Modelling of sealing structures for radioactive waste repositories in clay formations (en ce compris une introduction à la méthodologie de la recherche)

Abstract

To meet the climate goals established in the Paris agreement, decarbonisation of global energy production is necessary. To do so, nuclear energy has a key role to play. However, the nuclear industry faces several challenges. Among the latter, the production of high level radioactive waste represents a threat to the biosphere; their safe management is thus of great importance. Currently, deep geological disposal systems are widely recognised as the safest long-term option. It consists of the burial of the waste in a stable geological formation, as well as the implementation of an engineered barrier system. Nevertheless, the latter is subject to complex coupled hydro-mechanical phenomena, particularly due to the presence of bentonite, a clay exhibiting a significant swelling capacity upon hydration. Understanding these phenomena is thus essential to ensure the safety of this solution. Accordingly, in this work, the finite element code LAGAMINE is employed to numerically simulate the lifetime of the sealing structure. A progressive modelling strategy is adopted, starting with small scale benchmark exercises before a gradual evolution of the complexity of the models, up to the representation of the entire sealing structure. The emphasis is first placed on the behaviour of bentonite and on a sensitivity analysis of its mechanical parameters. Then, its behaviour under a situation closer to reality is studied through a larger scale 2D axisymmetric model, with a first quantification of several variables, such as the swelling pressure and the displacement of the bentonite sample for instance. Lastly, the behaviour of the entire sealing structure is studied, still through a 2D axisymmetric model. In this last model, different phases of the lifetime of the gallery are modelled, from the excavation works to the end of the saturation of the structure. In terms of results, this work first showed that the employed numerical model is capable of reproducing the experimental data corresponding to laboratory tests of a bentonite sample. Then, it showed that the first type of bentonite used in this work develops too much swelling pressure, making it necessary to switch to a different mixture of bentonite, composed of 20% of powder and 80% of pellet. Subsequently, it demonstrated that this new mixture effectively reduces the swelling pressure, thereby satisfying the requirements of the repository site. Finally, the full-scale model yielded results that are consistent with those presented before, confirming that the requirements are also satisfied for the entire sealing structure.