[HCG28-P08] Hydrogeological structures of hydraulic conductive fractures at a depth of 300 m
Keywords:geological disposal, hydrogeological structure, hydraulic conductive fracture, Toki Granite, Mizunami Underground Research Laboratory, high-level nuclear waste
In geological disposal of high-level nuclear waste, hydraulic conductive fractures in crystalline rock such as granite have been considered as a main pathway for radionuclides. Diffusion and absorption of radionuclide through microstructure in rock matrix can make mass transport to retard. Because the change of the characteristics of microstructure in rock matrix by structural movement and hydrothermal alteration have an effect on mass transport, it is important to understand the distribution and the structures of hydrogeological continuity of fractures as well as their characteristics of microstructure in rock matrix. There are three directions of fractures commonly distributed in the western and the eastern rock around GL –300 m Measurement Niche off Ventilation Shaft of Mizunami Underground Research Laboratory of JAEA: NW strike and high dip angle fracture (NW fracture), NE strike and high dip angle fracture (NE fracture) and low dip angle fracture, which is distributed around the Upper fracture zone in Toki Granite. The hydraulic conductive fractures across multiple boreholes to the west sides, which mainly show NW fracture have two types of characteristics: euhedral calcite-filled fractures with highly matrix chloritization including sphalerite and epidote and the fractures with poor filling minerals which is formed along cataclastic vein texture. In the former case, it is possible that the druse, which was formed after mineralization, plays a water channel. In the latter case, the fracture was formed along the cataclastic texture consisting of fractured rock-forming minerals. Each NW fracture is well continuous in hydrogeological aspects between the boreholes. Since being measured hydraulic pressure response between these NW fractures, it can be presumed that there is low dip angle fracture to connect the water channel according to the result of the in-situ resin injection test. The transmissibility coefficients in both types of fractures are measured about 10-7-10-5m2/s. NE fractures in the western side are poorly permeable because of highly alteration in matrix filling with sericite, chlorite and smectite. On the other hand, the east side tends to have a larger number of fractures compared to the west side, especially in low dip angle fractures. In the northeast region, the fracture density is high and the fractures have filling minerals such as calcite and smectite with alteration in matrix. This is interpreted as being close to the damage zone associated with the formation of the main shaft fault (Tsuruta et al., 2012) and being affected by alteration along the fault. Hydraulic conductive fractures on the east side are NW fractures, NE fractures and low dip angle fractures, however, there is no remarkably continuous in geological characteristics between the boreholes. Low dip angle fractures have poorly engagement between fracture surfaces in the core samples and the druse forming needle-like or plate-like euhedral calcite crystals. The water pressure response among the three holes in the east side tends to be dispersed over a wide range, not only in the fractures with a large amount of water during drilling. The transmissibility coefficients in both types of fractures are measured about 10-8-10-5m2/s. As a summary, in the rock around GL –300 m Measurement Niche off Ventilation Shaft, on the west side, NW fractures with hydrogeological continuity form higher-permeability zone through low dip angle fractures. On the east side, a large number of low dip angle fractures intersect between the fractures which has a large amount of spring water and may form a network of hydraulic conductive fractures. In addition, the results of the behavior of diffusion and sorption in rock matrix around the hydraulic conductive fractures will be reported. This study is commissioned by the Agency for Natural Resources and Energy in the Ministry of Economy, Trade and Industry. Core sampling were conducted as a part of a collaborative research with JAEA.