Diffusion of Carbon Dioxide Through Layers of the Unsaturated Zone at Yucca Mountain

by M. S. Ali, North Carolina A&T State Univ, Greensboro, United States,
T. Bardakci, North Carolina A&T State Univ, Greensboro, United States,
S. Ilias, North Carolina A&T State Univ, Greensboro, United States,
F. G. King, North Carolina A&T State Univ, Greensboro, United States,

Document Type: Proceeding Paper

Part of: High Level Radioactive Waste Management 1993


The estimation of diffusive transport of gases through tuffs is important in order to asses the Yucca Mountain as an environmentally safe and acceptable repository for spent fuel and high-level radioactive waste. The purpose of this study is to provide effective diffusivity data which may be used to calculate the amount of diffusion of carbon dioxide through Yucca Mountain tuff layers to the environment. The effective diffusivity of carbon dioxide was measured for five tuff samples from different layers of the USW-G4 drillhole, which is located in the central part of the proposed repository horizon. The total depth of the USW-G4 is about 905 m. The tuff samples studied are from the following layers and depths: Vapor Phase (83.6 m), Upper Lithophysal (130.1 m), Middle Nonlithophysal (210.1 m), Lower Lithophysal (297.3 m), and Lower Nonlithophysal (356.3 m). A steady state counter-diffusion method, using gas chromatographic analysis, was used to determine the effective diffusivity of carbon dioxide through tuff samples. The effective diffusivity increases linearly with temperature between 25 degrees C and 125 degrees C. With increasing porosity of the tuffs, The effective diffusivity also increases with increasing porosity of the tuffs for all the samples studied except the Vapor Phase tuff. To provide an explanation for low diffusion coefficient of carbon dioxide of Vapor Phase tuff, further study on pore structures and pore inner-connectivity is required. From the limited data of USW-G4 drillhole, it appears that the diffusion takes place primarily in the combined Knudsen and bulk diffusion regimes.

Subject Headings: Diffusion (porous media) | Radioactive wastes | Carbon dioxide | Porosity | Fuels | Drilling | Waste disposal

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