Development of new techniques and assessment of existing methodologies for investigating how particle size influences the behaviour of bentonite colloids under conditions relevant to those likely to be found in Geological Disposal Facilities

Dackombe, Lauren Jennifer (2019). Development of new techniques and assessment of existing methodologies for investigating how particle size influences the behaviour of bentonite colloids under conditions relevant to those likely to be found in Geological Disposal Facilities. University of Birmingham. Ph.D.

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Abstract

An increase in the global demand for low-carbon energy generation has resulted in renewed interest in nuclear power over recent years, making it more important that a safe means by which to dispose of long-lived radioactive waste be implemented. Two geological disposal facilities (GDFs) are currently in either the planning or construction stages in Scandinavia, but more will be needed globally in order to deal with the stockpiles held by many nations around the world.

Bentonite clay is very likely to be used as a backfill in these GDFs because its properties make it particularly well-suited for nuclear waste disposal: it swells on contact with water, thereby inhibiting the migration of dissolved contaminants, and sorbs many radionuclides strongly. However, colloids produced in this backfill may enhance the movement of some contaminants in the environment, as colloids that are eroded from the buffer and carried towards the surface by groundwater can transport radionuclides adsorbed onto them. The role of colloids in radionuclide migration therefore has potential implications for the performance of a GDF. It is crucial that these implications are understood thoroughly in order to best inform performance assessments for the facility, and the aim of this project was to develop, use, and assess experimental techniques which would lead to greater understanding of how particle size affects the behaviour and transport of bentonite colloids, so as to expand the existing knowledge that is available to inform performance assessments for GDFs.

In order to do this, suspensions of bentonite colloids were fractionated to produce samples with distinct size distributions, and then characterised. A flow system was constructed, and the colloid fractions were used in flow experiments. New methodologies were developed, and existing ones critiqued when the fractionation and characterisation techniques used during these steps presented unexpected challenges. Finally, the adsorption and desorption of Eu, Cs and Ni, onto and off the colloids was investigated.

The outcome of these experiments was that against expectations after reviewing the literature, the size of the colloids did not appear to influence their behaviour in the flow experiments. Additionally, this project highlighted that producing and characterising suspensions of clay colloids with narrow, distinct size distributions is very difficult, and that approaches described in the literature may not be suitable for bentonite. If further investigation into the influence size has on the behaviour of bentonite colloids is necessary in order to satisfy waste management organisations that it is unlikely to present problems in a GDF, more advanced laboratory and modelling approaches need to be developed.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):