Characterizing deep fracture zones within the natural barrier: Insights from borehole data around the KAERI underground research tunnel

High-level radioactive waste repositories rely on multi-barrier systems, including natural and engineered barriers, crucial for long-term safety through isolation and delay functions. Improving the performance of natural barriers poses challenges, necessitating thorough evaluation via ongoing field surveys and lab tests. Natural barriers are categorized and analyzed based on lithological and structural characteristics. Structural elements such as fractures, joints, and faults play a crucial role in performance assessment as they can serve as pathways for groundwater flow. Existing studies lack a unified criterion for defining structural boundaries, prompting a systematic review to establish standards suitable for site characteristics by applying various structural factors to study area boreholes. Considering the deep location of the disposal site, borehole data plays a pivotal role. The KAERI Underground Research Tunnel (KURT) is a small-scale Underground Research Laboratory (URL) with a maximum depth of 120 m excavated within the Korea Atomic Energy Research Institute (KAERI). The boreholes used in the study were drilled to depths ranging from approximately 500 to 1,000 m within the site, including KURT. The preliminary fracture zone was identified through frequency analysis of gaps between fractures in the borehole images. As reviewed previously, the methods for defining the fracture zone can broadly emphasize the background fracture and presenting exclusion conditions or emphasize the dense area of fractures and presenting boundary conditions. For each method, the match rate with the preliminary fracture zone was validated. To confirm the correlation between the match rate and the natural state, it was compared with the actual fracture density found in the drill core. By integrating a series of processes, we could quantify standards for defining deep fracture zones using boreholes. The results of this study will contribute to a three-dimensional model of the deep brittle structure within the natural barriers. Future research can explore the identified fracture zone, surface fracture zone, and the definition of fault zones with fault rocks.

Keywords: Deep fracture zones, background fracture, brittle structure, Borehole data, KURT

Acknowledgements: This research was supported by the Institute for Korea Spent Nuclear Fuel (iKSNF) and National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (MSIT) (2021M2E1A1085200).