This is a 4-year PhD studentship at the University of Strathclyde, offered in partnership with Nuclear Waste Services (NWS) and part of the EPSRC Centre for Doctoral Training (CDT) in Skills And Training Underpinning a Renaissance in Nuclear (SATURN).

Context of the Research

Around the world, many countries are developing deep geological disposal facilities designed to safely dispose radioactive waste for hundreds of thousands of years. These facilities consist of networks of tunnels deep below the surface, where canisters containing radioactive waste will be emplaced to permanently isolate the waste from the surface environment. A series of engineered and natural barriers will be employed to contain the waste over long timescales, ensuring environmental safety is maintained.
Bentonite, a type of natural clay, is one of the key candidate engineered barrier materials being considered to surround the canisters, to isolate them from the surrounding environment and contain the release of radionuclides. The main advantage of bentonite is that it swells when in contact with water, forming a tight, protective barrier around the waste. This barrier helps to contain the radioactive waste by minimising the flow of groundwater towards the waste and the subsequent release of radionuclides to the surrounding environment.

Over time, gases will naturally form inside a disposal facility, for example, as the metal waste containers slowly corrode. If gas is generated at sufficient rates and quantities, gas pressures could lead to the generation of temporary cracks or pathways that allow the gas to move through the bentonite. Normally, once the gas escapes, these cracks seal themselves again, restoring the integrity of the barrier.

However, some potential geological disposal facilities, including those being considered in the UK, have highly saline groundwater. The presence of salt is a game changer: the behaviour of bentonite when wetted with saline water changes dramatically, as its swelling and sealing capabilities are reduced. Yet, it is not clear whether salinity also makes the clay mechanically stronger, potentially allowing it to resist the formation of dilatant pathways or fractures. At the same time, the presence of salt alters the molecular interactions at the gas-water-clay interface, easing the passage of gas through the bentonite pore network, hence reducing the potential of fracturing the bentonite barrier.

Project Overview

This PhD project will investigate the fundamental processes that control how gas pressure, bentonite integrity, and resealing ability interact when the bentonite clay is exposed to high-salinity conditions.
The research will address a multiphysics problem, since gas transport through bentonite can occur through several fundamental mechanisms, such as dissolution and diffusion, capillary breakthrough, or mechanical fracturing of the clay. Each of these mechanisms is influenced differently by salinity.

Given this complex scenario, key questions remain unanswered:

• How does high salinity affect gas movement through bentonite?
• Does it allow gas to pass more easily without damaging the barrier, or does it create larger, longer-lasting fractures that prevent resealing?
• Can the bentonite barrier be engineered so that gas moves through it in a controlled and favourable way?

You will work on a custom-designed gas injection apparatus that allows for the creation of the bentonite barrier, simulate water arrival and gas pressure increase within the same cell, minimizing experimental artifacts and ability of in-situ monitoring of the barrier parameters.

Post-mortem samples will undergo a multiscale characterization to investigate the interaction between gas transport and bentonite clay microstructure. Micron and submicron scale imaging such as X-ray Computer Tomography or Scanning Electron Microscopy will be used along side analytical methods such as Mercury Intrusion Porosimetry.

In selected scenarios, in-situ X-ray computed tomography will be employed on smaller samples to monitor the evolution of super-micron sized fractures during gas transport and subsequent re-saturation with the possibility to extend this into some Synchrotron time at national or international facilities.

Training

As part of the Saturn CDT, you will engage in an intensive taught programme during the first six months of the studentship. This programme will cover core topics in nuclear science and engineering, alongside targeted skills training.

Throughout the entire duration of the studentship, you will receive ongoing support and training in advanced experimental techniques and procedures, with a particular focus on cutting-edge methods for macro- and microstructural characterization of soil–gas interactions.