Overview of the stability and barrier functions of the granitic geosphere at the Kamaishi Mine: relevance to radioactive waste disposal in Japan

The Kamaishi in situ studies were carried out in order to provide a technical basis for the studies of the geological disposal of High-level Radioactive Waste (HLW) in Japan. The studies were conducted in drifts located at ca. 300 m and ca. 700 m below the surface in the former Kamaishi iron ore mine, situated in the Kurihashi granodiorite, an early Cretaceous granitic rock (110∼120 Ma) in the Kitakami area, Iwate Prefecture, northeastern Japan. The fractures identified in the Kurihashi granodiorite were mainly formed during the early stages of the cooling process. The mineralogical and dating data suggest that almost no new fractures have been generated since 70∼60 Ma ago. The fractures have high-porosity fracture fillings with relatively high sorption values that imply the fractured granitic rock could have a significant retardation capability for contaminant transport through the granite. Monitoring of hydrological and geochemical conditions of groundwater for 10 years showed that even fractured granitic rock close to the drift is not thoroughly oxidized, and there are only negligible changes of hydrological conditions and groundwater geochemistry due to earthquakes. This information is of considerable importance for the practical design of a HLW repository and to assess any impact of the pre-closure conditions on the post-closure performance of the geological repository. Finally, the results of the study methodology and tools developed help increase confidence in the technical feasibility of geological disposal of HLW in Japan.