Barriers For Radioactive Waste Disposal Research Articles

Effect of drying cracks on swelling and self-healing of bentonite-sand blocks used as engineered barriers for radioactive waste disposal

Experiments were conducted to evaluate the healing of drying cracks in air-dried bentonite-sand blocks after hydration and swelling in groundwater, providing justifications to simplify the protection of blocks prior to installation in a high-level radioactive waste repository. Synthetic groundwater was prepared to represent the geochemistry of Beishan groundwater, and was used to hydrate the blocks during the swelling pressure and swelling strain measurements, as Beishan is the most promising site for China's repository. Healing of the surface cracks was recorded by photography, and healing of the internal cracks was visualized by CT images and hydraulic conductivity of air-dried blocks. The results indicate that the maximum swelling pressure and swelling strain are primarily affected by the geochemistry of Beishan groundwater, but not affected by the drying cracks. The maximum swelling pressure and swelling strain of air-dried blocks are comparable to or even higher than the pressure and strain of fresh blocks. The maximum swelling pressure measured in strong (i.e. high ion strength) Beishan groundwater was 44% of the pressure measured in deionized (DI) water, and the maximum swelling strain was reduced to 23% of the strain measured in DI water. Nevertheless, the remained swelling of the blocks hydrated in strong Beishan groundwater was sufficient to heal the surface and internal drying cracks, as demonstrated by the pictures of surface cracks and CT images. The hydraulic conductivity of the air-dried block permeated with strong groundwater was comparable (3.7× higher) to the hydraulic conductivity of the fresh block, indicating the self-healing of drying cracks after hydration and swelling in groundwater. A simplified method of protecting the block with plastic wraps before installation is recommended, since the remained swelling of the block hydrated in Beishan groundwater is sufficient to heal the drying cracks.

Saturated hydraulic conductivity of compacted bentonite–sand mixtures before and after gas migration in artificial seawater

To understand the self-healing property of an engineered barrier for radioactive waste disposal, the hydraulic conductivity of compacted bentonite–sand mixtures saturated with artificial seawater (SW) before and after gas migration was examined. Na- and Ca-bentonites were mixed with fine sand at a ratio of 70% bentonite in dry weight. Two aspects were considered during the experiment: the hydraulic conductivity of the specimen that was resaturated after gas migration and the distribution of water content immediately after gas migration to study gas migration pathways. The gas migrated through the entire cross-section of the specimen, and gas breakthrough occurred in the equilibrium swelling pressure range approximately. Subsequently, the gas flow rate reached a sufficient large value when the gas pressure was approximately twice the equilibrium axial pressure (the sum of swelling and confining pressures), which excluded the back pressure. Although the gas migration pathway was not visible when the specimen was observed immediately after gas migration, the water content distribution showed that several parts of the specimen with lower water content were connected in the direction of gas migration. After resaturation, the change in permeability was within a limited range—two to three times larger than that before gas migration for each type of bentonite in SW. This slight change suggests that gas migration creates a pore structure that cannot be sealed via crystalline swelling of montmorillonite in SW, even if highly compacted bentonite is used under a constant-volume condition.© 2022 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). • Gas migration of compacted bentonite–sand mixtures in seawater. • Saturated hydraulic conductivity after gas migration in seawater. • Breakthrough pressure increased along equilibrium axial pressure. • Low permeability of compacted bentonite did not change substantially.

Mechanistic insights into the dynamics of radionuclides retention in evolved POFA-OPC and OPC barriers in radioactive waste disposal

Cementitious barriers are widely used in all radioactive waste disposal facilities to ensure the fulfillment of several safety functions. A realistic quantification of radionuclide retention characteristics into these barriers under the dynamic changes that occur in the disposal facility is necessary to optimize the design of the facility and ensure the reliability of these safety functions. Herein, the beneficial use of Palm Oil Fuel Ash (POFA) as supplementary material to tailor the radionuclide retention characteristics of Ordinary Portland Cement (OPC) is investigated. The characterizations of evolved OPC and POFA-OPC samples revealed that the use of POFA slows down the carbonation process; enhances the formation and polymerization of low Ca/Si C-S-H and reduces the degradation state of the barrier. The effects of the progressive release and cooling down of Cs sources and progressive water intrusion on the retardation coefficients, relative contaminant retention, and capacity factors were investigated assuming the validity of the local linear and Langmuir based models. In comparison with reference OPC samples, it was found that POFA improved greatly the containment performance, where the retardation coefficients in the supplemented material are in the range (33–35.42) under the coupled effects of progressive release and cooling down of Cs sources. The retardation coefficients and the barrier capacities increased largely in very diluted systems. The adaptation of the local linear isotherm to describe the dynamic changes in the disposal facility was found to be associated with considerable uncertainties that need to be thoroughly identified.

Effect of interfaces on gas breaktrough pressure in compacted bentonite used as engineered barrier for radioactive waste disposal

In a deep geological nuclear waste repository gas can be generated by different processes. Understanding the gas transport mechanisms across the engineered and natural barriers in a repository is relevant for its security assessment, both in terms of mechanical stability and of radionuclide transport. The engineered barrier may be composed of compacted blocks of bentonite and the interfaces between these blocks might evolve into preferential fluid pathways, in particular for the gas generated around the waste canisters. Small-scale laboratory tests were performed in sound samples and in samples crossed by an interface to determine gas breakthrough pressure values after saturation and the effect on them of the interface. The FEBEX bentonite, a Spanish bentonite composed mainly of montmorillonite, was used in the tests. The gas breakthrough pressure of the saturated compacted samples increased with dry density and was higher than the swelling pressure of the bentonite. Gas breakthrough could take place either in an instantaneous or in a gradual way, the difference between both modes being the flow rate, much higher in the first case. The gas transport mechanism would be microscopic pathway dilation, with microfracturing in the case of the instantaneous episodes. A sealed interface along the bentonite did not seem to affect the breakthrough pressure or gas permeability values, since the behaviour patterns were similar in both kinds of samples, depending mostly on the bentonite dry density.

Identification of log units in clay rock formations based on local and spatial statistics of well-log properties: application to the Opalinus claystone in the Benken borehole

AbstractDue to their particularly good mechanical and self-healing properties combined with exceptionally efficient cation adsorbents and exchanger capacities, clay minerals and clay rock formations are considered as suitable geological barriers for radioactive waste disposal. The Middle Jurassic Opalinus Clay Formation has been identified as a potential host rock. Logging data were measured at the Benken borehole drilled through this formation in northern Switzerland. This paper presents a statistical methodology to improve the description of the physical properties of the clay rock based on the well-log data. The methodology involves the classification of a set of local statistics, calculated from a reduced number of principal components computed from well-log properties. The use of a kernel-based method to calculate local statistics allows an analysis of spatial variability to be carried out at different scales, and with different scale effects. The first-order layering was found to be robust and independent of kernel size (i.e. observation scale), while preserving small-scale heterogeneities that are useful for further interpretation. The log units can be more clearly interpreted in terms of stationary or transitional log units, depending on the behaviour of local statistics. Finally, the derived spatial variability of the log-units properties are compared with earlier lithological descriptions and stratigraphic data.

Development of a mathematical model for gas migration (two-phase flow) in natural and engineered barriers for radioactive waste disposal

AbstractIn a deep geological repository (DGR) for the long-term containment of radioactive waste, gases could be generated through a number of processes. If gas production exceeds the containment capacity of the engineered barriers or host rock, these gases could migrate through these barriers and potentially expose people and the environment to radioactivity. Expansive soils, such as bentonite-based materials, are currently the preferred choice of seal materials. Understanding the long-term performance of these seals as barriers against gas migration is an important component in the design and long-term safety assessment of a DGR. This study proposes a hydro-mechanical linear poro-elastic visco-capillary mathematical model for advective-diffusive controlled two-phase flow through a low-permeability expansive soil. It is based on the theoretical framework of poromechanics, incorporates Darcy's Law for both the porewater and poregas, and a modified Bishop's effective stress principle. Using the finite element method (FEM), the model was used to numerically simulate 1D flow through a low-permeability expansive soil. The results were verified against experimental results found in the current literature. Parametric studies were performed to determine the influence on the flow behaviour. Based on the results, the mathematical model looks promising and will be improved to model flow through preferential pathways.

Evaluation on Mechanical Stabilities of Clay-Sand Mixtures Used as Engineered Barrier for Radioactive Waste Disposal

Evaluation on Mechanical Stabilities of Clay-Sand Mixtures Used as Engineered Barrier for Radioactive Waste Disposal

Influence of water density on the water-retention curve of expansive clays

There is increasing evidence from researchers on clay mineralogy that the density of water attached to clay minerals may be greater than 1·0 Mg/m3. This fact becomes especially evident in compacted highly expansive clays close to water saturation. This kind of material is being considered in the design of engineered barriers for radioactive waste disposal, because of its sealing and retention properties. From a geotechnical point of view, most of the analyses required to check the performance of the barrier are sensitive to the value considered for the water density. This is the case for the unsaturated flow calculations, which depend on the water-retention properties of the soil. This paper presents, first, a review of measurements performed at microscopic level. A description of the hydration process of expansive clay considering the micro and macrostructure is included. Then a method to obtain an average water density as a function of suction for sodium and calcium bentonites is proposed. The method is used to correct the water-retention curves expressed in terms of the degree of saturation.

Modeling the competition between solid solution formation and cation exchange on the retardation of aqueous radium in an idealized bentonite column

Clays and clay rocks are considered viable geotechnical barriers in radioactive waste disposal. One reason for this is the propensity for cation exchange reactions in clay minerals to retard the migration of radionuclides. Although another retardation mechanism, namely the incorporation of radionuclides into sulfate or carbonate solid solutions, has been known for a long time, only recently has it been examined systematically. In this work, we investigate the competitive effect of both mechanisms on the transport of radium (Ra) in the near-field of a low- and intermediate level nuclear waste repository. In our idealized geochemical model, numerical simulations show that barium (Ba) and strontium (Sr) needed for Ra sulfate solid solutions also partition to the cation exchange sites of montmorillonite (Mont), which is the major mineral constituent of bentonite that is used for tunnel backfill. At high Mont content, most Ra tends to attach to Mont, while incorporation of Ra in sulfate solid solutions is more important at low Mont content. To explore the effect of the Mont content on the transport of radium, a multi-component reactive transport model was developed and implemented in the scientific software OpenGeoSys-GEM. It was found that a decrease of fixation capacity due to low Mont content is compensated by the formation of solid solutions and that the migration distance of aqueous Ra is similar at different Mont/water ratios.

Clay barriers in radioactive waste disposal

Deep geological is one of the preferred options for the disposal of high level radioactive waste. In most designs, the canisters placed in drifts or boreholes are surrounded by an engineered barrie...

Physicochemical relationships during a KCl-bentonite hydrothermal reaction

AbstractA dioctahedral bentonite was treated hydrothermally with KCI solutions (0.025-1 M) at different temperatures (60-200°C) and times (1-360 days) to evaluate its use as a barrier for radioactive waste disposal. Equations relating adsorbed K on smectite with initial and equilibrium K concentrations in solution, temperature and time have been obtained. These equations are useful for modelling the influence and behaviour of these variables and contribute to the performance assessment calculations for radioactive waste repositories. Likewise, the thermodynamic parameters of the K adsorption process have been calculated. Other relationships, including silica in solution and pH, are also shown.

Physicochemical Relationships During a KCl-Bentonite Hydrothermal Reaction

A dioctahedral bentonite was treated hydrothermally with KCI solutions (0.025-1 M) at different temperatures (60-200°C) and times (1-360 days) to evaluate its use as a barrier for radioactive waste disposal. Equations relating adsorbed K on smectite with initial and equilibrium K concentrations in solution, temperature and time have been obtained. These equations are useful for modelling the influence and behaviour of these variables and contribute to the performance assessment calculations for radioactive waste repositories. Likewise, the thermodynamic parameters of the K adsorption process have been calculated. Other relationships, including silica in solution and pH, are also shown.