SKB Task Force GWFTS: Task 9 - Increasing the realism of solute transport modelling in fractured media

Abstract:

SKB and several other waste management organizations have established the international SKB Task Force on Modelling of Groundwater Flow and Transport of Solutes (TF GWFTS) to support and interpret field experiments. An important objective of the task force is to develop, test and improve tools for conceptual understanding and simulating groundwater flow and transport of solutes in fractured rocks. Work is organized in collaborative modelling tasks. Task 9 focuses on realistic modelling of coupled matrix diffusion and sorption in heterogeneous crystalline rock matrix at depth. This is done by inverse and predictive modelling of different in-situ transport experiments. The ultimate aim is to develop models that in a more realistic way represent retention in fractured rock.

Posiva’s REPRO (rock matrix REtention PROperties) experimental campaign has been performed at the ONKALO rock characterization facility in Finland. The two REPRO experiments considered were the Water Phase Diffusion Experiment, addressing matrix diffusion in gneiss around a single borehole interval (modelled in Task 9A), and the Through Diffusion Experiment, which is performed between sections of three boreholes and addressed by modelling in Task 9C.

The Long-Term Diffusion and Sorption Experiment (LTDE-SD) was an in-situ radionuclide tracer test performed at the Äspö Hard Rock Laboratory at a depth of about 410 m below sea level. The experimental results indicated a possible deeper penetration of tracers into the rock matrix than expected and the shape of the penetration profiles were not according to theory. This experiment was modelled and interpreted in Task 9B.

Task 9D is addressing the possible benefits of the detailed modelling of the experiments in safety assessment calculations. The task is performed by upscaling of Task 9A to conditions applicable for performance assessments of nuclear waste repositories.

Of additional interest is the collective work performed by the task force to conceptually understand and interpret the field experiments, and at the same time increase the realism in solute transport modelling. This study would not have been possible without the support from the waste management organizations and the work by the multiple modelling teams.