Challenges and best practices for modelling fractures in geological repositories
Authors: Müller Christian , Burlaka Victoria , Flügge Judith , Gafoor Ajmal , Hassanzadegan Alireza , Herold Philipp , Johnen Marc , Zhao Hong 
Published: September 2023

Abstract. Crystalline rock formations, which are considered potential host rock formations for geological repositories in Germany, often
exhibit fractures that complicate the generation of safety assessment
models. To quantify the criteria outlined in the Ordinance on Repository
Safety Requirements accurately, it is necessary to take these fractures into
account and to consider both intact rock and rock mass properties in
numerical models. The consideration of fractures in numerical models is a challenge due to the
complex three-dimensional nature of the fractures. In the literature, various
methods to approach this challenge are documented, but the combination of
assumptions and simplifications often associated with these methods can lead
to significant uncertainties in numerical calculations. To address this
challenge, we evaluated the suitability of various modelling approaches in
fractured media, based on criteria such as accuracy, computational
efficiency, ease of implementation, and suitability for specific types of
problems. Comparative calculations (benchmarks) are used to develop the
corresponding numerical models based on previously defined generic models
that contain fractures. In this way, it is possible to clarify the influence
of different numerical approaches on modelling results, thus providing a
basis for assessing the modelling uncertainties required in the safety case. We would like to give an overview of two R&amp;D projects, SUSE (Safety analyses for repository systems in crystalline rocks) and PRECODE (Investigating the impacts of mining activities at great depths on the integrity of crystalline rock in the context of high-level radioactive waste disposal).
SUSE addresses hydraulic problems, specifically groundwater flow in
fractured crystalline rock. Analysing groundwater flow direction and rate is
crucial to assess the transport of radionuclides in the geosphere, as they
are most likely to be transported by groundwater. Different approaches were
tested, such as approaches that consider the fractures in an explicit manner
and approaches that try to upscale the heterogeneous fracture properties, and were compared with each other via benchmark cases. The cases
specified differ mainly in the number of fractures considered (single
fracture and fracture network) and the physical processes to be simulated
(fluid flow and solute transport). PRECODE, on the other hand, focuses on
mechanical problems, specifically the mechanical integrity of the geological
barrier. The integrity must be shown by demonstrating that the dilatant
strength and expected fluid pressure do not exceed the strength of the host
rock. Different approaches were tested, such as approaches that explicitly
represent fractures using discontinuum and continuum methods as well as
upscaling approaches that consider only the integral strength of a rock
mass, and were compared with each other via benchmark cases. The assumed
cases differ mainly in the respective scale considered, such as borehole,
tunnel, and repository scales. Overall, this research provides insight into the challenges and best
practices for modelling fractures in geological repositories and for
improving the safety of high-level radioactive waste disposal.