Understanding of a post-closure geological environment around a large underground facility is important for the safety assessment of geological disposal of high-level radioactive waste. Japan Atomic Energy Agency (JAEA) has performed the GREET (Groundwater REcovery Experiment in Tunnel) at the Mizunami Underground Research Laboratory (MIU) to evaluate the environmental recovery process after closure.
In the GREET, a mock up test drift (Closure test drift; CTD) was filled with in-situ groundwater as a simulation experiment of drift closure. The CTD is located in fractured crystalline rock at 500m depth below ground surface and has a floodable volume of approx. 900m3. Information of fracture distribution has been obtained by borehole investigation and mapping of gallery walls. The change of hydraulic pressure, hydrochemical condition and rock deformation have been monitored in and around the CTD during the tunnel excavation and water-filling. In parallel to the in-situ investigation, we perform a Hydro-Mechanical-Chemical (HMC) coupled simulation to enhance the understanding of the recovery process in fractured granite.
This study presents the simulation results of excavation stage. The sensitivity analysis were conducted with homogeneous model based on the investigations before excavation of CTD for the rough setting of simulation conditions. In addition, we try to reproduce the detail of HMC process with heterogeneous models generated by discrete fracture network model. Comparison of simulated results with observed data leads to the conclusion that the range of change in inflow during excavation can be predicted. However, model update is necessary for prediction of groundwater chemistry and spatial distribution of abrupt change in the drawdown.
The Mizunami Underground Research Laboratory (MIU) is being operated by the Japan Atomic Energy Agency (JAEA), in the Cretaceous Toki Granite in the Tono area, Central Japan. The MIU project is a broad-based, multi-disciplinary study of the deep geological environment, providing a scientific basis for the research and development of technologies needed for geological disposal in crystalline rock. The MIU design consists of two shafts, and several horizontal research galleries (Figure 1). The geological settings around MIU site are summarized in e.g. Ishibashi et al., (2016).
