fourth-lecture-npre442-sp11 - Environmental Impacts of...

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Unformatted text preview: Environmental Impacts of Radioactive Waste- Derived Radionuclides “One thing, however, caused us great uneasiness–our water reserve was already half exhausted.” —Henry Lawson in “Journey to the Center of the Earth” by Jules Verne. Environmental Impacts The environmental impacts of radionuclides in air, water, soil, and plants animals and depends on: The chemical properties of the specific radionuclide. The geologic characteristics of the area impacted. Environmental Fate The hydrogeology and climate that determines water availability and movement. Atmospheric dispersion depends on meteorological conditions, topography, plume depletion caused by precipitation, decay. and interaction with the ground. The type of organism, and habitat. Environmental Pathways The physicochemical form of the source of the radionuclide will be discussed in a later lecture. For now . . . Environmental Pathways Ideally, wastes should remain immobile and unreactive, but if water reaches the waste package, some radionuclides may dissolve or leach and move from the disposal area (more about leaching later). Radionuclides may be transported by water in the vadose zone (water unsaturated material). Environmental Pathways Radionuclides may be transported by water to the saturated zone (“water table”) and migrate with ground water flow. Depending on the type of radionuclide, the composition of the subsurface materials, the rate of groundwater movement, and time, a plume of contaminated groundwater may spread, and contaminate wells and surface water. Environmental Pathways Depending on the type of radionuclide, the gas permeability and water content of the subsurface materials, volatile radionuclides can diffuse from the disposal area into the atmosphere such as Rn. As radionuclides move away from the disposal area, their concentrations may be attenuated (lessened in severity) by a number physical and chemical processes. Attenuation Mechanisms Diffusion: the spontaneous movement of radionuclides (or any chemical or molecule) from an area of greater concentration to that of a lesser concentration. Diffusion will occur if there is a concentration gradient (the Second Law of Thermodynamics: entropy increases). Diffusion At some (molecular) level nothing is completely impermeable. Diffusion from waste packages: Steel barrels and canisters Ceramic and vitrified waste forms. Diffusion from disposal sites: Movement through compacted clays and soils, and rock formations. Movement in groundwater even if the water is not moving. Diffusion through a solid mass 99 Tc 99 Tc 99 Tc 99 Tc Thickness d area A, and donor reservoir volume V d Diffusion through a solid mass D = (V d x d)/(Ac d ) x (c i ” – c i ’)/t” –t’) where D = the diffusion coefficient c d = radionuclide concentration in donor tank c i = concentration in receptor side at time t Diffusion coefficients (cm 2 /sec) Soil (based on 50-year field data) 137 Cs: 0.6 to 3 x 10-4 90 Sr: 1.6 to 2.6 x 10-9 Cement and concrete (lab meas.) 3 H: 1.5 x 10-7 36 Cl: 3.0 x 10-7 137 Cs: 2.71 x 10-15 Diffusion coefficients (cm...
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fourth-lecture-npre442-sp11 - Environmental Impacts of...

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