If the flow rate is increased still further the particles will be blown completely out of the bed. Since the drag force on the particle is proportional to its area while its mass is proportional to its volume, small particles are subject to a greater aerodynamic drag per unit mass than large particles. A particle will leave the bed only when the aerodynamic drag exceeds the "weight" of the particular particle. In space the gravitational force is simulated by centrifugal acceleration. The proposed working fluid for the reactor and Brayton cycle is a helium-xenon mixture with an average molecular weight of 8. Since naturally occurring xenon has an atomic mass of 131.3 and helium has an atomic mass of 4, the atom percent of xenon in the mixture is 3.14%. Xenon occurs naturally as a mixture of 9 stable isotopes as listed below. breeder can be estimated from earlier studies of gas core breeders. A typical UFg breeder reactor configuration^ has a critical mass of 250 Kg. This is lower than the 1109 Kg for the Clinch River LMFBR or the 1468 Kg for the MSBR because of the absence of structural materials within the core. Since critical mass calculations have not been performed for rotating particle-fueled breeders, reactor parameters will be calculated for critical masses in the range of 250 to 2000 kg. Promising uranium-bearing fuels are: The mass density of uranium in these fuelsis 10.2 for UC2, 9.64 for UO2, and 13.5 for UN. For the case of UO2 particles with an 80% void fraction, the inner diameter of the cylindrical reactor vessel is calculated assuming its length equal to the diameter and a fluidized bed thickness of 5% of the diameter. The results are: The average cross section of natural xenon is 24.5 barns, as compared with less than 0.007 barns for helium. The average cross section of the heliumxenon mixture (M = 8) is 0.78 barns. This is close to the 0.534 barns cross section of sodium and the 0.66 barn cross section of water, thus the average thermal neutron absorption cross section for the helium-xenon mixture is close to that of other conventional reactor coolants. Since the helium- xenon mixture is a gas, its atom density at projected reactor operating temperatures is only about 1% that of liquid water or sodium, so its macroscopic cross section will be about 100 times less. Thus, the relatively high microscopic cross sections of some of the xenon isotopes are not expected to present any problem as far as the neutronics of the reactor is concerned. Work to date on rotating fluidized bed reactors has been concerned with non-breeders only, and no nuclear calculations for fluidized bed breeders are available. However, the critical mass of such a The pressure drop per unit thickness of a rotating fluidized bed is given by5,6
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