[H-CG21] Nuclear Energy and Geoscience

[HCG21-08] Comparing the sizes of HLW repository and geological structures

*Masahiro Chigira¹ (1.Fukada Geological Institute)

Keywords:High-level radioactive waste, Geological disposal, Rock types, Size of a repository, Homogeneity of rocks

For the siting of high-level radioactive waste repository in Japan, long-term geological stability has been taken seriously but geological structures have not been well addressed. Geological structures, however, are the basis for hydrological and mechanical interpretation and performance evaluation of a repository. There are now 2500 pieces of vitrified radioactive waste and 26000 pieces more if the 19000 ton of stored used fuels are reprocessed. Japanese government plans to make one repository for 40000 pieces of it at one location, which needs an underground area 3 km by 2 km. I examined the space in terms of actual geological structure for Neogene volcanic rocks, Neogene sedimentary rocks, accretionary complexes, and granite. Neogene volcanic rocks are generally heterogeneous and lavas and intrusives usually take irregular shapes (Yamagishi, 1973; Kano, 2016), so that a repository likely encounter complex structures that have not been characterized beforehand. Mudstone would be good as a host rock but most of mudstone in Japan is associated with sandstone and may not be large enough to install a repository. Abnormally high formation pressures and mud volcanoes could also be obstacles. Accretionary complexes need to be considered to have much more thrust faults than have been reported. For example, investigation of the Shimanto accretionary complex along the Kumano River, which provided nearly continuous outcrops because of the flood during a typhoon in 2011, revealed there are 71 thrust faults that have incohesive crush zones within 12.9 km interval; eight faults with a crush zone thicker than 80 cm. One of the crush zones has been reported that it separated the groundwater into two domains above and below the crush zone (Arai and Chigira, 2018). The Shimanto accretionary complex is a typical accretionary complex in Japan, which suggests there could be so many thrust faults in other accretionary complex in Japan. However, distribution and geometry of thrust faults could not be characterized without dense investigation. Granite, which have been previously considered to be rich in fractures, may have wide areas with less fractures in the depths of large batholiths.