Advanced Modelling for Deep Burial Structures in France

In the heart of France’s Callovo-Oxfordian (COx) claystone, a complex challenge unfolds—how do we safely seal radioactive waste for thousands of years? Enter Cigéo: the French National Radioactive Waste Management Agency’s ambitious project for long-term deep geological storage. At 500 meters below ground, sealing structures will protect humanity and the environment by limiting water flow and radionuclide migration. These engineered barriers are more than plugs—they are active systems interacting with the surrounding rock for millennia.

Central to the sealing strategy is bentonite, a swelling clay known for its low permeability. Confined between two concrete plugs, the bentonite core expands when hydrated, pressing against the excavation-damaged zone (EDZ) to recompress fractures and restore the rock’s original tightness. This self-sealing action supports the natural barrier function of the COx claystone.

Understanding how this system evolves over thousands of years isn’t easy. That’s where simulations step in. Using the finite-element software CODE_BRIGHT, researchers from UPC and Andra modelled the entire lifecycle—from excavation to post-closure equilibrium. They considered complex variables: swelling pressure, dry density, saturation, and the movement of concrete plugs.

The model showed that after 2,000 years, the bentonite core becomes fully saturated. Around 3,000 years in, the system reaches hydro-mechanical equilibrium. Swelling pressures varied depending on position within the core, highlighting the importance of uniform core density. Even under stress relief and partial deconfinement, the central core maintained its designed pressure—crucial for EDZ recompression.

What stood out? The sealing core’s ability to “breathe” with its environment. Concrete plug performance, backfill stiffness, interface shear strengths—each played a role in maintaining structural balance. Interestingly, even when plug-lining friction was reduced, the core still exerted meaningful pressure. EDZ recompression occurred only where the lining was removed, reinforcing the importance of direct host rock-core interaction.

This work confirms the power of advanced numerical modelling in designing repositories that work with nature—not against it. As the world searches for permanent radioactive waste solutions, projects like Cigéo provide a blueprint of scientific rigour, geotechnical innovation, and environmental responsibility.

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