Modelling the behaviour of compacted bentonite for nuclear waste disposal

压实膨润土被认为是地质处置设施工程屏障系统(EBS)的一部分。作为核废料罐和宿主形成物之间的缓冲器，它们将在与后者的界面上水化，并在与前者的界面上经受高温。EBS设计的目标是进行水化，以促进膨润土缓冲液的膨胀，从而增加其体积，以密封罐和主体地层之间的结构空隙。与膨润土的模拟有关的是对其在压实状态下的双重孔隙结构的认识，包括粘土集合体内的微观孔隙度和粘土集合体之间的宏观孔隙度。这种结构随着水化作用的减弱而减小，导致材料在完全水化(饱和)时具有单一的孔隙率。伦敦帝国理工学院(ICL)最近的一项博士研究为压实膨润土开发了一种新的双结构本构模型(IC DSM；Ghiadistri等人，2018；Ghiadistri，2019)。该模型是巴塞罗那扩展框架(Gens&Alonso，1992)的扩展和推广版本，并在定制的计算平台ICFEP(帝国理工学院有限元程序；Potts&Zdravkovic，1999,2001)中实现，该平台运行控制有限元方程的完全热-流体-机械(THM)耦合公式(崔等人，2018)。该模型已经成功地应用于压实膨润土(如Dueck等人，2014年)的实验室规模膨胀压力实验(Ghiadistri等人，2019a)和大型野外实验(如FEBEX实验(Enresa，2000年))的模拟，自2017年以来，这些建模工具还被用于Beacon Eurtom项目(批准号745942)，模拟压实膨润土块和颗粒的实验室膨胀压力实验，以及其他大型野外实验。现有研究的大部分(实验、野外、关于压实膨润土与核废料处置有关的行为的研究(数值)考虑了其在高达100摄氏度的温度下的暴露。拟议研究的目的是通过使用ICFEP软件对与瑞士Grimsel试验场的HotBENT实验有关的缓冲区和主岩的热、水力和机械演化进行预测建模，来探索温度高于100摄氏度的膨润土缓冲区的行为。这项研究的目的是根据GDF内地下金库和沉积孔网络的足迹来帮助优化GDF的设计。这项研究将首先利用已发表的文献，对现有的关于膨润土在高温下行为的实验证据进行审查。与背景研究中描述的方法类似，将首先模拟小规模实验室实验，以验证建模工具在100摄氏度以上的性能。然后，数值工具将应用于大型HotBENT实验的模拟，在该实验中，膨润土缓冲液暴露在250摄氏度以上的温度下。对膨润土THM演化的数值预测将与本实验收集的现场测量结果进行比较。现场数据将由英国放射性废物管理(RWM)提供，该机构也将担任该项目的工业监督。数值模拟将进一步研究宿主地层中的近场效应。特别是，这将涉及对工程屏障周围地面的渗透性和孔隙水压力状况因温度而可能发生的变化，以及与单个沉积孔/拱顶相关的这些变化的程度进行量化。

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.