Séverine Levasseur

Over the past decade, the Belgian nuclear research centre (SCK CEN) has gathered extensive laboratory data on diffusion coefficients of dissolved gases in water-saturated Boom Clay (BC). Since gas transport around a geological disposal facility occurs at metre scales, a key question is whether centimetre-scale laboratory values remain valid under in situ conditions. To address this, SCK CEN, EURIDICE, and ONDRAF/NIRAS launched the NEMESIS (NEon diffusion in MEgaS In-Situ) experiment in September 2023 at the HADES Underground Research Laboratory in Belgium. Its purpose is to confirm and refine understanding of gas diffusion in BC at realistic scales. NEMESIS reuses four piezometers from the 1990s MEGAS experiment. Dissolved neon is injected into the clay through a vessel–microtube–filter system, and its migration is monitored at three observation filters. This paper outlines the experimental design, strategy, initial findings, and early interpretations. The setup has proven effective under in situ conditions and provides new insights into the coupling between diffusion and water flow. Preliminary best-fit diffusion coefficients from Phase 1 (in-diffusion) are broadly consistent with laboratory measurements. Phase 2 (through-diffusion), to be conducted over the coming years, will deliver additional data to validate results, refine interpretations, and strengthen the characterisation of gas diffusion in Boom Clay.

In many countries, geological disposal facility is considered as an option for the long-term management of radioactive waste. In the French, Swiss, and Belgian concepts, clays have been selected as suitable host formations. Considerable amounts of gas can be generated within geological disposal facilities – mainly hydrogen from corrosion of metallic elements of the repository or of the waste themselves. Although gas production will be slow, it is important to assess how gas will accumulate, migrate, and be released within the disposal system and what could be the potential system perturbations caused by gas production and pressurisation in order to properly evaluate the functioning of a geological disposal facility. To address this question, the EURAD-GAS project under the umbrella of the European Joint Programme EURAD (2019–2024) studied gas transport mechanisms in clays. The project aimed to enhance understanding of gas movement through low-permeability materials and assess its implications for barrier integrity and repository performance. This paper provides an overview of the state of knowledge on gas transport processes collected within this project and illustrates how this knowledge can be used to assess gas transport at the scale of a geological repository in clay formations.

La démonstration de la faisabilité technique de construction de galeries dans une argile peu indurée constitue un objectif majeur du programme belge de Recherche, Développement et Démonstration (RD&D) relatif au stockage géologique des déchets radioactifs de haute activité et/ou de longue durée de vie. Ce programme s’appuie sur le laboratoire souterrain HADES, construit dans l’Argile de Boom à Mol à une profondeur de 225 mètres, en plusieurs étapes depuis les années 1980. L’étude analyse l’évolution des convergences de deux galeries construites à des périodes et selon des techniques différentes : le « Test Drift » (1987, excavation semi-manuelle) et la galerie de liaison (2002, tunnelier). Les mesures montrent que les diamètres des deux galeries continuent à évoluer et ne sont donc pas stabilisés, respectivement 37 ans (pour le « Test Drift ») et 22 ans (pour la galerie de liaison) après leur construction. Leurs modes de déformation diffèrent significativement : le « Test Drift » subit une compression dans toutes les directions tandis que la galerie de liaison présente une compression verticale associée à une extension horizontale. L’évolution des convergences du « Test Drift » est analysée via l’introduction d’un modèle viscoélastique du type Kelvin-Voigt. Son application aux mesures de convergences de cette galerie a permis de reproduire les observations in situ sur plusieurs décennies, confirmant la nécessité de développer un modèle intégrant l’évolution temporelle des différents matériaux présents et de leurs interactions afin de bien comprendre le comportement à long terme du revêtement de galeries creusées dans une argile peu indurée. Les résultats de cette étude présentent un intérêt majeur pour la conception et l’optimisation des galeries de stockage géologique en milieu argileux peu induré.

In the geological disposal of high-level radioactive waste in argillaceous rocks, studying the barrier integrity after gas transport and the pathway closure thanks to self-sealing capacity is a crucial aspect for the safety assessment. This paper presents experimental research in Boom Clay (a potential host rock in Belgium) to evaluate the effectiveness of self-sealing and possible fissure reactivation during a second gas invasion event. Initial water permeability under oedometer conditions was first measured on samples at two bedding orientations, being higher the sample with bedding planes parallel to flow, highlighting marked anisotropy. Then, gas injection tests at a constant volume rate were performed. Results indicated that Boom Clay underwent expansion and degradation during gas injection due to the development of fissures that were quantified using microstructural techniques. The computed effective gas permeability was not significantly dependent on bedding orientation and was slightly larger than the initial intrinsic water permeability. The re-saturation of the samples led to a recovery of the initial water permeability for both orientations, replicating the original anisotropy. The microstructural analyses confirmed the gas pathways’ closure, indicating good self-sealing and the regaining of the hydraulic barrier function. However, a small volume of large unconnected pores was detected on undrained unloading before the microstructural study. An additional gas injection after the self-sealing resulted in a higher effective gas permeability and a larger increase in pore volume, suggesting the reopening of fissures generated during the first injection. Finally, the experimental data were compiled within a multi-scale phenomenological model to relate the microstructural information to macroscopic flow transport properties capturing the intrinsic permeability increase on gas invasion and its recovery during self-sealing.

Gas production is expected in radioactive-waste storage structures. This will induce a slow increase in gas pressure, which necessitates the study of gas transfer at a low pressure. In this special case, calculations of the flow through storing materials while solely using permeability and Darcy’s law are likely to be inadequate, as diffusion may play a crucial role in the process. The gas permeability and gas diffusion coefficient of industrial concrete have then been measured on the dry material. Diffusion tests were performed with a new device, specially designed for this study. The diffusion coefficient was directly measured with the use of the first Fick’s law, as the test was analyzed under a steady state. Using some simplified hypotheses, it was then possible to compare the proportion of flow occurring due to diffusion with the one occurring due to permeation. The tendency is very clear and unambiguously shows that diffusion is predominant at a very low injection pressure but becomes negligible as soon as the gas pressure exceeds a moderate value.

Deep geological disposal aims to contain and isolate radioactive waste from the biosphere. Repository systems are made of multiple barriers working together, typically comprising the natural geological barrier provided by the repository host rock and its surroundings and an engineered barrier system. Due to their excellent properties for the confinement of contaminants, including low permeability, high sorption capacity, and swelling/self-sealing capacity, clayey materials are considered as engineered and/or natural barriers in most repository designs under development in Europe. During the lifetime of the repository, clay barriers will be exposed to perturbations, among which those are resulting from gas and heat production within the system. It is important to verify that these perturbations will not be detrimental to the good functioning of these barriers. In this paper, it is shown how the two EURAD R&D work packages, GAS and HITEC use a combination of experimental and modelling approaches to increase the understanding and predictability of the impact on clay barriers of the fundamental processes and their couplings related to gas and heat transport respectively, providing building blocks to support the evaluation of the robustness of the repository concepts.

During recent decades, argillaceous sedimentary formations have been studied as potential host formations for the geological disposal of long-living and heat-emitting radioactive waste—Boom Clay in Belgium and Opalinus Clay and Brown Dogger in Switzerland. A significant issue in the long-term performance of these potential host rocks concerns the generation and transport of gases. The pressure resulting from the generation of gas in an almost impermeable geological medium in the near field of a repository will increase. Under high gas pressures, the mechanical and hydraulic properties of the host rock are expected to change significantly. Preferential gas pathways may develop which exploit material heterogeneity, anisotropy (bedding planes), rock discontinuities, or interfaces between the different components of the repository, and may eventually lead to the release of the produced gases. Gas flow through these clayey rocks is investigated on the basis of laboratory work. Priority has been given to studying the volume change response of these initially water-saturated materials through relatively fast and controlled volume-rate gas injections. The effect of the gas injection rate, the confining pressure and the bedding orientation on the gas transport properties have been studied with particular attention paid to the coupling with strain behaviour. The results have shown features common to the three formations concerning the gas transfer process through preferential pathways, despite their initially differential properties.

In many countries, the preferred option for the long-term management of high- and intermediate level radioactive waste and spent fuel is final disposal in a geological repository. In this geological repository, the generation of gas will be unavoidable. In order to make a correct balance between gas generation and dissipation by diffusion, knowledge of the diffusion coefficients of gases in the host rock and the engineered barriers is essential. Currently, diffusion coefficients for the Boom Clay, a potential Belgian host rock, are available, but the diffusion coefficients for gases in the engineered concrete barriers are still lacking. Therefore, diffusion experiments with dissolved gases were performed on two concrete-based barrier materials considered in the current Belgian disposal concept, by using the double through-diffusion technique for dissolved gases, which was developed in 2008 by SCK CEN. Diffusion measurements were performed with four gases including helium, neon, methane and ethane. Information on the microstructure of the materials (e.g., pore size distribution) was obtained by combining N₂-adsorption, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and water sorptivity measurements. A comparison was made with data obtained from cement-based samples (intact and degraded), and the validity of existing predictive models was investigated.