17:15 〜 18:45
[HCG20-P10] Influence of Heterogeneity on Ions and HDO Diffusion in Pre-Neogene Sedimentary Rocks
キーワード：地層処分、先新第三紀堆積岩、拡散、透過曲線、曲線フィッティング
1. Introduction
This study examines pre-Neogene sedimentary rocks, prevalent in the Japanese basement and considered potential host rocks for high-level radioactive waste (HLW) repositories. These rocks, characterized by their block-in matrix structures, present unique challenges in understanding the diffusion behaviors of radionuclides. Given their lithofacies, vein patterns, and fracture characteristics, an approach is required to clarify diffusion behaviors, based on their structural characteristics. The research employs through-diffusion experiments with Cs+, I-, and HDO to address this. The obtained breakthrough curves are modeled with the help of X-ray Computed Tomography (X-ray CT) scanning data. These investigations are pivotal in assessing the barrier performance of pre-Neogene sedimentary rocks for effective long-term containment of HLW.
2. Materials and methods
Core samples (φ116mm × 25m length) from the Chichibu belt were processed into disks (φ 20 mm × 5 mm thick) for through-diffusion experiments using simulated groundwater. Tracer solutions with 1×10-5 M Cs+, 1×10-4 M I- and 5 wt% D2O were used, and the samples were analyzed via X-ray CT, featuring 3600 projections, with a voxel size of 16 µm. Characterization identified two mélange-type rocks with block-in-matrix structures, and one coherent-type rock with a relatively homogeneous sandstone matrix, all containing lithofacies such as quartz, albite, and clay minerals. Diffusion breakthrough curve fitting was conducted by adjusting diffusion coefficients, porosities, and distribution coefficients, assuming multiple diffusion pathways, reflecting the results of X-ray CT.
3. Result and discussion
Diffusion experiments indicated that the ratio of effluent concentration to influent concentration of the ions and HDO ranged from 1×10-4 to 4×10-3. The absence of noticeable retardation suggests minimal sorption. The variability in the slopes of the breakthrough curves implies the presence of multiple pathways for diffusion, reflecting differences in the lithophanes’ porosity, size, and distribution of fractures. X-ray CT analyses have revealed the existence of veins and fractures, penetrative fractures and partially filled fractures with minor lithofacies. These veins and fractures affect both matrix diffusion and fracture transport. These observations have led to the initiation of a model development, which will incorporate adjustments to different parameters. The discussion at the conference will involve fitting such models to the breakthrough curves and the associated sensitivity analyses. The findings and their implications for radionuclide diffusion in heterogeneous geological formations are to be further elaborated at the conference.
This study examines pre-Neogene sedimentary rocks, prevalent in the Japanese basement and considered potential host rocks for high-level radioactive waste (HLW) repositories. These rocks, characterized by their block-in matrix structures, present unique challenges in understanding the diffusion behaviors of radionuclides. Given their lithofacies, vein patterns, and fracture characteristics, an approach is required to clarify diffusion behaviors, based on their structural characteristics. The research employs through-diffusion experiments with Cs+, I-, and HDO to address this. The obtained breakthrough curves are modeled with the help of X-ray Computed Tomography (X-ray CT) scanning data. These investigations are pivotal in assessing the barrier performance of pre-Neogene sedimentary rocks for effective long-term containment of HLW.
2. Materials and methods
Core samples (φ116mm × 25m length) from the Chichibu belt were processed into disks (φ 20 mm × 5 mm thick) for through-diffusion experiments using simulated groundwater. Tracer solutions with 1×10-5 M Cs+, 1×10-4 M I- and 5 wt% D2O were used, and the samples were analyzed via X-ray CT, featuring 3600 projections, with a voxel size of 16 µm. Characterization identified two mélange-type rocks with block-in-matrix structures, and one coherent-type rock with a relatively homogeneous sandstone matrix, all containing lithofacies such as quartz, albite, and clay minerals. Diffusion breakthrough curve fitting was conducted by adjusting diffusion coefficients, porosities, and distribution coefficients, assuming multiple diffusion pathways, reflecting the results of X-ray CT.
3. Result and discussion
Diffusion experiments indicated that the ratio of effluent concentration to influent concentration of the ions and HDO ranged from 1×10-4 to 4×10-3. The absence of noticeable retardation suggests minimal sorption. The variability in the slopes of the breakthrough curves implies the presence of multiple pathways for diffusion, reflecting differences in the lithophanes’ porosity, size, and distribution of fractures. X-ray CT analyses have revealed the existence of veins and fractures, penetrative fractures and partially filled fractures with minor lithofacies. These veins and fractures affect both matrix diffusion and fracture transport. These observations have led to the initiation of a model development, which will incorporate adjustments to different parameters. The discussion at the conference will involve fitting such models to the breakthrough curves and the associated sensitivity analyses. The findings and their implications for radionuclide diffusion in heterogeneous geological formations are to be further elaborated at the conference.