Fluid Flow and Energy Transport in a High-Level Radioactive Waste Repository in Unsaturated Alluvium
Pollock, David Warren
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https://hdl.handle.net/2142/71124
Description
Title
Fluid Flow and Energy Transport in a High-Level Radioactive Waste Repository in Unsaturated Alluvium
Author(s)
Pollock, David Warren
Issue Date
1982
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Engineering, Environmental
Abstract
Many parts of the Great Basin have thick zones of unsaturated alluvium which might be suitable for disposing of high-level radioactive wastes. This thesis examines the possible effects of a strong heat source on temperature and the movement of water (vapor and liquid) in a repository located in unsaturated alluvium. The partial differential equations governing the transport of water, dry air, and energy in an unsaturated porous medium are developed. These equations were solved numerically using finite difference techniques. An idealized repository was considered which consisted of an array of cylindrical canisters buried at a depth of 20 meters in isolated drill holes. To similify the modeling process, the three-dimensional transport system was approximated by a single axisymmetric "cell" centered around an individual canister. This approach takes advantage of the fact that an approximate no-flux symmetry boundary exists midway between adjacent canisters. The waste form was assumed to be 10-year-old reprocessed high-level waste with a half-life of 30 years. Initial liquid saturations ranging from 0.15 to 0.35 were considered. The thermal load was adjusted to produce a maximum temperature near 100(DEGREES)C. The temperature in the simulated repository increased rapidly during the first few years, reached a peak after about 20 years, and then decreased slowly to within a few degrees of the initial temperature after a few hundred years. A circulation system developed near the canisters in which vapor moved away from the canisters and liquid water flow toward them. A dry zone formed which extended throughout the repository and from one to five meters above and below the repository, depending on the initial saturation conditions. The dry zone reached its maximum extent between 50 and 100 years after burial. As the temperature in the repository decreased, liquid flow into the repository caused liquid saturations in the dry zone to increase once again. The effects of the dry zone persisted for periods ranging from approximately 100 years to more than 1000 years, depending on the mobility of liquid in the unsaturated alluvium.
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