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A pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed as a candidate sealing material in a French deep geological repository for high-level radioactive waste. An in-depth understanding of the hydraulic property of this material is essential for accessing the over-pack corrosion and nuclide migration along the gallery. In this study, water flow in the compacted bentonite/claystone mixture was investigated by carrying out infiltration and water retention tests under constant-volume conditions alongside detailed microstructure observation. The evolution of total water hydraulic conductivity with suction was experimentally determined based on the instantaneous profile method. It was observed that the measured total hydraulic conductivity increased slowly at high suctions, then quickly at low suctions and finally approached the saturated value. A new method was developed based on the evolving pore structure, accounting for both vapour and liquid-water hydraulic conductivities. The total hydraulic conductivity predicted by the proposed method agreed well with the measured value, indicating the reliability of the predictive method. The vapour hydraulic conductivity was larger than the liquid-water one at a suction larger than 15·2 MPa, while liquid-water hydraulic conductivity became dominant at a suction lower than 15·2 MPa. In addition, based on the predicted vapour and liquid-water hydraulic conductivities, a numerical model was set up to simulate the water infiltration process. Comparison between the simulation and the measurement showed close agreement, validating the two-phase water flow mechanism in the bentonite/claystone mixture. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been considered as a promising sealing/backfilling material in deep geological repository for high-level radioactive waste in France. When the pre-compacted blocks are emplaced in the gallery, technological voids can remain. After the infiltration of groundwater from the host rock, they will swell freely, filling the technological voids, and then undergo further hydration under constant-volume conditions. From the perspective of the storage safety, it is essential to understand the effects of technological voids and hydration time on the hydro-mechanical behaviour of such pre-compacted blocks. In this work, a series of infiltration tests at various hydration times was carried out on compacted MX80 bentonite/COx claystone mixture with different technological voids. The evolutions of the swelling pressures in axial and radial directions as well as the hydraulic conductivity were monitored while wetting. After the predetermined hydration times, the dry density, water content and suction at different positions were determined, together with the microstructure investigation using mercury intrusion porosimetry. It was observed that the soils close to initial voids swelled and filled the initial voids upon contact with water, with significant increases in large-pore and medium-pore void ratios. From the variation of dry density profile with time, compression and swelling zones could be identified: in the compression zone, the soils corresponding to the initial soils were subjected to compression, with decrease in large-pore and medium-pore void ratios over time, whereas in the swelling zone, the soils with a higher dry density than the expected final one underwent further swelling, with large-pore void ratio increasing until saturation and then a slight decrease due to water redistribution in the soil. Owing to the soil density heterogeneity, the axial swelling pressure and hydraulic conductivity of samples with voids were slightly larger than those of samples without voids; contrarily, the radial swelling pressure was lower than that of the samples without voids. [ABSTRACT FROM AUTHOR]

Pellet-based expansive clay materials are considered as a sealing material for closing the galleries in radioactive waste disposal concepts. In repository conditions, the granular mixture progressively homogenises upon hydration by the host rock pore water. The present study focuses on the material behaviour before homogenisation. A grain-scale experimental characterisation is first performed in the laboratory. A model describing the hydromechanical behaviour of a pellet is proposed based on the experimental results. Then, suction-controlled swelling pressure tests are performed in the laboratory. Using Discrete Element Method (DEM) and the model proposed for a single pellet, the tests are successfully simulated. It is highlighted that (i) the swelling pressure evolves in two phases in the investigated suction range, controlled by the granular structure of the mixture; (ii) wall effects at the laboratory scale affects the material response; (iii) measurement variability associated to the sensor diameter is non-negligible; (iv) DEM is a valuable tool able to provide insight into the material behaviour. [ABSTRACT FROM AUTHOR]

The corrosion of metals contained in intermediate-level long-lived waste (ILW) under reduced chemical condition will lead to the production of hydrogen gas during the post-closure phase of a deep geological repository for radioactive waste. According to previous investigations by Talandier et al. (Proceedings of TOUGH symposium 2006, Berkeley, ), the period of concern covers several 1,000 years after closure of a repository in a clay host rock (Callovo-Oxfordian). The limited hydrogen transport efficiency of the host rock will lead to significant saturation of the concrete waste canister pore space and voids with a gas phase and pressure build-up within the emplacement drifts. On the other hand, the water availability is limited as a result of (i) the low permeability of the clay host rock and (ii) the desaturation of the rock mass close to the drift wall due to the ventilation of the drifts during the operational phase of the repository. In former numerical simulations it was assumed that under the reducing chemical conditions prevailing in the repository, the corrosion rate would be a function of the available metal surface and the temperature only. In this paper, simulation results based on new phenomenological functions are presented, which were implemented in TOUGH2. These allow taking into account (i) a water saturation dependency of the hydrogen generation rate, (ii) the water consumption due to the corrosion process, and (iii) the total metal mass available for corrosion. The paper presents results of 1D radial and 2D vertical simulations of a typical cross-section of a waste emplacement drift and the surrounding rock mass. The interactions between water availability from the low permeable clay, the dependency of the hydrogen generation rates from the water saturation of the waste package and the hydrogen migration in the host rock are demonstrated. [ABSTRACT FROM AUTHOR]

Within the framework of feasibility studies for a reversible, deep geological repository of high- and intermediate-level long-lived radioactive waste (HLW, IL-LLW), the French National Radioactive Waste Management Agency (Andra) is investigating the Callovo-Oxfordian (COx) claystone formation near Bure (northeast France) as a potential host formation for the repository. In 2000, Andra initiated construction of the Meuse Haute-Marne Underground Research Laboratory (MHM URL). Concurrently with on-site construction work, an extensive research program has been conducted, including theoretical and experimental studies and constitutive modelling. In-situ experiments provide an extensive data set for performance evaluation of models describing the thermo-hydro-mechanical behaviour of COx claystone under different conditions (excavation, heat, etc.). In 2012, a benchmark exercise was initiated to evaluate proposed models, including basic assumptions, mathematical descriptions, input variables and parameters, and to determine how these assumptions influence model outcomes with respect to in-situ experimental observations. The present paper outlines the main features of the hydromechanical behaviour of COx claystone and presents a series of laboratory test results (triaxial compression and creep tests) providing model development teams with a coherent database for model calibration and validation. [ABSTRACT FROM AUTHOR]

Summary The Excavation Damaged Zone (EDZ) surrounding a drift, and in particular its evolution, is being studied for the performance assessment of a radioactive waste underground repository. A specific experiment (called CDZ) was designed and implemented in the Meuse/Haute-Marne Underground Research Laboratory (URL) in France to investigate the EDZ. This experiment is dedicated to study the evolution of the EDZ hydrogeological properties (conductivity and specific storage) of the Callovo-Oxfordian claystone under mechanical compression and artificial hydration. Firstly, a loading cycle applied on a drift wall was performed to simulate the compression effect from bentonite swelling in a repository drift (bentonite is a clay material to be used to seal drifts and shafts for repository closure purpose). Gas tests (permeability tests with nitrogen and tracer tests with helium) were conducted during the first phase of the experiment. The results showed that the fracture network within the EDZ was initially interconnected and opened for gas flow (particularly along the drift) and then progressively closed with the increasing mechanical stress applied on the drift wall. Moreover, the evolution of the EDZ after unloading indicated a self-sealing process. Secondly, the remaining fracture network was resaturated to demonstrate the ability to self-seal of the COx claystone without mechanical loading by conducting from 11 to 15 repetitive hydraulic tests with monitoring of the hydraulic parameters. During this hydration process, the EDZ effective transmissivity dropped due to the swelling of the clay materials near the fracture network. The hydraulic conductivity evolution was relatively fast during the first few days. Low conductivities ranging at 10 –10 m/s were observed after four months. Conversely, the specific storage showed an erratic evolution during the first phase of hydration (up to 60 days). Some uncertainty remains on this parameter due to volumetric strain during the sealing of the fractures. The hydration was stopped after one year and cross-hole hydraulic tests were performed to determine more accurately the specific storage as well as the hydraulic conductivity at a meter-scale. All hydraulic conductivity values measured at the injection interval and at the observation intervals were all below 10 –10 m/s. Moreover, the preferential inter-connectivity along the drift disappeared. Specific storage values at the observation and injection intervals were similar. Furthermore they were in agreement with the value obtained at the injection interval within the second hydration phase (60 days after starting hydration). The graphical abstract synthesizes the evolution of the hydraulic/gas conductivity for 8 intervals since the beginning of the CDZ experiment. The conductivity limit of 10 –10 m/s corresponds to the lower bound hydraulic definition of the EDZ and it is demonstrated that EDZ can be sealed. This is a significant result in the demonstration of the long-term safety of a repository. [ABSTRACT FROM AUTHOR]

Compacted block composed of 70% crushed Callovo-Oxfordian claystone and 30% MX80 bentonite has been proposed as a possible material for sealing a potential repository of radioactive waste. In this study, the compaction property of the bentonite/claystone mixtures with various bentonite fractions and water contents were first investigated to define an appropriate protocol for the fabrication of blocks. Results show that the increase of bentonite fraction decreased the overall compressibility and the achievable dry density at a given stress. As the water content increased, the compressibility at a relatively low vertical stress increased due to the increasing deformability of aggregates and the decreasing inter-aggregate friction; contrarily, at a high vertical stress, a lower compressibility was observed for the samples with a higher water content because a larger stress was required to squeeze water within aggregates. During demoulding, the required demoulding pressure increased with the increase of bentonite fraction and decreased with the increasing water content. Then, to assess the suitability of the material, the swelling pressures and hydraulic conductivities of the compacted blocks were determined. It was found that the swelling pressure increased and the hydraulic conductivity decreased with the increases of bentonite fraction and dry density. By contrast, as the water content increased, more water was intercalated in the interlayer spaces and the inter-aggregate pore volume decreased, resulting in reductions of the swelling pressure and hydraulic conductivity. • Compressibility, rebound property, demoulding pressure, swelling pressure, and hydraulic conductivity were investigated; • Effects of bentonite fraction, dry density and water content were identified; • The effect of water content on the compressibility depended on vertical stress; • The increase of water content reduced the swelling pressure and the hydraulic conductivity. [ABSTRACT FROM AUTHOR]

During the excavation of drifts within the French Underground Research Laboratory (URL) in the Callovo-Oxfordian claystone (COx) formation, an excavation damaged zone (EDZ) was formed, which is expected to have adverse effects on the long-term safety of the radioactive waste repository. In this study, high pressure oedometer tests with controlled suction were performed to investigate the hydro-mechanical behaviour of damaged COx claystone. The hydraulic conductivity was measured under different vertical stresses. The suction and damage effects were analysed in terms of swelling, compression, hydraulic conductivity and creep. Besides, the evolution of micro-cracks inside the damaged claystone sample was also analysed through X-ray computed tomography (CT) and Mercury intrusion porosimetry (MIP). The oedometer results indicate that the damaged claystone has a lower swelling strain than the intact one, due to the effect of macro-pores created by damage. The damaged claystone with a higher suction exhibits a lower compressibility and larger yield stress. There is a linear correlation between hydraulic conductivity and vertical stress at zero suction. Compared with the intact claystone, the damaged one presents a lower compression index C c. Besides, step compression index C c * and secondary consolidation coefficient C α both increase with the increase of vertical stress and with the decrease of suction, indicating the dependence of the compression and creep behaviours on stress and suction. A satisfactory linear relationship is observed between C α and C c * at each suction and the corresponding slope depends on suction. Microstructure observations from CT and MIP tests show that the micro-cracks inside the damaged sample after loading are more significant with the increase of suction, suggesting that the micro-cracks were closed more easily by loading at a lower suction. A damaged coefficient was identified to quantitatively analyse the evolution of micro-cracks. • The swollen clay minerals filled the preexisting micro-cracks, leading to a decrease of the swelling potential. • The compression and creep behaviours depend on stress and suction. • A key stress 16 MPa is related to the closure of micro-cracks and grain breakage. • The micro-cracks became more significant with the increase of suction after oedometer compression. [ABSTRACT FROM AUTHOR]

Bentonite-based materials are employed in some nuclear waste disposal concept designs to seal underground tunnels and shafts. In some cases, these barriers consist of two parts: highly compacted blocks and granular buffer material consisting of pellets. These two components have highly distinct initial properties in terms of dry density, water content, and microstructure, but, at full water saturation, those tend to homogenise. However, the simultaneous application of pellets and blocks in the same barrier section creates challenges for understanding and modelling system performance that must be tackled. Therefore, this work merges experimental and computational approaches to better understand the hydro-mechanical processes that take place throughout the interaction of the various assemblies with one another and with the environment during hydration. Hence, the role of a variety of material configurations, considering simultaneously compacted blocks and pellets, and hydration boundary conditions is analysed giving great insight about intermediate saturation states. Discrepancies between experimental measurements and model results were explained by the effect of friction after a sensitivity analysis performed with the finite element code LAGAMINE. Results from experiments and calculations were in good agreement and offer supplementary knowledge about a relevant amount of the numerous phenomena (for instance related to dry density evolution and water distribution inside the sample) taking place during initial heterogeneous bentonite samples resaturation in isochoric conditions. • Employment of barcelona basic model for very different densities and pore structures materials simultaneously. • Friction development numerical study during experimental tests and influence on pressure measurements during saturation tests. • In-depth description of homogeneisation process during bentonite hydration in isochoric conditions. [ABSTRACT FROM AUTHOR]

An experimental investigation on the hydromechanical behaviour of pellet-powder mixtures, candidate materials for sealing the galleries in radioactive waste disposal concepts, was described. Imbibition experiments were performed using a specially-designed column, with square cross-section and glass sides allowing pictures to be taken by a camera. Relative humidity and swelling pressure were measured at different elevations of the column. Three mixtures with an identical pellet volume fraction and various powder volume fractions were used to study the influence of heterogeneities of density on the hydromechanical behaviour of the mixture under repository conditions. Results demonstrated that hydration of the material was dominated by water vapour flux, with pellets and inter-pellet pores not at hydraulic equilibrium. In mixtures with low powder content, the evolution of swelling pressure was controlled by contact forces between pellets. The material progressively lost the initial granular structure upon hydration. Note that hydration was made by imbibition, instead of injecting liquid water at high pressure, to avoid unrealistic liquid water flow rates in the initial large inter-pellet voids. In addition, sensors four times larger than the pellets were used to measure a representative value of swelling pressure. • Experiments on the hydromechanical behaviour of pellet-powder mixtures were described. • Material's dry density did not influence the water flow entering the samples. • Measured pressure was controlled by inter-pellet forces for low dry density material. • Imbibition changed the material structure progressively from granular to continuous. • Pellets and macropores were not at hydraulic equilibrium during imbibition. [ABSTRACT FROM AUTHOR]

Abstract: Deep geological barriers, such as Callovo-Oxfordian argillites, display suitable characteristics for a high level radioactive waste repository. Their low permeability is a key parameter to retain radionuclides locally. However, once the system is closed, gases produced by corrosion of the waste canisters can represent a risk to the installation safety. Gas pressure will build up with time, partially and locally desaturating Callovo-Oxfordian argillites. The pressure rise may fracture the argillites locally if the gas released through the host barrier is not evacuated fast enough. The present work focuses on porous-network identification with regard to gas transfer process. Mercury intrusion experiments were performed to evaluate how argillite reacts to gas intrusion. A sharp threshold is observed for pore diameters of 20 nm. Measured mercury porosity is 14% and is mainly made of mesopores (diameter<100 nm). These experiments were complemented by water adsorption isotherms. Due to the evapo-condensation phenomenon, the results were different from the mercury intrusion curves. The method reveals that a larger trapped porosity exists (diameters between 20 nm and 1 μm). Pores of 20 nm diameter represent the capillary barrier to gas breakthrough. A network model, based on percolation, namely XDQ was built based on the connectivity and pore size distribution obtained previously. It predicts gas permeability of 10−18 m2 at atmospheric pressure. The Klinkenberg effect greatly increases the gas permeability compared to water one. Gas permeability measurements confirm XDQ results. According to the XDQ model, permeability depends mainly on the percolation threshold and saturation. The model shows that partially desaturated network have no connected pathways for gas migration. In any case, pore connectivity is the key factor in gas transfer issues. [Copyright &y& Elsevier]

The Callovo-Oxfordian (COx) claystone has been considered as a possible host rock for deep geological repository of high-level radioactive waste in France. When constructing the repository, it is unavoidable to generate a fractured zone in the vicinity of excavated repository walls. In the assessment of the repository performance, it is important to investigate the role of the fractures on the swelling capacity of COx claystone and to further characterize the fractured zone evolution. In this study, the effect of fracture voids on the swelling behaviour of COx claystone was investigated by simulating the fracture voids with a space created by a limited amount of grains sandwiched between claystone disks, and by performing a series of swelling tests in oedometer under a low vertical stress (0.05 MPa) and swelling pressure tests under constant-volume condition. Results showed that, with water infiltrating into the sample with fracture voids, the grains collapsed before the swelling of claystone disks. The whole sample was composed of swelling zones corresponding to the claystone disks with higher dry density than the expected final value, and of compression zones corresponding to the initial fractures with lower dry density. Moreover, the axial strain and swelling pressure decreased as the fracture void increased. The maximum fracture voids that might be fully filled by the swelling of COx claystone (sealing) were found to be 5.14%–7.65% without considering the long-term creep contribution in the field condition. • Fractures were simulated with soil grains of defined sizes. • Water infiltration led to collapse of soil grains first prior to claystone swelling. • The swelling zones corresponded to claystone disks. • The compression zones corresponded to initial fractures. • A maximum fracture voids that might be sealed was 5.14%–7.65%. [ABSTRACT FROM AUTHOR]

Bentonite pellet-powder mixtures are candidate materials for sealing the galleries in deep geological repositories for radioactive waste. In the present work, swelling pressure tests are performed on pellet-powder mixtures with different powder contents. Results highlight the influence of the initial granular structure on the mechanical behaviour of pellet-powder mixtures. In mixtures with a low density powder in the inter-pellet porosity, the macroscopic response of pellet-powder mixtures is identical to that of a pellet assembly with no powder. A model is proposed to describe the hydromechanical behaviour of pellet-powder mixtures. The formulation considers two distinct states of the material, Granular and Continuous. In the Granular state, the pellets control the mechanical behaviour of the mixture. In the Continuous state, both pellets and powder contribute to the mechanical behaviour of the mixture. In the Granular domain, the material behaviour is described by constitutive laws proposed after a numerical study using Discrete Element Method. In the Continuous domain, the material behaviour is described by a modified Barcelona Basic Model. Transition between the two domains depends on the density of the powder phase and suction. The model is implemented in a Finite Element Method code, and the swelling pressure tests performed in the laboratory are simulated with a single set of parameters. These results improve the knowledge on the behaviour of bentonite pellet-powder mixtures during hydration in repository conditions. • Swelling pressure tests have been carried out on three different bentonite pellet-powder mixtures. • The initial granular structure controls the evolution of swelling pressure if the density of the powder phase is low. • The behaviour of pellet assemblies has been studied under isotropic compression through DEM simulations. • Based on experimental and DEM results, a conceptual model for pellet-powder mixture has been proposed. • The numerical results generally reproduced very well the experimental results, using a single set of parameters. [ABSTRACT FROM AUTHOR]

Bentonite is considered for engineered barriers in radioactive waste geological repositories due to its low permeability, swelling pressure and radionuclide retention properties. The French concept favors the use of bentonite under granular form - pellets and crushed pellets mix - for an easier building of plugs and seals relatively to bricks. However, this introduces a significant degree of macroscopic initial heterogeneity (at several scales) that is expected to disappear via mass redistribution during resaturation in order to obtain homogeneous hydraulic and mechanical performance. This also may have practical effects on the reliability of results from tests with small volumes of bentonite compatible with the time scales manageable at the laboratory. In this study, isochoric swelling pressure tests at various densities and sample sizes were carried out and evidenced bias mainly due to the size of the cell compared to the size of the pellets, although the average density is equivalent, pointing towards incomplete homogenization. Large-scale swelling pressure tests, however, converged towards values previously measured for more finely grained materials. Local pressure measurements also evidenced some stress heterogeneities that do not seem to disappear at medium time-scales. These stress heterogeneities could be correlated to local density variations of the final state assessed using destructive measurements and X-ray computed tomography. • A number of variable size swelling tests on granular MX80 bentonite are presented. • Stress and dry density heterogeneities of the final state are demonstrated. • First results of a metric-scale resaturation experiment are presented. [ABSTRACT FROM AUTHOR]

MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed as a sealing/backfilling material in a deep geological repository of radioactive waste in France. A good understanding of the swelling behaviour of this mixture is essential when evaluating the long-term performance of the repository. In this work, the swelling pressure of MX80 bentonite/COx claystone mixture was investigated by the constant-volume method for a full range of bentonite fraction. Results show that the swelling of claystone in the mixture can be inhibited by bentonite and its contribution to the global swelling pressure depends on the bentonite fraction. For the mixture with >70% bentonite, claystone behaves as an inert material, and its contribution to the global swelling pressure can be ignored. However, for the mixture with less bentonite, the swelling of claystone will significantly contribute to the global swelling pressure. A method was proposed allowing the swelling pressure of bentonite/claystone mixture to be predicted in the full range of bentonite fraction. • Claystone's contribution to swelling pressure depends on the bentonite fraction. • The effect of bentonite on claystone's swelling can be described by an inhibition factor. • A method is proposed to predict the swelling pressure of bentonite/claystone mixture. [ABSTRACT FROM AUTHOR]