Swelling characteristics of bentonite buffers under high

Although nuclear waste exists in significant quantities and is continuously produced worldwide, waste management represents a great challenge which has yet to be resolved, but which is becoming pressing under the interest that energy and sustainability-related issues attract. Deep geological repositories are scientifically considered to be the only feasible way to dispose of nuclear waste. Although different countries have developed different geological disposal concepts, depending on regional geology, most concepts share the idea of engineering a barrier (buffer) of a compacted, highly expansive clay (e.g. MX80 bentonite) between the canister containing the waste and the host formation. As no appropriate site for a repository has been selected in the UK, research on the topic remains generic, with the thermo-hydro-mechanical behaviour of bentonite buffers being a major area of investigation. Issues of concern identified by UK policy makers RWM Ltd include temperature effects on strength and stiffness (shear and bulk), swelling potential, homogenisation, permeability, water retention capability and resaturation bentonite buffers.
Being unsaturated at the time of placement, the buffer is expected to absorb ground water from the host formation and in doing so, to swell into gaps left open during placement, providing sealing and protection to the canisters bearing nuclear waste. This renders the swelling potential of the clay one of the key parameters in assessing its suitability as a buffer material in nuclear waste disposal. Nonetheless, the high temperatures generated by the waste may alter the swelling its characteristics, compromising its ability to perform its designated function, rendering it vulnerable to cracking, thus compromising the safety of the nuclear waste repository, which typically is expected to have a design life of over 10000 years.
The aim of the project is to characterise the effect of temperature on the swelling behaviour of compacted MX80 bentonite, proposed to be used as buffer in the SKB concept for deep geological disposal, due to its swelling and self-healing capability. The scope of the proposed PhD project is: (a) to measure the swelling potential of MX80 at ambient temperature and at temperatures up to 85oC; (b) to provide experimental insight as to how the as-compacted properties of MX80 may affect its swelling potential at high temperatures; (c) to optimise the initial properties of MX80 in order to improve its performance. This is associated with optimising both the as-compacted state of the buffer and the gaps left during its placement in deposition holes. To this end, a newly designed oedometer cell will be commissioned and utilised. The equipment was funded by EPSRC (EP/J021199/1) and forms part of a series of equipment which includes the following cells: (a) temperature-controlled oedometer; (b) suction-controlled oedometer; (c) temperature-controlled triaxial; (d) temperature-controlled isotropic cell; and (e) set of four desiccators with RH control.
The project forms part of coordinated efforts within the Geotechnics section to tackle challenging topics related to Energy Geotechnics.