AbstractAt the present time, the solution considered as the most favourable and sustainable by the international community for the last stage of higher activity waste (HAW) management, whether consisting of irradiated fuel elements or vitrified radioactive waste, is storage at great depths, in underground facilities, within stable geological formations. This solution involves isolating the HAW from the biosphere by means of interposing a series of natural and engineered barriers, in the storage facilities at depths ranging between 200-1000m (UK). HAW is first embedded into extremely durable, corrosion-resistant containers. These are then arranged in galleries in stable geological formations surrounded by a buffer material, characterised by low permeability and high retention capacity. The isolation and confinement of HAW are therefore provided by the container, the buffer material and the natural barrier or host rock.
Bentonite is internationally considered as the most suitable buffer and backfill material. Nonetheless, for bentonite to accomplish the safety functions for which it has been selected, as a buffer and backfill material, it has to preserve its physicochemical characteristics over the expected timescales of a geological disposal facility. Under geological disposal conditions, the unpreventable corrosion of the containers, thermal gradients and mineral content of groundwaters will give rise to a highly-dynamic environment at the contact interface between the bentonite and containers. As a result, interactions between the clayey matrix and the released by-products of corrosion are expected to occur, inducing bentonite evolution via alteration and/or transformation into new mineral phases that may compromise the functionality of the buffer.
This research focuses on the physicochemical evaluation of different highly-compacted bentonite samples retrieved from the contact interface between bentonite and a simulated container. This research investigated the mineralogical evolution of the bentonite barrier to evaluate the presence of neoformed mineral phases, possibly arising from interactions between bentonite and corrosion by-products, and considering thermo-hydro-chemical-mechanical factors. A detailed characterisation of the samples was performed, and the results were used to assess the degree of alteration experienced by bentonite after 18 and 6.5 years, and 61, 122, 183 and 211 days of simulated experimental exposure respectively. This work has highlighted the key factors that could have altered the geotechnical barrier functionality after a relatively long-term exposure, as well as a short-term exposure to simulated geological conditions. This research has also highlighted the need to investigate the impact of a fully-reduced environment, as expected during mid-life of a repository, since a fully-reduced environment could accelerate clay alteration and/or transformation, potentially inducing the alteration of 2:1 phyllosilicates (smectite group) into 1:1 phyllosilicates (kaolinite-serpentine group) or Fe-rich smectites, which eventually will lead to a complete loss of the swelling capacity of bentonite and, therefore, its self-healing ability.
|Date of Award||23 Jan 2020|
|Sponsors||Radioactive Waste Management|
|Supervisor||Thomas Bligh Scott (Supervisor)|