Modelling the behaviour of compacted bentonite for nuclear waste disposal

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.

It cannot be overstated how important reducing CO2 emissions are in both electricity production for homes and industry but also in reducing road pollution by replacing petrol/diesel cars with electric cars in the next 20 years. These ambitions will require a large growth in electricity production from low carbon sources that are both reliable and secure and must include nuclear power in this energy mix. Such a future will empower the vision of a prosperous, secure nation with clean energy. To do this the UK needs more than 100 PhD level people per year to enter the nuclear industry. This CDT will impact this vision by producing 70, or more, both highly and broadly trained scientists and engineers, in nuclear power technologies, capable of leading the UK new build and decommissioning programmes for future decades. These students will have experience of international nuclear facilities e.g. ANSTO, ICN Pitesti, Oak Ridge, Mol, as well as a UK wide perspective that covers aspects of nuclear from its history, economics, policy, safety and regulation together with the technical understanding of reactor physics, thermal hydraulics, materials, fuel cycle, waste and decommissioning and new reactor designs. These individuals will have the skill set to lead the industry forward and make the UK competitive in a global new build market worth an estimated £1.2tn. Equally important is reducing the costs of future UK projects e.g. Wylfa, Sizewell C by 30%, to allow the industry and new build programme to grow, which will be worth £75bn domestically and employ tens of thousands per project.

We will deliver a series of bespoke training courses, including on-line e-learning courses, in Nuclear Fuel Cycle, Waste and Decommissioning; Policy and Regulation; Nuclear Safety Management; Materials for Reactor Systems, Innovation in Nuclear Technology; Reactor Operation and Design and Responsible Research. These courses can be used more widely than just the CDT educating students in other CDTs with a need for nuclear skills, other university courses related to nuclear energy and possibly for industry as continual professional development courses and will impact the proposed Level 8 Apprenticeship schemes the nuclear industry are pursuing to fill the high level skills gap.

The CDT will deliver world-class research in a broad field of nuclear disciplines and disseminate this work through outreach to the public and media, international conferences, published journal articles and conference proceedings. It will produce patents where appropriate and deliver impact through start-up companies, aided by Imperial Innovations, who have a track record of turning research ideas into real solutions. By working and listening to industry, and through the close relationships supervisory staff have with industrial counterparts, we can deliver projects that directly impact on the business of the sponsors and their research strategies. There is already a track record of this in the current CDT in both fission and fusion fields. For example there is a student (Richard Pearson) helping Tokamak Energy engage with new technologies as part of his PhD in the ICO CDT and as a result Tokamak Energy are offering the new CDT up to 5 studentships.

Another impact we expect is an increasing number of female students in the CDT who will impact the industry as future leaders to help the nuclear sector reach its target of 40% by 2030.
The last major impact of the CDT will be in its broadening scope from the previous CDT. The nuclear industry needs to embrace innovation in areas such as big data analytics and robotics to help it meet its cost reduction targets and the CDT will help the industry engage with these areas e.g. through the Bristol robotics hub or Big Data Institute at Imperial.

All this will be delivered at a remarkable value to both government and the industry with direct funding from industry matching the levels of investment from EPSRC.