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Physical Properties of the“Once Through Then Out” Pebble-Bed Reactor

U. Hansen, R. Schulten, E. Teuchert

Nuclear Science and Engineering / Volume 47 / Number 1 / Pages 132-139

January 1972


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A new and promising operating modus for the pebble-bed reactor has been investigated. Instead of circulating the fuel balls several times through the core they are moved only once slowly from the top to the bottom. Due to the increasing depletion toward the lower core area, there is a substantial axial tilt of the power density, and the downward flow of the cooling gas ensures for the system an optimal heat removal. The reduced power generation in the hot core area and the absence of hot spots enable achievement of a higher power density than in the known pebble-bed type and make possible a rise in the average gas outlet temperature up to 950°C. For a UO2-fueled reactor the life history is followed for several years by means of a two-dimensional calculational approach. Apart from the advantages in thermodynamics, the new system is marked by a very short and smooth running-in period, by a high sensibility of reactivity to control poison changes inside the upper reflector, and by an ideal accommodation of the burnup in the balls running with different flow speeds in different radial positions. The spatial distribution of the power density can be flexibly manipulated by changes in the fuel cycle speed, in fuel ball layout, or by the use of a higher feed enrichment in the outer core region. A brief parameter study and a discussion of technological aspects give an outline of the developing potential of that new type.

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