Current status and future prospects of the Horonobe International Project

Aoyagi, Kazuhei; Ozaki, Yusuke; Hayano, Akira; Ono, Hirokazu; Tachi, Yukio

Nihon Genshiryoku Gakkai-Shi ATOMO, 67(6), p.354 - 358, 2025/06

https://doi.org/10.3327/jaesjb.67.6_354

Japan Atomic Energy Agency launched the Horonobe International Project (HIP) utilizing the Horonobe Underground Research Laboratory. The main objectives of this project are to develop and demonstrate advanced technologies to be used in repository design, operation and closure and a realistic safety assessment in deep geological disposal, and to encourage and train the next generation of engineers and researchers. In this review, an overview of the HIP is presented.

Rock strength and stress dependence of local flow-path connectivity within faults or fractures; A Preliminary overview of virtual and in-situ hydraulic tests

Ishii, Eiichi; Ozaki, Yusuke; Aoyagi, Kazuhei; Sugawara, Kentaro*

Hydrogeology Journal, 33(1), p.63 - 85, 2025/02

https://doi.org/10.1007/s10040-024-02859-y

This study performed virtual packer tests on modeled single fractures on computer and derived the relationship between flow dimension and mappable indicator, DI, which is defined by the mean stress, groundwater pressure, and rock tensile strength. The greater DI results in the smaller flow area in faults or fractures, subject to fracture-normal closure. Comparing the derived relationship with results from in situ hydraulic tests on natural faults in rock with few fracture-mineral-fillings revealed that flow-path connectivity is high (flow dimension 1.5) when DI was 2.0 while was low (flow dimension 1.5) when DI was 2.0. This relationship was valid even when DI was varied, or faults were sheared, during injection tests on faults, and even in rock with abundant fracture-mineral-fillings. However, flow-path connectivity in minor fractures far from faults could be also low even when DI was 2.0 probably due to poor connection to the main fault network or sealing effects of fracture-mineral-fillings. When the permeability of intact rock is high, flow-path connectivity in fractures was high even when DI was 2.0. These findings can be helpful to map the spatial distribution of flow-path connectivity in faults or fractures from limited borehole data.

Excavation damaged zone around a simulated disposal pit for high level radioactive waste disposal excavated in soft sedimentary rock

Aoyagi, Kazuhei; Ozaki, Yusuke; Ono, Hirokazu; Ishii, Eiichi

Dai-16-Kai Iwa No Rikigaku Kokunai Shimpojiumu Koen Rombunshu (Internet), p.269 - 274, 2025/01

We investigated the development of the excavation damaged zone (EDZ) induced by the excavation of modeled disposal pit which was excavated as a part of the full-scale engineering barrier experiment at 350 m depth. Seismic and electric tomography surveys, observation of rock core samples, borehole televiewer surveys and three-dimensional excavation analysis were performed to evaluate the extent of the EDZ around the pit. It was clarified that the EDZ was developed 0.8 to 1.6 m from the wall of the pit at a relatively shallower depth caused by the effect of the EDZ induced around the floor of the gallery. The extent of the EDZ was gradually reduced along the depth, and the maximum extension was 0.3 m from the wall of the pit at the deeper section.

Impact of earthquakes occuring around the Horonobe Underground Research Center to the excavation damaged zone around the shaft

Niunoya, Sumio*; Hata, Koji*; Aoyagi, Kazuhei; Matsui, Hiroya

Tonneru Kogaku Hokokushu (CD-ROM), 34, p.IV-1_1 - IV-1_6, 2024/12

A multi-optical measurement probe consisting of an optical fiber AE sensor, a pore pressure sensor, and a thermometer was installed at a depth from 350 to 370 m of the East access shaft. Using this measurement system, we continuously monitored the EDZ around the shaft and changes in pore water pressure. In this report, we summarized the measured AE and pore pressure data during the maintenance period after the completion of excavation to a depth of 350 m, and analyze the waveform data when earthquake occurred. We also investigated whether the measurement system can capture the effect of seismic motion to the EDZ. As a result, the AE count was slightly increased and pore pressure was slightly changed owing to the seismic motion, however, there is little impact to the EDZ.

Transmissivity prediction of the Excavation Damaged Zone fracture around the gallery at 500 m at the Horonobe Underground Research Laboratory

Aoyagi, Kazuhei; Ozaki, Yusuke; Tamura, Tomonori; Ishii, Eiichi

Proceedings of 4th International Conference on Coupled Processes in Fractured Geological Media; Observation, Modeling, and Application (CouFrac2024) (Internet), 10 Pages, 2024/11

In high-level radioactive waste disposal, it is crucial to estimate the transmissivity of gallery excavation-induced fractures, i.e., excavation damaged zone (EDZ) fractures, because EDZ fractures can be a radionuclide migration pathway after the backfilling of the facility is completed. From previous research, the transmissivity of the fracture can be estimated through the empirical equation using the parameter ductility index (DI), which corresponds to the effective mean stress normalized to the tensile strength of the rock. In this research, we performed a hydromechanical coupling analysis of a gallery excavation at the Horonobe Underground Research Laboratory to estimate the transmissivity of the EDZ fracture before the excavation. At first, we simulated the gallery excavation at 350 m and showed that the measured transmissivity was within the range of the estimated transmissivity using the DI. After that, we also predicted the excavation of a gallery at 500 m by setting the hydromechanical parameters acquired from the laboratory tests before the excavation. The estimated transmissivity at 500 m was one order of magnitude less than that at 350 m. This result might be related to the closure of the fracture under high-stress conditions and low rock strength.

Collection of measurement data obtained in the extension of 350 m galleries at the Horonobe Underground Research Laboratory Project

Sakurai, Akitaka; Aoyagi, Kazuhei; Murakami, Hiroaki; Tamura, Tomonori; Fujieda, Daigo; Togase, Kazuki

JAEA-Data/Code 2024-005, 48 Pages, 2024/07

JAEA-Data-Code-2024-005.pdf:6.54MB
JAEA-Data-Code-2024-005-appendix(DVD-ROM).zip:1029.9MB

We currently focus on the three tasks that were identified as "key R&D challenges to be tackled" in the "Horonobe Underground Research Plan for the fiscal year 2020 Onwards". These tasks include "Study on near-field system performance in geological environment", "Demonstration of repository design options", and "Understanding of buffering behaviour of sedimentary rocks to natural perturbations". To implement these tasks, we will excavate shafts and galleries to the 500 m depth. From fiscal year 2023, we will start extension of the 350 m gallery excavating 3 horizontal gallery (Niches No.6, No.7, and Niche of the East Shaft No.1) and excavation of the shaft to the 500 m depth and excavation of 500 m gallery. This report summarizes the measurement data acquired at Niches No.6, No.7, and Niche of the East Shaft No.1 in fiscal year 2023 to accumulate the basic data for carrying out the Observational Construction Program for the excavation of the shaft to the 500 m depth and galleries at the 500 m depth.

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 occurrence of the phenomena in the field. The water containing the clay minerals was injected into the rock at the 200m stage of the Mizunami Underground 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 occurred in the real field.

Evaluation of excavation damaged zones (EDZs) in Horonobe Underground Research Laboratory (URL)

Hata, Koji*; Niunoya, Sumio*; Aoyagi, Kazuhei; Miyara, Nobukatsu*

Journal of Rock Mechanics and Geotechnical Engineering, 16(2), p.365 - 378, 2024/02

https://doi.org/10.1016/j.jrmge.2023.05.010

Excavation of underground caverns, such as mountain tunnels and energy-storage caverns, may cause the damages to the surrounding rock as a result of the stress redistribution. In this influenced zone, new cracks and discontinuities are created or propagate in the rock mass. Therefore, it is effective to measure and evaluate the acoustic emission (AE) events generated by the rocks, which is a small elastic vibration, and permeability change. The authors have developed a long-term measurement device that incorporates an optical AE (O-AE) sensor, an optical pore pressure sensor, and an optical temperature sensor in a single multi-optical measurement probe (MOP). Japan Atomic Energy Agency has been conducting R&D activities to enhance the reliability of high-level radioactive waste (HLW) deep geological disposal technology. In a high-level radioactive disposal project, one of the challenges is the development of methods for long-term monitoring of rock mass behavior. Therefore, in January 2014, the long-term measurements of the hydro-mechanical behavior of the rock mass were launched using the developed MOP in the vicinity of 350 m below the surface at the Horonobe Underground Research Center. The measurement results show that AEs occur frequently up to 1.5 m from the wall during excavation. In addition, hydraulic conductivity increased by 2 to 4 orders of magnitude. Elastoplastic analysis revealed that the hydraulic behavior of the rock mass affected the pore pressure fluctuations and caused micro-fractures. Based on this, a conceptual model is developed to represent the excavation damaged zone (EDZ), which contributes to the safe geological disposal of radioactive waste.

Resin-injection testing and measurement of the shear displacement and aperture of excavation-damaged-zone fractures; A Case study of mudstone at the Horonobe Underground Research Laboratory, Japan

Aoyagi, Kazuhei; Ishii, Eiichi; Chen, Y.*; Ishida, Tsuyoshi*

Rock Mechanics and Rock Engineering, 55(4), p.1855 - 1869, 2022/04

https://doi.org/10.1007/s00603-022-02777-z

Safety assessments related to the long-term migration of radionuclides in high-level radioactive waste disposal repositories need to consider the evolution of permeability associated with excavation-damaged-zone (EDZ) fractures. We observed EDZ fractures preserved by resin injection around a gallery in the Horonobe Underground Research Laboratory in Japan with the aim of assessing the sensitivity of aperture to shear displacement (i.e., dilation angle) in EDZ fractures. To date, shear displacement along EDZ fractures has not been quantified despite its importance for estimating the evolution of fracture aperture around excavations after the repository is backfilled. Enlarged photographs of EDZ fractures fixed by resin were examined to obtain reliable and accurate measurements of the shear displacement and aperture of EDZ fractures without additional disturbance. Measured shear displacement and fracture aperture are poorly correlated, meaning that fracture aperture is insensitive to shear displacement after fracture formation. This insensitivity is closely reproduced by a previously reported empirical relationship and is attributed to the relatively high levels of normal stress acting on fracture surfaces at 350 m depth, which suppress shear-induced dilatation. Considering this insensitivity between fracture aperture and shear displacement, shear-induced dilation of EDZ fractures is estimated to be negligible even if the fractures are shear-displaced after the repository is backfilled, so long as the normal stress acting on fracture surfaces does not reduce substantially. The applied resin injection and fracture observations constitute a useful tool for helping to assess the likelihood of shear-induced dilation after the repository is backfilled.

Numerical simulation of oxygen infusion into desaturation resulting from artificial openings in sedimentary formations

Miyakawa, Kazuya; Aoyagi, Kazuhei; Akaki, Toshifumi*; Yamamoto, Hajime*

Dai-15-Kai Iwa No Rikigaku Kokunai Shimpojiumu Koen Rombunshu (Internet), p.609 - 614, 2021/01

Desaturation is expected due to excavation of an underground repository, especially in the newly created fractures zone (EDZ). During the construction and operation of facilities, the air in the gallery infuses into the rock around the gallery though the excavation affected area and causes oxidation of host rock and groundwater, which increase nuclide mobilities. In the Horonobe underground research laboratory (HURL), which is excavated in the Neogene sedimentary formations, no pyrite dissolution or precipitation of calcium sulfates was found from the cores drilled in the rock around the gallery. The reason for no oxidation is estimated that the release of dissolved gases from groundwater due to pressure decrease flows against the air infusion. In this research, the mechanism of O intrusion into the rock was investigated by numerical multiphase flow simulation considering advection and diffusion of groundwater and gases. In the simulation, only Darcy's and Henry's laws were considered, that is, chemical reaction related to oxidation was not handled. The effects of dissolved gas and rock permeability on O infusion into the rock were almost identical. Decreasing humidity with relatively low permeability leads to extensive accumulation of O into the EDZ even though with a relatively large amount of dissolved gas. In the HURL, the shotcrete attenuates O concentration and keeps 100% humidity at the boundary of the gallery wall, which inhibits O infusion. Without the shotcrete, humidity at the gallery wall decreases according to seasonal changes and ventilation, which promotes O intrusion into the EDZ but the chemical reaction related to O buffering such as pyrite oxidation consumes O.