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 technic al basis for the studies of the geological disposal of High-level Radioacti ve 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 min e, situated in the Kurihashi granodiorite, an early Cretaceous granitic roc k (110 similar to 120 Ma) in the Kitakami area, Iwate Prefecture, northeast ern Japan. The fractures identified in the Kurihashi granodiorite were main ly formed during the early stages of the cooling process. The mineralogical and dating data suggest that almost no new fractures have been generated s ince 70 similar to 60 Ma ago. The fractures have high-porosity fracture fil lings with relatively high sorption values that imply the fractured graniti c rock could have a significant retardation capability for contaminant tran sport through the granite. Monitoring of hydrological and geochemical condi tions of groundwater for 10 years showed that even fractured granitic rock close to the drift is not thoroughly oxidized, and there are only negligibl e changes of hydrological conditions and groundwater geochemistry due to ea rthquakes. This information is of considerable importance for the practical design of a HLW repository and to assess any impact of the pre-closure con ditions on the post-closure performance of the geological repository. Final ly, the results of the study methodology and tools developed help increase confidence in the technical feasibility of geological disposal of HLW in Ja pan. (C) 2000 Elsevier Science B.V. All rights reserved.