Contribution of Mont Terri URL to the development of the Cigéo project in France

The framework for managing long-lived radioactive waste in France was established by French laws adopted in 1991 and 2006. The 1991 law initiated a 15-year research programme investigating three options: long-term storage, a deep geological repository (DGR) and separation/transmutation. As part of this programme, Andra studied several potential sites and, in 1998, selected Callovo-Oxfordian claystone as a potential host rock for a DGR. The Meuse/Haute-Marne underground research laboratory (MHM URL) was built in Bure in 2000 to support experiments. At the end of the fifteen-year research period, the 2005 report confirmed the feasibility of a deep geological repository in the Callovo-Oxfordian claystone (COx) in the Meuse/Haute-Marne area. Subsequently in 2006, Andra initiated the Cigéo project (also known as the industrial geological disposal centre) to design the repository and complete the safety assessment. The application for authorisation to create the facility was submitted in 2023. Experiments conducted at the MHM URL have played a crucial role in demonstrating the project's robustness and refining its design (Armand et al. 2015). These studies serve two main purposes: improving our understanding of host rock behaviour to ensure long-term safety, and developing the technologies required for construction, operation and closure. As Delay et al. (2014) have highlighted, underground research laboratories (URLs) have evolved significantly over time. Around 25–30 years ago, their main purpose was to identify and address key scientific and technical challenges. At that time, priorities included: (a) defining research programmes focused on the confinement provided by the host rock and engineered barrier systems, (b) developing experimental equipment and methodologies, (c) carrying out preliminary feasibility studies, and (d) acquiring fundamental geological data. Since the early 2000s, and increasingly in recent years, URLs have shifted towards developing full-scale demonstrators. These demonstration experiments are designed to represent, test and optimise potential disposal systems and repository components under realistic conditions (Armand et al. 2015). In parallel with the development of the MHM URL, and even before, Andra participated in experiments in other URLs in claystones (Boom clay at HADES/Belgium, Opalinus Clay at Mont Terri/Switzerland), and even in granite (Grimsel Test Site/Switzerland), focusing on seal experiments. This was driven by a dual objective: (a) preparing its own experimental programmes at the MHM URL (methodology, experimental devices, protocols, etc.), and (b) gaining general knowledge of the similarities and differences between various clay-rich rocks. In the 1980s, HADES was the first URL dedicated to studying the geological disposal of radioactive waste in a deep plastic clay formation. Therefore, Andra relied on the work carried out in HADES through cooperation projects (Armand et al. 2023). However, the COx claystone is an indurated claystone, similar to Opalinus Clay, with comparable hydromechanical behavior. As a result, a major research programme was launched at the Mont Terri URL at the end of the 1990s. Moreover, given the time required to construct the access shafts to the MHM URL and the construction delays, as well as the 2005 milestones established under the 1991 law, the early implementation of a range of experimental studies at Mont Terri enabled the development of experimental concepts that were subsequently successfully deployed at the MHM URL. This paper aims to demonstrate Mont Terri's contribution to Andra's research and development programme. Different examples of added value are presented to illustrate some of the outcomes, specifically relating to geomechanics and geochemistry. These include the characterisation of claystone behaviour and seals (HM, THM and damaged zone by excavation, etc.), which are useful for the design of underground structures (constructability, lining/support, etc.), as well as for sealing in deep clay formations (constructability, lining/support, etc.). Other issues include safety assessments relating to the migration of radionuclides and interactions with materials.