Bentonite Swelling into Voids: Different Modelling Approaches for Hydration with Technological Gaps

Bentonite-based materials have emerged as a highly promising choice for engineered barriers in nuclear waste deep geological disposal. These materials are characterised by low permeability, high swelling capacity and effective radionuclide retardation, making them suitable for sealing underground galleries and canisters containing nuclear waste. However, the presence of technological gaps within the bentonite or the host rock can significantly influence their hydromechanical behaviour, potentially creating preferential pathways for radionuclide migration, thus affecting the overall performance of the engineered barrier. In this study, two different modelling strategies (namely, "gap" and "no-gap") to reproduce technological gaps and their effect on the hydromechanical behaviour of bentonite-based materials during intermediate saturation stages are proposed. The numerical model is used to simulate laboratory tests, and the numerical results are compared with experimental data coming from hydration test conducted under overall constant volume (isochoric) conditions. It is noteworthy that the specimen used in the experimental study is characterised by a localised gap between its side and the cell wall. The paper highlights the benefits of the "gap" numerical model, which employs interface elements to reproduce technological gaps at the side of the cell and exhibits satisfactory capabilities in reproducing the experimental swelling pressure evolution during bentonite hydration, especially during the transient wetting stages. Significant implications are expected for predicting site performance of engineered barrier systems in nuclear waste disposal applications. The effect of technological gaps on the hydro-mechanical behaviour of bentonite-based materials for nuclear waste disposal is investigated.Two different modelling strategies, "gap" and "no-gap", are proposed to simulate the presence of technological gaps in the bentonite.The numerical results are compared with experimental data from a hydration test under constant volume conditions.The advantages of the "gap" numerical model, which can reproduce the experimental swelling pressure evolution more accurately, are demonstrated and its implications are discussed for the performance of engineered barrier systems.