The NDA Radioactive Waste Management Directorate is developing a Disposal System Safety Case (DSSC) that will set out the safety arguments that would be made to support the development of a Geological Disposal Facility (GDF) in the range of different geological environments that may need to be considered in the UK.
Quintessa has developed system descriptions for a range of generic geological environments that will form an input for the post-closure component of the DSSC. An initial study assessed the modifications that might need to be made to the Phased Geological Repository Concept (PGRC) for ILW were it to be implemented in each of the different environments. A companion study considered the potential compatibility of a range of disposal concepts for HLW and Spent Fuel with the different environments.
This work has illustrated the approaches that might be used to model the groundwater pathway for each environment using the level of information that would typically be available at a desk study stage, such as those envisaged in the on-going MRWS programme, prior to any actual site investigation. The modelling illustrated the role that the geosphere would be likely to play in the post-closure safety case for each environment and how the type of information that would be required by a performance assessment model might be derived for each of the different environments. This included the development of a new method for representing diffusion-dominated transport in a very simple performance assessment model.
A number of tools were used including analytical solutions, FEFLOW and GoldSim. Illustrations of two of the simple models developed using FEFLOW are shown: a 3D flow and transport model for a fractured host rock, and a 2D flow and transport model for an environment in which solute transport is dominated by diffusion.