Using THM modelling to evaluate the potential reactivation of faults in a generic nuclear waste repository in crystalline rock

Assessing safety in a deep geological repository for radioactive waste requires a thorough evaluation of coupled thermal, hydraulic and mechanical (THM) processes. This allows the analysis of a possible reduction of the containment capacity of the host rock.  For this purpose, numerical modeling is regarded as a necessary and powerful tool.  Moreover, a distinctive characteristic of crystalline rock is its fractured nature. These fracture networks are expected to influence both the hydraulic and mechanical behavior of the system. Therefore, for a safety concept in crystalline rock, the influence of fractures on the containment of the radionuclides has to be considered. Here, the repository concept in fractured crystalline rock presented in (2), in which multiple smaller containment rock zones are used for the disposal of the nuclear waste.  

This contribution aims to extend the numerical analysis concept for host rock integrity proposed in (2) by including an assessment of the possible risk of fracture reactivation. To this end, the numerical results obtained in (2) are taken as starting point, in particular the temporal evolution of the stress field. The calculated stresses are used to evaluate the deformation behavior at fracture locations (1), specifically for a comparison between dilation and shear. In this context, the evaluation of the German integrity criterion, which focuses on the safety-relevant dilatant behavior of the containment providing rock zone, is extended by the consideration of discrete fractures. Results show that, due to the emplacement of the nuclear waste in the crystalline rock, a change in the potential fault reactivation can be expected. The intensity of this change depends, among others, on fracture orientation and the development of the temperature field and is therefore transient.

References

1. Ferrill, D. A., Smart, K. J. and Morris, A. P. (2020): Resolved stress analysis, failure mode, and fault-controlled fluid conduits. Solid Earth, 11, 899–908, https://doi.org/10.5194/se-11-899-2020.

2. Guevara Morel, C., Thiedau, J. and Maßmann, J. (2025): Numerical assessment of the barrier integrity for a generic nuclear waste repository in crystalline rock. International Journal of Rock Mechanics and Mining Sciences 197, 106326, https://doi.org/10.1016/j.ijrmms.2025.106326.