Radioactive Waste Disposal System Research Articles

Advective gas flow in bentonite: Development and comparison of enhanced multi-phase numerical approaches

Understanding the impact of repository gas, generated from degradation of waste and its interaction with the host rock, is essential when assessing the performance and safety function of long-term disposal systems for radioactive waste. Numerical models based on conventional multi-phase flow theory have historically been applied to predict the outcome and impact of gas flow on different repository components. However, they remain unable to describe the full complexity of the physical processes observed in water-saturated experiments (e.g., creation of dilatant pathways) and thus, the development of novel representations for their description is required when assessing fully saturated clay-based systems. This was the primary focus of Task A within the international cooperative project DECOVALEX-2019 (D-2019) and refinement of these approaches is the primary focus of this study (Task B in the current phase of DECOVALEX-2023).This paper summarises development of enhanced numerical representations of key processes and compares the performance of each model against high-quality laboratory test data. Experimental data reveals that gas percolation in water-saturated compacted bentonite is characterised by four key features: (i) a quiescence phase, followed by (ii) the gas breakthrough, which leads to a (iii) peak value, which is then followed by (iv) a negative decay. Three models based on the multiphase flow theory have been developed. These models can provide good initial values and reasonable responses for gas breakthrough (although some of them still predict a too-smooth response). Peak gas pressure values are in general reasonably well captured, although maximum radial stress differences are observed at 48 mm from the base of the sample. Here, numerical peak values of 12.8 MPa are predicted, whereas experimental values are about 11 MPa. These models are also capable of providing a reasonable representation of the negative pressure decay following peak pressure. However, other key specific features (such as the timing of gas breakthrough) still require a better representation. The model simulations and their comparison with experimental data show that these models need to be further improved with respect to model parameter calibration, the numerical representation of spatial heterogeneities in material properties and flow localisation, and the upscaling of the related physical processes and parameters. To further understand gas flow localisation, a new conceptual model has been developed, which shows that discrete channels can possibly be induced through the instability of gas-bentonite interface during gas injection, thus providing a new perspective for modeling gas percolation in low-permeability deformable media.

Concepts of heat dissipation of a disposal canister and its computational analysis

The stability of engineered barriers in high-level radioactive waste disposal systems can be influenced by the decay heat generated by the waste. This study focuses on the thermal analysis of various canister designs to effectively lower the maximum temperature of the engineered barrier. A numerical model was developed and employed to investigate the heat dissipation potential of copper rings placed across the buffer. Various canister designs incorporating copper rings were presented, and numerical analysis was performed to identify the design with the most significant temperature reduction effect. The results confirmed that the temperature of the buffer material was effectively lowered with an increase in the number of copper rings penetrating the buffer. Parametric studies were also conducted to analyze the impact of technical gaps, copper thickness, and collar height on the temperature reduction. The numerical model revealed that the presence of gaps between the components of the engineered barrier significantly increased the buffer temperature. Furthermore, the reduction in buffer temperature varied depending on the location of the gap and collar. The methods proposed in this study for reducing the buffer temperature hold promise for contributing to cost reduction in radioactive waste disposal.

Simulating the Hydraulic and Volume Change Behavior of Compacted Highly Expansive Soil under Potential Field Stress and Seasonal Climatic Variation

A sustainable design of some engineering applications, such as earth dam cores, landfill liners, clay barriers, and radioactive waste disposal systems, that utilize compacted expansive soils requires simulation for probable field conditions. This study investigated the hydraulic and volume change (H-VC) behaviors of highly expansive compacted soils in Al-Qatif city under different seasonal climatic variations for a wide range of stress conditions, aiming for more economical and rational design and practices. The extent of the effect of the start cycle condition of the cyclic wetting and drying (W/D) process on the examined properties is examined, as well. Two testing series of the cyclic W/D process, representing the probable seasonal climatic variations, were executed for varied axial stress conditions. The H-VC behaviors of expansive soils are affected by the simulated seasonal variation (i.e., cyclic W/D process), with the first cycle of W/D being the most effective cycle and an elastic state being attained by the third to fourth cycle. Swell fatigue is noted for both testing series, and this is attributed to the initial placement condition. Analysis of results recommends exposure of the compacted expansive soil layers in the field to drying after compaction to reduce their equilibrium wetting potential. As a consequence of the noted shrinkage accumulation, a reduction tendency of saturated hydraulic conductivity (ksat) with repeated W/D cycles is reported for both series under all the stress states applied. Finally, it is recommended for clay barrier projects to be submerged once compacted to obtain barriers with the lowest values of hydraulic conductivity.

Ab Initio Study of the Electronic Structure and Lattice Dynamics of Scheelite Type AgTcO4

To build a safe, sustainable, and reliable disposal system for radioactive waste from the nuclear industry, researchers need to understand the physical and chemical aspects of technetium (Tc)‐based oxides. Here, structure–property correlation studies of scheelite‐type are performed using ab initio calculations. The full phonon dispersion studies reveal the dynamical stability of this compound. The partial phonon density of states, including the complete mode assignment of the vibrational frequencies, shows the maximum involvement of silver (Ag) atoms in the low‐frequency range, while high contribution of oxygen atoms and average contribution from Tc atoms are found throughout the frequency region. The electronic band structure shows that the main contribution to the bottom of the conduction band is Tc d states with a minor contribution from oxygen p states, while the top of the valence band is from Ag d states, which are responsible for the electronic transitions. The frequency‐dependent dielectric functions and computed optical properties show near‐isotropy behavior and, together with absorption peak in the low‐energy range, indicate that may be a good phosphor when doped with a suitable activator. The estimations are grounded by the available experimental data and also deliver direction for distinct and futuristic applications of .

Mineralogy and Geochemistry of a Bentonite Pellets Column Heated for 10 years

A common design of a high-level radioactive waste (HLW) disposal system consists of the waste canisters emplaced in tunnels or shafts, with the space between the heat-emitting canisters and the surrounding rock filled with a bentonite-based material. Understanding the behavior of this barrier, in particular the effect of prolonged heating on its properties, is important to assess the barrier’s long-term performance. The objective of the present study was to add to this understanding and to supply experimental data about the state of bentonite hydrated and heated for a long period of time. To that end, a 50 cm long column of Wyoming-type bentonite pellets was heated at its base at 140 °C (simulating the waste canister) while a synthetic sodium-chloride-rich groundwater including sulfate, calcium, and magnesium was supplied through its upper surface for almost 10 y. At the end of the experiment the upper half of the column was saturated, but in the lower half the water content decreased sharply toward the heater, where it was close to 0%. No relevant mineralogical changes occurred, and the main component of the bentonite continued to be a predominantly sodium montmorillonite with no structural changes with respect to the initial one. In the area where the temperatures were >60°C and the material very dry, however, the smectite was not able to completely develop the 1-layer hydrate after 48 h of stabilization at RH 55%, although its expandability was fully recovered when the smectite was hydrated with liquid water. The ions solubilized as a result of the water-content increase were transported toward the heater and concentrated in two distinct areas: sodium and calcium chlorides closer to the heater than calcium and sodium sulfates. At the heater contact, the bentonite microstructure was of dense packets with carbon and Na-S-coated cavities. Precipitation of calcite and calcium sulfates and possibly dissolution of silica minerals also took place close to the heater.

A numerical study on THM coupled behavior in the high-level radioactive waste disposal system

It is essential to securely isolate high-level radioactive waste from the biosphere, and geologic disposal of it at a deep underground repository is considered the most effective method. Therefore, it is crucial to research the complex thermo-hydro-mechanical-chemical (THMC) coupled behavior in geological disposal systems parallel with numerical simulation. DECOVALEX is an international cooperating project to efficiently develop numerical methods and models and validate through test results for predicting the THMC interactions in the disposal systems. In Task C of DECOVALEX-2023, the modeling teams focus on understanding pore pressure development and THM interactions in the host rock and buffer material during the FE experiment, a 1:1 scale in-situ heater test based on Nagra’s Reference repository design. We used OGS-FLAC3D for the numerical simulation, combining OpenGeoSys for TH simulation and FLAC3D for M simulation. At the first phase of the task, a simple two-dimensional benchmark problem was defined to set up the numerical model. THM coupled processes in the bentonite were simulated with a two-phase flow system, and we investigated the temperature and pressure variations on the given monitoring position. Vaporization induced by a temperature increase in the bentonite was observed at the heater’s vicinity, and flow occurred by capillarity, and pressure difference was well simulated. Additionally, the flow process was dominant in the near field of engineering barrier, while we observed thermal pressurization in the far-field area. We plan to apply the developed model to a full-scale three-dimensional numerical simulation for the next phase.

Solid-Water Interface Interaction of Selenium with Fe(II)-Bearing Minerals and Aqueous Fe(II) and S(-II) Ions in the Near-Field of the Radioactive Waste Disposal System.

Selenium can be highly toxic in excess for both animals and humans. However, since its mobile forms can be easily adsorbed with ferric minerals, its mobility in the natural oxic environment is generally not an issue. Still, the removal and immobilization of the long-lived radioactive isotope 79Se from the contaminated anoxic waters is currently a significant concern. 79Se can be accessible in the case of radionuclides' leaching from radioactive waste disposals, where anoxic conditions prevail and where ferrous ions and Fe(II)-bearing minerals predominate after corrosion processes (e.g., magnetite). Therefore, reductive and adsorptive immobilizations by Fe(II)-bearing minerals are the primary mechanisms for removing redox-sensitive selenium. Even though the information on the sorptive interactions of selenium and Fe(II)-bearing minerals seems to be well documented, this review focuses specifically on the state of the available information on the effects of the redox properties of Fe(II)-bearing solid phases (e.g., ferrous oxides, hydroxides, sulfides, and carbonates) on selenium speciation via redox transformation and co-occurring coprecipitation.

Selenium Nanowire Formation by Reacting Selenate with Magnetite.

The mobility of 79Se, a fission product of 235U and long-lived radioisotope, is an important parameter in the safety assessment of radioactive nuclear waste disposal systems. Nonradioactive selenium is also an important contaminant of drainage waters from black shale mountains and coal mines. Highly mobile and soluble in its high oxidation states, selenate (Se(VI)O42-) and selenite (Se(IV)O32-) oxyanions can interact with magnetite, a mineral present in anoxic natural environments and in steel corrosion products, thereby being reduced and consequently immobilized by forming low-solubility solids. Here, we investigated the sorption and reduction capacity of synthetic nanomagnetite toward Se(VI) at neutral and acidic pH, under reducing, oxygen-free conditions. The additional presence of Fe(II)aq, released during magnetite dissolution at pH 5, has an effect on the reduction kinetics. X-ray absorption spectroscopy analyses revealed that, at pH 5, trigonal gray Se(0) formed and that sorbed Se(IV) complexes remained on the nanoparticle surface during longer reaction times. The Se(0) nanowires grew during the reaction, which points to a complex transport mechanism of reduced species or to active reduction sites at the tip of the Se(0) nanowires. The concomitant uptake of aqueous Fe(II) and Se(VI) ions is interpreted as a consequence of small pH oscillations that result from the Se(VI) reduction, leading to a re-adsorption of aqueous Fe(II) onto the magnetite, renewing its reducing capacity. This effect is not observed at pH 7, where we observed only the formation of Se(0) with slow kinetics due to the formation of an oxidized maghemite layer. This indicates that the presence of aqueous Fe(II) may be an important factor to be considered when examining the environmental reactivity of magnetite.

Thermodynamic Studies on the Hydrolysis of Trivalent Plutonium and Solubility of Pu(OH)3(am).

The temperature-dependent reaction properties of actinide elements are of particular interest in the safety assessment of high-level radioactive waste (HLRW) disposal systems. In this study, the hydrolysis of Pu(III) and the solubility of Pu(OH)3(am) were investigated at various temperatures (10-40 °C) in 0.1 M NaClO4. A strong reducing condition for maintaining the oxidation state of Pu(III) while slowly increasing the pH of the solution was realized by electrolysis. The formation constants of the first hydrolysis species, log *β1', and the solubility products of Pu(OH)3(am), log *Ks,0', at 10, 17, and 40 °C were experimentally determined using spectrophotometry, laser-induced breakdown detection, and radiometry. The enthalpy and entropy changes for these reactions were estimated using the van't Hoff equation. The first hydrolysis of Pu(III) is endothermic (ΔrHm° = 34.10 ± 4.48 kJ mol-1), and the dissolution of Pu(OH)3(am) is exothermic (ΔrHm° = -294.29 ± 23.05 kJ mol-1) with negative entropy changes. These thermodynamic data will contribute to improving the reliability of the safety assessment of HLRW disposal facilities and understanding the geochemical behavior of Pu under reducing or anoxic aqueous conditions at elevated temperatures.

DECOVALEX-2019: An international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems

The DECOVALEX Project is an international research collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. DECOVALEX stands for “DEvelopment of COupled Models and VALidation against EXperiments”. The creation of this international initiative, now running for almost 30 years, was motivated by the recognition that prediction of these coupled effects is an essential part of the performance and safety assessment of geologic disposal systems for radioactive waste and spent nuclear fuel, and also for a range of other sub-surface engineering activities. DECOVALEX emphasizes joint analysis and comparative modeling of state-of-the-art field and laboratory experiments, across a range of host rock options and repository designs. Participating research teams are from radioactive waste management organizations, national research institutes, regulatory agencies, universities, and consulting groups, providing a wide range of perspectives and solutions to these complex problems. The most recent phase of the DECOVALEX Project, here referred to as DECOVALEX-2019, started in 2016 and ended in 2019. Modeling teams from 13 international partner organizations participated in the comparative evaluation of seven modeling tasks involving complex field and/or laboratory experiments. This Virtual Special Issue on DECOVALEX-2019 provides an in-depth overview of these collaborative research efforts and how these have advanced the state-of-the-art of understanding and modeling coupled THMC processes.

An analytical model for 3D consolidation and creep process of layered fractional viscoelastic soils considering temperature effect

The behavior of long-term deformation of saturated soil under external loads has been one of the research focuses in geological engineering and geotechnical engineering. The temperature change, especially in the fields of geological radioactive waste disposal and geothermal system, usually significantly influence the consolidation and creep process of soils. This paper presents an analytical three-dimensional (3D) model for investigating the consolidation and creep behavior of layered soils considering temperature effect. Based on the analytical layer element elastic solution, fractional calculus theory and elastic-viscoelastic correspondence principle, an analytical 3D model is built by introducing the coefficients of thermal expansion and viscosity varying with temperature into the fractional Merchant model (FMM). Verifications against published analytical and numerical results are provided, followed by extensive parametric studies to investigate the effects of type of viscoelastic model, temperature change, fractional order and viscosity of FMM, layered characteristic of soils on consolidation and creep process. It’s evident that the present model predicts long-term behavior of layered viscoelastic soils well and can be used to evaluate the effect of temperature change on consolidation and creep process in geological engineering and geotechnical engineering.

Modelling of the long-term evolution and performance of engineered barrier system

Components of the so-called “multiple-barrier system” from the waste form to the biosphere include a combination of waste containers, engineered barriers, and natural barriers. The Engineered Barrier System (EBS) is crucial for containment and isolation in a radioactive waste disposal system. The number, types, and assigned safety functions of the various engineered barriers depend on the chosen repository concept, the waste form, the radionuclides waste inventory, the selected host rock, and the hydrogeological and geochemical settings of the repository site, among others. EBS properties will evolve with time in response to the thermal, hydraulic, mechanical, radiological, and chemical gradients and interactions between the various constituents of the barriers and the host rock. Therefore, assessing how these properties evolve over long time frames is highly relevant for evaluating the performance of a repository system and safety function evaluations in a safety case. For this purpose, mechanistic numerical models are increasingly used. Such models provide an excellent way for integrating into a coherent framework a scientific understanding of coupled processes and their consequences on different properties of the materials in the EBS. Their development and validation are supported by R&D actions at the European level. For example, within the HORIZON 2020 project BEACON (Bentonite mechanical evolution), the development, test, and validation of numerical models against experimental results have been carried out in order to predict the evolution of the hydromechanical properties of bentonite during the saturation process. Also, in relation to the coupling with mechanics, WP16 MAGIC (chemo Mechanical AGIng of Cementitious materials) of the EURAD Joint Programming Initiative focuses on multi-scale chemo-mechanical modeling of cementitious-based materials that evolve under chemical perturbation. Integration of chemical evolution in models of varying complexity is a major issue tackled in the WP2 ACED (Assessment of Chemical Evolution of ILW and HLW Disposal cells) of EURAD. WP4 DONUT (Development and improvement of numerical methods and tools for modeling coupled processes) of EURAD aims at developing and improving numerical models and tools to integrate more complexity and coupling between processes. The combined progress of those projects at a pan-European level definitively improves the understanding of and the capabilities for assessing the long-term evolution of engineered barrier systems.

Decrease of radionuclide sorption in hydrated cement systems by organic ligands: Comparative evaluation using experimental data and thermodynamic calculations for ISA/EDTA-actinide-cement systems

The sufficient retardation of radionuclide release by sorption is a central issue in assessing the safety of radioactive waste disposal systems and risks posed by contaminated sites. Various types of radioactive wastes and natural environments contain organic substances that can stabilize aqueous radionuclide complexes and therefore lead to a decrease of sorption. This effect can be quantified by sorption reduction factors (SRFs). The present study focuses on testing a methodology to quantify SRFs in the presence of organic ligands for cement systems. Three approaches for the estimation of SRFs were compared for a wide range of systems including representative organic ligands (iso-saccharinic acid: ISA and ethylenediaminetetraacetic acid: EDTA) and selected key radionuclides (trivalent-, tetravalent- and hexavalent-actinides): 1) analogy with solubility enhancement factors (SEFs) derived from published solubility data, 2) calculation based on radionuclide speciation as predicted by thermodynamic calculations, and 3) direct quantification from published experimental sorption data in ternary systems. Our approach allows to critically evaluate the dependence of SRFs in various systems on the chosen method of quantification, in accordance with the data availability for a given system. Our comparisons suggest that reliable SRFs can only be derived from sorption measurements in ternary systems, i.e, where the effect can be directly measured in the presence of solid surface, radionuclide and organic ligand. On the other hand, SRFs often need to be derived in the absence of such direct evidence, and approximations based on analogies and speciation information can be applied to estimate SRFs in such cases. However, such estimates may be subject to substantial uncertainties. This should not be seen as a fundamental hindrance to the use of approximations, but uncertainties need to be clearly acknowledged and an SRF should reflect this e.g. by providing (additional) conservative estimates.

International collaboration in disposal research: comparative modeling of coupled processes in the DECOVALEX project

Abstract. This presentation gives an overview of an international research collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. The creation of the international DECOVALEX project, now running for more than 25 years, was motivated by the recognition that prediction of these coupled effects is an essential part of the performance and safety assessment of geologic disposal systems for radioactive waste and spent nuclear fuel. DECOVALEX emphasizes joint analysis and comparative modeling of state-of-the-art field and laboratory experiments, across a range of host rock options and repository designs. Participating research teams are from radioactive waste management organizations, national research institutes, regulatory agencies, universities, and consulting groups, providing a wide range of perspectives and solutions to these complex problems. The presentation provides examples of the research contributions made collectively in past DECOVALEX tasks and also touches on the unique modeling challenges tackled in the ongoing project phase, referred to as DECOVALEX-2023. The current phase comprises 17 partner organizations, about 50 modeling teams, and 7 modeling tasks, which cover a broad portfolio from fundamental studies on gas migration to full-scale in situ heater experiments in different host rocks to performance assessment studies. Together, these examples illustrate that the insight and scientific knowledge gained within the DECOVALEX project would not have been possible if one group had studied these complex THMC modeling challenges alone rather than within a truly collaborative setting.

Behavior of compacted Ca-bentonite subjected to HMC loading: observations and interpretation

This study presents result of HMC experimental test and discussions of chemical effort in Cabentonite. SWCC test, swelling pressure test and unconfined compression test as hydro-mechanical-chemical (HMC) test were applied to Cabentonite-sand mixture. Na-type bentonite has been studied to predict safety and confidence for artificial barrier layer consist of bentonite that many experimental research reports are represented, and can offer technology advanced comprehensive performance due to mathematical simulation models associated to physical parameters on thermal-hydration-mechanical-chemical properties. This study proposed a significant property for calcium bentonite to relate evaluating safety in radioactive waste disposal system, which emphasised much influence of suction and salinity chloride to hydration-mechanical behaviour for Calcium bentonite.

Swelling deformation of Gaomiaozi bentonite under alkaline chemical conditions in a repository

The safety assessment for the long-term operation of a deep repository is an essential work for the engineering of geological disposal for high-level radioactive waste (HLW). Bentonite selected as a barrier/backfill material in repositories, can be exposed to the chemical effects of alkaline site water owing to cement degradation, which poses a great threat to the long-term safety of the repository. This work is focusing on the swelling property of bentonite with alkaline pore water in order to investigate the evolution of vertical strain of compacted Gaomiaozi bentonite under the simulated long-term environments of Chinese HLW repository. Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy tests were utilized to analyze the bentonite after it reacted with the pore water solutions. Results reveal that the swelling strain is significantly influenced by the composition and concentration of the solutions, and that swelling time decreases as temperature increases. In the secondary swelling stage of the bentonite hydrated by solutions containing K+, the swelling strain decays because K ions enter the montmorillonite crystal layer and generate low-swelling or non-swelling minerals. The solutions (particularly high alkaline solutions) containing K ions strongly decrease the swelling performance of bentonite, suggesting that alkaline cement water can degrade the swelling property of the buffer material in a high-level radioactive waste (HLW) disposal system.

Thermochemical changes on swelling pressure of compacted bentonite

Bentonite has been considered as a potential buffer material in engineering barriers of high-level radioactive waste disposal systems. The intrusion of groundwater and heat from the waste change the temperature and chemical composition of the bentonite, consequently can alter the mechanical properties of the bentonite. In this study, thermochemical effects on the swelling behaviors were observed in compacted Ca-bentonite samples. Laboratory tests were conducted on Korean Gyeongju bentonite blocks that were compacted at different dry densities. Swelling pressure tests at various temperatures between 30 °C and 90 °C were carried out by supplying de-ionized water, NaCl solution, and CaCl2 solution. The study revealed that at higher temperature the bentonite swells faster, but the final swelling pressure decreases in the higher temperature condition. As for the chemical effect, the final swelling pressure at the same temperature shows an order: CaCl2 > De-ionized water > NaCl. In addition, the swelling behavior of the bentonite during a thermal loading cycle was experimentally investigated. The swelling pressure at the same temperature revealed different values depending on whether the bentonite was heated up or cooled down. The result demonstrated that the temperature change history should be a significant factor in determining the swelling pressure of bentonite buffers.

A Prediction of Saturated Hydraulic Conductivity for Compacted Bentonite Buffer in a High-level Radioactive Waste Disposal System

A geological repository comprises a natural barrier and an engineered barrier system. Its design components consist of canisters, buffers, backfill, and near-field rock. Among the engineered barrier system components, bentonite buffers minimize the groundwater flow from near-field rock and prevent the release of nuclide. Investigation of the hydraulic conductivity of the buffer to groundwater flow is an important factor in the performance evaluation of the stability and integrity of the engineered barrier of the repository. In this study, saturated hydraulic conductivity tests were performed using Gyeongju bentonite at various dry densities and temperatures, and a hydraulic conductivity prediction model was developed through multiple regression analysis using the 120 result sets of hydraulic conductivity. The test results showed that the hydraulic conductivity tends to decrease as the dry density increases. In addition, the hydraulic conductivity increased with increasing temperature. The multiple regression analysis results showed that the coefficient of determination (R2) of the hydraulic conductivity prediction equation was as high as 0.93. The hydraulic conductivity prediction equation presented in this study could be used for the design of engineered barrier systems.

Evaluation of direct shear strength of compacted bentonite having pore-water pressure

Safety of great deep repository design has been investigated for high-level radioactive waste disposal system in several countries such as Belgium, Canada, China, France, Germany, Japan, Sweden and Switzerland. The repository of the disposal is in most cases based on the concept of a multi-barrier system using the host rock barrier formation and a man-made barrier formation. The man-made barrier consists of high expansive bentonite. Thermal-hydro-mechanical behaviour simulation models were developed, including some parameters described by experimental works. The complex phenomena due to the transition into saturation and chemical reactions at the bentonite barrier system have been explained. This study focused on direct shear strength for compacted bentonite related to some factors induced by uncertainty problems such as hydration effect and pore-water pressure. Measured shear strength properties of compacted bentonite had been determined at high suction values. Also, it is clear that there is some influence of direct shear speed on direct shear strength both under unsaturated-saturated conditions. A modified direct shear apparatus was used in this study to observe changes in shear strength with increments of pore-water pressures.

Adsorption of Eu(III) onto Gaomiaozi bentonite corroded by cement waters: Effect of cement solutions on the long-term sorption performance of bentonite in the repository conditions

Abstract The adsorption of europium (Eu(III)) onto Gaomiaozi (GMZ) bentonite corroded by cement water at different stages was studied by batch experiments considering the in-situ conditions in a high-level radioactive waste (HLW) repository. Before this, the corrosion tests were carried out on compacted GMZ bentonite specimens with two cement solutions (Young Cement Water -YCW and Evolved Cement Water -ECW) simulating the early and late cement degradation waters during the long-term operation of a repository. X-Ray diffractometry (XRD) tests and scanning electron microscope (SEM) tests were performed to identify the characteristics of corroded GMZ bentonite. The results indicate that the adsorption capability of YCW corroded bentonite is similar to that of ECW corroded bentonite, and both of them are higher than that of raw GMZ bentonite. Cement solutions impact the adsorption performance of bentonite mainly by increasing pH of pore water in bentonite and altering its mineralogy (forming secondary minerals). Increasing pH implies increasing the sorption percentage of nuclide ions. Besides, most of the secondary minerals in bentonite have a good adsorption capacity. As a consequence, the cement solutions have positive effect on the long-term adsorption performance of bentonite, suggesting that GMZ bentonite is a good retention material in a high-level radioactive waste (HLW) disposal system.