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[en] The investigation into a full-scale 27 m high, by 6 m wide, thermosyphon loop. The simulation model is based on a one-dimensional axially-symmetrical control volume approach, where the loop is divided into a series of discreet control volumes. The three conservation equations, namely, mass, momentum and energy, were applied to these control volumes and solved with an explicit numerical method. The flow is assumed to be quasi-static, implying that the mass-flow rate changes over time. However, at any instant in time the mass-flow rate is constant around the loop. The boussinesq approximation was invoked, and a reasonable correlation between the experimental and theoretical results was obtained. Experimental results are presented and the flow regimes of the working fluid inside the loop identified. The results indicate that a series of such thermosyphon loops can be used as a cavity cooling system and that the one-dimensional theoretical model can predict the internal temperature and mass-flow rate of the thermosyphon loop.
[en] This paper presents the development of a one-dimensional mathematical model for simulate the processes of mass and energy transfer during cooling of the molten nuclear fuel relocated to the vessel bottom during a severe accident. The cooling effect is produced by the thermosyphon phenomenon, in which removal of heat from nuclear decay is carried out by the water that possibly penetrates between the bottom wall of the vessel and the molten material. The water heats up and changes phase producing steam. This steam generates a pressure strong enough to lift the molten material into a cavity. Through this cavity, water vapor flows to keep the molten material partially cooled. The mathematical model includes a model of the molten material and the remaining water. The growth of the crust of the molten material and its temperature profile, with heat generation by decay, are described by the approach Stefan problem in one dimension. The results show that for powers less than 4.15 k W m-3 is possible to solidify and cool the molten material in less than 2.5 x 106 s, that is in about 29 days. (author)