LONG-TERM CHANGE IN URANIUM MIGRATION PROCESSES IN A HIGHLY ERODED GRANITE, DEMONSTRATED BY URANIUM SERIES DISEQUILIBRIUM IN FRACTURE FILLING MATERIALS

Mineralization and mobilization of uranium (U) are strongly associated with subsurface redox processes. This implies that the safety of the deep geological disposal of nuclear waste depends on the redox conditions that is one of key factors indicated by the U inventory. The Korea Atomic Energy Research Institute (KAERI) Underground Research Tunnel (KURT) is located in an active groundwater recharge area in granitic bedrock. In this study site, oxic meteoric water has infiltrated the subsurface through permeable zones of highly weathered granite, thereby, forming a complicated redox system. Carbonate minerals are the dominant fracture filling materials (FFMs) and the FFMs are enriched in U, with concentrations up to 48.96 mg/kg. We examined long-lived radioactive isotopes of ²³⁰Th, ²³²Th, ²³⁴U, ²³⁵U, and ²³⁸U and redox sensitive tracers such as Fe, δ⁵⁶Fe and rare-earth elements (REEs) in the FFMs to understand fluctuations in redox conditions in the granitic fracture system. The main focus is on U migration processes, and to confirm the relative depths of the eroded granitic zone infiltrated by meteoric water over geological time scales. U accumulation and overall increase of other redox sensitive elements occurred at relatively shallow depths (~180 m below ground level, bgl). This implies that the U distribution in the FFMs was associated with redox processes in paleo-groundwater. In the dynamic redox transition zone, the mobilization and immobilization of U from the depths of 50 m to 203 m, bgl, occurred during 120–205 Ka. The results give new insights into long-term paleo-redox processes in complex granitic fracture system analogs for potential deep-seated geological repositories for high-level radioactive waste.