Assessment of Retention Processes for Transport in a fractured system at Äspö (Sweden) Granitic Site: from Short-Time Experiments to Long-Time Predictive Models

Overview & Synopsis

We present results obtained within Task 6 modelling project hosted by SKB (Swedish Nuclear Fuel and Waste Management Company) Task Force on Numerical Modelling of Flow and Solute Transport in fractured granitic rock. Modelling the transport of the radionuclide in natural fractured media aims at characterizing and assessing the performance of a potential deep geologic repository. Task 6 modelling exercise objective is to provide a bridge between models based on detailed site investigation data and calibrated against tracer experiments (month scale) and models corresponding to a hypothetical repository post-closure time scale (hundred thousands of years). Latter models capture the most significant features and processes and allow for a sensitivity analysis of uncertain parameters. Two features involved in tracer experiments (TRUE-1 and True Block Scale-TBS) were studied. The first is a 10 m single fracture while the second is a 200 m semi synthetic fractured block. The study focused on matrix zone heterogeneity, their influence on the retention processes and their level of identification from tracer tests. Results show that tracer tests poorly constrain the systems. A model suited for Performance Assessment was built for the single fracture geometry. A smeared fracture approach to model transfers in a fractured block is presented. The approach is qualified and validated on synthetic test cases and applied to Task 6 fractured block. Results show that it is appropriate for Performance Assessment time scale purposes. Nevertheless, the approach would require further developments to be applied to systems incorporating lower scale heterogeneities under experimental conditions.