Ryuji Takeuchi

Deposition of radiocesium on the river flood plains around Fukushima

The environment in the area around Fukushima Daiichi Nuclear Power Plant has been contaminated by widely deposited significant amount of radioactive materials, which were released to the atmosphere caused by the Fukushima Daiichi Nuclear Power Plant accident due to the Great East Japan Earthquake, which occurred on March 11, 2011. The radiocesium released in the accident mainly affects radiation dose in the environment. Decontamination work in the contaminated area except a mountain forests has been conducted to decrease the radiation dose. However, there are concerns that the redistribution of this radiation due to water discharge will occur due to the resulting transport of radiocesium. In particular, the deposition of soil particles containing radiocesium on the flood plains in the downstream areas of Fukushimas rivers can potentially increase the local radiation dose. Therefore, it is important to understand the influence of the deposition behavior of radiocesium on the radiation dose. Investigations of rivers have been performed to enhance the understanding of the mechanisms by which radiocesium is deposited on these flood plains. It was found that the spatial distribution of the radiocesium concentration on the flood plain along the river is heterogeneous with a dependence on the depositional condition and that the number of points with high air dose rates is limited. In detail, the radiocesium concentration and air dose rates in flood channels are higher than those at the edges of the river channels. Based on these heterogeneity and hydrological events, the deposition and transport mechanisms of the radiocesium due to water discharge at rivers were also interpreted, and a conceptual model was constructed.

Hydrogeological Characterization Based on Long Term Groundwater Pressure Monitoring

The Mizunami Underground Research Laboratory (MIU) is now under construction by the Japan Atomic Energy Agency in the Tono area of central Japan. The MIU project is being implemented in three overlapping Phases: Surface-based Investigation (Phase I), Construction (Phase II) and Operation (Phase III). The changes of groundwater pressure due to shaft excavation can be considered analogous to a large-scale pumping test. Therefore, there is the possibility that the site scale groundwater field (several km square) can be approximated by the long-term groundwater pressure monitoring data from Phase II. Based on the monitoring observations, hydrogeological characteristics were estimated using the s-log(t/r2 ) plot based on the Cooper-Jacob straight line method. Results of the s-log(t/r2 ) plots are as follows. The groundwater flow field around the MIU construction site is separated into domains by an impermeable fault. In other words, the fault is a hydraulic barrier. Hydraulic conductivity calculated from s-log(t/r2 ) plots are in the order of 1.0E−7(m/s). The above results from the long term monitoring during Phase II are a verification of the hydrogeological characteristics determined in the Phase I investigations.Copyright

Hydrogeological Characterization on Surface-Based Investigation Phase in the Mizunami Underground Research Laboratory Project, in Japan

The Mizunami Underground Research Laboratory (MIU) project is being carried out by Japan Atomic Energy Agency in the Cretaceous Toki granite in the Tono area, central Japan. The MIU project is a purpose-built generic underground research laboratory project that is planned for a broad scientific study of the deep geological environment as a basis of research and development for geological disposal of nuclear wastes. One of the main goals of the MIU project is to establish comprehensive techniques for investigation, analysis, and assessment of the deep geological environment. The MIU project has three overlapping phases: Surface-based Investigation (Phase I), Construction (Phase II) and Operation (Phase III). Hydrogeological investigations using a stepwise process in Phase I have been carried out in order to obtain information on important properties such as, location of water conducting features, hydraulic conductivity and so on. Hydrogeological modeling and groundwater flow simulations in Phase I have been carried out in order to synthesize these investigation results, to evaluate the uncertainty of the hydrogeological model and to identify the main issues for further investigations. Using the stepwise hydrogeological characterization approach and combining the investigation with modeling and simulation, understanding of the hydrogeological environment has been progressively improved.Copyright

Hydrogeological Conceptual Model Determined From Baseline and Construction Phase Groundwater Pressure and Surface Tiltmeter Data at the Mizunami Underground Research Laboratory, Japan

A hydrogeological conceptual model has been developed based on pressure responses observed at multilevel pressure monitoring zones in seven boreholes and surface tilt data in and around the Mizunami Underground Research Laboratory site. Pressure changes caused by some earthquakes, cross-hole hydraulic testing, and shaft excavation activities are considered. Surface tilt has been measured from the half way of the shaft excavation phase. The shaft excavation has been commenced from July 2003 with two shafts (Main shaft and Ventilation shaft). By the end of October 2005, discharging of water in the shafts has been halted at the depths of 172m and 191m respectively to allow modifications to be made to the water treatment facility due to an excess of F and B concentration in the water. This results in the recovery of the groundwater levels and filling of the underground workings. Beginning in February 2006 pumping has been resumed and the underground workings have been re-occupied. Continuous groundwater pressure and surface tilt measurements with some numerical analysis during the shaft excavation phase show the existence of the flow barrier fault predicted from the surface-based investigation phase and hydraulic parameter around the shafts.Copyright

Technical Know-How of Selection Process for the Horonobe Underground Research Laboratory Area and Site

This study demonstrates the selection process for the Horonobe URL based on surveys of existing information and geophysical surveys on a regional scale. In addition, preliminary requirements on the geological environment, safety (during construction of the underground facility) and social and environmental constraints were taken into consideration. The technical know-how utilised through the experiences for the site selection is described here. The proposed Horonobe URL site required the existence of argillaceous sedimentary formations and associated groundwater. Further fundamental requirements were appropriate rock mechanical properties and low gas content in the host rock to meet safe underground construction and operation regulations. This led to a stepwise narrowing down from several potential URL areas located completely within the Horonobe District to one candidate URL area and, finally, to a specific URL site. In the URL investigation area (ca. 3 km × 3 km) the main surface-based investigations were conducted as the first step to choosing the actual URL site. This was selected based on establishing fundamental factors related to the geological environment, safety and societal issues. This paper provides an outline of the process utilised in selecting the URL site by taking into consideration technical and social requirements. Thus stepwise approach and experience in selecting the URL site will be applicable when NUMO needs to select a site through literature surveys, and preliminary and detailed investigations in the future.Copyright

Current Status of Phase II Investigations: Mizunami Underground Research Laboratory (MIU) Project

The Mizunami Underground Research Laboratory (MIU) Project, a comprehensive research project investigating the deep underground environment in crystalline rock, is being conducted by the Japan Atomic Energy Agency at Mizunami City, Central Japan. The MIU Project is being carried out in three overlapping phases: Surface-based Investigation (Phase I), Construction (Phase II), and Operation (Phase III), with a total duration of 20 years. The overall project goals of the MIU Project from Phase I through to Phase III are: 1) to establish techniques for investigation, analysis and assessment of the deep geological environment, and 2) to develop a range of engineering techniques for deep underground application. Phase I was completed in March 2004, and Phase II investigations associated with the construction of the underground facilities are currently underway. Phase II investigation goals are to evaluate the geological, hydrogeological, hydrogeochemical and rock mechanical models developed in Phase I and to assess changes in the deep geological environment caused by the construction of underground facilities. Geological mapping, borehole investigations for geological, hydrogeological, hydrochemical and rock mechanical studies are being carried out in shafts and research galleries in order to evaluate the models. Long-term monitoring of changes in groundwater chemistry and pressure associated with the construction of the underground facilities continue in and around the MIU site, using existing boreholes and monitoring systems. This report summarizes the current status of the MIU Project on results of the Phase II investigations to date.Copyright