Geological characterisation of the active Atera Fault in central Japan: Implications for defining fault exclusion criteria in crystalline rocks around radioactive waste repositories

Abstract

In the orogenic field of the Japanese archipelago, faults are inevitably distributed in all types of potential host rocks for high-level radioactive waste (HLW) repositories. These faults could have an adverse impact on a repository if they were to provide a short-circuit to the biosphere, so allowing any radionuclides released from the repository a more rapid transport pathway to the surface than would be the case otherwise. Associated with many faults is a damage zone with fault gouge and crushed rock developed in and alongside the main fault plane. The specific characteristics of a damage zone are considered to be a reflection of the process of faulting and may be used to understand the influence of fault movement on the surrounding host rocks. In order to clarify the geological characteristics by which potentially transmissive features of faults can be avoided when siting a radioactive waste repository for HLW in a crystalline host rock, the active Atera Fault, which is located in central Japan, has been studied. Relationships between the morphological features of the fault, its associated damage zone and the fracture frequency have been investigated. Detailed mapping of the damage zone shows that the fractures formed by later faulting activity can be distinguished from the early fractures developed in the rock by using the microscopic textures and the differences between fracture filling minerals. In particular, a network of shortly transected fractures filled by carbonate and iron-oxyhydroxide minerals was preferentially formed in the damage zone along the Atera Fault, probably by recent fault movement. The density of fractures also suggests that the mechanical damage zone formed by faulting extends for up to ca. 200m on each side of the main fault plane. However, the geochemically influenced zone has been spatially restricted by geochemical buffering reactions involving crushed fracture-filling materials along the fault. The results provide a basis for defining exclusion zones around faults that might be identified in a future crystalline repository host rock. The methodology is applicable to site characterization and the appropriate allocation of repository panels and deposition holes for waste canisters.