Development of a numerical modeling strategy used to assess safe radioactive waste disposal in crystalline rock

The safe disposal of heat-generating radioactive waste in deep geological repositories is a major challenge that requires a thorough understanding of the host rocks integrity and its ability to contain radionuclides over extended time periods. Crystalline rock is one of the potential rocks under consideration in Germany for hosting heat-generating nuclear waste. Fractures and other types of discontinuities usually characterize crystalline rock. It is therefore expected, that the presence of fracture networks influences both the hydraulic and mechanical system behavior. This will affect transport mechanisms in the system and could potentially impact the rocks integrity and therefore its containment capacity. Thus numerical modeling approaches to assess the thermal, hydraulic and mechanical (THM) processes require an appropriate representation of the fracture networks.

This contribution aims to provide an overview of the recent work done by the BGR towards the development of a numerical strategy for the safety assessment of crystalline rock in the context of deep geological repositories [1, 2, 4]. The modeling strategy focuses on the assessment of host rock integrity and the description of flow and transport in fractured crystalline. This comprises the numerical analysis of the complex interactions due to the coupled THM processes triggered by the decay heat of the disposed nuclear waste. The dominant influence of fracture networks in particular on flow and transport processes is also included. Based on the statistical characterization of fracture networks their properties are incorporated in numerical simulations by a combination of upscaling to an equivalent porous medium and explicit representation on lower dimensional elements. The strategy has also been partly developed within the framework of the DECOVALEX-2023 Task F joint project [2]. The open-source finite element code OpenGeoSys version 6 [3] has been used for numerical solutions in both the coupled THM problem as well as in the transport from radionuclides. We will present the developed strategy and its application to generic nuclear repository systems in crystalline rock.

References

[1] Guevara Morel, C.; Thiedau, J. & Maßmann, J. (2024): Advances in the numerical modeling strategy (concept) of a generic nuclear waste repository in crystalline rock. EGU General Assembly 2024, 14–19 Apr 2024, Vienna, Austria. DOI: https://doi.org/10.5194/egusphere-egu24-9854.

[2] Leone, R. et al. (2025) 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.

[3] Bilke et al. (2025): OpenGeoSys (OGS 6.5.4), January 2025, https://doi.org/10.5281/zenodo.14672997.

[4] Thiedau, J., Maßmann, J., Guevara Morel, C., Weihmann, S. & Alfarra, A. (2021). CHRISTA-II - Analysen zur Integrität von geologischen Barrieren von Endlagersystemen im Kristallin. BGR, Ergebnisbericht, B3.5/B50112-52/2021-0003/001: 118 S., Hannover.