promising for satellite power are given in Table 6. Of these the RPBR is considered as the most practical for 1991 technology availability. The very high temperature gas cooled reactor (VHTR) has a high core temperature but suffers from poor reactor life and the difficulties associated with refueling of a solid core reactor. One additional possibility is the transparent partition (light bulb) reactor wherein energy is transported through transparent tubes (e.g., quartz) into an opaque gas (e.g., seeded helium). The study focus was therefore shifted to the RPBR, since this reactor in effect "side steps" the material compatibility problem. The RPBR has been also studied for space nuclear propulsion. The fuel consists of uranium carbidezirconium carbide particles which are retained by centrifugal force in a rotating drum, which is a porous metal cylinder through which the working fluid (helium) circulates into the particle bed. The gas flows radially inward through the particle. Finally the heated gas passes out axially along the drum. Primary questions for the RPBR are how low a pressure drop can be achieved across the bed, since this influences the Brayton cycle efficiency, and whether the helium mass flow is sufficient to cool the drum. A fluidized bed reactor utilizes nuclear fuel consisting of small spherical or granular particles slightly suspended by a gas stream. Such reactors permit continuous fuel recycling and can be designed to heat gases to higher temperatures than would be possible for reactors using solid fuel elements. Since the fuel is in the form of small particles (typically 0.5 mm) the temperature differential between the center and edge of the particle is small compared to the temperature differentials inherent in a standard high temperature reactor core; this permits the working fluid gas to be heated to a temperature approaching the maximum operating temperature of the nuclear fuel. In addition to the high temperature capability of such reactors, the potential of such reactors for breeding with continuous fuel recycle is of considerable interest both for terrestrial and space applications. For example, ERDA is currently supporting work at Georgia Tech^ on the design and analysis of the Fluidized Carbon Coated Particles Reactor (FCCPR) and its associated fuel cycle to reduce the potential for nuclear proliferation. This particular project is designing a nuclear electric power station in the few hundred MWe range for export to third world countries and includes reactor physics and fuel cycle characteristics of a sustainer reactor (breeding ratio about 1.0) and advanced conversion technologies, evaluation of the safety of the system including handling and transportation of the wastes, economic assessment, assessment of problems areas, and a development plan. Fluidized bed reactors operate by suspending the fuel particles in the gas stream, but not entraining the particles in the gas. Figure 4-38 illustrates the basic principles. i A Fig. 4-38. Particle Bed Reactor Concept If a gas is blown upward through a packed bed of particles, then at low flow rates the particles will remain packed and the gas will simply flow through the bed of particles and out through the upper surface. When the flow rate is increased beyond the minimum flow rate required for fluidization, the particles are no longer packed but are buoyed upward to form an expanded bed of agitated particles which behave as a liquid. This is an important difference between a fluidized bed and a packed bed—a packed bed does not behave as a liquid whereas a fluidized bed does, since the individual particles in a fluidized bed behave similarly to molecules in a liquid, interacting with nearby particles through collisions rather than by resting upon the nearest particles. The pressure drop across a fluidized bed is equal to the weight of the bed divided by the area of the supporting surface.
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