, which provides critical insight for design and performance of engineered barrier system in a DGR concept.

Investigation of different buffer layer impact on AlN/GaN/AlGaN HEMT using silicon carbide substrate for high-speed RF applications

The AlN/GaN heterostructure on the AlGaN back barrier with different buffer layer structures using a silicon carbide (SiC) substrate was investigated in this work. This study mainly focused on selecting a cost-effective buffer layer with a thickness of more than 1 μm and fewer defects for improving DC/RF performance. The extra epitaxial growth process steps can be avoided by choosing an appropriate, high-quality buffer material called β-Ga2O3 (β-gallium oxide). It has a good lattice match with AlGaN alloys and SiC. It leads to fabricating cost-effective, reliable HEMTs. The GaN buffer has a poor lattice match with SiC; the nucleation layer must be used between GaN and SiC for lattice match to avoid stress defects and dislocations, which helps to reduce buffer traps. The growth of thick GaN crystals leads to high costs and more defects. Creating a solid conduction band barrier with the help of a back-barrier (AlGaN) leads to a boost in the confinement of electrons in both devices. Still, the traps and defects in the GaN buffer do not effectively halt the leakage current compared to the β-Ga2O3 buffer. The T-shaped gate with the impact of LGD (gate to drain length) and LGS (gate to source length) scaling in lateral dimensions was also studied and the device performance was reported. The HEMT with β-Ga2O3 buffer delivers significant results in drain current (2.3 A/mm), transconductance (603 S/mm), and cut-off frequency (486 GHz) for the scaled lateral lengths LG (Gate Length) of 60 nm,LGD of 0.9 μm, and LGS of 0.4 μm. This excellent performance helps to use this HEMT device in cost-effective future high-speed RF applications.

Membrane behavior of clay under mixed solution conditions

Compacted clay is employed as the buffer material for landfills, and multiple ions are dissolved in the leachate restricted by the compacted clay layer. The membrane efficiency is an important indicator to assess the barrier properties of the compacted clay layer and is measured through membrane tests. However, most membrane tests are currently conducted with a single solute solution, which does not reflect the mixed solution characteristics of leachates. To assess the membrane efficiency of compacted clay under mixed solution conditions, 13 membrane tests were conducted on a bentonite-amended soil using KCl–NaCl mixed solutions, KCl–CaCl2 mixed solutions, and KCl–AlCl3 mixed solutions with different mixing ratios at a total concentration of 20 mM. Nuclear magnetic resonance (NMR) tests were conducted on the soil specimen after the membrane tests to investigate the micromechanism of the membrane behavior under mixed solution conditions. Results indicate that the membrane efficiency increased with the mixing ratio of Na+ but decreased with the mixing ratio of Ca2+ or Al3+. In the 13 membrane tests, the lowest membrane efficiency was achieved when the specimen was tested with pure AlCl3 solution. The relationship between the membrane efficiency and mixing ratio was also investigated at the microscopic scale. As the ion valence increases, the diffuse double layer thickness is smaller and the proportion of macropores is larger, as verified by NMR tests.

The influence of isotropic loading and unloading on anisotropic evolution of saturated hydraulic conductivity of bentonite-sand mixtures in a cube triaxial permeameter

Anisotropic permeability of bentonite mixtures governs water flow regime in buffer materials such as landfill cover systems, cutoff walls, and high-level waste repositories. Because of its widespread application, a series of 48 permeability tests were conducted in this study on specimens prepared from bentonite-sand mixtures with three bentonite contents. The tests were run along both isotropic loading and unloading paths to explore the evolution of anisotropic ratio from three distinct fabric anisotropy states. Accordingly, a new cubic sample-type, large-scale triaxial permeameter was introduced and employed. The results revealed an enhancement of as-compacted anisotropy ratio from 1 to almost 7 with bentonite contents. The anisotropy ratio was suppressed with isotropic loading for 40% and 70% bentonite while it was enhanced for 10% bentonite content. Unloading was revealed to reduce the anisotropy of higher bentonite contents due to the enhancement of fissures along horizontal and vertical directions. The practical implication in terms of the long-term performance of a landfill liner with medium to high bentonite contents is that despite a 10- to 30-fold reduction in permeability with loading, the leachate flow would be less anisotropic compared with the initial condition as supported by a reduction in the anisotropy ratio up to 3.5.

Towards Reliable Barrier Systems: A Constrained XGBoost Model Coupled with Gray Wolf Optimization for Maximum Swelling Pressure of Bentonite

Bentonite and bentonite mixtures are used as buffer material for deep geological radioactive waste repositories. The proper determination of bentonite maximum swelling pressure is important as it influences the long-term stability and sealing characteristics of the barrier system. We developed a constrained machine learning model based on extreme gradient boosting (XGBoost) algorithm tuned with grey wolf optimization (GWO) to determine the maximum swelling pressure of bentonite and bentonite mixtures. We integrated a penalty term into the XGBoost algorithm’s loss function to constrain the model during the training phase ensuring meaningful predictions. To this end, we compiled a dataset containing 305 experimental data of bentonite maximum swelling pressure and other relevant soil properties including montmorillonite content, liquid limit, plastic limit, plasticity index, initial water content, and soil dry density to develop the model. The performance of the hybrid GWO-XGBoost model was benchmarked against feed-forward and cascade-forward neural network models. The GWO-XGBoost model showed a better performance in estimating the experimental values comparing to the other models, thereby demonstrating its effectiveness and reliability in determining the maximum swelling pressure of bentonite and bentonite mixtures.

Impact on repository footprint of blending spent mixed oxide fuels with UO2 fuels during reprocessing

The heat generated by Am-241 poses a challenge in the vitrification of spent mixed oxide (MOX) fuel after reprocessing. When the buffer material temperature exceeds the limit temperature of 100℃ due to the heat generated by vitrified waste, the buffer material quality is degraded, thereby affecting nuclide migration. Therefore, the waste-loading ratio of vitrified waste is estimated by heat generation, which determines the number of glass canisters and repository footprint. An option to reduce heat generation is to blend MOX high-level waste (HLW) with UO2 HLW. This study evaluated the thermal impact of blending MOX HLW with UO2 HLW while considering the cooling period and blending ratio. The maximum loading ratio was calculated for each blending condition in the range of buffer material temperatures below 100 °C, and the results were used to estimate the number of glass canisters and assess the repository footprint.Graphical abstract

Influence of oxygen content on the properties of In2(OxS1−x)3 used as buffer material in Cu(In,Ga)Se2 solar cells

We investigate magnetron-sputtered In2(OxS1−x)3 compounds acting as an alternative buffer system to the solution-grown CdS or Zn(O,S) buffer layers in Cu(In,Ga)Se2 (CIGS) thin-film solar cells. The influence of the oxygen content on the solar cell performance, microstructure of the mixed systems, bandgap, and band offsets to CIGS is investigated experimentally and also characterized by calculations based on density functional theory. Samples in a series with different chemical compositions ranging from In2S3 to In2O3 are either directly deposited from ceramic targets or from a pure In2S3 target by reactive sputtering by adding O2 in the Ar sputtering gas. The binary compounds In2S3 and In2O3 sputtered at 220 °C substrate temperature from ceramic targets exhibit a crystalline structure, whereas the ternary In2(O,S)3 compounds are either nanocrystalline in the case of In2(O0.25S0.75)3 or amorphous for In2(O0.5S0.5)3 and In2(O0.75S0.25)3. For [O]/([O] + [S]) ratios above 0.25, the cell efficiencies decrease drastically, mainly due to lower open-circuit voltages (VOC). This behavior can be explained by an increase of the negative conduction band offset between the CIGS absorber and the oxygen-rich In2(OxS1−x)3 or In2O3 buffer, resulting in pronounced VOC losses. Adding oxygen to In2S3 with optical bandgap energies of around 2 eV results in a bowing of the values to below 2 eV and finally reaching values of around 2.7 eV for In2O3 if an indirect band transition is assumed. In summary, our results reveal that pronounced oxygen incorporation in In2S3 is not beneficial in terms of CIGS device efficiency because oxygen is electronically inactive and poorly miscible.

Electrocatalytic Effects of Au Nanoclusters in Its Electrochemiluminescence Enhanced by Coreactant Tertiary Amines

AbstractAtomically precise metal nanoclusters are increasingly exploited in electrocatalysis such as water electrolysis and CO2 reduction. The catalytic effects are generally evaluated by conventional electrochemical methods or end‐product analysis. Herein, near infrared electrochemiluminescence (ECL) of the nanoclusters is introduced as a new signaling mechanism to complement classical voltammetric analysis for elucidating essential electrocatalytic parameters. Coreactant ECL from an aqueous soluble Au22 nanocluster (NCs) enhanced by a common pH buffer materials 4‐(2‐hydroxyethyl)‐1‐piperazineethanesulfonic acid (HEPES) and by a piperazine drug hydroxyzine (HDZ) is studied under oxidative reduction pathways. Foot of the wave analysis on voltametric features and the kinetics profiles in potential‐step experiments are studied in different nanocluster and coreactant concentrations. Benchmark parameters such as rate constants are determined under EC (Electron‐transfer‐Chemical‐reaction) mechanism as feasibility validations. Because the luminescence properties of AuNCs are sensitive to the surface ligands or adsorbents, ECL is proposed as a new signal readout for operando studies providing insights for governing reaction mechanism and kinetics during catalytic reactions. Retrospectively, mechanistic understanding of the complex multi‐step reactions during ECL generation can provide guidance for the evaluation and optimization of ECL property for better quantitative applications.

Coupled multiphase flow and viscoelastic mechanics modeling of gas injection in a compacted bentonite buffer

Bentonite is chosen as a suitable buffer material for a deep geologic repository for radioactive waste. Thus, understanding the behavior of gas migration in the buffer layer is key to the safety assessment and functioning of such a repository. Based on the gas-injection experiments performed by the British Geological Survey (BGS), the modeling of gas migration in a compacted bentonite is carried out by the multi-phase-flow module (H) coupled with the viscous-elastic geomechanics module (M) of a fully coupled model, called THMC 7.1. Two laboratory scenarios for gas injection into compacted and saturated bentonite confined in a pressure vessel are considered in this simulation study. Injected gas (Helium) accumulates, entering the saturated bentonite after reaching a critical pressure to be detected by the gas filters to show the timing of the “breakthrough”. It is found in the experiments that the total stress reaches the maximum value right after breakthrough but does not exceed the gas injection pressure. It is found that our simulation results can capture the peak lab-test values of total stress and porewater pressure as well as the lab-test timing of breakthrough. Moreover, the decay patterns of both total stress and porewater pressure are well described in the simulations. A comparison of the simulation results with the experimental data shows that our HM coupled modeling can qualitatively and quantitatively model the gas migration behavior and its mechanical contribution to the buffer response. According to the presented simulations, the further improvement of the viscous-elastic geomechanical modelling is also discussed.

Multifunctional ytterbium oxide buffer for perovskite solar cells.

Perovskite solar cells (PSCs) comprise a solid perovskite absorber sandwiched between several layers of different charge-selective materials, ensuring unidirectional current flow and high voltage output of the devices1,2. A 'buffer material' between the electron-selective layer and the metal electrode in p-type/intrinsic/n-type (p-i-n) PSCs (also known as inverted PSCs) enables electrons to flow from the electron-selective layer to the electrode3-5. Furthermore, it acts as a barrier inhibiting the inter-diffusion of harmful species into or degradation products out of the perovskite absorber6-8. Thus far, evaporable organic molecules9,10 and atomic-layer-deposited metal oxides11,12 have been successful, but each has specific imperfections. Here we report a chemically stable and multifunctional buffer material, ytterbium oxide (YbOx), for p-i-n PSCs by scalable thermal evaporation deposition. We used this YbOx buffer in the p-i-n PSCs with a narrow-bandgap perovskite absorber, yielding a certified power conversion efficiency of more than 25%. We also demonstrate the broad applicability of YbOx in enabling highly efficient PSCs from various types of perovskite absorber layer, delivering state-of-the-art efficiencies of 20.1% for the wide-bandgap perovskite absorber and 22.1% for the mid-bandgap perovskite absorber, respectively. Moreover, when subjected to ISOS-L-3 accelerated ageing, encapsulated devices with YbOx exhibit markedly enhanced device stability.

Analytical modeling of bentonite swelling with experimental validations through hydrothermal reaction and free swelling index measurement

Bentonite has been widely used as buffer and backfill materials for nuclear waste disposal in last few decades. The main reasons of bentonite being selected for buffer and backfill materials include its low hydraulic permeability and high plasticity, which can restrict the transport of nuclear waste through the barrier and maintain the sealing ability of barrier, respectively, even with the deformation of bentonite. However, illitization process reduces the amount of montmorillonite in the buffer and its swelling ability, which may result in the loss of sealing ability and subsequent leakage of nuclear waste. This study presents an advanced analytical solution to describe the movement of swelling clay after absorbing water, combined with Darcy's law and constitutive laws of stress and strain with the consideration of changing permeability and porosity with time. The developed analytical solution has been validated with experiment results of two different types of bentonites—Gyeongju bentonite and Bentonil. Free swelling index with different montmorillonite/illite fractions were measured and compared with the analytical solution, as showing the average errors of lower than 5%.

Study on coupled heat-water-vapor transfer in buffer material based on SPH method

To study the hydrothermal evolution law in buffer materials, based on the heat balance equation of unsaturated soil and the quality control equation of water migration, a coupled heat-water-vapor model of unsaturated soil is proposed. This model comprehensively considers the transition process of water vapor, the heat transfer caused by water flow and vapor migration, as well as the influence of temperature potential on water flow and vapor migration. The smoothed particle hydrodynamics (SPH) method can be used to calculate their evolution process conveniently. In the solution, the vapor content of heat balance equation is solved first, and then the water content and temperature field are solved, so as the coupling of temperature field between water and vapor field is implemented. The rationality of the model is verified by the tested results of soil column test. According to the established theoretical model, the heat-water-vapor three-field coupling problem of the buffer material in the repository was simulated and studied, and the hydrothermal evolution law inside the buffer material was revealed. On this basis, the influence of the change of groundwater pressure and heat source temperature on the hydrothermal evolution law of buffer material was further analyzed.

Learning Module on Buffer Material with a CRT Approach to Solve Problem Topics in Jombang Typical Coffee Beer-Drink

Objective: Learning with the CRT approach is suitable for realizing contextual learning in the Independent Curriculum. So, research was carried out to develop a PBL model learning module using a CRT approach that is ideal for application in teaching chemistry regarding buffer solutions. Method: The research carried out was of a qualitative descriptive type. Open module development uses the Research and Development (R&D) method. The research stages include definition, design, development, and dissemination. Results: The trial of the teaching application module was carried out at 3rd State Senior High School Jombang. The results show that the Learning module developed is suitable for learning Buffer material using the CRT approach, which is indicated by a validity score of 4 in the outstanding category and can complete 35 students with an average increase in learning outcomes in the N-Gain score in the medium category. Novelty: The novelty of this research lies in the CRT approach, which is essential to apply to the independent curriculum combined with the PBL learning model to train students to analyze a problem in a buffer material.

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.

Application of Buffer Layer to Improve the Efficiency of Organic Solar Cells

Organic solar cells (OSCs) are widely studied for their advantages such as simple production, low cost, good flexibility, and large‐area printing. The buffer layer between electrodes and the photoactive layer has a significant impact on the efficiency and stability of OSCs. In order to lower the energy barrier height at the interface, provide an Ohmic contact with lower series resistance, and increase the charge collection efficiency of the corresponding electrodes for either holes or electrons, suitable electrode buffer materials can be chosen. The selection of electrode buffer layer has an important effect on the photoelectric conversion efficiency. Therefore, the study of buffer layer characteristics has certain guiding significance for the improvement of device structure and performance optimization. At present, the buffer layer materials used in OSCs are mainly organic polymer materials, organic small molecule materials, metal fluoride, metal oxides, and so on. Herein, the structure and working principle of OSCs are introduced first. Then, the properties and function of buffer layer in OSCs are summarized and discussed. Finally, the application of common buffer layer materials in OSCs is summarized, and how to improve the performance of the device is described.

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.

Temperature-dependent microbial reactions by indigenous microbes in bentonite under Fe(III)- and sulfate-reducing conditions

Bentonite is a promising buffer material for constructing spent nuclear fuel (SNF) repositories. However, indigenous microbes in bentonite can be introduced to the repository and subsequent sealing of the repository develops anoxic conditions over time which may stimulate fermentation and anaerobic respiration, possibly affecting bentonite structure and SNF repository stability. Moreover, the microbial activity in the bentonite can be impacted by the heat generated from radionuclides decay. Therefore, to investigate the temperature effect on microbial activities in bentonite, we created microcosms with WRK bentonil (a commercial bentonite) using lactate as the electron donor, and sulfate and/or ferrihydrite (Fe(III)) as electron acceptors with incubation at 18 ℃ and 50 ℃. Indigenous WRK microbes reduced sulfate and Fe(III) at both temperatures but with different rates and extents. Lactate was metabolized to acetate at both temperatures, but only to propionate at 18 ℃ during early-stage microbial fermentation. More Fe(III)-reduction at 18 ℃ but more sulfate-reduction at 50 ℃ was observed. Thermophilic and/or metabolically flexible microbes were involved in both fermentation and Fe(III)/sulfate reduction. Our findings illustrate the necessity of considering the influence of temperature on microbial activities when employing bentonite as an engineered buffer material in construction of SNF repository barriers.

Improving Students' Analytical Skills and Learning Interest Through the Tarl and CRT Approaches with The PBL Model on Chemical Buffer Material

Classroom action research was conducted to determine the increased analytical skills and students' interest in 2 cycles. Learning is carried out by applying the Problem-Based Learning (PBL) model with the Culturally Responsive Teaching (CRT) and Teaching at The Right Level (TaRL) approaches. Learning is carried out referring to the Kurikulum Merdeka. The research carried out was of the type of classroom action research. The research subjects were 26 students from the same class and school. The research was carried out in two cycles. The first cycle carried out learning using the PBL model with a CRT approach. The same learning was carried out in the second cycle using the TaRL approach. Data collection techniques are obtained from students' pre-tests and post-tests on students' analytical skills and learning interests. The research method is quantitative and descriptive qualitative, with an analysis technique using N-Gain and categorized based on the Likert scale. Research shows an increase in students' analytical skills to 0.46 or in the medium category. The classical increase in students' learning interest is as much as 95% in the high category.

Free swelling behavior of MX80 bentonite under elevated temperatures up to 200 °C

This paper focuses on understanding trends in the swelling potential of MX80 bentonite under temperatures up to 200 °C using a high-pressure cell. The free swelling behavior of expansive clays under high temperature and high fluid pressure conditions that may be encountered in geological repositories for high-level radioactive waste is important as the swelling potential is closely linked with key transition points on the physical and chemical properties of these clays. Free swell tests performed at temperatures ranging from 22 to 200 °C under sufficient pressure to ensure that the pore water remains as a superheated liquid were performed to assess whether the swell index of bentonite follows similar non-monotonic trends with temperature as observed in the literature for the cation exchange capacity. The measured swell indices follow an increasing-decreasing trend with a transition close to 100 °C. The experimental results can be used to guide parameter selection in long-term simulations on the buffer behavior of the buffer material, which requires an understanding of temperature effects on the coupled thermal-hydraulic-mechanical properties governing these processes.