1:45 PM - 3:15 PM
[HCG21-P10] Accumulation of the data of geological environment characteristics in Pre-Neogene accretionary sedimentary rocks
Keywords:Radioactive waste, Geological disposal, Pre-Neogene sedimentary rocks, Accretionary sedimentary rocks, Geological environment characteristics
1. Introduction
To achieve the safe geological disposal of high-level radioactive waste etc., NUMO has published the NUMO Pre-siting SDM-based Safety Case (NUMO, 2021), which is described as a generic safety case without specifying the site. In the paper, the Pre-Neogene sedimentary rocks, which are widely distributed at depths deeper than 300 m than the disposal depth in the Japan Islands, have a problem that the quality-assured geological environment data available from previous information has been limited. Therefore, in order to acquire and expand the geological environment characteristics data of the Pre-Neogene sedimentary rocks necessary to ensure the technical reliability of the paper, borehole investigation and sampling and analysis of groundwater were carried out at a site in the underground tunnel of the Kannagawa Hydropower plant of TEPCO Renewable Power, Incorporated which is located in the bedrock of Pre-Neogene accretionary sedimentary rocks.
2. Content of implementation items
Two boreholes (All-core drilling, Total Length 25 m, Vertical Downward, Core Diameter 92 mm) were drilled in the underground tunnel, and geophysical loggings, fluid logging, and hydraulic packer tests were carried out in these boreholes. By using the core samples obtained from the boreholes, core observation, thin section observation, XRD, XRF, thermal conductivity test, specific heat test, density test, effective porosity test, ultrasonic wave propagation velocity measurement, uniaxial compression test, radial compression test, triaxial compression test, and through-diffusion test were carried out. Groundwater sampling were carried out from 4 inflow points in the underground tunnel, and groundwater quality analysis, organic analysis, and colloid analysis were carried out.
3. Results
Based on the geological and environmental data acquired this time, it was found that the Pre-Neogene accretionary sedimentary rocks at the site consisted of a mixed rock containing fragments of mudstone, tuffaceous mudstone, fine-grained sandstone, medium-grained sandstone, chert, limestone and tuff, and that the rock was very dense and solid (True density of about 2.7 g/cm3, Effective porosity of about 1.3%). It also shows extremely low permeability (hydraulic conductivity: order of 10-11 m/s to 10-14 m/s), which are lower than the hydraulic conductivity (Statistic: Median 6.7 x 10-7 m/s, Logarithmic mean 4.7 x 10-7 m/s) of the bedrock of the Pre-Neogene sedimentary rocks in the NUMO Pre-siting SDM-based Safety Case (NUMO, 2021). While the hydraulic conductivity (Statistic: Median 1.8 x 10-10 m/s, Logarithmic mean 5.4 x 10-10 m/s) obtained by laboratory tests using rocks of the Pre-Neogene sedimentary rocks are presented in the paper, and compared with the aforementioned hydraulic conductivity of rocks, these values are more consistent with the hydraulic conductivity data obtained at the site. It is inferred that this is caused by the absence of significant groundwater-path structure in the packer intervals.
From the results of isotope analysis of groundwater collected at four locations in the underground tunnel, it was found that there were young groundwater that had infiltrated underground for less than several decades or groundwater that had infiltrated underground for at least several tens of thousands of years, and from the examination based on the results of water quality analysis, it was found that there was a possibility that it originated from paleoseawater at the time of forming stratum.
4. Conclusion
Through this study, the quality assured geological environment characteristics data were obtained at the Pre-Neogene accretionary sedimentary rocks from the same geological environment. It was also confirmed that a combination of existing technologies could obtain data from Pre-Neogene accretionary sedimentary rocks in which geological structure, thermal, hydraulic, mechanical, and chemical environments could be interrelated.
To achieve the safe geological disposal of high-level radioactive waste etc., NUMO has published the NUMO Pre-siting SDM-based Safety Case (NUMO, 2021), which is described as a generic safety case without specifying the site. In the paper, the Pre-Neogene sedimentary rocks, which are widely distributed at depths deeper than 300 m than the disposal depth in the Japan Islands, have a problem that the quality-assured geological environment data available from previous information has been limited. Therefore, in order to acquire and expand the geological environment characteristics data of the Pre-Neogene sedimentary rocks necessary to ensure the technical reliability of the paper, borehole investigation and sampling and analysis of groundwater were carried out at a site in the underground tunnel of the Kannagawa Hydropower plant of TEPCO Renewable Power, Incorporated which is located in the bedrock of Pre-Neogene accretionary sedimentary rocks.
2. Content of implementation items
Two boreholes (All-core drilling, Total Length 25 m, Vertical Downward, Core Diameter 92 mm) were drilled in the underground tunnel, and geophysical loggings, fluid logging, and hydraulic packer tests were carried out in these boreholes. By using the core samples obtained from the boreholes, core observation, thin section observation, XRD, XRF, thermal conductivity test, specific heat test, density test, effective porosity test, ultrasonic wave propagation velocity measurement, uniaxial compression test, radial compression test, triaxial compression test, and through-diffusion test were carried out. Groundwater sampling were carried out from 4 inflow points in the underground tunnel, and groundwater quality analysis, organic analysis, and colloid analysis were carried out.
3. Results
Based on the geological and environmental data acquired this time, it was found that the Pre-Neogene accretionary sedimentary rocks at the site consisted of a mixed rock containing fragments of mudstone, tuffaceous mudstone, fine-grained sandstone, medium-grained sandstone, chert, limestone and tuff, and that the rock was very dense and solid (True density of about 2.7 g/cm3, Effective porosity of about 1.3%). It also shows extremely low permeability (hydraulic conductivity: order of 10-11 m/s to 10-14 m/s), which are lower than the hydraulic conductivity (Statistic: Median 6.7 x 10-7 m/s, Logarithmic mean 4.7 x 10-7 m/s) of the bedrock of the Pre-Neogene sedimentary rocks in the NUMO Pre-siting SDM-based Safety Case (NUMO, 2021). While the hydraulic conductivity (Statistic: Median 1.8 x 10-10 m/s, Logarithmic mean 5.4 x 10-10 m/s) obtained by laboratory tests using rocks of the Pre-Neogene sedimentary rocks are presented in the paper, and compared with the aforementioned hydraulic conductivity of rocks, these values are more consistent with the hydraulic conductivity data obtained at the site. It is inferred that this is caused by the absence of significant groundwater-path structure in the packer intervals.
From the results of isotope analysis of groundwater collected at four locations in the underground tunnel, it was found that there were young groundwater that had infiltrated underground for less than several decades or groundwater that had infiltrated underground for at least several tens of thousands of years, and from the examination based on the results of water quality analysis, it was found that there was a possibility that it originated from paleoseawater at the time of forming stratum.
4. Conclusion
Through this study, the quality assured geological environment characteristics data were obtained at the Pre-Neogene accretionary sedimentary rocks from the same geological environment. It was also confirmed that a combination of existing technologies could obtain data from Pre-Neogene accretionary sedimentary rocks in which geological structure, thermal, hydraulic, mechanical, and chemical environments could be interrelated.