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

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.