Radioactivity and the Environment

HydroFrame PhDs

Development of coupled processes numerical models of tracer and colloidal transport in an underground research laboratory beneath the Swiss Alps

Ben Harvey, Birmingham University

This project will develop techniques that model the processes involved with the migration of colloids and radionuclides from geological disposal facilities (GDFs). This work is important in order to gain a better understanding of how the geosphere provides a barrier for these facilities, which is an important part of the safety case that needs to be made in the development of GDFs.

My project will use numerical modelling techniques to solve equations such as Darcy's law and the Advection-Dispersion equation, as well equations that describe processes such as sorption and blocking. I plan to use COMSOL Multiphysics software for the majority of my modelling work, modelling column experiments carried out in Birmingham, as well as experiments undertaken as part of the CRR and CFM experiments at the Grimsel Test site in Switzerland. These experiments have been carried out in fractured granite, as it is a potential geological location for future GDF sites.

Do microbes and natural organic matter lead to increased actinide mobility in fractured rocks?

Matthew Kirkby, Imperial College

Metamorphic/igneous rocks are one possible host rock type for a potential UK repository. These rocks are often complex and highly fractured, leading to high fracture permeability. Much of this terrain is covered by peat and this blankets the weathered bedrock. Consequently, transport of organic materials in the form of microorganism and organic ligands into the host rocks and groundwater aquifer is likely and a possible major controlling factor of nuclide mobility in these environmental. Biogeochemical processes at the water-mineral interface and especially in the fractured rocks needs to be quantified and understood.

Evaluation of effective media versus discrete fracture representations: Implications for describing the seismic and geomechanical response for nuclear waste repositories

Emmanouil Parastatidis, University of Leeds

Summary: One method for modelling the seismic and geomechanical behaviour of fractured rock is to use an equivalent medium representation of the underlying fracture network. This approach has limitations, such as the applicable frequency range, the types of fracture properties that can be studied, and inability to handle non-uniform effects due to the stress field. The alternative is to model the individual fractures explicitly. This approach readily allows the models to capture the influence of stress state, as well as specific fracture properties such as fracture size, filling and stiffness. However, such methods are numerically intensive. The aim of my project is to establish the most suitable representations of fracture networks for describing seismic and geomechanical behaviour of fractured rock masses around nuclear waste repositories. In particular, I will explore and determine under what conditions effective medium representations are able to capture sufficient detail of the effective behaviour of the fracture networks. I will do this by comparing explicit fracture models to their effective medium representations, using the WAVE finite difference wave propagation code, and the 3DEC rock mechanics code.

Hydraulic transmissivity of fracture networks generated through geomechanical fracture-growth simulations

Robin Thomas, Imperial College

Most previous modelling of the field-scale transmissivity of fractured rock masses has been based on stochastically generated fracture networks, with lengths, orientations and apertures taken from statistical distribution functions. The purpose of my PhD project will be to inject more "geological realism" into this process. To achieve this, I will be using an in-house, three-dimensional geomechanical finite element simulator to generate my fracture networks. The fracture orientations and shapes, and their apertures, will therefore be governed by the laws of fracture mechanics, rather than being stochastically generated from some statistical distribution.

The transmissivities of the resulting networks will then be calculated explicitly by the same geomechanical simulator, and compared to the values predicted using either Leung’s analytical method for 2D networks, or the 3D extension that is being developed within HydroFrame by post-doctoral researcher Anozie Ebigbo. My calculations will serve to test these approximate methods, and should shed light on the effect that rock properties, such as elastic moduli, fracture toughness, and mechanical heterogeneity, have on field-scale hydraulic transmissivities.

Integrated thermo-hydro-mechanical simulation to predict the seismic response of a potential underground nuclear repository site

Sam Parsons, Leeds University

This project focuses on the usual granitic-type repository sites and coupling thermal stresses within established integrated hydro-mechanical elastic models (Itasca FLAC3D software). The project would use elastic constitutive models to understand and predict the influence of thermal stresses and repository behaviour with time. I am also developing subsurface model representative of potential nuclear repository sites, using extensive databases of geo-mechanical and fluid properties for the region.

Integrating thermal coupling within a geomechanical multiphase model framework for the study of fractured rock masses in the vicinity of a geological disposal facility for radioactive waste

Clement Joulin, Imperial College

My PhD aim to improve a numerical tool to get a better grasp of the physical processes occurring in fractured rocks in the vicinity of a nuclear waste repository. I aim to add a thermal effect modelling feature to an existing finite-discrete element code, which will be further coupled with a fluid flow simulator. Thermally-induced stresses, extending up to 100 metres around the repository, are of critical importance, as this will undoubtedly have an effect on fracture characteristics, new fracturing and the potential for radionuclides to be transported upwards and into the biosphere. My work is required to be flexible and generic enough to be used in any fractured geological formation that might be investigated as a potential location of a geological repository in the UK.