Submitted by Paula Block on Wed, 23/04/2025 - 11:35
Talk Title: The Deep Geological Repository in Switzerland - Some Critical Engineering Challenges
Speaker Bio: Dr Alexandros Nordas is a Senior Scientific Associate and Lecturer in the Chair of Underground Construction at ETH Zurich. His research focuses on life-cycle simulation and hazard assessment of underground systems, squeezing ground conditions and related time-dependent processes (creep, consolidation), mechanised TBM tunnelling, constitutive modelling, as well as the development of design aids for engineering practice. Over the past years, Alexandros has also been actively involved as an expert technical consultant in underground infrastructure projects, predominantly the deep geological repository for radioactive waste in Switzerland overseen by Nagra. Since September 2024, he has also undertaken the position of Senior Specialist for Rock Mechanics and Underground Construction at Nagra, alongside his position at ETH Zurich. Alexandros holds a 5-year Diploma in Civil Engineering from the National Technical University of Athens (2013; Honours), along with an M.Sc. in Earthquake Engineering(2014; Distinction) and a Ph.D. in Computational Structural Mechanics (2019) from Imperial College London. He has authored and co-authored more than 40 expert technical consulting reports and scientific publications in international, peer-reviewed journals and conference proceedings. He also serves as a peer reviewer for the Journal of Rock Mechanics and Geotechnical Engineering (Elsevier), Rock Mechanics and Rock Engineering (Springer), and several others. Throughout the years, Alexandros has received numerous honours and awards in recognition of his work and teaching, including the Telford Premium Prize by the Institution of Civil Engineers (ICE), as well as the Letitia Chitty Centenary Memorial Prize and the Patrick J. Dowling Prize in Advanced Structural Engineering from Imperial College London.
Talk Abstract: Nuclear energy plays a central role in the decarbonisation pathway towards net zero, providing a reliable and scalable source of electricity with virtually no direct emissions. Nevertheless, its widespread deployment poses significant challenges, foremost among them being the safe, long-term management of radioactive nuclear waste. The concept of deep geological disposal originated in the 1960s and has since evolved into an internationally recognised solution for long-term waste containment. It entails isolating the waste from the human habitat by emplacing it in repositories embedded in stable geological formations several hundred metres below the ground surface. The repository’s safety over the thousands of years required for radioactive decay relies on a multi-barrier system, comprising engineered barriers as well as the host rock, which acts as a natural barrier. Deep geological repository initiatives are currently advancing at varying levels of technical and regulatory maturity in many countries across the globe.
In Switzerland, the construction of a deep geological repository is foreseen over the next years. Nagra – the Swiss National Cooperative for the Disposal of Radioactive Waste – has identified Opalinus Clay as the most suitable host rock, due to its favourable properties for long-term waste containment: extremely low hydraulic permeability, self-sealing capacity via swelling, and waste radionuclide retention capacity. Since 2008, intensive geological investigations have been conducted to identify the most suitable siting region for the repository. These efforts culminated in Nagra’s 2024 siting proposal for Nördlich Lägern, a site in northern Switzerland offering an optimal combination of depth (ca. 900 m), thickness (ca. 100 m), and lateral extent of the Opalinus Clay formation. At Nördlich Lägern, a combined repository is currently planned, encompassing 3.5 m-diameter emplacement drifts for high-level waste (HLW), and 14 m-diameter emplacement caverns for low- and intermediate-level waste (L/ILW).
The design and construction of the repository’s underground structures pose significant technical challenges, inter alia due to the substantial depth, the prevailing hydrogeological conditions, and the geomechanical behaviour of the host rock. This presentation delves into some critical engineering challenges addressed over the past years, including: (i) the comprehensive geotechnical characterisation of Opalinus Clay based on an extensive suite of laboratory experimental investigations; (ii) the development of a design-oriented constitutive modelling framework for Opalinus Clay, capable of reproducing intricate aspects of its hydromechanical behaviour and accounting for key uncertainties; and (iii) the development of high-fidelity computational models for the assessment of hazards related to the construction and long-term structural safety of the emplacement tunnels, considering their intended purpose. The presented advancements highlight critical design considerations for the subsequent project stages, but also reveal novel aspects and offer broader guidance for the development of underground infrastructure in comparably demanding environments.