Modelling the behaviour of compacted bentonite for nuclear waste disposal

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Compacted bentonite clays are envisaged as part of engineered barrier systems (EBS) in geological disposal facilities (GDF). Placed as a buffer between the nuclear waste canister and the host formation, they will be subjected to hydration, at their interface with the latter, and to high temperatures, at their interface with the former. The objective of the EBS design is for hydration to promote the swelling of the bentonite buffer and hence increase its volume to seal construction voids between the canister and the host formation. Pertinent to the modelling of bentonite is the recognition of its double-porosity structure in the as-compacted state, comprising the micro-porosity within the clay aggregates and macro-porosity between the clay aggregates. This structure diminishes with hydration, leading to a single-porosity material at full hydration (saturation). A recent PhD research at Imperial College London (ICL) developed a new double-structure constitutive model for compacted bentonite clays (IC DSM; Ghiadistri et al., 2018; Ghiadistri, 2019). The model is an extended and generalised version of the Barcelona Expansive framework (Gens & Alonso, 1992) and is implemented in the bespoke computational platform ICFEP (Imperial College Finite Element Program; Potts & Zdravkovic, 1999, 2001), which operates a fully thermo-hydro-mechanically (THM) coupled formulation of the governing finite element equations (Cui et al., 2018). The model has been successfully applied to simulations of both laboratory-scale swelling pressure experiments (Ghiadistri et al., 2019a) on compacted bentonite (e.g. Dueck et al., 2014) and large-scale field experiments (Ghiadistri et al., 2019b) such as the FEBEX experiment (ENRESA, 2000), exposed to temperatures under 100^o C. Since 2017 these modelling tools have also been used in the BEACON Euratom project (grant no. 745942), simulating laboratory swelling pressure tests on compacted bentonite blocks and pellets, as well as other large-scale field experiments.Most of the existing research (experimental, field, numerical) on the behaviour of compacted bentonite, in relation to nuclear waste disposal, has considered its exposure to temperatures of up to 100^o C. The objective of the proposed research is to explore the behaviour of bentonite buffers at temperatures above 100^o C, by conducting predictive modelling with the software ICFEP, of the thermal, hydraulic and mechanical evolution of the buffer and host rock, associated with the HotBENT experiment at Grimsel Test Site in Switzerland. The research is aimed at helping optimisation of GDF design in terms of the footprint of the network of underground vaults and deposition holes within a GDF. The research will first conduct a review of existing experimental evidence on bentonite behaviour under high temperatures, using published literature. Similar to the methodology described in the Background research, small-scale laboratory experiments will be simulated first to verify the performance of the modelling tools at temperatures over 100^o C. The numerical tools will then be applied to simulations of the large-scale HotBENT experiment in which bentonite buffers are exposed to over 250^o C temperatures. The numerical predictions of the bentonite's THM evolution will be compared to field measurements collected from this experiment. The field data will be provided by the Radioactive Waste Management (RWM), UK, who will also act as industrial supervisor for the project. The numerical modelling will further investigate the near-field effects in the host formation. In particular, this will involve quantification of the likely changes, due to temperature, in the permeability and the pore water pressure regime in the ground around the engineered barrier, as well as the extent of these changes in relation to a single deposition hole / vault.