Rare earth element (REE) contents of hydrothermally altered dacitic and basaltic rocks in the Kuroko mining area were analyzed. It was found that geochemical features of REE (Light REE/Total REE ratio, La/Sm ratio, Eu anomaly) are related to K₂O index (= K₂O / (K₂O + Na₂O + CaO + MgO) × 100) and oxygen isotopic composition (δ¹⁸O). This relation indicates that LREE (light rare earth element) did not remove from the rocks to hydrothermal solution and were probably added to the rocks from hydrothermal solution at and near the site of hydrothermal ore deposition. This geochemical behavior of LREE which are considered to be chemical analogue elements for Am and Cm suggests that Am and Cm would not remove from the high level nuclear waste glass if the hydrothermal solution ascent to the disposal site and contacted with the high level nuclear waste.

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  Multi-barrier concept utilizes both engineered barrier system (EBS) and natural barrier system (NBS) to guarantee long-term safety of geological disposal. For this purpose an appropriate disposal concept has to be adopted based on the information obtained in the site characterization at each stage, which includes enhancement or modification of some components of the EBS. Although properties of geological environment often exhibit significant spatial heterogeneity, in most of the existing studies, nuclide release from a single waste package surrounded by an “average” host rock is calculated and the result is regarded as a representation of the whole heterogeneous repository region. However, spatial heterogeneity in the repository region implies that there exist sub-regions that are more favorable from the safety perspective and the ones that are less favorable. In other words safety performance of an entire repository cannot be determined without knowing how many wastes can be accommodated within the more favorable sub-regions and how thoroughly hydrological connection between favorable and less favorable sub-regions can be mitigated by design and engineering solutions.
  In this study, as an alternative to the strategy based on homogeneous approximation, an approach to optimizing repository layout by maximizing number of waste packages to be accommodated in the favorable sub-regions in a heterogeneous geological environment and to evaluate its contribution to improving safety performance of the entire system is proposed.

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  The flow rate of underground-water in deep geologic environment is a key parameter to evaluate the performance of the repository for high-level radioactive waste disposal. The preliminary estimation however suggested the flow rate is much lower than the detection limit using conventional measurement techniques. This situation may underestimate the repository function because of the lack of the actual measured data. Thus the authors developed a new technique for the three-dimensional flow measurement by using the ultrasonic echo reflected from solid particle tracers suspended in groundwater. The moving track is traced by the matrix arrayed sensor and this technique enables us to measure the wide range of flow rates of 10^-10～10^-5ｍ/s. The preliminary experiments hence suggested the tracer must be stably suspended for a period not shorter than two days. To realize this requirement, the disturbance of sedimentation due to density difference between the solid particle and groundwater and Brownian motion of the particle must be estimated and eliminated. We designed and fabricated the solid particle tracer having a core covered with a shell of different density and the resulting density is easily adjustable. The stability of the tracer was examined by in-situ experiment carried out in 500m depth bore hole and successfully measured the flow rates of 10^-10～10^-5ｍ/s.

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