European Underground Research Infrastructure for Disposal of nuclear waste in a Clay Environment

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.

Water content may have a crucial effect on the petrophysical and mechanical properties of high porosity rocks such as chalk. Consequently, the water fluctuations may significantly influence the behaviour of engineering structures that are constructed in chalks. It is the aim of this study to illustrate how the elastic and strength properties of chalk can evolve considering water content variation using P-wave velocity and analytical analyses. To study the influence of water on the ultrasonic compressional wave velocity (VP), measurements of travel time of three samples subjected to different water content were performed. The Uniaxial Compressive Strength (UCS) in dry and saturated chalk specimens was also estimated. To clarify the possible influence of lithology on the physicomechanical properties of rocks, three samples of phosphatic chalk were studied in thin sections. Automated mineral analysis and textural imaging of the samples were performed using an FEI QEMSCAN®. The analyses of the Ultrasonic tests reveal that changes in the water content are associated with variations in the P-wave velocity. Based on P-wave velocity changes as a function of water content, analytical models have been used to predict the elastic and strength properties. The data indicate that the presence of water significantly reduced the elastic and strength parameters of the chalk, a result that is in agreement with the UCS laboratory tests. The derived equations can be used for predicting the elastic and strength properties of high-porous chalk from the P-wave velocity as a function of water content. This approach may avoid the necessity for time-consuming laboratory testing.

Abstract The option of disposing of radioactive waste deep underground has been studied in Belgium by SCK CEN since the 1970s. This led in 1980 to the construction of the HADES underground research laboratory (URL) in a clay formation, the Boom Clay, at a depth of 225 m under the premises of SCK CEN in Mol. Over the last four decades, many in situ experiments have been conducted in the HADES URL. These have made a significant contribution to ONDRAF/NIRAS’ research, development and demonstration (RD&D) efforts demonstrating that disposal in Boom Clay can offer a safe solution for the long-term management of high-level and/or long-lived radioactive waste. Moreover, the construction of the HADES URL itself is a demonstration that shafts and galleries can be constructed in clay at that depth. However, the HADES URL did not only contribute to the Belgian programme. Many of the in situ experiments have been part of international research and the laboratory has provided valuable input to the research programmes of other URLs, such as the Meuse/Haute-Marne URL in France and the Mont Terri rock laboratory in Switzerland. This paper gives a brief overview of the main contributions of the HADES URL to both national and international research into geological disposal.

In 1168, the first silver was found in the Saxon Erzgebirge. Mining began shortly thereafter and continues to this day at varying intensities. About 250 miners are permanently engaged in rehabilitation work at old mining sites so that houses and roads are safe and the mine water can drain through galleries. The radon potential in the Erzgebirge is high. Despite radiation protection measures, exposures to more than 6 mSv occurred among 22 miners in 2016 alone. For this reason, the miners are monitored with personal radon dosimetry. The dosimeters consist of diffusion chambers with CR-39 nuclear track detectors. Outside the miners' working hours, the dosimeters must be stored together with a reference dosimeter. After a 3 mo wearing period, the dosimeters are replaced and the miners' exposures are determined. For this purpose, the exposures of the reference dosimeters that have occurred outside working hours are subtracted from the exposures of the miners' dosimeters. In mining practices, this method has caused problems. The conditions of use (storing dosimeters along with a reference dosimeter) were often not met by the miners. To solve the problems, reference dosimeters were replaced by radonproof boxes with activated carbon cartridges inside. The miners stored their dosimeters in the boxes outside working hours. A uniform reference value for the radon activity concentration of 50 Bq m was used for all boxes to determine the exposure of miners during working hours. The associated error was small. In future, reference dosimeters and the measurement of a reference value will not be necessary.

SummaryDeep excavation in some geological media needs lining of the gallery. This could limit the extent of the so‐called excavation damaged zone and the resulting convergence of the material due to tunneling. Boom clay, the reference potential host rock in Belgium for disposal of high‐level radioactive waste, is one of these media for which lining of the gallery walls is essential. A correct simulation of the lining behavior in the course of the excavation process, where the rock comes into contact with the lining, and in the long term, remains a significant challenge in analysis of the whole coupled phenomena of rock in interaction with the lining. This study aims to numerically model the lining behavior. The main objective is to develop a model that could realistically simulate the behavior of a discontinuous lining made of concrete segments. We propose to numerically analyze the response of the blocks in contact with each other and in interaction with rock, with the use of zero‐thickness interface elements. To validate the developed model and a proposed approach, a particular analysis compares the obtained results with the available in situ measurements. This study then discusses the deficiency of the simplistic model that considers a continuous lining. In addition, regarding the contact mechanism on the interface between the rock and the lining, the obtained results demonstrate an interesting relation between the contact phenomena and the shear banding within the rock around the gallery. Copyright © 2017 John Wiley & Sons, Ltd.

The mechanical behaviour of Boom Clay has been studied for many years in the context of geological disposal of radioactive waste in Belgium. The aim of this study is to investigate the anisotropic behaviour of Boom Clay in terms of compressibility and hydraulic conductivity. Oedometer tests (with effective vertical stress (\( \sigma_{v}^{{\prime }} \)) up to 32 MPa) were carried out on samples of various orientations: parallel, perpendicular and inclined 45° to the bedding plane. The compressibility index (C c ) and swelling index (C s ) were compared. Only a slight difference between these parameters was observed, suggesting that the anisotropic behaviour of Boom Clay cannot be revealed under the test conditions adopted. The hydraulic conductivity (k) was also determined by the Casagrande’s method for different values of vertical effective stress (\( \sigma_{v}^{{\prime }} \)). Unlike compressibility, the hydraulic conductivity, however, showed a clear anisotropic behaviour with k ver < k inc < k hor .

Boom Clay is considered to be a potential host formation for a geological repository of radioactive waste in Belgium. In this context, a thorough understanding of its mechanical behaviour is thus important. In this study, the anisotropy of the shear modulus of natural Boom Clay samples was investigated using the bender element method. Seven samples located at different distances from the connecting gallery of the HADES underground research laboratory were tested for shearing in three different directions with respect to the bedding plane. The results obtained confirm the mechanical anisotropy of Boom Clay. Furthermore, the values of small-strain shear modulus in the near field of the gallery were found to be lower than those in the far field, suggesting that the mechanical properties of Boom Clay remain affected by gallery excavation damage that occurred more than 10 years ago.