Abstract Cretaceous limestones near Maastricht (SE Netherlands) have been quarried at least since Roman times. In the late eighteenth century, scientific interest developed in their macrofossil content and specimens were illustrated for the first time. Amongst the early discoveries was a partial skull of a large predatory vertebrate that would play an important role in the emergence of modern palaeontology and our understanding of the concept of extinction. After decades of scientific debate, this animal was recognized as a large extinct marine relative of monitor lizards (varanoids) and named Mosasaurus. A detailed lithostratigraphy of Upper Cretaceous (Santonian–Maastrichtian) rocks was established in the Maastrichtian type area during the mid-1970s, which resulted in a renewed interest in fossil hunting by professional and amateur palaeontologists alike. During recent decades, both micro- and macrofossils have enabled a refinement of biozonations, correlations within the basin and with sections elsewhere, a greater insight into taphonomic processes and updated taxonomic interpretations. A new age model and chemostratigraphical framework is the most recent addition, permitting the placement of geoheritage in a larger frame and intensifying outreach to the public, including also virtual and augmented reality and hands-on experience to visitors of museum and (disused) quarries alike.

Abstract In southern Belgium, 23% of abstracted groundwater volumes are from chalk aquifers, representing strategic resources for the region. Due to their specific nature, these chalk aquifers often exhibit singular behaviour and require specific analysis. The quantitative evolution of these groundwater resources is analysed for the Mons Basin and Hesbaye chalk aquifers as a function of past evolution, in the short and long term. Groundwater level time series exhibit decreases when analysed over different periods. This is particularly visible for the Hesbaye chalk aquifer when comparing the 1960–90 and 1990–2020 periods. Such decreases are associated with observed temperature increases and precipitation decreases, inducing a decrease of aquifer recharge, and a probable increase of groundwater abstraction in the adjacent catchment. Past evolution is also discussed considering recent winter and summer drought events. The aquifers exhibit long delays in response to recharge events, particularly where the thickness of the partially saturated zone plays a crucial role in observed delays. Regarding future evolution, simulations of the impact of climate changes using medium–high emission scenarios indicate a probable decrease of the groundwater levels over the Hesbaye chalk aquifer.

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.

Abstract When assessing a rock formation for its suitability as a potential host rock for the disposal of radioactive waste, knowledge about its pore water chemical composition is essential. When the HADES underground research laboratory became operational in the early 1980s, it offered the possibility of extracting pore water from Boom Clay. At the time, however, there was almost no experience of sampling pore water from deep clay formations. The low hydraulic conductivity of clays makes it difficult to extract pore water and the sampling process itself can induce changes in the observed chemical characteristics due to oxidation and re-equilibration with the ambient air. In the past decades, significant progress has been made in the techniques and protocols to sample and monitor pore water. The use of nitrogen instead of compressed air to drill boreholes and installing piezometers limited the disturbances induced by oxidation of the clay. Furthermore, an advanced system was developed to simultaneously sample pore water and dissolved gases and measure some key geochemical parameters such as pH, pCO 2 and redox potential under in situ conditions. This has resulted in a more reliable characterization of the Boom Clay pore water and a better understanding of perturbing processes such as oxidation.

Abstract To examine the impact of the heat generated by high-level radioactive waste on Boom Clay, two heater tests have been launched in the HADES underground research facility: the small-scale ATLAS Heater Test and the large-scale PRACLAY Heater Test. The major objective of these tests is to confirm and refine the thermo-hydro-mechanical (THM) constitutive models and associated parameter values obtained from a laboratory characterization programme. This paper presents the observations from the ATLAS and PRACLAY heater tests and the combined numerical modelling of these tests. To characterize the excavation damaged zone in the clay around these tests, a mechanical model with a strain-dependent elastic modulus is introduced for the Boom Clay. The consistency between the observations from laboratory tests and in-situ tests and the outcomes from the numerical models strengthen the confidence in our understanding of the THM behaviour of Boom Clay. They also enabled us to validate the mechanical model and produce a set of anisotropic THM property values for both intact and damaged Boom Clay.

Abstract Since the 1990s, SCK CEN, EIG EURIDICE and ONDRAF/NIRAS have been investigating the impact of gas generation on the Boom Clay and the engineered barriers. Several experiments have been performed to study gas transport in Boom Clay at laboratory scale and in the HADES URL. This paper gives an overview of these experiments. The transition from the laboratory to the in-situ scale is still a challenging task. It is our ambition to address these issues for Boom Clay, starting with the diffusive transport of dissolved gas. A large set of gas diffusion coefficients in Boom Clay from small-scale lab experiments (centimetre scale) is already available, and in order to validate these for use on a larger (metre) scale, an in-situ diffusion experiment with dissolved gas will be performed in the HADES URL, using the existing boreholes. In this new experiment, called NEMESIS, dissolved neon gas will be injected in one filter, and its diffusion will be monitored by three other filters. By re-using existing boreholes dating from the 1990s, the NEMESIS experiment will continue to provide new diffusion data for the next five years.

Abstract A wide range of metals have been studied as a candidate container material to be considered in the geological disposal concept of high-level radioactive waste and spent fuel in Belgium. More than 40 years ago, SCK CEN started studies on the corrosion of these metals. The HADES underground research laboratory (URL) played an important role in the corrosion research as it enabled in situ corrosion experiments to be carried out that simulate realistic disposal conditions as closely as possible. These experiments consisted of placing metallic coupons on a steel support tube, heated from the inside, that was installed in Boom Clay, thereby exposing the coupons to various conditions representative of the disposal concept that was considered at that time. Test durations lasted from 6 months to approximately 7.5 years. This paper summarizes the results from the various corrosion studies and discusses their implications in the choice of disposal concept. One of the main outcomes of these experiments was a change of rationale regarding the choice of the container material from carbon steel (corrosion-allowance) to stainless steel (corrosion-resistant). The main arguments for this change were the need to avoid severe pitting corrosion during the aerobic period and to minimize the generation of hydrogen gas during the subsequent anaerobic period.

Abstract Assessing the long-term safety of a deep geological repository for the disposal of radioactive waste depends on an adequate understanding of the processes governing radionuclide transport. From the early days of the research on geological disposal in clay in Belgium, large-scale, long-term in situ migration experiments were started to test whether our knowledge acquired about small-scale samples can be scaled up in time and space. These experiments use multi-filter piezometers to introduce radiotracers in a ‘source filter’ and monitor their breakthrough in ‘monitoring filters’. The CP1 experiment started in 1988 and used HTO as a tracer, while the Tribicarb-3D started in 1995 and used a cocktail of HTO and H 14 CO 3 − . At the start of these experiments, blind predictions were made based on lab-derived parameters and a simple representation of the hydrological system. Several decades later, these blind predictions still describe the data remarkably well. These tests provide valuable data for upscaling and validating the transport models in Boom Clay and allow us to estimate transport parameters at a larger scale. They provide strong arguments that the radiological safety of a deep geological repository in a clay rock can be guaranteed.

Abstract Twenty-five years ago, a unique long-term and large-scale in situ experiment with 14 C-labelled natural organic matter (NOM) was set up at the HADES underground research facility in Mol (Belgium) to study its migration behaviour. Natural organic matter plays an important role in the mobility of various safety-relevant radionuclides, which is critical in the context of Safety & Performance Assessment (SA/PA) calculations for a possible nuclear waste repository. The objective of this work is to enlarge the confidence in current NOM transport models by validating them with the in situ experiment, which is still continued to this day. Stepwise adding more complexity to the model resulted in a 10-parameter model with which excellent fits to the data are obtained. The model considers two different fractions that are transported by advection and diffusion and can be subject to both irreversible and reversible immobilization processes. The associated fitted parameter values compare well with values determined on small-scale migration experiments. This builds confidence in the NOM transport model, which in turn contributes to the confidence in the outcome of the radionuclide migration calculations performed in the context of SA/PA. These results again highlight the incredible value of such long-running experiments at underground research facilities like HADES.

Abstract When the Belgian Nuclear Research Centre (SCK CEN) launched a research, development and demonstration programme on geological disposal in the 1970s, it was not certain if a deep geological repository could be constructed in poorly indurated clay. This was tested by constructing the HADES underground research laboratory (URL) in Boom Clay, 225 m below SCK CEN's site in Mol. The construction history of the URL reflects how the understanding of the Boom Clay increased and how the excavation techniques and design of the gallery lining improved. It demonstrated that shafts, galleries and crossings between galleries can be constructed using industrial techniques. It also allowed characterization of the hydromechanical response of the clay and the clay disturbances induced by the excavation. This increased understanding is also reflected in the evolution of the geological disposal concept considered in Belgium. The current disposal concept foresees the installation of seals in the shafts and galleries. The HADES URL also offered the opportunity to test possible seal designs and develop a better understanding of the behaviour of bentonite, a possible seal material, owing to its swelling capacity, under in situ conditions.

Abstract Demonstrating the feasibility of constructing tunnels in deep clay formations is an important goal of the Belgian RD&D programme on the geological disposal of radioactive waste. In 2002 a major achievement was reached when the HADES Underground Research Laboratory (URL) in Boom Clay was extended with the construction of the Connecting Gallery. This demonstrated that it is feasible to construct galleries in poorly indurated clays using industrial techniques. To monitor the mechanical behaviour of the gallery and assess its stability, strain gauges were embedded in the segmental gallery lining and prisms were installed on the segments. These sensors provide valuable information that will support the design of future galleries. This paper presents 20 years of monitoring data in the Connecting Gallery and a first analysis of these data in terms of Boom Clay behaviour. In addition, the key findings are compared with those of a similar analysis performed by Andra (the French Radioactive Waste Agency) at the Meuse/Haute-Marne URL. The latter URL is excavated in the Callovo-Oxfordian claystone. The comparison identifies general trends and highlights similarities between the behaviour of tunnels in poorly indurated clay (Boom Clay) and in claystone.

Abstract A monitoring plan is an important part of a disposal programme. Monitoring a deep geological repository for the disposal of radioactive waste faces several challenges. These may arise from the technically demanding environment in which the monitoring equipment must operate or from the potentially long period of time during which they must operate. Over the past decades, a lot of experience has been gained in monitoring experiments in underground research laboratories (URL). Since the HADES URL became operational in the 1980s, thousands of sensors have been installed. To document the experience gained in this context, ONDRAF/NIRAS launched a research project to evaluate the performance of the monitoring equipment implemented in the HADES URL. This required developing a method to assess the performance of sensors in a consistent way. The methodology is explained in this paper and illustrated for the instruments installed to monitor the THM response of Boom Clay to the large-scale PRACLAY in situ experiment.

Abstract Since 1975, the European Commission has supported research in the field of radioactive waste management and geological disposal through the Euratom Research and Training Programme. During the first two community programmes (1975–85), the research activities focused on basic knowledge, feasibility and safety assessments of such geological disposal repositories. It was during this first decade of collaborative research activities that the site characterization, preliminary design studies and the application for authorization to construct the first underground research laboratory, the High Activity Disposal Experimental Site facility, took place.

Abstract The collaboration between HADES and the Mont Terri rock laboratory started in 1995, when granite was considered the most suitable host rock for radioactive waste disposal in Switzerland. When an alternative host formation was looked for by NAGRA, the choice of clay as host rock became rapidly clear. It was then decided to build an underground research laboratory in the Opalinus Clay taking advantage of the excavation of a motorway tunnel crossing the Mont Terri anticline. Exchanges of knowledge and experience on the behaviour of argillaceous formations were focused on the feasibility of constructing a safe geological repository for high-level radioactive waste: from excavation and gallery lining techniques to geochemistry, diffusion of radionuclides and coupled thermo-hydro-mechanical processes. The research programme at Mont Terri and the growing numbers of partners from four up to nine countries worldwide led to successive extensions of the galleries. The scientific and technical advances made possible by the collaboration between both laboratories dealt with the development of common methods and joint experimental and modelling efforts. This review article summarizes the main scientific lessons learned during these exchanges, stressing the added values of the knowledge transfer between partners and the overall cross-fertilization between HADES and Mont Terri.

Abstract In order to fulfil its mission, the Belgian Regulatory Body (constituted in Belgium by FANC and Bel V) has set up its own Research & Development programme, independent from the Waste Management Organisation's one. The Regulatory Body's Research & Development programme is mostly intended to investigate safety issues with the objective to be able to assess if the safety concept developed by the Waste Management Organisation fulfils the defined safety requirements. It includes the development and the follow-up of in situ experiments in underground research laboratories (e.g. in the Mont Terri project in Switzerland and at Tournemire in France). Such activities are essential to maintain and improve the scientific and technical skills, to strengthen the independence from the Waste Management Organisation and to build public confidence in the regulatory function. More generally, Research & Development in underground research laboratories is important to the Regulatory Body as it allows investigating processes, technologies and activities important to safety under in situ conditions. In particular, it is essential to accurately identify and characterize the processes upon which the safety functions of a disposal system rely, as well as processes that may affect the system performance. It is also necessary to demonstrate that construction and operational methods and technologies are technically feasible and can be implemented as assumed in the safety case. Such research and development activities are of great importance to properly manage some of the key uncertainties associated with a disposal programme and in particular to identify, characterize and reduce them where needed.