Christophe Grenier

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

We present results obtained within the Task 6 modeling project hosted by the Swedish Nuclear Fuel and Waste Management Company (SKB) Task Force on Numerical Modeling of Flow and Solute Transport in fractured granitic rock. Modeling the transport of radionuclides in natural fractured media is done to characterize and assess the performance of a potential deep geologic repository. The Task 6 modeling exercise objective provides a bridge between models based on detailed site investigation data and calibrated against tracer experiments (month scale), and models corresponding to a hypothetical repository postclosure time scale (hundred thousands of years). The latter postclosure models capture the most significant features and processes of radionuclide transport, and allow for a sensitivity analysis of uncertain parameters. Two features from the tracer experiments (TRUE-1 and True Block Scale-TBS) are studied. The first is a 10 m single fracture, the second is a 200 m semisynthetic fractured block. The study focuses on matrix zone heterogeneity, its influence on retention processes, and its level of identification from tracer tests. Results show that tracer tests poorly constrain the systems. In addition, we build a model suited to performance assessment for single fracture geometry and present a smeared-fracture approach to model transfers in a fractured block. The model is calibrated and validated on synthetic test cases and applied to the Task 6 fractured block. Results show that this approach is appropriate for performance-assessment time-scale modeling. Nevertheless, the approach requires further development to be applied to systems incorporating small-scale heterogeneities affecting flow and transport conditions.