Search

International audience

[Abstract:] Data science (digitalisation and artificial intelligence) became more than an important facilitator for many domains in fundamental and applied sciences as well as industry and is disrupting the way of research already to a large extent. Originally, data sciences were viewed to be well-suited, especially, for data-intensive applications such as image processing, pattern recognition, etc. In the recent past, particularly, data-driven and physics-inspired machine learning methods have been developed to an extent that they accelerate numerical simulations and became directly usable for applications related to the nuclear waste management cycle. In addition to process-based approaches for creating surrogate models, other disciplines such as virtual reality methods and high-performance computing are leveraging the potential of data sciences more and more. The present challenge is utilising the best models, input data and monitoring information to integrate multi-chemical-physical, coupled processes, multi-scale and probabilistic simulations in Digital Twins (DTw) able to mirror or predict the performance of its corresponding physical twins. Therefore, the main target of the Topical Collection is exploring how the development of DTw can benefit the development of safe, efficient solutions for the pre-disposal and disposal of radioactive waste. A particular challenge for DTw in radioactive waste management is the combination of concepts from geological modelling and underground construction which will be addressed by linking structural and multi-physics/chemistry process models to building or tunnel information models. As for technical systems, engineered structures a variety of DTw approaches already exist, the development of DTw concepts for geological systems poses a particular challenge when taking the complexities (structures and processes) and uncertainties at extremely varying time and spatial scales of subsurface environments into account. This work has been financed within the framework of EURAD, the European Joint Programme on Radioactive Waste Management (Grant Agreement No 847593) and PREDIS (Pre-disposal management of radioactive waste, Euratom research and training programme, grant agreement No 945098). The contribution of Javier Samper (UDC) was partly funded by Project PID2019-109544RB-I00). These supports are gratefully acknowledged. Open Access funding enabled and organized by Projekt DEAL.

International audience; The geochemical distribution of U and associated major and trace elements (As, Li, Pb, Sr, Zn) was studied at the former Rophin U mine (Puy-de-Dôme, France). Three zones of contrasting radiological settings were identified and sampled: a background area (1000 nSv·h−1). Sediment and water samples were collected both in a streambed located in the background area and in the streambed downslope the former mining area, and a soil sample was collected in the wetland downslope. Using both sequential and selective chemical extractions, and quantifying also the chemical reservoirs of As, Li, Pb, Sr, and Zn, it could be concluded that, in the streams, U was mainly bounded to primary ore minerals (phosphates) that were transported through particulate transport. It was also bound, to a lower extent, to clays, Mn oxyhydroxides and organic matter, certainly due to the sorption of aqueous U originating from partial dissolution or leaching of primary ore materials. Ore minerals remaining stable in the stream sediments were certainly included in a quartz matrix and hence were not in accessible for dissolution. In the wetland soil, selective extractions evidenced that U was about evenly distributed between humid/fulvic acids and organic matter unaffected by NH4OH 1M.

International audience

Euratom Research and Training in 2022: challenges, achievements and future perspectives [Abstract:] 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. Euratom research and training program 2014–2018; 745942 Ministerio de Ciencia e Innovación; PID2019-109544RB-I00

A HPx model that jointly considered the flow, transport, gas diffusion and reactivity processes induced by exposure to the atmosphere of a recently excavated shale was used for the interpretation of a weathering experiment in a lysimeter. The lysimeter was filled with a mechanically disaggregated shale presenting preferential pathways for water and a hydraulic conductivity at saturation of 100 cm/day. The water content and the seasonal saturation and desaturation cycles were identified as the main driving mechanisms of shale alteration. The water content determined the diffusion of gaseous oxygen in the shale’s unsaturated porosity, which gave rise to a zonation of the oxidation of pyrite, contained at 1 wt% in the shale. The oxidation of pyrite induces a release of sulphates, cations, iron and trace metals (Pb, Ni, Zn, Co, Cu and As). The acidification due to pyrite oxidation is buffered by calcite dissolution. While the formation of iron (oxy-)hydroxide is efficient for sorbing trace metals, whose content remained at low concentrations in the drainage water. Seasonal precipitation of gypsum during the summer desaturation at the top of the lysimeter was also identified. The hydraulic, chemical and mineralogical observations made in the lysimeters were reproduced using HPx reactive transport code under unsaturated conditions. It was possible to account for the gas diffusion where O2 availability controlled the reactivity with the shale, depending on the meteorological conditions and the drainage at the base of the lysimeter.

Mineral precipitation affects the pore structure and thus transport properties of porous media. In this study, we investigated the pore-scale dynamics of precipitation in diffusion controlled systems and the resulting impacts on the effective diffusivity, using a micro-continuum reactive transport model. Forty two-dimensional pore structures representing both idealized and realistic geometries were simulated with consideration of different precipitation scenarios and rates. A homogeneous nucleation scenario reproduced patterns observed in previous experimental study showing mixing-induced precipitation, and a surface growth scenario captured the pattern for mineral precipitation on a substrate with the same or similar mineral structures. In all cases, local precipitation resulted in the reduction in the average porosity of the domain (Φ) until the cessation of diffusive transport and the termination of precipitation. The minimum porosity reached was referred to as the critical porosity (Φc). The effective diffusivity (Deff) decreased with Φ and dropped sharply to effectively zero, that is, the critical effective diffusivity (Deffc), as Φc was reached. These pore-scale dynamics can be captured by a revised (Formula presented.) relationship that explicitly considers the critical porosity and the corresponding effective diffusivity, and the pre-exponential coefficient and the exponent of the relationship varied with initial pore structure and the precipitation kinetics. Overall, the homogeneous nucleation scenario results in systematically larger Φc and coefficients that give rise to a sharper decrease in diffusivity as Φc is approached, compared to the surface growth scenario. The revised relationship was also implemented at continuum scale and used to examine column scale diffusivity change and reactions.

International audience; Water in calcium silicate hydrate (C-S-H) is one of the key parameters driving the macroscopic behavior of cement materials for which water vapor partial pressure has an impact on Young's modulus and the volumic properties. Several samples of C-S-H with a bulk Ca/Si ratio ranging between 0.6 and 1.6 were characterized to study their dehydration/hydration behavior under water-controlled conditions using 29 Si NMR, water adsorption volumetry, X-ray diffraction, and Fourier-transform near-infrared diffuse reflectance under various water pressures. Coherent with several previous studies, it was observed that an increase in the Ca/Si ratio is due to the progressive omission of Si bridging tetrahedra, with the resulting charge being compensated for by interlayer Ca, and that water conditioning influences the layer-to-layer distance and the achieved NMR spectral resolution. Water desorption experiments exhibit one step toward low relative pressure, accompanied by a decrease in the layer-to-layer distance. When sufficient energy is provided to the system (T ≥ 40°C under vacuum) to remove the interlayer water, the shrinkage/swelling is partially reversible in our experimental conditions. A change in layer-to-layer distance of less than 3 Å is measured in the C-S-H between the wet and dried states. When the bridging SiO 2 tetrahedra are omitted, interlayer Ca interacts with layer O and water interacts with the cations and potentially with the surfaces. This structural organization is interpreted as a mid-plane monolayer of water in the interlayer space, this latter accounting for about 30% of the volume of C-S-H particles.

The mechanisms of selenate (SeO42–) sorption by Friedel’s salt (AFm-Cl2), a layered double hydroxide phase found in (altered) cement-based materials, were studied using a combination of wet chemist...

International audience; ernadite is a nanocrystalline manganese oxide, which controls the fate of many trace elements in soils and sediments through sorption and oxidative-degradation mechanisms. This exceptional reactivity directly results from its crystal structure, which may however evolve upon contact with redox-sensitive species. Understanding these changes is a prerequisite to predict and model the geochemical cycle of trace elements in the environment. Here, the structural and morphological modifications affecting synthetic nanocrystalline vernadite (δ-MnO2) upon contact with increasing concentrations of Mn2+ were investigated using wet chemistry, synchrotron X-ray diffraction and transmission electron microscopy. Fresh δ-MnO2 crystals had an Mn oxidation state of 3.94 ± 0.05 and a ~10 A layer-to-layer distance. Crystal size was ~10 nm in the layer plane, and ~1 nm perpendicular to that. Upon contact with aqueous Mn2+ under anoxic conditions, δ-MnO2 crystals underwent several morphological and mineral evolutions, starting with the stacking, perpendicular to the layer plane, of d -MnO2 crystals to form crystals ~10 nm 2 nm which were then subjected to oriented aggregation both along and perpendicular to the layer plane to form lath-like crystals with dimensions o ~100 nm 20 nm. Finally, these laths stacked perpendicular to the layer plane to form synthetic feitknechtite (β-MnOOH) crystals with sizes up to ~100 nm 500 nm when the Mn2+ loading reached 31.9 mmol g1. Structural transformation from d -MnO2 to synthetic feitknechtite was detected at Mn2+ loading equal to or higher than 3.27 mmol g-1. These mechanisms are likely to influence the geochemical fate of trace elements in natural settings where Mn2+ is abundant. Firstly, the systematic increase in crystal size with increasing Mn2+ loading may impact the sorption capacity of vernadite and feitknechtite by reducing the density of reactive edge sites. Secondly, the fate of trace elements initially sorbed at the vernadite surface is unclear, as they could either be released in solution or incorporated into the feitknechtite lattice.

International audience; A five-year-old interface between a Low Alkali Concrete (LAC) formulation (CEM III/ B containing 66% slag and 10% nano-silica) and Opalinus Clay (OPA) from a field experiment at Mont Terri Underground Rock Laboratory in Switzerland (Jenni et al., 2014) has been studied to decipher the textural, mineralogical and chemical changes that occurred between the two reacting materials.Reactivity between LAC concrete and OPA is found to be limited to a similar to 1 mm thick highly porous (ca. 75% porosity) white crust developed on the concrete side. Quantitative mineralogical mapping of the white crust using an electron microprobe and infrared spectroscopy on the cement matrix provides evidence of a Mg-rich phase accounting for approximatively 25 wt % of the matrix associated with 11 wt % of calcite, calcium silicate hydrate (C-S-H) and other cement phases. EDX analyses and electron diffraction combined with transmission electron microscopy of the Mg-rich phase provide evidence for a tri-octahedral 2: 1 phyllosilicate with mean composition:(Ca-0.5 +/- 0.2) (Mg-2.0 +/- 0.4, Fe-0.2 +/- 0.1, Al-0.5 +/- 0.3,square(0.3 +/- 0.3)) (Al-0.9 +/- 0.2, Si-3.1 +/- 0.2) O-10 (OH)(2), where square represents vacancies in the octahedral site.Apart from this reactive contact, textural, mineralogical and chemical modifications at the contact with the LAC concrete are limited. OPA mineralogy remains largely unmodified. X-ray micro-fluorescence and EPMA mapping of major elements on the OPA side also provides evidence for a Mg-enriched 300-400 mu m thick layer. The cation exchange capacity (CEC) values measured in the OPA in contact with the LAC concrete range between 153 and 175 meq kg(-1) of dry OPA, close to the reference value of 170 +/- 10 meq kg(-1) of dry OPA (Pearson et al., 2003). Changing cation occupancies at the interface with LAC concrete are mainly marked by increased Ca, Mg and K, and decreased Na. Leaching tests performed on OPA with deionized water and at different solid to water ratios strongly suggest that Cl and SO4 have either conservative behaviour or are constrained by the solubility of a precipitated sulfate phase. The Cl and SO4 concentrations measured at 2 cm from the interface are close to concentrations of undisturbed OPA pore waters (SO4: 4.5 +/- 1.5 mmol kg(-1) of dry OPA; Cl: 7.5 +/- 2.1 mmol kg(-1)of dry OPA), and increase towards the interface with the concrete. The SO4 to Cl ratio also increases towards the interface, suggesting that the increasing anion concentrations are not related to porosity variations but rather to a concentration gradient and sulfate phase precipitation near the interface.

Diagenetic smectite dehydration and smectite-to-illite transition in clay-rich sediments can contribute to the generation of overpressure in sedimentary basins, because of the release of water associated with these mineralogical reactions. However, the challenge of deciphering the contribution of smectite dehydration and illitisation to fluid pressure amongst other mechanisms generating overpressure requires the different mechanisms at play to be jointly considered. Here, we developed a coupled chemical and thermo-hydro-mechanical code, named SURP, to calculate the overpressure generation in a sedimentary pile because of sediment compaction during its burial, temperature increase, water flow and because of the release of water by clay dehydration and illitisation. For this purpose, a revised thermodynamic model describing smectite dehydration by a solid solution approach was considered, together with the different thermo-hydro-mechanical processes occurring in sedimentary basins. The SURP code was applied to the case study of an overpressured passive margin in the Niger Delta, composed of thick shale layers in a sedimentary pile. In this geological scenario, smectite dehydration was simulated and could explain 10–30% (20–50 bar) of the present day overpressure in the main reservoirs, in addition to compaction disequilibrium.

International audience; Fougerite is a naturally occurring green rust, that is, a layered double hydroxide (LDH) containing iron (Fe). Fougerite was identified in natural settings such as hydromorphic soils. It is one of the few inorganic materials with large anion adsorption capacity that stems from the presence of isomorphic substitutions of Fe2+ by Fe3+ in its layers. The importance of anion adsorption in the interlayer of LDH has often been highlighted, but we are still missing a mechanistic understanding and a thermodynamic framework to predict the anion uptake by green rust. We combined laboratory and in operando synchrotron X-ray diffraction and scattering experiments with geochemical modeling to contribute to filling this gap. We showed that the overall exchange process in green rusts having nanometer and micrometer sizes can be seen as a simple anion exchange mechanism without dissolution–recrystallization or interstratification processes. A thermodynamic model of ion exchange, based on the Rothmund and Kornfeld convention, made it possible to predict the interlayer composition in a large range of conditions. This multiscale characterization can serve as a starting point for the building of robust and mechanistic geochemical models that will allow predicting the role of green rust on the geochemical cycle of ions, including nutrients, in soils.

Silicate glasses are durable materials but laboratory experiments reveal that elements that derive from their environment may induce high corrosion rates and reduce their capacity to confine high-level radioactive waste. This study investigates nuclear-glass corrosion in geological media using an in situ diffusion experiment and multi-component diffusion modelling. The model highlights that the pH imposed by the Callovo–Oxfordian (COx) claystone host rock supports secondary-phase precipitation and increases glass corrosion compared with pure water. Elements from the COx rock (mainly Mg and Fe) form secondary phases with Si provided by the glass, which delay the establishment of a passivating interface. The presence of elements (Mg and Fe) that sustain glass alteration does not prevent a significant decrease in the glass-alteration rate, mainly due to the limited species transport that drives system reactivity. These improvements in the understanding of glass corrosion in its environment provide further insights for predictive modelling over larger timescales and space.

International audience

19 pages; International audience; The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l−1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the

The evaporation of seawater in arid climates is currently the main accepted driving mechanism for the formation of ancient and recent salt deposits in shallow basins. However, the deposition of huge amounts of marine salts, including the formation of tens of metres of highly soluble types (tachyhydrite and bischofite) during the Aptian in the South Atlantic and during the Messinian Salinity Crisis, are inconsistent with the wet and warm palaeoclimate conditions reconstructed for these periods. Recently, a debate has been developed that opposes the classic model of evaporite deposition and argues for the generation of salt by serpentinization. The products of the latter process can be called “dehydratites”. The associated geochemical processes involve the consumption of massive amounts of pure water, leading to the production of concentrated brines. Here, we investigate thermodynamic calculations that account for high salinities and the production of soluble salts and MgCl2-rich brines through sub-seafloor serpentinization processes. Our results indicate that salt and brine formation occurs during serpentinization and that the brine composition and salt assemblages are dependent on the temperature and CO2 partial pressure. Our findings help explain the presence and sustainability of highly soluble salts that appear inconsistent with reconstructed climatic conditions and demonstrate that the presence of highly soluble salts probably has implications for global tectonics and palaeoclimate reconstructions.

Concrete is the most used material for civil and industrial infrastructures. In the context of a geological disposal of nuclear waste, concrete performance may be degraded when in contact with the host formation. The ability to monitor the changes of the concrete properties using the spectral induced polarization (SIP) method is therefore of great interest. In our study, SIP laboratory measurements in the 100 mHz to 45 kHz frequency range were carried out on low-pH cement and concrete. These materials exhibited a very high resistivity (mostly above 10 kΩ m) and a high phase shift (mostly above 100 mrad) between measured voltage and injected current. The complex resistivity measurements were interpreted using electrochemical and microstructural membrane and Maxwell-Wagner polarization models. Relevant information on the mean pore size, pore size distribution, and connected porosity was obtained from SIP, opening up the possibility of using our approach to monitor concrete stability in-situ.

The construction of a repository for high-level radioactive waste (HLW) in a deep clay formation will produce large volumes of excavated rock that must be stored of at the surface. This material will be weathered for decades to century, and its evolution with time must be assessed in order to decide on its future technological use (i.e. backfill materials of HLW repository) and to evaluate its impact on the surrounding environment (e.g. acid drainage). Predictive modelling that required a consistent set of kinetic data must be used for such evaluation. To this end, the kinetics of mineralogical transformations were investigated on an argillaceous rock from the tailings stored at the surface of an underground research laboratory (Callovian-Oxfordian – COx – argillaceous rock, Bure, France). Mineralogical analysis and flow-through experiments were combined in order to quantify the oxidation kinetics of this material. Deionized water was brought into equilibrium with the atmosphere and was percolated through a suspension of powdered COx rock. Examination of the samples prior to and after the experiments showed that the main transformations were pyrite oxidation associated with calcite and dolomite dissolution. The acidity produced by pyrite oxidation was totally buffered by the dissolution of carbonate minerals that in turn affected the composition of the clay exchanger. Geochemical modelling was used to confirm the relevance of a selected mineralogical assemblage with associated kinetic parameters in order to evaluate the extent of COx rock weathering with time.

Amongst all cement phases, hydrated calcium aluminate (AFm) plays a major role in the retention of anionic species. Molybdenum (Mo), whose 93Mo isotope is considered a major steel activation product, will be released mainly under the form of MoO42− in a radioactive waste repository. Understanding its fate is of primary importance in a safety analysis of such disposal. This necessitates models that can both predict quantitatively the sorption of Mo by AFm and determine the nature of the sorption process (i.e., reversible adsorption or incorporation). This study investigated the Cl−/MoO42− exchange processes occurring in an AFm initially containing interlayer Cl in alkaline conditions using flow-through experiments. The evolution of the solid phase was characterized using an electron probe microanalyzer and synchrotron high-energy X-ray scattering. All data, together with their quantitative modeling, coherently indicated that Mo replaced Cl in the AFm interlayer. The structure of the interlayer is described with unprecedented atomic-scale detail based on a combination of real- and reciprocal-space analyses of total X-ray scattering data. In addition, modeling of several independent chemical experiments elucidated that Cl−/OH− exchange processes occur together with Cl−/MoO42− exchange. This competitive effect must be considered when determining the Cl−/MoO42− selectivity constant.

International audience

Large amounts of radioactive waste await final disposal worldwide. Repository facilities that will host this waste must be capable of ensuring very long-term isolation to protect the environment and ensure the safety of the future generations. Because of this need, the concept of geological disposal of radioactive waste emerged in the late 1950s and is now considered, in many countries, to be the best option to ensure safe, long-term containment of radioactive waste. A combination of waste overpacks (e.g. metal canisters, concrete), engineered barriers such as bentonite, and natural barriers such as clay rocks, constitutes the elements of the so-called "multiple-barrier system" between the waste matrix and the biosphere. These barrier properties will evolve with time in response to the physical and chemical interactions between the various constituents of the barriers and the surrounding environment. Consequently, predicting how these properties evolve is important for performance evaluations of the repository concepts. In recent decades, reactive transport modeling codes have become more and more efficient and are used as a bridge between current process knowledge and predictive capabilities. After describing the basic principles that have guided how these reactive transport models are constructed, their usefulness will be exampled on the modelling of clay-concrete interaction. The complexity of the multispace and temporal-scale issues will be tackled by modelling a ten years in-situ experiment and reviewing predictive modelling that have been carried out for timescales that are not accessible to experimentation.

A new in situ experiment is proposed for observing and understanding well integrity evolution, potentially due to changes that could occur during a well lifetime. The focus is put on temperature and pressure stresses. A small section of a well is reproduced at scale 1:1 in the Opalinus Clay formation, representative of a low permeable caprock formation (in Mont Terri Underground Rock Laboratory, Switzerland). The well-system behavior is characterized over time both by performing hydro-tests to quantify the hydraulic properties of the well and their evolution, and sampling the fluids to monitor the chemical composition and its changes. This paper presents the well integrity assessment under different imposed temperature (17–52°C) and pressure (10–28 bar) conditions. The results obtained in this study confirm the ability of the chosen design and observation scale to estimate the evolution of the well integrity over time, the characteristics of the flow along the well-system and the reasons of the observed evolution. In particular, the estimated effective well permeability is higher than cement or caprock intrinsic permeability, which suggest preferential flow pathways at interfaces especially at the very beginning of the experiment; the significant variations of the effective well permeability observed after setting pressure and temperature stresses indicate that operations could influence well integrity in similar proportions than the cementing process.

International audience; The Callovian-Oxfordian (COx) clay formation in the Paris Basin (France) has been the target of many studies investigating the feasibility of deep nuclear waste disposal in a reduced clay formation. To determine the mobility of radionuclides in the host rock formation, modeling of the porewater chemistry, particularly iron solute concentrations, is necessary. Notably, this study aims to understand the supersaturation of Fe(III) oxyhydroxides given by models. Fe(III) oxyhydroxides have been identified magnetically in unpreserved Callovian-Oxfordian samples. In this study, a set of magnetic measurements are used to detect the Fe-bearing magnetic minerals present in the COx clay formation. A core sample from the borehole FOR 1118, preserved from air since its collection, is the target of this study. The magnetic measurements performed show that magnetite and goethite are the main magnetic minerals (

International audience; We present here the global results of an eight-year project, where we attempted to complete the ThermoChimie database for cement minerals, zeolites and clay minerals. This work reports the methodology adopted to ensure the consistency between the three groups of minerals. Our first step concerns the relations between the three groups of minerals from field observations and experimental results gathered from the literature. A summary of the selection process is then given for clay and cement minerals and a specific focus is proposed for zeolites, with examples of analcime and mordenite. The global consistency of the selection is investigated with respect to the main chemical tendencies found for cement/clay interactions, especially considering the “alkalinity” of the minerals. Consistency is checked by establishing predominance diagrams in the chemical sub-systems of interest for the three groups of minerals, then comparing the phase relations with respect to experimental results or field observations. The methodology is illustrated using the case of gismondine and zeolite P(Ca).

Both a cured cement paste and a paste undergoing in situ hydration have been characterized using state-of-the-art synchrotron diffraction tomography. The mineralogy, macroporosity and carbonation phenomena of the samples have been determined., To understand the main properties of cement, a ubiquitous material, a sound description of its chemistry and mineralogy, including its reactivity in aggressive environments and its mechanical properties, is vital. In particular, the porosity distribution and associated sample carbonation, both of which affect cement’s properties and durability, should be quantified accurately, and their kinetics and mechanisms of formation known both in detail and in situ. However, traditional methods of cement mineralogy analysis (e.g. chemical mapping) involve sample preparation (e.g. slicing) that can be destructive and/or expose cement to the atmosphere, leading to preparation artefacts (e.g. dehydration). In addition, the kinetics of mineralogical development during hydration, and associated porosity development, cannot be examined. To circumvent these issues, X-ray diffraction computed tomography (XRD-CT) has been used. This allowed the mineralogy of ternary blended cement composed of clinker, fly ash and blast furnace slag to be deciphered. Consistent with previous results obtained for both powdered samples and dilute systems, it was possible, using a consolidated cement paste (with a water-to-solid ratio akin to that used in civil engineering), to determine that the mineralogy consists of alite (only detected in the in situ hydration experiment), calcite, calcium silicate hydrates (C-S-H), ettringite, mullite, portlandite, and an amorphous fraction of unreacted slag and fly ash. Mineralogical evolution during the first hydration steps indicated fast ferrite reactivity. Insights were also gained into how the cement porosity evolves over time and into associated spatially and time-resolved carbonation mechanisms. It was observed that macroporosity developed in less than 30 h of hydration, with pore sizes reaching about 100–150 µm in width. Carbonation was not observed for this time scale, but was found to affect the first 100 µm of cement located around macropores in a sample cured for six months. Regarding this carbonation, the only mineral detected was calcite.

International audience; This study is focused on the hydromechanical behaviour of a compacted swelling clay upon wetting. More precisely, a multi-scale investigation of the swelling behaviour of a purified bentonite under constrained conditions was carried out by a monitoring of the swelling pressure and the porosity evolution at various scales (inter-aggregate, inter-particle, interlayer). A specific oedo-tomometer, has been designed to monitor both the swelling pressure and the inter-aggregate porosity evolution deduced from microfocus X-ray computed tomography measurements (size higher than 5 µm). The results showed the influence of hydration on specimens with an increase of the swelling pressure and a decrease of the inter-aggregate pore size during hydration. Therefore, a coupling between swelling pressure and textural reorganisation at inter-aggregate pore scale was highlighted. This coupling was also based on hydration process at interlayer scale.; L'hydratation d'une argile gonflante provoque une augmentation de sa pression de gonflement associée à une réorganisation profonde de sa microstructure à toutes les échelles. L'objectif de ce travail est de caractériser et de comprendre les processus couplés chémo-hydromécaniques multi-échelles dans les argiles soumises à la circulation d'un fluide. Un dispositif spécifique, oedo-tomomètre, a été conçu pour assurer le suivi couplé de la pression de gonflement en corrélation avec l'évolution de la porosité inter-agrégat identifiable par microtomographie de rayons X (taille > 5 µm). Les résultats ont permis d'établir une corrélation entre le développement de la pression de gonflement et la réorganisation progressive de la porosité d'une argile gonflante compactée. L'utilisation de différents fluides a permis de montrer le couplage entre l'évolution de la pression de gonflement et la réorganisation de la porosité inter-agrégats, couplage lui-même dépendant des processus d'hydratation induits à l'échelle interfoliaire (gonflement cristallin ou osmotique).

International audience; In-situ kinetics and mechanisms of Ni2+ uptake by synthetic vernadite were determined at pH 5.8 and I = 0.1 M NaCl using wet chemistry, atomic-resolution scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) and synchrotron high-energy X-ray scattering (HEXS) in both the Bragg-rod and pair distribution function formalisms. The structural formula of the initial solids was (Mn0.053+Na0.23+)-Mn-Tc(H2O)(0.69)H+ (0.06) [(Mn4+ (0.86)Mn(0.04)(3+)vac(0.1))O-2), where species under brackets form the layer having "vac" layer vacancies, and where other species are present in the interlayer, with TC standing for "triple corner sharing" configuration. According to HEXS and STEM-EELS, adsorbed Ni2+ adopted mainly a TC configuration, and had a Debye-Waller factor about four times higher than layer Mn. Steady-state was reached after similar to 2.2 h of contact time, and the final structural formula of the solid was (Ni0.122+Mn3+)-Ni-Tc-Mn-Tc Na-0.05(+) 0.12H2O0.36H0.01+(Mn(0.87)(4+)vac(0.13))O-2]. Atomic-scale imaging of the solids also evinced the presence of minor Ni adsorbed at the crystal edge. The retention coefficient R-D = 10(3.76 +/-) (0.06) L kg(-1), computed from PDF data modelling and solution chemistry results, was in agreement with those available in the literature.

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO ) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms - edge sorption, interfacial dissolution-reprecipitation - are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe., This work has been supported by a grant from Labex OSUG@2020 (Investissements d’avenir - ANR10 LABX56). B.M., A.F.-M., L.C., S.G. and F.C. thank the NEEDS program from the CNRS for funding support. B.M. also thanks the financial support from the China Scholarship Council (CSC).

The Cement–Opalinus Clay Interaction (CI) Experiment at the Mont Terri rock laboratory is a long-term passive diffusion–reaction experiment between contrasting materials of relevance to engineered barrier systems/near-field for deep disposal of radioactive waste in claystone (Opalinus Clay). Reaction zones at interfaces of Opalinus Clay with two different types of concrete (OPC and “low-pH”/ESDRED) were examined by sampling after 2.2 and 4.9 years. Analytical methods included element mapping (SEM, EPMA), select spot analysis (EDAX), 14C-MMA impregnation for radiography, and powder methods (IR, XRD, clay-exchanger characterisation) on carefully extracted miniature samples (mm). The presence of aggregate grains in concrete made the application of all methods difficult. Common features are a very limited extent of reaction within claystone, and a distinct and regularly zoned reaction zone within the cement matrix that is more extensive in the low-alkali cement (ESDRED). Both interfaces feature a de-calcification zone and overprinted a carbonate alteration zone thought to be mainly responsible for the observed porosity reduction. While OPC shows a distinct sulphate enrichment zone (indicative of ingress from Opalinus Clay), ESDRED displays a wide Mg-enriched zone, also with claystone pore-water as a source. A conclusion is that substitution of OPC by low-alkali cementitious products is not advantageous or necessary solely for the purpose of minimizing the extent of reaction between claystone and cementitious materials. Implications for reactive transport modelling are discussed.

International audience; Editorial handling by Prof. M. Kersten Keywords: Cement-based materials Radionuclide and toxic element retention Sorption Solubility Hydrolysis Diffusion CEBAMA A B S T R A C T Cement-based materials are key components in radioactive waste repository barrier systems. To improve the available knowledge base, the European CEBAMA (Cement-based materials) project aimed to provide insight on general processes and phenomena that can be easily transferred to different applications. A bottom up approach was used to study radionuclide retention by cementitious materials, encompassing both individual cement mineral phases and hardened cement pastes. Solubility experiments were conducted with Be, Mo and Se under high pH conditions to provide realistic solubility limits and radionuclide speciation schemes as a prerequisite for meaningful adsorption studies. A number of retention mechanisms were addressed including adsorption, solid solution formation and precipitation of radionuclides within new solid phases formed during cement hydration and evolution. Sorption/desorption experiments were carried out on several anionic radionuclides and/or toxic elements which have received less attention to date, namely: Be, Mo, Tc, I, Se, Cl, Ra and 14 C. Solid solution formation between radionuclides in a range of oxidation states (Se, I and Mo) with the main aqueous components (OH À , SO 4 À 2 , Cl À) of cementitious systems on AFm phases were also investigated.

Cement-based materials are key materials used in repositories of low, intermediate and high-level radioactive waste in any host-rock concept. These materials are used for waste conditioning, liners, seals as well as structural components. The CEBAMA (CEment-BAsed MAterials, properties, evolution, barrier functions) project (2015–2019) has been an initiative granted by the European Commission under the Horizon 2020 Research and Training Programme of the European Atomic Energy Community (EURATOM), focused on the study of cementitious materials. The project has addressed key issues of relevance for long-term safety and key scientific questions related to the use of cement-based materials in nuclear waste disposal applications. This publication presents the advances resulting from the research undertaken in the CEBAMA project, with special emphasis on their contribution to decrease uncertainties in the Safety Case and the Performance Assessment of radioactive waste repositories. The analysis is presented in three different axes, in coincidence with the lines of research of the project: degradation of cement-based materials and its impact on cement/clay interfaces; retention of safety relevant radionuclides and toxicants on cement-based materials and modelling advances in cementitious related systems. The research investigations have provided important new insights on process understanding and model developments which have significantly decreased the level of uncertainty.

On a geological carbon dioxide storage site, wells (decommissioned or active) drilled through low-permeable caprock are potential connections between the CO2 storage reservoir and overlying sensitive targets like aquifers. The integrity of wells over time is therefore essential for the fluids confinements (brine with or without dissolved CO2 or buoyant gaseous CO2). Well integrity can be defined as its capacity to maintain the isolation of fluids in the subsurface reservoirs. To ensure this isolation, the well casing and the caprock are bonded by a cement sheath; after abandonment, a cement plug is used to avoid upward migration within the casing. However, according to the literature wellbore integrity might be compromised by a) operational defects (during the drilling and cementing processes, during the operations where pressure and temperature changes can modify stress conditions, during the abandonment process) or b) chemical changes especially in the context of CO2 storage where different chemical environments can be possibly found. The evolution of the well integrity is therefore a complex combination of several physical processes (hydrological, thermal, mechanical and chemical at least) on the different materials and elements (formation, cement, casing, interfaces, annuli). The issue addressed in this study is the behaviour of the complete wellbore system in different conditions and especially under influence of CO2. An in situ experiment has been designed and is being run in an underground rock laboratory (Mont Terri, Switzerland) at an intermediate scale between the laboratory experiments (which offer the opportunity to assess specific phenomena over time) and field observations (which allow an assessment of the entire system in subsurface at a specific time). The purpose is here to follow the evolution of the integrity of the whole well system both for hydrogeological and geochemical aspects. Modelling and lab experiments, including material characterization are performed in parallel to enable a better interpretation and understanding of the observations. The experimental setup has been designed as follows: a small section of a wellbore is reproduced in the Opalinus Clay of the underground rock laboratory (caprock-like formation) at scale 1:1 (5.5 '' casing and Ø198 mm borehole), using carbon steel for the casing and class G cement. Below and above the well section, two different intervals have been designed for a continuous monitoring of the pressure and temperature conditions or for fluid injection and extraction (for fluid sampling for instance). The experimental protocol contains several stages: 1- Drilling, relaxation and well completion; 2- Initial state characterization; 3- Temperature increase in the system and characterisation; 4- Injection of pore water with dissolved CO2 and tracers, and characterization. The characterization of the system includes both performing hydro-tests (continuous and constant-head tests) to quantify the hydraulic properties of the well and their evolution over time, and sampling the fluids to analyse the geochemical composition and changes. Stages 1, 2 and 3 are now completed and stage 4 is in progress. The first results (of stage 2 and 3) confirm the ability of the chosen design to assess well integrity over time. It also shows first trends in the evolution of integrity especially under influence of the temperature: the equivalent permeability of the wellbore decreased significantly during that stage, improving the well integrity. Three modelling methods are used to predict the results of the experiment or to use the recorded data: 1) flow and transport modelling, 2) geochemical modelling, and 3) reactive transport modelling (TOUGH2 used for 1, and PhreeqC and TOUGHREACT for 2 and 3). Flow modelling has been associated with the hydraulic tests in order to quantify the well integrity in terms of equivalent permeability during stage 2 and 3. Temperature effects have been included in the models to understand the changes that occurred in terms of effective permeability during stage 3. Geochemical models have been developed to evaluate the validity of chemical sealing as a potential explanation for the increase of well integrity observed during stage 3. The models are also used as predictive tools on the potential evolutionary scenarios for stage 4, regarding water chemistry variations and mineralogical changes. They have been notably useful to calibrate the conditions of the dissolved CO2 in terms of tracers used, and of sampling and in-line fluid analysis protocol. Each of those models will be progressively refined during the CO2-dissolved injection (stage 4) with the aim of aiding the interpretation of the observed processes and phenomena. Finally this field experiment-modelling integrated framework is completed by laboratory characterizations and experiments (in autoclave principally) carried out to provide input data for the modelling with regards to cement, caprock and their reactivity with pore water and CO2.

The determination of reliable weathering/dissolution rates for cement phases is of fundamental importance for the modelling of the temporal evolution of both radioactive waste repositories and CO2 geological storage sites (e.g. waste matrix, plug in boreholes). Here, the dissolution kinetics of AFm-Cl (hydrated calcium aluminates containing interlayer Cl) has been studied using flow-through experiments conducted at pH values ranging from 9.2 to 13. Mineralogical (XRD) and chemical (EPMA, TEM) analyses have been performed to determine the evolution of the phases during the dissolution experiments. For pH values between 10 and 13, the dissolution of AFm-Cl is congruent (i.e. Ca/Al ratios close to 2 both for solids and outlet concentrations). In contrast, the precipitation of amorphous Al-phases and possibly amorphous mixed Al/Ca phases is observed at pH 9.2, leading to Ca/Al ratios in the outlet solutions higher than those of the initial solid. Therefore, at pH 9.2, even if Cl–/OH– exchange occurs, estimation of dissolution rate from released Cl appears to be the best proxy. Dissolution rates were normalized to the final specific surface areas (ranging from 6.1 to 35.4 m2 g−1). Dissolution rate appears to be pH-independent and therefore the far-from-equilibrium dissolution rate at room temperature is expressed as: logR(mol m–2 s–1) = –9.23 ± 0.18

International audience; Large amounts of nuclear waste await final disposal worldwide. A combination of waste overpacks (e.g. metal canisters, concrete), engineered barriers such as bentonite, and natural barriers such as clay rocks, constitutes the elements of the so-called " multiple-barrier system " between the waste matrix and the biosphere. The number, types and assigned safety functions of these various barriers depend on the chosen repository concept, the waste form, the radionuclide inventory in the waste, the selected host rock, the hydrogeological and geochemical settings of the repository site, etc.(Apted and Ahn, 2010). These barrier properties will evolve with time in response to the physical and chemical interactions between the various constituents of the barriers and the surrounding environment. Consequently, predicting how these properties evolve is of prime importance for performance and safety evaluations of the repository concepts. As a prerequisite, initial properties of the materials used in the disposal have to be understood, to better predict their long term behavior. Although micro imaging techniques are more and more sophisticated and powerful (Gaboreau et al., 2016), few techniques allow in-situ characterization of both the evolution of the different phases's mineralogy and their 3D spatial arrangement. To tackle this issue, X-Ray diffraction computed tomography (XRD-CT, see Fig1) that allows to record in each voxel of the recorded volume an X-ray diffraction pattern, has been successfully applied to investigate hydration and microstructural development in cements (Voltolini et al., 2013). Here we present results obtained by synchrotron XRD-CT on a cement paste formulation, which is constituted of blended Portland, fly ash, blast furnace slag cement (Chen et al., 2012), foreseen to be used for nuclear waste disposal application. The mineralogy (including spatial distribution) of a cured cement paste and of a cement paste undergoing in-situ and time-resolved hydration will be compared and discussed. In addition, special care will be taken to analyze the evolution of cement porosity as a function of time and of associated spatially-and time-resolved carbonation mechanism.

International audience; Anthropogenic radionuclides (RN) are generated by a wide range of industrial and medical activities. In the contexts of waste storage, the quantification of RN migration is of paramount importance. RN migration is partly ruled by the interaction of RN with the solid surfaces. Usually experiments are conducted at various scales from laboratory to the field in order to measure retention and retardation parameters of radiotracers. Whereas this experimental approach is mandatory to tackle the issue of RN migration, the understanding of the natural speciation of stable isotopes that are analog to RN brings additional useful information. In particular, the RN natural speciation sheds light on RN isotopic exchange and " irreversible " trapping mechanisms. This study aims at overviewing the association of natural trace elements (U, Th, Ni, I, Sr and Zn) with carbonate minerals in the Callovian-Oxfordian sedimentary formation that is under consideration for deep nuclear waste disposal in France. The combined use of sequential extraction techniques, microscopic and spectrometric techniques, as well as laser-ablation coupled to chemical analysis techniques made it possible to establish the distribution of I, Sr, U, Th and Ni in the various mineral and organic phases present in the clay rock. I and Sr and in a less extent U and Th are mainly carried by carbonates while Ni is distributed in a variety of phases including pyrite, sphalerite, chlorite, organic matter and muscovite.

Quantitative analysis of the X-ray pair distribution function collected on calcium silicate hydrates having Ca/Si ratios ranging between 0.57 and 1.47 was applied. With increasing Ca/Si ratio, Si bridging tetrahedra are omitted and Ca(OH)2 is detected at the highest ratios., The structural evolution of nanocrystalline calcium silicate hydrate (C–S–H) as a function of its calcium to silicon (Ca/Si) ratio has been probed using qualitative and quantitative X-ray atomic pair distribution function analysis of synchrotron X-ray scattering data. Whatever the Ca/Si ratio, the C–S–H structure is similar to that of tobermorite. When the Ca/Si ratio increases from ∼0.6 to ∼1.2, Si wollastonite-like chains progressively depolymerize through preferential omission of Si bridging tetrahedra. When the Ca/Si ratio approaches ∼1.5, nanosheets of portlandite are detected in samples aged for 1 d, while microcrystalline portlandite is detected in samples aged for 1 year. High-resolution transmission electron microscopy imaging shows that the tobermorite-like structure is maintained to Ca/Si > 3.

The structure of nanocrystalline calcium silicates hydrates (C–S–H) having Ca/Si ratios ranging between 0.57 ± 0.05 and 1.47 ± 0.04 was studied. All samples are nanocrystalline and defective tobermorite. An increase of the Ca/Si ratio resulting from omission of bridging Si in the Si chains and incorporation of interlayer Ca was observed., The structure of nanocrystalline calcium silicate hydrates (C–S–H) having Ca/Si ratios ranging between 0.57 ± 0.05 and 1.47 ± 0.04 was studied using an electron probe micro-analyser, powder X-ray diffraction, 29Si magic angle spinning NMR, and Fourier-transform infrared and synchrotron X-ray absorption spectroscopies. All samples can be described as nanocrystalline and defective tobermorite. At low Ca/Si ratio, the Si chains are defect free and the Si Q 3 and Q 2 environments account, respectively, for up to 40.2 ± 1.5% and 55.6 ± 3.0% of the total Si, with part of the Q 3 Si being attributable to remnants of the synthesis reactant. As the Ca/Si ratio increases up to 0.87 ± 0.02, the Si Q 3 environment decreases down to 0 and is preferentially replaced by the Q 2 environment, which reaches 87.9 ± 2.0%. At higher ratios, Q 2 decreases down to 32.0 ± 7.6% for Ca/Si = 1.38 ± 0.03 and is replaced by the Q 1 environment, which peaks at 68.1 ± 3.8%. The combination of X-ray diffraction and NMR allowed capturing the depolymerization of Si chains as well as a two-step variation in the layer-to-layer distance. This latter first increases from ∼11.3 Å (for samples having a Ca/Si ratio

International audience; A specific oedometer cell has been set up to measure the swelling pressure of compacted montmorillonites at constant volume and to concomitantly visualise the evolution, upon wetting, of how the microstructure is organised through X-ray microtomography. The swelling pressure experiments were conducted with solvents of various natures. In addition to conventionally used water and saline solutions, we used an organic solvent (methyl methacrylate – MMA). We chose this to explore the effect of its different physical and chemical properties, and to differentiate the respective roles of crystalline and osmotic pressures on macroscopic swelling behaviour.The results, which combined both swelling pressure measurements and quantification of microstructure evolution upon hydration for the two different solutes, give sound understanding on the development of osmotic and/or crystalline swelling and their relative impact both on the microstructure and on the magnitude of the macroscopic swelling pressure of compacted montmorillonites.