Radioactivity and the Environment

HydroFrame PDRAs

Understanding the influence of pH, organics, mineralogy, and bacteria on uranium migration in fractured rock

Dr Janice Kenney, Imperial College

I am working to develop a better understanding of how actinides are transported through fractured rock in the subsurface at the high pH environments predicted in nuclear waste disposal. The end goal of this work is to determine the groundwater chemistry, mineralogy, and level of bacterial activity which would be the most efficient to keep nuclear waste immobilized. Since the disposal in the UK is still undecided, we will text various rock and groundwater compositions and decide which combinations will be most beneficial in inhibiting actinide transport from Intermediate-level waste (ILW) disposal. ILW will be encased in cement and embedded in a steal barrel. When the steal barrel corrodes, the groundwater will begin to leach the cement, creating a very high pH environment (pH 11.8-13.6). However, in order to understand completely and furthermore, develop predictive models of what happens in high pH environments, we must understand the properties of the actinides in water-rock environments across a much larger pH range.

To do this we will be preforming titrations on minerals and rocks to determine the surface functional groups responsible for the binding of protons and/or actinides, then we will use the acidity constants and site concentrations from this and later apply them to surface complexation models of actinides such as U adsorbed onto the surfaces of those minerals and rocks. These titrations will also be done with various ground water components in solution, including various concentrations of sodium bicarbonate and sodium chloride, since these components are very likely to occur in high concentrations in many UK groundwaters.

Time-lapse seismic monitoring of nuclear waste repository sites

Dr Hannah Bentham, Leeds University

For my research, I observe stress changes in a geological disposal facility (GDF) that can lead to fracture growth and fluid saturation in the host rock using both active (reflection) and passive (microseismic) seismic methods. I explore suitable seismic monitoring strategies that enable the integration of thermo-hydro-mechanical monitoring with seismic measurements repeated over time (time-lapse/4D), and microseismic observations. Furthermore, I develop data processing methodologies suitable for integrated thermo-hydro-mechanical, 4D seismic and microseismic monitoring.

Currently I have been producing realistic synthetic seismograms using fractured geological models to explore the characteristics of seismic waveforms that could be used to site a GDF. I will explore the effect of the type of seismic survey and survey design on the detection of fractures to aid siting and monitoring of the storage facility through construction and operational stages.

Seismic forward modelling of fracture response to inform survey design for repositories

Dr Sofya Titarenko, Leeds University

The main purpose of my work package is to improve survey design using wave-fracture models. To achieve that, I am using a code previously developed and successfully applied in the field (WAVE). The WAVE program is designed to solve models with large numbers of explicitly modelled fractures with varied fracture properties. Complexity in scale has been achieved through reduced geometrical complexity. However, it is very important to advance the realism of the fracture zone and provide more meaningful results for repository investigations. One requirement is to have realistic distributions in fracture size, since fracture lengths follow a logarithmic scale. It means improving the current program to work with larger models and to run efficiently on powerful multiprocessor computers. Another requirement is to advance the geometrical complexity of the fracturing. To achieve that, I will try a number of numerical approaches so as to increase the complexity of the models while keeping them efficient overall and capable of solving very large scale problems.

Modelling of transport in fractures from tests performed at Grimsel

Dr Lindsay McMillan, University of Birmingham

The safety case for any Geological Disposal Facility (GDF) must be supported by detailed computer modelling that demonstrates an understanding of experiments and observations made in the GDF. As the UK does not currently have an underground research facility, the aim of my project is to improve UK-based understanding of how to characterise and model migration of contaminants around a GDF by undertaking computer modelling of various subsurface tests conducted at the underground research laboratory in Grimsel, Switzerland.

Hydromechanical processes in fractured rocks around a repository

Dr Xiang Jiansheng, Imperial College

This work package will develop Open Source coupled numerical models using a new rigorous mesh-adaptive finite element framework for flow in discrete fractures and matrix blocks. The computational physics challenge is to translate four recent proof-of-concept formulations into a reliable computationally efficient coupled framework that includes, in addition, thermal effects. The proposed research will concentrate on study domains at 1-10 metre scale at different locations in the repository near-field and far-field, and develop fluid flow models in discrete fracture systems. In this work, mesh-adaptivity resolves down to examine inside and around fractures. Refining the computational mesh to will allow the multiphase representation of the rock mass to follow maximum pressure gradients. Within each rock volume, the multiphase flow mesh coarsens and refines to capture key flow processes, whilst preserving the surface-based representation of geological heterogeneity with remarkable computational efficiency, due to anisotropic mesh adaptivity.

Hydraulic transmissivity of geologically realistic fracture networks

Dr Anozie Ebigbo, Imperial College

Fractures can serve as flow paths for fluids through otherwise impervious rocks. Since mobile fluids in the vicinity of nuclear-waste-storage sites can be detrimental to the environmental, it is important to be able to quantify the conductance of ­rock masses at these sites. Fracture networks in rocks are usually characterised statistically using information gathered from outcrops (surface exposure of once-buried rock) or generated using geomechanical models. Given these characteristics, is it possible to deduce the conductance (or transmissivity) of fractured rock masses with simple mathematical expressions? Such analytical expressions would be very useful in the stochastic assessment of environmental risk or site suitability. Their derivation for realistic, three-dimensional fracture networks is the goal of this project.