Effect of high temperature on chemo-mechanical degradation of compacted clays intended for the isolation of HLW and SNF

高放核废料(HLW)和乏核燃料(SNF)的地质处置设施(GDF)基于多屏障概念，由金属罐(包裹HLW/SNF)、工程粘土屏障(用作罐周围的缓冲)和作为天然屏障的宿主岩石组成。不饱和压实膨润土是建造工程屏障系统(EBS)的常用材料。EBS将经历复杂的热-水-机械和化学(THMC)过程，触发过程包括HLW/SNF释放的热量、粘土(来自围岩)的水化、高度受限条件下压实膨润土的逐渐润湿和膨胀导致的应力增加以及化学相互作用。目前对温度(T)如何影响粘土缓冲器的流体力学和化学行为的了解主要基于涉及温度高达100摄氏度的研究。然而，来自世界各地负责开发和交付GDF的不同国家的当局认识到，在远高于100摄氏度(例如200摄氏度)的温度下实现安全运行将使设计、就位策略、临时存储和GDF成本得到更好的优化。已观察到钙基膨润土(即饱和二价阳离子)的膨胀压力(SP)随温度的增加而减小，而钠基土(即饱和一价阳离子)的膨胀压随温度的增加而增加。这种不同行为背后的物理化学现象尚待详细研究。这是一个关键的研究组成部分，考虑到钠基膨润土和钙基膨润土都被认为是分离HLW/SNF的潜在屏障材料。达到目标SP是提供所需机械保护的关键，在为GDF选择合适的膨润土时，准确预测这一性能将是至关重要的。首要目标是进行必要的基础研究，以优化T高达200℃的EBS设计中膨润土障的类型和特性。这需要更好地了解用作屏障的钠和钙膨润土在高温下的行为，特别是在GDF条件下。该项目将结合在大学(TAMU和ICL)以及国家实验室(SNL和BGS)工作的美国和英国的研究人员，进行基础、实验和数值研究，以促进对用于EBS的钠和钙膨润土在非常高的T，高达~200摄氏度时行为的当前理解。

Geological Disposal Facilities (GDFs) for high-level nuclear waste (HLW) and spent nuclear fuel (SNF) are based on the multibarrier concept, consisting of a metallic canister (encapsulating the HLW/SNF), an engineered clay barrier (that serves as a buffer around the canister), and the host rock, which serves as a natural barrier. Unsaturated compacted bentonite is the material generally selected to build the engineered barrier systems (EBS). The EBS will be subjected to complex thermo-hydro-mechanical and chemical (THMC) processes triggered by the heat released by the HLW/SNF, the hydration of the clay (from the surrounding rock), increments in stresses induced by progressive wetting and swelling of the compacted bentonite under highly confined conditions, and chemical interactions. Current understanding of how temperature (T) affects the hydromechanical and chemical behaviour of the clay buffer is primarily based on studies involving T up to 100 degrees C. However, authorities from different countries around the world tasked with developing and delivering GDFs recognise that enabling safe functioning at T much higher than 100 degrees C (e.g., 200 degrees C) would allow better optimisation of the design, emplacement strategies, interim storage and GDF costs. It has been observed that the swelling pressure (SP) of a Ca- bentonite (i.e., saturated with divalent cations) will tend to decrease with increasing T, but the SP of a Na- bentonite (i.e., saturated with monovalent cations) will tend to increase with increasing T. The physicochemical phenomena behind this dissimilar behaviour have yet to be investigated in detail. This is a critical research component considering both Na- and Ca-bentonites which are envisaged as potential barrier materials for the isolation of HLW/SNF. Achieving a target SP is a key to providing the mechanical protection required, and accurate prediction of this property will be essential when selecting suitable bentonites for a GDF.The overarching aim is to conduct the fundamental research necessary to optimise the type and properties of the bentonite barrier in the design of EBS at T as high as 200 degrees C. This requires better understanding of the behaviour at high T of Na- and Ca- bentonites intended as barriers, particularly when subjected to GDF conditions. This project will combine researchers from US and UK working at universities (TAMU and ICL) and national laboratories (SNL and BGS) to conduct fundamental, experimental and numerical investigations to advance the current understanding of the behaviour of Na- and Ca-bentonites intended for EBS, when subjected to very high T, up to ~200 degrees C.