Geological Disposal Of Nuclear Waste Research Articles

Diffusion of radioactive anion and cation in silver-calcium-bentonite

AbstractIn the geological disposal of nuclear waste, the simultaneous reduction of diffusion rate of cationic and anionic radioactive species is essential. Natural calcium-bentonite only reduces the diffusion rate of cations, not anions. In this study, the half of calcium ions is bentonite was exchanged to silver ions to reduce diffusion rates of both of radioactive cation (137Cs+) and anions (36Cl−, 131I−) at the same time. This is due to the precipitation of silver halides in the interlayer space. The diffusion rate of both anionic and cationic radioactive species are small in compacted the silver-calcium bentonite.

Shear-induced fluid localization, episodic fluid release and porosity wave in deformable low-permeability rock salt

Understanding fluid distribution and migration in deformable low-permeability rock salt is critical for geologic disposal of nuclear waste. Field observations indicate that fluids in a salt formation are likely compartmentalized into relatively isolated patches and fluid release from such a formation is generally episodic. The underlying mechanism for these phenomena remains poorly understood. In this paper, a hydrological-mechanical model is formulated for fluid percolation in a rock salt formation under a deviatoric stress. Using a linear stability analysis, we show that a porosity wave (a train of alternating high and low porosity pockets) can emerge from positive feedbacks among intergranular wetting, grain boundary weakening and shear-induced material dilatancy. Fluid localization or episodic release can be viewed as a stationary or propagating porosity wave respectively. Fluid pockets transported via a porosity wave remain relatively isolated with minimal mixing between neighboring pockets. We further show that the velocity of fluid flow can be significantly enhanced by the emergence of a porosity wave. The concept and the related model presented in this paper provide a unified consistent explanation for the key features observed in fluid flow in rock salt. The similar process is expected to occur in other deformable low-permeability media such as shale and partially molten rocks under a deviatoric stress. Thus, the result presented here has an important implication to hydrocarbon expulsion from shale source rocks, radioactive waste isolation in a tight rock repository, and caprock integrity of a subsurface gas (CO2, H2 or CH4) storage system. It may also help develop a new engineering approach to fluid injection into or extraction from unconventional reservoirs.

Temporal Behavior of Glacially Induced Stresses and Strains at Potential Sites for Long‐Term Storage of Used Nuclear Fuel in Canada

AbstractContinental‐scale glaciations cause deformation, geopotential, rotation and stress changes of the Earth. Subsurface stress changes have implications to future activities such as carbon capture and storage, enhanced oil recovery and deep geological disposal of nuclear waste. We model glacially induced stresses, strain changes and deformation for North America, with emphasis on the two potential sites for long‐term storage of used nuclear fuel in Canada (Saugeen Ojibway Nation (SON)‐South Bruce area in southwestern Ontario and Wabigoon Lake Ojibway Nation (WLON)‐Ignace area in northwestern Ontario). We apply a revised, high‐resolution ice history of the past glacial cycle from the University of Toronto Glacial Systems Model, assumed to be representative for future glacial cycles, together with a set of seven different one‐ and three‐dimensional earth structures. We find that glacially induced stresses and strains can vary strongly throughout a glacial cycle, whereas especially the horizontal components can change from tensional to compressive in nature. Such changes can happen within a few 1,000 years, caused by drastic and rapid ice thickness increase or decrease above the potential site. Despite SON‐South Bruce being located further away from the ice sheet center than WLON‐Ignace and temporarily in the forebulge of the developing ice sheet during glaciation, stresses and strains are very similar in magnitude and range at both sites. We also see the potential that the glacially induced stresses can alter the direction of the pre‐existing maximum horizontal stress at SON‐South Bruce. These results will be incorporated in the site safety and site selection process.

Position paper on high fidelity simulations for coupled processes, multi-physics and chemistry in geological disposal of nuclear waste

This opinion paper describes the major coupled T(Thermal)-H(Hydro)-M(Mechanical)-C(Chemical) processes in geological repository systems and the frontier of related model development. Particular focus is made on the analysis of existing approaches and open research questions with respect to the further development of coupled codes and models for realistic multi-scale simulations of repository systems. These include the use of machine learning and artificial intelligence in acceleration of computer codes; sensitivity analysis, inverse modelling and optimisation; software engineering and collaborative platforms for model development.

Elastic modulus prediction for high-temperature treated rock using multi-step hybrid ensemble model combined with coronavirus herd immunity optimizer

Elastic modulus is a crucial mechanical parameter that measures the stiffness property of rock materials. In underground rock engineering, such as deep energy development and geological disposal of high-level nuclear waste, accurately determining the elastic modulus of engineering rock masses in high-temperature environments plays a vital role in understanding the instability-triggering conditions of hard and brittle rock masses in deep underground engineering and maintaining the safety and stability of deep underground engineering. Traditionally, conducting indoor experimental tests is time-consuming, labor-intensive, and costly. Therefore, developing a convenient and accurate rock elastic modulus prediction model based on machine learning technology holds significant importance. To this end, this study proposed a multi-step hybrid ensemble model (MHEM) for predicting high-temperature treated rock elastic modulus. The input parameters of the model include diameter, height, density, temperature, confining pressure, crack damage stress, and strength, while the output parameter is the elastic modulus. Subsequently, a coronavirus herd immunity optimizer (CHIO) intelligent optimization algorithm was employed to optimize the MHEM, and the CHIO-MHEM was established. Then, the performance of the developed models was compared and evaluated with eight other different prediction models. Finally, SHAP method and tree model feature analysis were used to quantify the feature importance of the prediction model. The research results indicate that, compared with other prediction models, the CHIO-MHEM exhibits superior performance, achieving accurate prediction of the elastic modulus of rocks treated at different high temperatures. Additionally, among all input parameters, rock density and temperature have the most significant influence on the rock elastic modulus.

GDSA framework, a computational framework for complex modeling problems in radioactive waste management

This paper details a computational framework to produce automated, graphical workflows, and how this framework can be deployed to support complex modeling problems like those in nuclear engineering. Key benefits of the framework include: automating previously manual workflows; intuitive construction and communication of workflows through a graphical interface; and automated file transfer and handling for workflows deployed across heterogeneous computing resources. This paper demonstrates the framework's application to probabilistic post-closure performance assessment of systems for deep geologic disposal of nuclear waste. However, the framework is a general capability that can help users running a variety of computational studies.

Retention of 226Ra in the sandy Opalinus Clay facies from the Mont Terri rock laboratory, Switzerland

The safety assessment of deep geological disposal of nuclear waste requires a sound understanding of radionuclide retention in the repository host rocks. Here, we investigate the retention behaviour of the safety relevant radionuclide 226Ra in the heterogeneous sandy facies of the Opalinus Clay (OPA-SF) at the Mont Terri underground rock laboratory. Various rock samples were selected from a drill core (BAD-1) located in the lower sandy facies, which were representative for its textural and mineralogical heterogeneity. The samples were carefully characterized with respect to mineralogy and chemistry using X-Ray diffraction and electron microscopy. The amounts of quartz, carbonates and clay minerals in the samples ranged between 58 wt.% – 65 wt.%, 13 wt.% – 35 wt.% and 5 wt.% – 25 wt.%, respectively, underpinning the mineralogical heterogeneity. Rare (Ba,Sr)-sulphate precipitates were identified in the clay matrix by electron microscopy. The uptake behaviour of Ra was evaluated using batch sorption experiments. Corresponding to the mineralogical variability of the rock samples, the measured distribution ratios Rd for the 226Ra retention varied between 20 L kg−1 and 260 L kg−1 after 120 days. The experimental results and thermodynamic modelling approaches suggest that various processes contribute to the uptake of 226Ra on different time scales: fast cation exchange and surface complexation reactions by clay minerals and the (slower) formation of solid solutions during recrystallisation of (Ba,Sr)-sulphates as well as retention by carbonate minerals; the latter mechanism still requiring further investigation.

Deep geological disposal of plastic waste: Pros and cons

Plastic production is growing, plastic waste is accumulating, and current waste management methods (recycling, incineration, etc.) are not yet able to solve this waste problem. Proposals and projects aimed at solving or facilitating the problem of plastic waste are therefore relevant. This article proposes a deep geological disposal method for plastic waste projects. The prototype for this project was the deep geological disposal of nuclear waste. Although plastic waste and nuclear waste are fundamentally different, the adequacy of this approach is justified by the fact that plastic waste, such as nuclear waste, has a a long degradation period and poses great danger to the environment, animals, and humans. The article is conceptual and examines the aspects necessary for the implementation of the project, such as the establishment of a special international UN agency for plastic waste management, specific sources of funding, giving plastic waste a monetary value, applying free market principles, and using existing potential opportunities for deep geological disposal of plastic waste. This article also discusses the proposed concept for finding more optimal options.

Atomistic Computer Simulations of Uranyl Adsorption on Hydrated Illite and Smectite Surfaces

A quantitative understanding of the molecular-scale mechanisms of radionuclide sorption on different clay minerals is crucial for the development and safe implementation of geological nuclear waste disposal technologies. We apply classical molecular dynamics (MD) computer simulations to study the adsorption of uranyl on the external basal surfaces of two typical clay models. In the illite model, negative charge is primarily localized in the tetrahedral sheets, while in the lower-charge smectite model, the isomorphic substitutions are introduced in the octahedral sheet. The comparison of atomic density distributions at the clay surfaces and adsorption-free energies profiles as a function of distance from these surfaces demonstrates that overall U behavior at the basal clay surface is quite similar for illite and smectite. Uranyl is sorbed as a mixture of outer-sphere aqua complexes [UO2(H2O)5]2+ and hydrolyzed aqua complexes [UO2(H2O)4–5OH]+ on both surfaces. The structural and compositional differences between the models do not greatly affect the uranyl’s nearest coordination environment and are mainly reflected in the specific localization and orientation of the uranyl ions at both surfaces and in the magnitude of the adsorption-free energies. The observed quantitative characteristics of uranyl interactions with illite and smectite surfaces will help to better understand U behavior during the sorption process on clay minerals for the entire range of mixed-layer illite–smectite structures. A comparison of two versions of the ClayFF force field in the simulations made it possible to more accurately and quantitatively evaluate some subtle features of the uranyl–clay interactions and to obtain a more precise composition of uranyl complex with the modified ClayFF force field (ClayFF-MOH).

Insight to the cesium entrapment by natural shale: implications to geological disposal of nuclear waste

ABSTRACT The knowledge of radionuclides sorption behavior of various site-specific heterogeneous geosystems is essential for developing different clean-up strategies and predicting the radiological risks for long term. The present study is aimed at understanding the mineralogical and textural characteristics of natural shale rock (sampled from Kachchh Basin of Gujarat, India) and its correlation with cesium (Cs) sorption behavior. The modal mineralogical composition, textural attributes, petrophysical properties, surface functionalities and morphology of shale were evaluated. Clay minerals (smectite, kaolinite and illite) constitute more than 40% of the shale along with the major component- quartz. A comprehensive Cs batch sorption study performed at varying experimental parameters demonstrated that the maximum Cs retention of shale was 34.80 ± 2.85 mg g−1 in the studied concentration range. Sorption equilibrium data agreed well with Freundlich isotherm, highlighting the heterogeneity of shale. Variation in solution pH (in the range of 6–10) during sorption demonstrated the buffering of solution in near neutral range indicating the role of cations (available in the interlayers of clay minerals) in Cs uptake. Alternatively, the presence of illite mineral in the shale with known frayed edge sites contributed to the irreversibility of sorption event. The present studies illustrate the potential of shale as host rock for its application in nuclear waste management.

A thermomechanical model for argillaceous hard soils–weak rocks: application to THM simulation of deep excavations in claystone

The paper presents the enhancement of an existing constitutive model for argillaceous hard soils–weak rocks to incorporate non-isothermal conditions, to be used in coupled thermo-hydro-mechanical (THM) simulations of underground excavations subjected to temperature variations within the context of deep geological nuclear waste disposal. The proposed thermo-elastoplastic extension accounts for the effect of temperature on the yield and plastic potential functions and on the elastic stiffness. The resulting model is validated through the simulation of relevant non-isothermal laboratory tests reported in the literature. The model is then applied to the coupled THM simulation of an in situ heating test conducted at the Meuse/Haute-Marne underground research laboratory in Bure, France, excavated in the Callovo-Oxfordian claystone. Results show that the incorporation of thermal effects into the constitutive description of the host rock plays a significant role in the behaviour of the excavation when subjected to thermal loading, particularly in the evolution of the excavation fractured zone.

What the Flux? – Water-Rock-Microbe Interactions and Crustal Gases in the Deep Subsurface

The deep, dark fracture zones of the continental crust host a fascinating interplay between water, rocks, and microbes, resulting in the production and consumption of gases, including methane, volatile organic compounds (VOCs), and hydrogen. Various geological factors influence the formation and release of these crustal gases, including the local rock type with its concentration of radioactive elements and carbon, temperature, and the connectivity and dynamics of fracture systems with each other and to the surface. To understand the formation, accumulation, and release of crustal gases, methodologies of hydrogeochemistry, biogeochemistry, and isotope geochemistry can be employed. Sample collection from drill holes and mines, coupled with on-line monitoring of gas flux rate and composition, provides important data. Furthermore, the integration of molecular biological methods enhances our understanding of the water-rock-microbe interactions that shape the deep subsurface gas realm. Crustal gases have crucial implications for life in extreme environments, including those outside of our planet Earth, but potentially also pose significant challenges to drilling, mining, and their environmental impact. Moreover, crustal gases hold relevance for the energy sector, contributing to both the long-term safety of geological disposal of nuclear waste, carbon footprint of geothermal wells, and the exploration of hydrogen as a sustainable energy resource.

Laboratory Study of Effective Stress Coefficient for Saturated Claystone

Claystone is potentially the main rock formation for the deep geological disposal of high-level radioactive nuclear waste. A major factor that affects the deformation of the host medium is effective stress. Therefore, studying the effective stress principle of claystone is essential for a stability analysis of waste disposal facilities. Consolidated drained (CD) tests were carried out on claystone samples to study their effective stress principle in this paper. Firstly, two samples were saturated under a specified confining pressure and pore pressure for about one month. Secondly, the confining pressure and pore pressure were increased to a specified value simultaneously and then reverted to the previous stress state (the deformations of the samples were recorded during the whole process). Different incremental combinations of the confining pressure and pore pressure were tried at this step. Finally, the effective stress coefficients of the samples were obtained through a back analysis. Furthermore, some potential influencing factors (the neutral stress and loading rate) of the effective stress coefficient were also studied through additional tests. Some interesting results are worth mentioning: (1) the effective stress coefficient of claystone is close to one; (2) the neutral stress and loading rate may have little effect on the effective stress coefficient of claystone.

Application of machine learning for modeling brønsted-guggenheim-scatchard specific ion interaction theory (SIT) coefficients

Machine learning methodologies can provide insight into Brønsted-Guggenheim-Scatchard specific ion interaction theory (SIT) parameter values where experimental data availability may be limited. This study develops and executes machine learning frameworks to model the SIT interaction coefficient, ε. Key findings include successful estimations of ε via artificial neural networks using clustering and value prediction approaches. Applicability to other chemical parameters is also assessed briefly. Models developed here provide support for a use-case of machine learning in geologic nuclear waste disposal research applications, namely in predictions of chemical behaviors of high ionic strength solutions (i.e., subsurface brines).

Real-time permeability evolution of limestone under high temperature and triaxial stresses

In deep underground geological disposal of nuclear waste, geothermal energy development, underground coal gasification, geothermal resources development and other underground high temperature projects, they are all closely related to the permeability and thermal expansion characteristics of rocks in high temperature high-pressure real-time state. In this experiment, the real-time evolution of permeability of limestone under high-temperature and triaxial stresses condition was studied using the HTHP rock mechanics triaxial test system and compared with the permeability of limestone after high temperature cooling treatment. The test results show that: the effect of temperature on limestone permeability facilitation under high temperature cooling condition is significant, and the permeability increases by 1–2 orders of magnitude relative to the initial permeability for every 100 °C increase in the preheat temperature after 300 °C. Under high-temperature and high-pressure in real time condition, the difference values of permeability at different temperature points relative to room temperature always do not more than one order of magnitude. During the heating process at 500 °C, the permeability curves of the triaxial pressures range from 15 to 35 MPa are very different. Under the pressure condition of 15 MPa, the permeability curve increases monotonically and rapidly with the increasing temperature; When the pressure reaches 25 MPa pressure, there is a peak value area in the permeability curve; the permeability decreases monotonically with temperature when the pressure up to 35 MPa. Compared with the only thermal effect, the thermal- mechanical coupling effect has a significant inhibitory effect on limestone permeability development, and the permeability disparity between the two types of heat treatment rocks become more and more significant with increasing temperature and hydrostatic pressure.

An experimental methodology to assess the impact of desaturation on gas diffusion in clay based materials

The transport of gas in clay-based materials is dominated by diffusion. SCK CEN has studied and recorded an extensive data set of diffusion coefficients of gases in various saturated clays (Boom Clay, Opalinus Clay, Callovo-Oxfordian claystone and bentonites) used in Europe as host formations or in engineered barrier systems in the context of deep geological disposal of high and intermediate level nuclear wastes. However, partially saturated conditions may exist during the life cycle of a radioactive waste repository. The current study aims at establishing an experimental method to measure the diffusion coefficients of various gases in partially saturated clay-based materials by means of double through diffusion tests (cross diffusion of gases across the porous sample, driven by concentration gradients) and to assess the impact of desaturation on gas diffusivity. Historically, gas diffusion measurements have been performed under dissolved conditions with continuous water and dissolved gas flow around the clay sample. However, since the saturation degree of the sample has to be constant, a novel setup has been designed and optimized to operate under a pure gas phase under controlled relative humidity (RH). The experimental concept consists of cross diffusion of gas mixtures from two reservoirs at a fixed relative humidity on either side of the sample cell, namely, the upstream and downstream reservoirs. The RH is controlled using the same saturated salt solutions in both reservoirs. During the experiment, gases will diffuse from the upstream to the downstream reservoir. By measuring the change in gas composition in the downstream reservoir with a gas analyzer equipped with a mass spectrometer (which expresses the gas composition in terms of partial pressure), the gas flux and hence the diffusion coefficient is estimated by applying steady state Fick’s first law. The experimental methodology has been validated by performing an experiment on a sandy-clay sample of known petrophysical properties, at full saturation.

Temperature effect on non-Darcian flow in low-permeability porous media

The Darcy’s law specifies a linear relation between the Darcy velocity of a fluid flow and the pressure gradient that drives the flow. However, in low-permeability porous media, the velocity of a fluid flow exhibits a nonlinear dependence on the imposed pressure gradient, which is referred to as the non-Darcian flow behavior. Temperature has been postulated to affect the threshold pressure gradient of a non-Darcian flow; however, the supporting data is very limited due to the challenges in conducting flow experiments in low-permeability porous media. In this study we for the first time report a systematic measurement of the threshold pressure gradient under various permeabilities and temperatures. The results show that a higher temperature leads to a lower threshold pressure gradient under the same permeability and a faster reduction of the threshold pressure gradient with increasing permeability. The experimental data are fitted to a two-parameter model we previously developed to determine the parameters, h0 and a, which characterize the interfacial fluid-solid interactions and the transition between the Darcy and non-Darcian regimes. Both h0 and a decrease with an increasing temperature. The lower value of h0 under a higher temperature leads to the faster decline of threshold pressure gradient with increasing permeability. This is the first comprehensive experimental study that measured the threshold pressure gradient under multiple different permeabilities and temperatures, which generated the first laboratory dataset of this kind. The experimental data and the model constrained here allow for systematic predictions of both Darcy and non-Darcian flows for a general set of low-permeability porous media under various temperatures and pressure gradients, which have important applications to geological disposal of nuclear waste, hydrocarbon extraction from shale, and contaminant remediation in clay-rich formations.

A Review of Techniques for Measuring the Biot Coefficient and Other Effective Stress Parameters for Fluid-Saturated Rocks

Abstract Predicting the behavior of a saturated rock with variations in pore fluid pressure during geo-energy production and storage, deep geological disposal of nuclear wastes, etc. with skeletal mechanical behavior in the linear elastic range is carried out using the isothermal theory of poroelasticity that incorporates Biot's effective stress principle. For conditions that are not within linear elasticity, other effective stress coefficients are used. Several experimental methods for determining Biot's and other effective stress coefficients have been documented in the literature. The objective of this study is to review the fundamentals of these techniques, their advantages and disadvantages, and to include several case studies. Current techniques for Biot's coefficient are based on different premises: jacketed and unjacketed bulk moduli or compressibility values; volume changes of the bulk and pore fluid from a drained triaxial test on a saturated sample; isotropic-isochoric compression tests on a saturated sample; matching volumetric strains for dry and saturated samples; estimation of the Biot coefficient from other poroelastic parameters; and approximation of the jacketed bulk modulus from ultrasonic wave velocities and/or unjacketed bulk modulus from the mineralogical compositions. Other effective stress coefficients are based on matching failure envelopes for dry and saturated samples and variations of rock properties (such as volumetric strain, permeability, and ultrasonic wave velocities) with respect to confining stress and pore pressure. This article discusses variations in Biot's and other effective stress coefficients produced using the different techniques and how factors such as pore geometry, test conditions, stress path, and test temperature affect the coefficients.

Bentonite swelling into a void under suction or water flow

In the context of the deep geological disposal of nuclear waste and to improve the understanding of the homogenisation process of bentonite barriers, in particular with regard to the filling of technological voids, tests were performed with compacted FEBEX bentonite samples hydrated under limited axial swelling conditions. The samples were saturated from the top surface using the vapour transfer technique (imposing suctions of 6 and 0.5 MPa), or with deionised water injected at a low flow rate either from the bottom surface or from the gap on top. The final water content of the samples saturated via vapour transfer was related to the suction imposed during the tests according to the water retention curve. Thus, the final water content of the samples tested under suction 6 MPa was lower and not enough to allow sufficient bentonite swelling to close the gap. In contrast, the samples saturated under suction 0.5 MPa reached higher water contents and were able to fill the gap before the equilibrium water content had been reached. In the tests performed with liquid water supplied through the gap, the samples were able to swell easily into the open void and the gap closed earlier than when hydration took place from the opposite end. In all the tests, the final water content of the bentonite was higher and the dry density lower towards the hydration surface. These gradients were more remarkable and persistent as the initial strain was larger. No completely homogeneous density or water content distribution was observed in any of the tests. These changes were also reflected in the pore size distribution. The pores of size larger than the upper limit of mercury intrusion porosimetry (~ 550 µm) were also quantified, which allowed a better representation of the pore size distribution of the more swollen samples. Although the void ratio corresponding to pores smaller than 200 nm (em) was initially higher, over time the volume of macropores (eM) increased more, giving place to an overall decrease in the em/eM ratio, which tended to be constant along the samples in the longest tests. In the first stages of hydration, the macropore void ratio and size considerably increased close to the gap in the tests with saturation from it.

Sensitivity analysis of generic deep geologic repository with focus on spatial heterogeneity induced by stochastic fracture network generation

Geologic Disposal Safety Assessment Framework is a state-of-the-art simulation software toolkit for probabilistic post-closure performance assessment of systems for deep geologic disposal of nuclear waste developed by the United States Department of Energy. This paper presents a generic reference case and shows how it is being used to develop and demonstrate performance assessment methods within the Geologic Disposal Safety Assessment Framework that mitigate some of the challenges posed by high uncertainty and limited computational resources. Variance-based global sensitivity analysis is applied to assess the effects of spatial heterogeneity using graph-based summary measures for scalar and time-varying quantities of interest. Behavior of the system with respect to spatial heterogeneity is further investigated using ratios of water fluxes. This analysis shows that spatial heterogeneity is a dominant uncertainty in predictions of repository performance which can be identified in global sensitivity analysis using proxy variables derived from graph descriptions of discrete fracture networks. New quantities of interest defined using water fluxes proved useful for better understanding overall system behavior.