GeoSafe Project

GeoSafe is organised into seven interconnected Work Packages (WPs), each of which is focused on answering one or more of the key questions highlighted in the original GeoDraw call. Each WP lead is supported by one or more co- Investigators. The bulk of the research is being carried out by PDRAs, each of whom is associated with one or more WPs. Roughly, WP1 and WP2 fall under the rubric of Challenge 1, WP3 and WP4 under Challenge 2, and WP5, WP6, and WP7 under Challenge 3, although this categorisation is not rigid, as some WPs will contribute to more than one Challenge. Each Work Package is supported by a dedicated Subject Matter Expert (SME) from NWS. The overall project is being supervised by Imperial College London.

Each of the three Challenges has a Challenge Lead, who will oversee the work covered within that Challenge, co-ordinate interactions between the three Challenge areas, and lead meetings between project researchers to facilitate this interaction. The three Challenge Leads are each assisted by a Subject Matter Expert from Nuclear Waste Services.

The seven individual WPs are described briefly below.

WP 1: Multi-scale (Å-scale to metre-scale) and multi-dimensional (2D and 3D) characterisation of LSSR heterogeneity. In this WP, we are applying a host of novel techniques to provide insights into characterisation and conceptualisation of geological complexity and heterogeneity at multiple length and time scales.

The objectives of WP 1 include: (a) To identify the key lithologies present, and the degree of macro-scale heterogeneity within the selected LSSRs, and the lithological and compositional nature of this heterogeneity. (b) To quantify the impact of macro- to micro-heterogeneity on hydrological and mechanical properties, and assess the applicability of the French knowledge base to similar UK lithologies. (c) To investigate the hydro-mechanical properties of selected end-member samples representing each lithology, and the influence of stress history and regional sedimentology. (d) 3D spatial quantification of microstructure (laminations, mineral distribution, orientation, grain size, interactions), and pore network (pore size, geometry, connectivity) heterogeneity from mm-scale down to A-scale. (e) Quantification of the degree of heterogeneity across multiple scales, categorisation of LSSR archetypes and the implications for their response to likely stress change scenarios during GDF evolution.

WP 2: Characterisation and evolution of critical flow paths. The aim of this WP is to understand how the permeability of LSSR changes with stress, and how it will evolve under different environmental conditions with time. The work will provide vital input to the numerical modelling described in WP 6 and WP 7.

The objectives of WP 2 include: (a) To identify and measure baseline mechanical and flow properties for LSSR lithologies. (b) To quantify the yield conditions for key LSSRs, and monitor the associated permeability changes, as they are loaded to, and beyond, failure. (c) To identify and quantify the propensity for self-healing of distributed (microfracture) and localised fracture damage (macroscopic fractures) as a function of time, temperature, pressure, and pore fluid saturation and chemistry.

WP 3: Geochemical Controls on Radionuclide Transport and Fate in Heterogeneous LSSR – from Atomic to Macro-scale. This WP focuses on identifying and assessing the key geochemical processes that control radionuclide speciation and transport within representative LSSRs, using a variety of lab-based experimental approaches combined with state-of-the-art analytical techniques, such as synchrotron based X- ray and high-resolution electron microscopy imaging.

The objectives of WP 3 include: (a) To determine bulk partitioning and sorption mechanisms of radionuclides in LSSRs as a function of geochemical conditions. (b) Determine the fate and transport rate of radionuclides within LSSRs. (c) To investigate radionuclide transport and the effect of microbes within the fractures and pore system in LSSRs and their interactions over time scales from seconds to months. (d) Develop geochemical models to predict radionuclide speciation and processes controlling transport in LSSRs.

WP 4: Geomicrobiological controls of radionuclide transport and fate. This WP will define how microbial processes contribute to the long-term evolution of the baseline LSSR and the Chemically-Disturbed Zone (CDZ) systems characterised in WP3, with a clear focus on quantifying microbial controls on contaminant mobility.

The objectives of WP 4 include: (a) To determine how the heterogeneity of LSSR affects its ability to support microbial activity through the provision of organic carbon and nutrient sources and to characterise the microbial communities within LSSRs. (b) Use microcosm batch cultures to quantify the impact of LSSR GDF-relevant microorganisms on the fate of priority radionuclides under a range of relevant biogeochemical conditions expected during LSSR evolution. (c) To determine the impact of microbial processes on radionuclide fate column experiments, designed to mimic the CDZ in a cementitious GDF environment.

WP 5: Pore-scale reactive transport modelling and upscaling. In this WP, we are developing a multi-scale numerical framework to model geochemical reactive transport from the nanometre-scale to the centimetre-scale, and will provide quantitative models that describe how fracture permeabilities evolve as a result of radionuclide sorption.

The objectives of WP 5 include: (a) To use numerical simulations and machine learning to upscale radionuclide transport from the nm scale to the mm scale. (b) To develop digital twins of mm-scale LSSR samples. (c) Use the digital twin geometries to upscale radionuclide transport and sorption from the mm to the cm scale. (d) To upscale radionuclide reactive transport and sorption from the cm scale to the reservoir scale.

WP 6: Modelling of damage caused by excavation process and thermal effects from waste. Simulations of the deformation around underground excavations and tunnels in a GDF constructed within LSSR will be conducted using the Imperial College Geomechanics Toolkit (ICGT), making use of rock property values and heterogeneity characterisation from WP1 and WP2. Constitutive modelling of the LSSRs will implement modified Cam-Clay type models, calibrated with data from WP1 and WP2.

The objectives of WP 6 include: (a) To numerically simulate the deformation and damage that will occur around underground excavations and boreholes in a GDF constructed within an LSSR formation, due to the excavation process. (b) To compare the damage predictions made using different constitutive models such as Cam-Clay, Drucker-Prager, elastic-brittle, etc. (c) To numerically simulate the deformation and damage that will occur around underground excavations and boreholes in a GDF constructed within an LSSR formation, due to thermal effects.

WP 7: Transport in LSSR Rocks at the GDF scale. This WP will quantify how subsurface heterogeneities at different scales, as characterised in WP1, affect deformation of the rock or cause damage that may influence the transport of water and gas in the vicinity of the GDF, thereby perhaps permitting the migration of radionuclides. Numerical models will include different potential disposal sites, including the modelling of different rock units, depths, in situ stresses, GDF configurations and deposition scenarios.

The objectives of WP 7 include: (a) Simulation of GDF LSSR base case scenarios of interest, to be selected in collaboration with NWS. (b) Identifying GDF, rock properties, and in situ condition variations of interest, as well as large-scale heterogeneities to incorporate into the models. (c) Numerically quantify damage development as a function of multi-scale heterogeneities, due to mechanical, fluid- driven, and thermal stresses. (d) Simulation of permeability changes in the LSSRs and their effect on transport properties. (e) Systematic investigation of LSSR property ranges to develop an understanding of the effects of heterogeneities and in situ conditions on permeability changes during deposition and storage.

To further promote close collaboration between the various Work Packages, and to ensure that the major outputs of the project will align with the needs of NWS’ GDF programme, two Cross-Cutting Themes have been identified, each with a Lead from within the GeoSafe research consortium. Cross-Cutting Theme 1 is “Conceptualisation of compositional and physical heterogeneity, and its relation to Radionuclide Transport”. Cross-Cutting Theme 2 is “Fracture/flow pathways across spatial and temporal scales”.