Influence of clay-doped water on permeability in granite rock mass

Nara, Yoshitaka*; Kashiwaya, Koki*; Oketani, Kazuki*; Fujii, Hirokazu*; Zhao, Y.*; Kato, Masaji*; Aoyagi, Kazuhei; Ozaki, Yusuke; Matsui, Hiroya; Kono, Masanori*

Zairyo, 73(3), p.220 - 225, 2024/03

https://doi.org/10.2472/jsms.73.220

The fractures in the rock are the main pass of groundwater flow and solute transport. The filling of fine-grained particle, such as clay minerals, was confirmed to decrease the permeability of rock by laboratory experiment. This research aimed to verify the occurance of the phoenomena in the fild. The water containing the clay minerals was injected into the rock at the 200m stage of the Mizunami undearground research laboratory. The hydraulic conductivity decreased two order before and after the injection. This result suggested that the decrease of hydraulic conductivity by the filling of fine-grained particle in the fractures occured in the real field.

Results of environmental impact investigations as part of environmental monitoring investigation in backfilling of shafts and tunnels of Mizunami Underground Research Laboratory (2022)

Takeuchi, Ryuji; Nishio, Kazuhisa*; Kokubu, Yoko

JAEA-Data/Code 2023-013, 74 Pages, 2024/01

JAEA-Data-Code-2023-013.pdf:4.2MB

The Tono Geoscience Center of Japan Atomic Energy Agency (JAEA) has been conducting the environmental monitoring investigation to confirm the environmental impacts associated with the backfilling of shafts and tunnels at the Mizunami Underground Research Laboratory (MIU). This report summarizes the results of the environmental impact investigations conducted as part of the environmental monitoring investigation around the MIU Site in FY2022, which include groundwater level measurement in wells, river flow rate measurement, water analysis of Hazama river, noise and vibration surveys, and soil survey.

Proposition of confirmation items on the borehole sealing for the disposal of radioactive waste

Murakami, Hiroaki; Nishiyama, Nariaki; Takeuchi, Ryuji; Iwatsuki, Teruki

Oyo Chishitsu, 64(2), p.60 - 69, 2023/06

https://doi.org/10.5110/jjseg.64.60

In order to confirm the quality control items for borehole closure in radioactive waste disposal projects, in-situ borehole sealing tests using bentonite material were conducted. As a result, the closure performance was successfully demonstrated by comparing the data of water injection tests conducted before and after the installation of the closure material. However, the breakthrough was observed after closing, probably due to high differential pressure applied to the seal section. Thus, it is important to ascertain throughout the entire operation that the borehole is adequately closed. The placement and specifications of the closure material should be determined according to the hydrogeological structure in the borehole. The confirmation items to use bentonite for sealing material are identified to be: to consider swelling and density loss in the borehole; to place the planned depth using appropriate emplacement technique; to be placed without damage to seals when use some backfilling materials, considering effect of permeability on adjacent seals.

Isotopic signals in fracture-filling calcite showing anaerobic oxidation of methane in a granitic basement

Mizuno, Takashi; Suzuki, Yohei*; Milodowski, A. E.*; Iwatsuki, Teruki

Applied Geochemistry, 150, p.105571_1 - 105571_11, 2023/03

https://doi.org/10.1016/j.apgeochem.2023.105571

Anaerobic oxidation of methane (AOM) affects both the redox conditions and carbon cycle in groundwater. However, examples of studies on crystalline rock deep in terrestrial subsurface as well as the potential host rock for geological disposal are few. Therefore, we conducted a paleohydrogeological study on fracture-filling calcite in the Toki Granite. The O value (-32.7 to -0.59 permil) revealed that the groundwater that precipitated the calcite was groundwater derived from hydrothermal fluid, freshwater that came from the surface, and seawater that penetrated during marine transgression. On the other hand, C (-56.6 to 6.0 permil) was wider than the isotopic range of DIC that originated from hydrothermal, freshwater, and seawater sources (-25 to 2 permil). Calcite with C that was lighter than -25 permil was believed to have precipitated DIC, which was provided by AOM. In contrast to previous studies, the Mizunami AOM calcite was precipitated in a freshwater environment, indicating that various processes could have generated AOM in crystalline rocks deep in the terrestrial subsurface.

Study on investigation method of fracture distribution based on data obtained at the Mizunami Underground Research Laboratory, central Japan

Sasao, Eiji

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 29(2), p.112 - 118, 2022/12

https://doi.org/10.3327/jnuce.29.2_112

Since fractures in granite act as pathways for groundwater flow and mass transport, understanding of fracture distribution is an important subject for the disposal of high-level nuclear waste. Fracture data obtained through the borehole investigation from ground surface contains errors due to the crossing angle between boreholes and fractures. I studied method of borehole investigation that can effectively characterize the fracture distribution based on fracture information obtained from geological investigation of shaft wall at the Mizunami Underground Research Laboratory. As a result, it was found that inclined boreholes captured larger number of fractures than vertical boreholes. Therefore, inclined boreholes are preferable for efficient characterization of the fractures. Fracture orientation at surface exposure is well concordant with that at the shaft, which implies that the drilling direction should be determined based on the result of surface fracture mapping.

Demonstration of the groundwater observation network system in backfilled underground facility

Murakami, Hiroaki; Takeuchi, Ryuji; Iwatsuki, Teruki

JAEA-Technology 2022-022, 34 Pages, 2022/10

JAEA-Technology-2022-022.pdf:3.47MB

Japan Atomic Energy Agency (JAEA) has been conducting the hydro-pressure and hydrochemical monitoring for more than two decades to understand the hydrochemical disturbance due to the excavation of tunnels at Mizunami Underground Research Laboratory (MIU). To understand the environmental influence due to the backfilling of research tunnels that started in 2019, environmental monitoring of groundwater has been performed and recovery status of groundwater is being confirmed. In order to observe the deep-groundwater environment from the ground, the groundwater pressure monitoring and sampling, which have been performed in the research tunnel, are to be performed from the ground. However, backfilling of a large-scale underground facilities such as MIU is globally unprecedented, thus it was necessary to develop a new observation system. Accordingly, we developed a new observation network to observe the environment around the research tunnels of the MIU. This system enables monitoring of groundwater pressure and water sampling of the backfilled tunnel from the ground while utilizing the existing-monitoring system installed in the tunnels. Accordingly, we demonstrated its technology through the environmental monitoring of groundwater. The results of the environmental monitoring and the existing groundwater data of MIU indicate that this system is able to monitor the groundwater environment in the backfilled tunnels.

Precipitation sequence of fracture-filling calcite in fractured granite and changes in the fractionation process of rare earth elements and yttrium

Mizuno, Takashi; Milodowski, A. E.*; Iwatsuki, Teruki

Chemical Geology, 603, p.120880_1 - 120880_16, 2022/08

https://doi.org/10.1016/j.chemgeo.2022.120880

This study has focused on the formation sequence and Rare earth elements with yttrium (REY) of fracture-filling calcite in the Toki Granite in the Mizunami area, central Japan. The morphological, chemical and isotopic characteristics of the calcite and chemistry of fluid inclusions reveal that the calcite in the Toki Granite can be differentiated into four discrete generations: Calcite I (oldest) to Calcite IV (most recent). The precipitation history of calcite reflects the changes in the hydrogeochemical regime of paleo-groundwaters, controlled by the evolution of groundwater by seawater infiltration associated with marine transgression and surface water infiltration associated with marine regression and uplift. The post-Archean average shale-normalized REY patterns in generations of calcite show no significant Ce anomaly, negative Eu anomaly, and light REY (LREY)-depleted pattern in dominates. These features are also common to the Toki Granite. The consistency of the features in each generation of calcite indicates that REY was supplied from the Toki Granite by water-rock interaction. The lack of a Ce anomaly in the calcite demonstrates that groundwaters have maintained reducing conditions during the calcite precipitation. However, the fractionation of LREY and heavy REY (HREY) in each generation of the calcite is more pronounced than in the granite. The fractionation process in the paleo-groundwaters from which each generation of calcite precipitated closely relates to the systematic variation of carbonate complex in REY series and/or pH in palaeo-groundwater. The findings of this study will be necessary for assessing the long-term safety of geological disposal of high-level radioactive waste.

Association of hydrothermal plagioclase alteration with micropores in a granite; Petrographic indicators to evaluate the extent of hydrothermal alteration

Yuguchi, Takashi*; Izumino, Yuya*; Sasao, Eiji

Journal of Mineralogical and Petrological Sciences (Internet), 117(1), p.220415_1 - 220415_12, 2022/00

https://doi.org/10.2465/jmps.220415

This study presents the use of petrographic plagioclase alteration indicators as a new method for quantitatively evaluating the extent of plagioclase alteration within granites, using the Toki granite, central Japan. Alteration indicators and areal fractions of microvoids in the plagioclases were obtained via BSE image analysis. The volume of the micropores in the altered plagioclase was characterized by the areal fraction of microvoids in the grains. The plagioclase alteration indicators were obtained as the ratio between the alteration product area and the original plagioclase area. In our previous study, we focused on biotite chloritization indicators. We found positive correlations between the plagioclase alteration and biotite chloritization indicators in the same sample, indicating that each alteration indicator can be used independently as a representative value for the sample. In the Toki granite, the plagioclase alteration was related to the biotite chloritization.

Genesis and development processes of fractures in granite; Petrographic indicators of hydrothermal alteration

Yuguchi, Takashi*; Izumino, Yuya*; Sasao, Eiji

PLOS ONE (Internet), 16(5), p.e0251198_1 - e0251198_17, 2021/05

https://doi.org/10.1371/journal.pone.0251198

This study analyzes the relationships among alteration indicators, areal microvoid fractions in chloritized biotite, and macroscopic fracture frequencies in the Toki granite, central Japan, to establish the genesis and development processes of fractures in granite. Petrographic alteration indicators using biotite chloritization as innovative methods are proposed to evaluate the extent of hydrothermal alteration and fracture frequency within granites. Samples with high macroscopic fracture frequencies correspond to a high number of areal microvoid fractions and large alteration indicators. The alteration indicators contribute to the characterization of present and future distributions of macroscopic fracture frequencies.

Excavation of shafts and research galleries at the Mizunami Underground Research Laboratory (Construction work of MIU Part VIII); Construction progress report

Safety and Facility Management Section, Tono Geoscience Center

JAEA-Review 2020-052, 116 Pages, 2021/03

JAEA-Review-2020-052.pdf:12.81MB

This progress report presents an outline compilation of construction activities, primary tasks performed, construction progress and safety patrol report at the Mizunami Underground Research Laboratory Construction Work Part VIII. The outline of construction activities is a summary based on the scope of work planned the main activities are based on the Tono Geoscience Center weekly reports; and the construction progress is based on the planned and actual schedules. The actual performance of the construction work of MIU part VIII carried out from April 1, 2018 until May 15, 2020 is described in this report.