Analysis of Evaporation in Nuclear Waste Boreholes in Unsaturated Tuff
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Analysis of Evaporation in Nuclear Waste Boreholes in Unsaturated Tuff W. Zhou, P. L. Chambr6, T. H. Pigford and W. W.-L. Lee, Department of Nuclear Engineering, University of California, Berkeley, California 94720-0001. Introduction We present an analysis of evaporation in a nuclear waste borehole in unsaturated tuff. In unsaturated tuff, water in contact with a waste container will evaporate due to the difference in vapor pressure between water in a fiat film and water held in rock pores with curved interfaces. Decay heat will also enhance evaporation. It is important to study evaporation in a potential geologic repository of nuclear waste in unsaturated rock because the corrosion of waste containers is increased with liquid water. For radionuclides other than gaseous ones, their release from waste solids requires liquid water. Formulation We study evaporation in the one-dimensional system shown in the inset in Figure 1. Plane 1 is the heated surface and plane 2 is the rock. If water comes in contact with the surface (plane 1), a thin film of thickness 6 is assumed to form on the surface. We assume that the film has a fiat interface with the gas consisting of air and water vapor. The space between the heated surface and the rock is L + 6. We make the following assumptions: 1. The air is at rest. Because air flow will enhance evaporation, neglecting air flow underestimates evaporation. 2. The total gas pressure is constant and equal to the ambient atmospheric pressure. 3. Vapor transport mechanism through still air is by molecular diffusion. 4. There are no sources and sinks for vapor inside the gas region. 5. The system is in quasi steady state. The liquid-gas interface is motionless. 6. The physical properties of water and the gas mixture are constant with temperature. 7. Gas components behave as ideal gases. Air is treated as a single component. 8. At the gas-liquid interface, water vapor and the liquid are in thermodynamic equilibrium. Inside the unsaturated rock, vapor is in local thermodynamic equilibrium with the liquid. 9. Radiative heat transfer has been neglected. For a fixed heating rate, if there is significant radiative heat transfer, then there would be less heat transferred by conduction and convection, reducing evaporation. This assumption overestimates evaporation. The energy equations determine the quasi-steady temperature distributions. Although these depend parametrically on time, we shall write only Te(x) and T,(x). In the liquid region, the heat transfer is by conduction only so that -A-
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