Development of coupled processes numerical models of tracer, colloid and radionuclide transport in field migration experiments

Harvey, Benjamin Edward (2019). Development of coupled processes numerical models of tracer, colloid and radionuclide transport in field migration experiments. University of Birmingham. Ph.D.

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Abstract

Deep geological disposal is widely recognised as the best method for disposing of higher-activity radioactive wastes. There is a need to increase understanding of processes that can lead to radionuclide migration from a geological disposal facility (GDF). Colloids are sub-microscopic particles that are ubiquitous in groundwater. If radionuclides become associated with colloids they can potentially be transported by different processes than would be predicted by traditional solute transport modelling.

Colloid-facilitated radionuclide transport is being investigated as part of the Colloid Formation and Migration (CFM) project at the Grimsel Test Site in Switzerland, where migration experiments investigating the transport of conservative tracers, bentonite colloids and radionuclides have been carried out in a well characterised shear zone within fractured granodiorite. This study produced a new modelling approach that describes colloid-facilitated radionuclide transport, which is applied to two migration experiments, modelling the migration of conservative tracers, bentonite colloids and radionuclides.

For the first time, a model that includes a detailed distribution of transmissivity generated by inverse modelling has been combined with a colloid-facilitated radionuclide transport model to describe these field experiments. The model was able to replicate successfully the transport of tracers, colloids and radionuclides in both experiments, using consistent parameters and processes to describe transport for most species, identifying important radionuclide transport processes in the different experiments. Certain radionuclides showed different transport behaviour in the two migration experiments due to their redox or sorption chemistry, and this study was able to model and highlight this as a further area for investigation. The colloids at least partially facilitated the transport of all of the radionuclides modelled. However, the sorption of the radionuclides to the colloids was shown to be reversible, with the rate of desorption forming a key control on their migration in these field experiments.
The methods and tools developed here can be applied to future migration experiments as part of CFM, and to assess the colloid-facilitated radionuclide migration at other sites, including a future GDF site.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):