Further development of a numerical modeling concept of the coupled THM behavior within a generic nuclear waste repository in crystalline rock

Crystalline rock is one of the potential rocks under consideration in Germany for hosting a heat-generating nuclear waste repository. The focus of this contribution is a repository system, which relies on the Containment Providing Rock Zone (CRZ) in a crystalline rock as the principal containment barrier. In compliance with the German safety requirements, the integrity of the CRZ is required, i.e. to keep its containment capabilities for a period of 1 million years. This implies that the formation of new pathways must be excluded, temperature development must not significantly impair the barrier effect and anticipated stresses and fluid pressures should not exceed the dilatancy strength and the fluid pressure capacity, respectively. To meet this requirement a safety-oriented assessment of mechanical, hydraulic, thermal and chemical processes, as well as their couplings, occurring in the host rock due to the storage of heat-generating nuclear waste, excavation and/or gas production among others, is needed and numerical modelling is an essential and powerful tool for it.

Fractures and other types of discontinuities, which usually characterize crystalline rock, are expected to influence the hydraulic behavior of the system. Thus, an adequate representation of the fracture network is required in order to capture its relevant properties, which will ultimately define the hydraulic boundary conditions surrounding the CRZs. Typically it is only possible to characterize fracture networks statistically. This requires a systematic investigation to quantify the influence of multiple realizations on the repository system.

The decay-heat produced by the heat-generating nuclear waste leads to an increase of the local temperature through which the host rock as well as the fluid will expand. Consequently, a change in the effective stresses is also expected. With the aim of obtaining a better representation of the temperature-induced fluid pressure increase a bi-directional coupling of the hydraulic-mechanical (HM) processes is being tested.

Previous work done by [1] based on a one-directional coupling between the HM processes for the estimation of the fluid pressure and one realization of a statistically generated generic geological model. This contribution aims to further develop the work from [1] by using a bi-directional coupling between the HM processes, implemented in the open-source finite element code OpenGeoSys version 6, to calculate the thermal induced fluid pressure increase and quantify the impact on the integrity criteria. Moreover, preliminary results for an approach used to evaluate the influence of multiple statistically equivalent fracture networks on the repository will be presented.

References

[1] Thiedau, J., et al.: CHRISTA-II - Analysen zur Integrität von geologischen Barrieren von Endlagersystemen im Kristallin. Ergebnisbericht, Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, 2021