Numerical flow and transport modeling of a generic nuclear repository in crystalline rock

An internationally accepted concept for the long-term containment of spent high-level nuclear waste (HLW) is its disposal using deep geological repositories. Therefore, the need arises to evaluate long-term safety and the efficiency of under-ground nuclear waste storage regarding among others radionuclide transport mechanisms. For this purpose, numerical modeling is an essential and powerful tool. This BGR contribution focuses on the performance assessment modeling of a generic nuclear repository in crystalline rock done within the framework of the DECOVALEX-2023 Task F joint project. The BGR-modeling strategy describes flow and transport in fractured crystalline rock using a combined Equivalent Continuous Porous Media (ECPM) and Discrete Fracture Network (DFN) approach. Using the open-source finite element code OpenGeoSys version 6, stationary flow and radionuclide transport is simulated based on the advection-dispersion equation.

Fractures and other types of discontinuities, which usually characterize crystalline rock, are expected to influence the hydraulic behavior of system and hence potentially influence transport mechanisms in the system. Therefore, their representation in numerical models is non-trivial. For this study, large connected fracture zones are represented as deterministic features.  Meanwhile smaller fractures, in which only statistical characterization can be obtained, are stochastically generated and represented as an ECPM with upscaled hydraulic properties. 

This contribution aims to propose an approach towards the performance assessment of a generic deep geological repository in fractured crystalline rock. Results regarding the obtained flow field as well as the corresponding radionuclide migration will be presented.

References

[1] Leone, R. et al., Comparison of performance assessment models and methods in crystalline rock: TASK F1 DECOVALEX-2023. Geomechanics for Energy and the Enviroment 2025; 41: 100629. https://doi.org/10.1016/j.gete.2024.100629.