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 The heat generated by high-level waste or spent fuel will create disturbances around a deep geological repository (DGR) containing these wastes. Since the 1990s, SCK CEN, EIG EURIDICE and ONDRAF/NIRAS have been characterizing the thermo-hydromechanical (THM) behaviour of Boom Clay and assessing the impact of the thermal disturbances. This research has included laboratory tests as well as in situ experiments in the HADES Underground Research Laboratory. The two types of tests have been complementary. Laboratory tests have allowed understanding of the THM behaviour and determination of associated values of the THM parameters of the clay under well-controlled boundary conditions and loading paths. This knowledge and the parameters were then validated and even improved by in situ tests which allowed investigation of the effects of temperature on the Boom Clay behaviour at large scales. This paper gives an overview of this research and presents the main findings. It also explains how the knowledge gained supports the design of a possible future DGR and contributes to assessing the extent and impact of the THM disturbances in the Boom Clay around a DGR.

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 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 In the 1980s, HADES (High-Activity Disposal Experimental Site) was the first underground research laboratory (URL) dedicated to the study of the geological disposal of radioactive waste in a deep clay formation, the Boom Clay. It was not until the early 2000s, after a siting process, that ANDRA implemented the Meuse/Haute-Marne URL, in the Callovo-Oxfordian formation at a depth of about 500 m in order to develop the Cigéo project (French industrial centre for geological disposal). ANDRA therefore relied heavily on the work carried out in HADES, through numerous co-operation projects (participation in in situ experiments) both between ANDRA and ONDRAF/NIRAS and SCK CEN (EURIDICE) and/or with Mont Terri consortium, and within European projects (CLIPEX, RESEAL, etc.). This was driven by a dual objective: (1) to prepare its own experimental programmes in the Meuse/Haute-Marne underground laboratory (methodology, experimental devices and protocols, etc.); and (2) to acquire general knowledge on the behaviour of argillaceous rocks, in particular in terms of similarity and differences between the various argillaceous rocks. This paper illustrates the contribution of HADES to the ANDRA programme. This concerns the characterization of the claystone behaviour, host rock and swelling clay-based seals (hydromechanical, thermo-hydromechanical, excavation damaged zone, etc.), and the design and the behaviour of underground structures and seals in deep clay formation (constructability, lining/support, etc.).

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 After decades of research, development and demonstration (RD&D), mature concepts for the geological disposal of long-lived and high-level radioactive waste exist and some are close to being implemented. Underground research laboratories (URLs) have made an essential contribution to this progress. They enable in situ characterization and testing of host rocks and the demonstration of technologies and component performances at representative scales and under realistic geological conditions. They also offer a tool for training personnel and show aspects of the geological disposal concepts to stakeholders, including the public. In this paper we will present the different types and roles of URLs and we will discuss how the RD&D role of URLs has evolved and how it is likely to evolve in the near future.

Abstract The timeframes involved in nuclear waste management often speak to the imagination, and even transcend it: what does it mean to isolate and contain human-made materials for periods up to hundreds of thousands or even a million years? In this article, we reflect on the role played by the HADES Underground Research Laboratory in making the distant future comprehensible today. Our argument starts by focusing on the pioneering role HADES played and plays in knowledge production on geological disposal. It highlights the heterogeneous nature of scientific experiments and experiences, and the performative role these play in defining matters of concern for research and development. Second, attention is directed to how HADES contributes to the defining of what is considered possible and imaginable, and how it therefore not only renders the future more predictable, but also contributes to the making of that future. We end the paper with a reflection on the implications of what ‘making the future’ could entail from an ethical perspective, discussing how the intergenerational responsibilities that come with these future-making capacities could be handled.

Abstract Geological disposal provides the safe long-term management solution for higher-activity radioactive waste. The development of a repository (or geological disposal facility) requires a systematic and integrated approach, taking into account the characteristics of the waste to be emplaced, the enclosing engineered barriers, and the host rock and its geological setting. Clays and clayey material are important in the development of many national geological disposal systems. Clays exhibit many interesting properties, and are proposed both as host rocks and as material for engineered barriers. Whatever their use, clays present various characteristics that make them high-quality barriers to the migration of radionuclides and chemical contaminants. As host rocks, clays are, in addition, hydrogeologically, geochemically and mechanically stable over geological timescales (i.e. millions of years).