ROCK TUNNELS | DEEP REPOSITORY R&D FOR RADIOACTIVE WASTE


MODELLING A SEALING CONCEPT FOR DEEP BURIAL


Research on plans for long-term deep burial of radioactive waste has modelled a sealing structure concept and the excavation damage zone in Cox claystone geology in France


M. Alonso1, J. Vaunat1 1 , M-N. Vu2 , J. Talandier2 , S. Olivella1 & A. Gens1


Department of Civil and Environmental Engineering, Universitat Politècnica de Catalunya, Barcelona, Spain 2


French National Radioactive Waste Management Agency (Andra), Paris, France


INTRODUCTION The French National Radioactive Waste Management Agency (Andra) is leading the design of a deep geological radioactive waste disposal to be located in the Callovo-Oxfordian formation. At the disposal main level of the proposed Cigéo repository, large-diameter galleries are to provide access to storage cells and connection between zones. After a long operational period, the disposal facility is to be closed by large diameter sealing structures, intended to prevent the flow of water and the migration of radionuclides over the


post-closure period (~ a few thousand years). The paper presents the assessment and simulation


of the phenomena underlying the response and performance of the sealing concept under real disposal conditions, looking at: 3D hydro-mechanical modelling of sealing off the galleries in clayey rock; the large geometries involved, plus interfaces and anisotropy; progressive hydration and swelling of the sealing core; longitudinal movement of concrete plugs while core density and swelling pressure do not significantly change; and, the swelling core resulting in some recompression of the excavation-induced damaged zone (EDZ) in the area of lining deposition.


PROJECT Due to its very low hydraulic conductivity, low effective diffusion, and significant radionuclide retention capacity, the Callovo-Oxfordian (COx) claystone is being considered to act as the host formation for geological disposal of radioactive waste. Andra is leading the design of Cigéo, a deep geological disposal to be implemented (if authorised) in eastern France at about 500m depth, in the Cox claystone (see Figure. 1). The main role of a deep geological disposal is


to isolate the radioactive waste from humans and biosphere. The long-term safety of such facilities is associated with their ability to protect the environment over the period of several millions of years (Gens 2004).


20 | February 2025 The long-term safety of Cigéo relies on the COx


claystone and the so-called sealing structures, positioned in key positions in shafts, ramps and horizontal drifts. They are intended to limit the flow of water and the migration of radionuclides. Excavation of underground drifts leads to significant


changes in the local stress field and generally causes damage to the rock around the openings. The area affected around the drifts is generally identified as the EDZ and the alterations usually imply significant changes in flow and transport properties (Blümling et al. 2007). Since water is one of the main transfer vectors of


radioactive substances, controlling the preferential pathway of groundwater through the fractures caused by excavation is a key objective. The sealing structures should be therefore designed not only to seal the drifts but also to reduce the EDZ to low permeability, re- creating conditions similar to those of the intact host rock. Performance assessment to confirm the design of


the sealing structures and to verify that requirements are fulfilled is complex, given the variety of materials and the large number of interacting processes and phenomena to evaluate, and the extremely long periods of time under consideration (Gens et al. 2021). Numerical simulation is a useful and powerful tool


to better understand the behaviour of the engineered barriers. The paper presents the work carried out to assess and simulate the phenomena underlying the response and performance of the large-diameter sealing concept under real disposal conditions.


SEALING STRUCTURE: CURRENT CONCEPT OVERVIEW


In the Cigéo concept, large-diameter galleries (~10m excavation diameter) would provide access to the storage cells and connection between different parts of the deep disposal facility. Sealing these tunnels is challenging. Due to its high swelling capacity when hydrated with water and its low permeability, bentonite


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