Abstract

Occurrence characteristics and existing forms of U-Th containing minerals in KURT (KAERI Underground Research Tunnel) granite are investigated to understand long-term behavior of radionuclides in granite considered as a candidate rock for the geological disposal of high-level radioactive waste. KURT granite primarily consists of quartz, feldspar and mica. zircon, REE(Rare Earth Element)-containing monazite and bastnaesite are also identified. Besides, secondary minerals such as sericite, microcline and chlorite including quartz vein and calcite vein are observed. These minerals are presumed to be accompanied by a post-hydrothermal process. U-Th containing minerals are mainly observed at the boundaries of quartz, feldspar and mica, mostly less than 30 μm in size. Quantitative analysis results using EPMA (Electron Probe Micro-Analyzer) show that 74.2 ~ 96.5% of the U-Th containing minerals consist of UO₂ (3.39 ~ 33.19 wt.%), ThO₂ (41.61 ~ 50.24 wt.%) and SiO₂ (15.43 ~ 18.60 wt.%). Chemical structure of the minerals calculated using EPMA quantitative analysis shows that the U-Th minerals are silicate minerals determined as thorite and uranothorite. The U-Th containing silicate minerals are formed by a magmatic and hydrothermal process. Therefore, KURT granite formed by a magmatic differentiation is accompanied by an alteration and replacement owing to a hydrothermal process. U-Th containing silicate minerals in KURT granite are estimated to be recrystallized by geochemical factors and parameters such as temperature, pressure and pH owing to the hydrothermal process. By repeated dissolution/precipitation during the recrystallization process, UTh containing silicate minerals such as thorite and uranothorite are formed according to the variation in the concentrated amount of U and Th.

Keywords high-level radioactive waste, KURT granite, U-Th containing silicate minerals, EPMA, hydrothermal process