considered, the overall system efficiency is approximately 12%. The thermionic devices are connected in a series/parallel network to minimize the impact of device failures on the total system electrical output. Reactor control is accomplished by movable rods located in the radial reflector. Ilie rods consist of a boron carbide poison section and a beryllium reflector section. When the reactor is shut down, the poison section of the rod is located in the reflector. To start the reactor, the rods are moved upward to place the beryllium section in the reflector. Radiation protection for the payload is provided by a separation boom, a lithium hydride neutron shield, and a zirconium hydride gamma shield. The boom length and shield thicknesses are optimized based on system mass. The mass of the STAR-C power system in the 10 to 50 kWe range can be decreased significantly by making some minor changes in the concept. These changes consist primarily of optimizing the core length to diameter ratio and maximizing the ratio of the inner to outer fuel radius (Ref. 3). This "optimized OTR" is included in the mass comparisons in Chapter 4. 2.2 TFE Based Power Systems Three in-core thermionic reactor concepts, which are based on the thermionic fuel element were evaluated in this study. These concepts include (a) an all-TFE reactor (Ref. 4), (b) a moderated TFE reactor (TOPAZ, Ref. 5), and (c) a TFE reactor with SNAP driver fuel (Ref. 6). All of these concepts incorporate rotating reflec- tor/control drums within the pressure vessel, surrounding the core. The TFE based reactor concepts employ cylindrical fuel/converter elements that are stacked on end in a manner analogous to dry cells in a flashlight (Figure 2.2). The converter stack is encased in a metallic cladding to form a thermionic fuel element (TFE). Each cell consists of a stack of annular uranium dioxide fuel pellets surrounded by a tungsten emitter, a cesium vapor-filled gap, a collector and an insulator sheath (Figure 2.3). During power operation, heat from the nuclear fuel boils electrons off the emitter surface (—1800 K). These electrons cross the interelectrode gap to the cooler (—1000 K) collector surface. The voltage potential between the emitter and collector is used to drive the current through the electrical load. Total power system efficiencies are typically on the order of 8.5%. Waste heat is carried from the cladding surface to a radiator by a flowing NaK coolant. Thermoelectromagnetic pumps and the SP-100 radiator design were assumed in our analysis. Radiation shields consisting of zirconium hydride plus lithium hydride layers and tungsten plus lithium hydride layers were both considered in our calculations. The all-TFE concept (Figure 2.4) is a fast reactor with no moderator or driver fuel. This concept is more suitable for higher power levels (>.100 kWe) since the critical mass requirements are relatively high. ITte absence of a moderator or driver fuel permits coolant temperatures up to 1000 K or more.
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