Auswahl der wissenschaftlichen Literatur zum Thema „Bentonite pellet/powder mixture“

Bentonite pellet-powder mixtures are candidate sealing materials in radioactive waste disposal concepts. The mixture is installed in galleries in dry state as a granular material. The material is progressively hydrated by the pore water of the host rock and becomes homogeneous. Before homogenisation, the granular structure controls the material behaviour. In the present work, a modelling approach able to address particular features of pellet-powder mixtures is introduced. Two domains are considered: i) granular, and ii) homogeneous. The material behaviour before homogenisation is studied through Discrete Element Method (DEM) simulations. Constitutive laws for the granular state are proposed from DEM results. The behaviour of the homogenised material is described by a modified Barcelona Basic Model (BBM). Transition from granular to homogeneous states depends on suction and relative volume fractions of pellets and powder. Swelling pressure tests performed in the laboratory are satisfactorily simulated using this approach.

MX80 bentonite pellet/powder mixture has been regarded as a candidate sealing material in deep geological high-level radioactive waste disposal. As single pellet plays an important role in the global hydromechanical behaviour of the mixture, a series of single pellet compression tests were conducted on cylindershape MX80 bentonite 32-mm pellet after reaching suction equilibrium. The changes in volume and water content during the suction equilibration with the vapour equilibrium technique allowed the water-retention properties to be investigated. X-ray μCT observations were also made on pellets after reaching the target suctions. Results showed that upon drying the water content, void ratio and volumetric strain decreased slightly, but increased significantly upon wetting. Moreover, upon drying the network of cracks presented insignificant change and the cracks were mainly located in the marginal areas of pellet, while on wetting, cracks substantially propagated from the marginal areas to the core of pellet. The uniaxial compression tests on single pellets showed that the axial strain εaxial contributed to the contractancy, while the lateral strain εlateral to the dilatancy. The uniaxial shear strength σa, max, the crack closure (CC) stress σcc, the crack initiation (CI) stress σci, the crack damage (CD) stress σcd and the Young’s modulus E were found to decrease with suction decreasing, evidencing the suction effect through softening and cracking. Because of the combined effect of suction and wetting-induced cracks, the Poisson’s ratios v increased until the suction decreased to 38-25 MPa, and then decreased with further suction decrease. Uniaxial loading closed the horizontal wetting-induced cracks at suctions ≤ 9 MPa, leading to a drop of εlateral before CC.

Bentonite mixtures of MX-80 (80% of high-density pellets and 20% of bentonite powder on a mass basis) have been recently proposed as a candidate material for sealing deep geological disposals of high-level radioactive waste. A loading/unloading oedometer test at constant water content has been performed on this mixture, which has been modelled using the finite element Code_Bright. The constitutive model used to represent the mechanical response is the Barcelona Expansive Model (BExM), since a multi-modal pore size distribution characterises the pore network of the mixture. During compression at constant water content, an increase in the degree of saturation and a consequent reduction of suction is induced. Consequently, two competing effects occur at different pore-size scales: (a) compression due to mean net stress increase; and (b) expansion on induced suction reduction that mainly affects the micro-porosity level inside aggregates. A sensitivity analysis has been performed to explore these effects, in which the elastic compressibility parameter at the micro-porosity scale for changes in mean effective stress plays an important role.

To better understand results of SEALEX in situ tests carried out at Tournemire Underground Research Laboratory, the hydromechanical behavior of a pellet/powder MX80 bentonite mixtures prepared at a dry density of 1.49 Mg/m3 were investigated by means of microfocus X-ray computed tomography (μ-CT) observations and laboratory small scale infiltration tests. Radial and axial swelling pressures as well as relative humidity were monitored while wetting. Two configurations were considered: for the first, a pellet/powder mixture was prepared following a specific protocol to minimize initial structural heterogeneity; the second one was specially designed to study a strong heterogeneous mixture distribution. μ-CT observations performed on the two samples during hydration revealed an apparently homogeneous sample for the first mixture after 100 days of hydration. For the second specimen, several voids were still observed after 40 days of hydration. A comparison was made between the in situ and mock-up tests. It was observed that the evolutions of radial and axial swelling pressures depend on the initial heterogeneous distribution of the mixture. This heterogeneity is due to the different dry density values at the vicinity of the different sensors. The final values of axial swelling pressures were different for both configurations for the same global dry density.

Binary mixtures of high-density MX-80 bentonite pellets (80%) and bentonite powder (20%) at hygroscopic water content have been recently considered as an alternative engineered barrier system for the long-term disposal of radioactive wastes. These mixtures display a dry density of around 1.49 Mg/m3 on pouring and present components with multi-modal pore size distributions that significantly evolve during the hydro-mechanical paths. To better understand the hydro-mechanical behaviour of this multiple porosity mixture, the contribution initially focuses on describing the initial state, as well as the main microstructural features determined by mercury intrusion porosimetry tests for the mix and each of the components (pellets and powder). Afterwards, hydro-mechanical results of both components and the mixture are presented. The hydraulic results focus on the water retention and water permeability, while the mechanical tests concentrate on the compressibility properties on loading.

Les matériaux à base de bentonite sont envisagés pour la réalisation des ouvrages de fermeture dans les concepts de stockage de déchets radioactifs. Dans ce contexte, les matériaux mis en place sous forme de mélanges de granules (pellets) de bentonite sont étudiés car ils présentent moins de contraintes d’installation comparés aux blocs préfabriqués. Les pellets sont disposés dans les galeries à l’état sec et forment un matériau granulaire. Le matériau est humidifié par l’eau issue de la roche encaissante et acquiert une texture homogène. Avant homogénéisation, le caractère granulaire du matériau contrôle son comportement.Ce travail se concentre sur la caractérisation expérimentale et la modélisation numérique d’un mélange de pellets et pellets concassés (poudre) de bentonite en proportion 70/30 en masse sèche, envisagé pour réaliser les ouvrages de fermeture dans le concept français de stockage de déchets radioactifs. La proposition, l’implémentation et la validation d’un modèle de comportement tenant compte de la nature granulaire initiale et d’éventuelles hétérogénéités locales de densité est l’objectif principal de ce travail.L’influence de la nature granulaire est mise en évidence par l’intermédiaire d’essais de pression de gonflement à succion contrôlée au laboratoire. Les essais sont réalisés sur des échantillons préparés à différentes teneurs en poudre. A partir d’une caractérisation expérimentale à l’échelle du pellet, un modèle simple décrivant le comportement hydromécanique du pellet est proposé et implémenté dans un code de calcul utilisant la méthode des Éléments Discrets (DEM). Les essais réalisés au laboratoire sur les échantillons sans poudre sont simulés par la DEM et le modèle proposé. Les résultats expérimentaux sont reproduits de manière satisfaisante. Cette méthode est utilisée pour simuler de larges assemblages granulaires de densités variées soumis à des sollicitations hydromécaniques. Les paramètres influençant le comportement macroscopique des assemblages de pellets à l’état « granulaire » sont identifiés à partir de ces simulations. Un ensemble de lois de comportement sont proposées pour modéliser le matériau comme un milieu continu.La transition de l'état granulaire à l’état homogène est décrite par des critères faisant intervenir la succion et les fractions volumiques relatives des pellets et de la poudre. Une version modifiée du Barcelona Basic Model est proposée pour modéliser le comportement du matériau à l’état « homogène ». Le modèle est implémenté dans le code de calcul par Éléments Finis (FEM) BIL. Les essais de pression de gonflement réalisés au laboratoire sur des échantillons contenant différentes teneurs en poudre sont reproduits de manière satisfaisante, sur l’ensemble du chemin d’humidification, en utilisant un unique jeu de paramètres.Le comportement du matériau au cours de l’humidification en conditions de volume constant est finalement étudié à plus grande échelle par la réalisation d’essais d’imbibition en modèle réduit au laboratoire, à différentes teneurs en poudre. Les cellules sont de section carrée avec une face vitrée. Une caméra permet l’observation de l’évolution de la texture du matériau au cours de l’humidification. La prédominance des transferts en phase vapeur dans le processus de saturation du matériau, l’influence et l’évolution de la structure granulaire au cours de l’humidification et l’influence de la teneur en poudre sur la réponse macroscopique du mélange sont mis en évidence. Des lois de transferts hydriques sont proposées à partir des résultats et observations expérimentales et implémentées dans le code BIL. La perspective principale de ce travail est la réalisation de simulations à plus grande échelle en utilisant le modèle développé. Ainsi, des problématiques inhérentes à l’utilisation des mélanges de pellets et poudre, notamment la nature granulaire initiale et les hétérogénéités locales de densité, pourront être prises en compte
Bentonite based materials are considered as a sealing material in radioactive waste disposal concepts because of their low permeability, radionuclide retention capacity and ability to swell upon hydration, thus filling technological gaps. Within this context, bentonite pellet mixtures have been studied owing to operational convenience. Pellets are laid in the galleries in a dry state as a granular assembly. The mixture progressively becomes more homogeneous upon hydration by the pore water of the host rock. Before homogenisation, the granular structure of the material controls the macroscopic behaviour of the mixture.The present work focuses on the experimental characterisation and numerical modelling of a mixture of bentonite pellet and crushed pellet (powder), in proportion 70/30 in dry mass, a candidate sealing material in the French concept of radioactive waste disposal. The proposition, implementation and validation of a new modelling framework, considering features such as the initial granular structure of the material or local heterogeneities of densities, is the main objective of this PhD work.The influence of the initial granular structure is evidenced by performing suction-controlled swelling pressure tests in the laboratory, using samples of various powder contents. From grain-level experimental characterisation, a simple model describing the hydromechanical behaviour of a pellet is proposed and implemented in a Discrete Element Method (DEM) code. Using DEM and the proposed model, aforementioned swelling pressure tests performed on samples containing no powder are satisfactorily simulated. The same method is used to model large granular assemblies of various pellet densities upon hydromechanical loadings. Relevant parameters involved in the macroscopic behaviour of pellet mixtures in “granular” state are identified from simulation results and constitutive laws are proposed to describe the hydromechanical behaviour of these materials using a continuum mechanics approach.The transition from “granular” state to “homogenised” state is described by criteria proposed from experimental results and data available in the literature and involves suction and relative volume fractions of pellet and powder. A modified version of the Barcelona Basic Model is proposed to model the material behaviour in “homogenised” state. The proposed model is implemented in the Finite Element Method (FEM) code BIL. Using a single set of parameters, all swelling pressure tests performed in the laboratory are satisfactorily reproduced in FEM simulations along the entire hydration path.The material behaviour upon hydration in constant volume condition is finally studied at a larger scale by performing mock-up imbibition tests, using various powder contents. Cells have a square section; a glass side and a camera allow the texture to be observed during hydration. The dominance of vapour transfers in the saturation process of the material, the influence and evolution of the granular structure upon hydration, and the influence of the powder content on the macroscopic response are notably identified. Transfer laws are proposed to describe the observed material behaviour in the mock-up tests and implemented in BIL.The realisation of larger scale coupled simulations using the proposed hydromechanical model is a perspective arising from this PhD work. Predictive simulations could be performed at the structure scale, considering relevant features such as the initial granular structure and local heterogeneities of density in the sealing plugs

Cette thèse porte sur le comportement hydromécanique d’un mélange de poudre et pellets de bentonite MX80 avec une proportion 80/20 en masse sèche. Il s’agit d’un matériau étudié par l’Institut de Radioprotection et de Sûreté Nucléaire (IRSN) dans le cadre du projet SEALEX qui a pour objectif principal la vérification de l’efficacité des dispositifs de scellement ou des barrières ouvragées dans le system du stockage géologique des déchets radioactifs. Le comportement hydromécanique du matériau à différentes échelles a été étudié par différents essais en laboratoire. Premièrement, les changements à l’échelle microstructurale d’un seul pellet de bentonite durant l’hydratation a été abordée à l’aide de deux techniques : la porosimétrie au mercure et la tomographie aux rayons-X. Les résultats ont montré que le gonflement d’un pellet peut être expliqué par deux mécanismes : la création des fissures surtout à des succions entre 38 et 9 MPa, et le gonflement des grains de bentonite, correspondant à l’hydratation des smectites à l’échelle nano. A des succions inférieures à 9 MPa, une diminution de l’épaisseur des feuillets d’argile et une augmentation du désordre des ceux-ci sont observées. Des essais de rétention d’eau, de pression de gonflement et de compression à l’odomètre à succion contrôlée ont été effectués sur le mélange de poudre et pellets. Les propriétés de rétention d’eau sous conditions de volume constant et pour un seul pellet sous conditions de gonflement libre apparaissent similaires pour des valeurs de succion supérieures à 4 MPa. Cela implique que la succion physico-chimique est prédominante devant la succion capillaire. Pour des valeurs de succions plus basses, une capacité de rétention plus faible a été observée sous conditions de volume constant, à relier à la disparition des macro-pores par le gonflement des grains de bentonite. Des valeurs de pression de préconsolidation plus petites que celles des mélanges de bentonite pure ont été obtenues pour des succions non-nulles, montrant l’effet granulaire des pellets dans le mélange. Deux colonnes d’infiltration ont été réalisés afin d’étudier deux cas extrêmes avec une densité sèche globale identique (1.49 Mg/m3). Avec la première colonne, un mélange de poudre et pellets relativement homogène, fabriqué en suivant un protocole spécial a été étudié. En revanche, un mélange fortement hétérogène a été fabriqué dans la deuxième colonne d’infiltration. Les résultats montrent que la pression de gonflement radiale dépend fortement de la distribution des pellets et de la poudre ainsi que de l’évolution du front d’hydratation. Une anisotropie de gonflement a été observée dans les deux cas, avec la pression de gonflement axiale inférieure à celle radiale. De plus, la valeur finale de pression de gonflement axiale est différente pour les deux colonnes, bien que les deux échantillons aient fabriquées avec la même densité sèche globale. En parallèle, plusieurs observations à la tomographie aux rayons-X ont été réalisées sur le mélange de pellets et poudre pendant l’hydratation. Un mélange complètement homogène a été observé après 100 jours d’hydratation à l’échelle étudiée (50 μm/voxel). Un nouveau modèle d’endommagement qui prend en compte des fissures observées au sein du pellet pendant hydratation a été développé en adaptant le Barcelona Expansive Model (BExM). L’essai d’infiltration sur l’échantillon relativement homogène a été simulé avec succès en utilisant le modèle développé. L’hétérogénéité initiale de la porosité a été aussi considérée dans la simulation afin de reproduire l’anisotropie de gonflement. Les résultats expérimentaux obtenus dans le cadre de cette étude permettent de mieux comprendre la réponse des ouvrages de scellement avec le mélange de pellets et poudre de bentonite dans le projet SEALEX. De plus, le modèle développé, qui prend en compte des fissures observées au sein du pellet et l’hétérogénéité initiale du matériau, permettra d’améliorer
The present investigation deals with the hydro-mechanical behavior of a mixture composed of pellets and powder of MX80 bentonite with a proportion of 80/20 in dry mass. This is one of the studied materials by the French Institute for Radiation protection and Nuclear Safety (IRSN) within the SEALEX project, which aims at investigating the long-term performance of swelling clay-based sealing systems in the context of geological high-level radioactive waste disposal. This study has been conducted by following an experimental program covering different scales. Firstly, the microstructure changes while wetting of a single pellet was investigated by combining MIP results with μ-CT observations. Results revealed that swelling of a pellet is due to the development of cracks, with significant development between 38 and 9 MPa of suction, combined to swelling of bentonite grains, which is governed by the hydration mechanisms of smectite at nano-scale. The application of suctions below 9 MPa leads to a significant decrease of the platelet thickness and to an increase in the disorder of the platelet assembly. Water retention tests, swelling pressure tests and suction controlled oedometer tests on the pellet/powder mixture were performed. Similar water retention properties were observed for the mixture under constant-volume condition and pellet under free swelling condition under suctions higher than 4 MPa, suggesting that physico-chemical suction prevails on capillary suction. At lower suctions, constant-volume condition defined a lower water retention capacity because of the disappearance of macro-pores. Lower yield stress values than the common pure bentonite mixtures were found for the pellet/powder mixture for non-zero suctions, showing that the volume change behavior is governed by the rearrangement and crushing of pellets, and the loss of the granular structure in the case of zero suction. Two mock-up tests were performed, aiming at studying two extreme cases at a global dry density of 1.49 Mg/m3: a homogeneous pellet/powder mixture fabricated by following a special protocol, and a strong heterogeneous sample. Results revealed that the radial swelling pressure depends strongly on the local pellet/powder distribution combined with the evolution of the hydration front. An anisotropy swelling was found in both cases, being the axial swelling pressure lower than the radial one. Moreover, different values of axial pressure were found between the two tests, even though they have the same global dry density of samples. In parallel, μ-CT observations were carried out on the mixture while wetting, revealing a homogeneous sealed sample after 100 days of hydration. No density gradients were identified at the investigated resolution (50 μm/voxel) after this long time of hydration. A new damage model, which takes into account the development of fissures within a pellet while wetting, was proposed an included to the well-IVknown double porosity Barcelona Expansive Model (BExM) to carry out numerical simulations of one mock-up test. The initial heterogeneous porosity distribution was also considered to reproduce the anisotropy swelling. The experimental results obtained in this study will greatly help well understand the response of seals made up of pellets/powder bentonite mixture in the SEALEX in situ experiment. Moreover, the constitutive model developed taking into account the pellet cracking damage and the initial sample heterogeneity allows significantly improving the prediction of hydomechanical behavior of seals/plugs made up of this mixture, constituting thus an useful tool for the safety assessment of the nuclear waste disposal system

Basic oxygen furnace slag (BOF slag) contains high amount of useful oxides such as CaO, MgO, SiO2, etc., therefore, a solution of the slag recycling would be used as a flux in pelletizing process. To understand role of slag in the pellet, this research examines the mechanical, chemical and microstructure properties of the pellets containing various amount of the slag. In the experimental process, the green pellets were made from a mixture of bentonite, iron ore and the slag. The green pellets were sintered in the resistance heating furnace maintaining temperature at 1200 °C for 120 minutes. Then the fired pellet was tested strength and observed microstructure with variation of the slag amount in the pellets. The result shown that cold compression strength of the pellets containing from 1.06 to 18.45 mass% slag was higher values than 200 kg/p, hence, the pellet was able to charge the pellets into the blast furnace. The highest strength was 342 kg/p with 5.08 mass% slag in the pellet. Besides, optical observed microstructure of the pellet indicated the phases of magnetite, hematite and silicate. In addition, the obtained results shown that there was a deterioration of porosity with increasing the slag amount. Accordingly, the porosity got a slight decrease when the slag amount increased from 1.06 to 5.08 mass% in the pellet. Once the amount of the slag was higher than 5.08 mass%, the porosity significantly reduced with increasing the slag amount; the porosity decreased from 23 to 12 %.

Abstract The industrial flame spraying process has been analyzed by three-dimensional Computational Fluid Dynamics (CFD) simulation. The actual process is employed at the Volvo Aero Corporation for coating of fan and compressor housings. It involves the Metco 6P gun where the fuel, a mixture of acetylene and oxygen, flows through a ring of 16 orifices, while the coating material, a powder of nickel-covered bentonite, is sprayed through the flame with a stream of argon as a carrier gas by a central orifice. The gas flow was simulated as a multi-component chemically reacting incompressible flow. The standard, two equations, k-e turbulence model was employed for the turbulent flow field. The reaction rates appeared as source terms in the species transport equations. They were computed from the contributions of the Arrhenius rate expressions and the Magnussen and Hjertager eddy dissipation model. The particles were modeled using a Lagrangian particle spray model. In spite of the complexity of the system, the complex geometry and the numerous chemical reactions, the simulations produced fairly good agreement with experimental measurements. The powder size distribution was found to play a critical role in the amount of unmelted fraction of particles. The modeling approach seems to give a realistic description of the physical phenomena involved in flame spraying, albeit some model refinement is needed.