Reactor. The particle bed reactor (PBR) has a unique core in that the individual fuel assemblies are contained between a porous outer cylinder through which the coolant enters (the cold frit) and a porous inner cylinder through which the heated gas exits (the hot frit). The very small fuel particles (500 micron) allow for very rapid thermal transients and high operating temperatures. Each particle is coated with a series of materials designed to contain fission gases, resist internal pressures, and protect the particle from chemical attack or erosion by the high temperature hydrogen. Figure 19 shows the reactor configuration and Fig. 20 shows various aspects of the fuel and core. The thermal hydraulic behavior of this reactor requires that particular attention be paid to the pressure drops across the core and across each frit. The small fuel particle beds have a very low pressure drop but the potential exists to have flow distribution problems, with possible local heating of the individual fuel elements, if the flow is not properly distributed and controlled along the length of the elements. Calculations by the developer show that this potential problem can be controlled by appropriate distribution of porosity along the length of the frits. Turbine. A 10-stage turbine turning at 16 000 rpm was chosen and is shown in Fig. 21. The design techniques employed in this turbine are typical for the aerospace industry and result in a very lightweight functional turbine. Alternator. The alternator chosen has an external rotating rotor and is shown in Fig- 22. General Atomics System description. General Atomics (GA) developed a closed cycle system in the 10s of MW range consisting of an in-core thermionic reactor and a bank of Alkaline fuel cells. The fuel cells are used to supply a burst of power and the reactor power is used to regenerate the fuel cell reactants after the power burst is completed. The water
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