1992-08-03

Dual-Radial Cell Thermionic Fuel Element 929131

There is considerable interest and increasing effort to achieve both higher electrical powers and increased energy conversion efficiency in thermionic power systems. These efforts are driven by potential NASA and DOD applications under consideration near the end of the present century or early in the next. In recent years, emphasis has been on finding compatible materials which will permit higher emitter surface temperatures, flattening axial temperature profiles to avoid near-zero power production at the ends of the emitters, studying fundamental issues related to the physics of the gap plasma, stacking cells vertically to achieve higher powers, and so on (1,2,3,4,5).
In the research reported in this paper, an attempt has been made to evaluate the potential of innovative thermionic fuel element (TFE) designs to achieve high energy conversion efficiencies and to quantify efficiency gains relative to the fuel mass of the system. In view of the high cost of enriched uranium (93w/o) electric power production (or a parameter proportional to it) per unit mass of fuel would serve as a meaningful efficiency figure of merit (FOM) to evaluate one design against another. Since electrical power produced is proportional to emitter surface area (Esa), a FOM can be defined as Esa per unit mass of uranium (Mu), that is, Esa/Mu. To compare different TFE designs, a baseline or reference critical reactor system is required.
In this paper one such design, called a dual-radial cell TFE, is considered. Fuel in the form of an annulus delivers heat to a pair of emitter-gap-collector cells arranged radially rather than as a vertical stack of individual single cells. The dual-radial cell has an inherent emitter surface area geometrical advantage. This suggests that a large increase in the TFE power production, relative to a single cell TFE at the same emitter temperature, may be achieved for a small increase in fuel mass. To evaluate a dual-radial cell TFE, relative to a single cell TFE, reference TFEs and a reference reactor system were defined.
Results indicate that the dual-radial cell TFE has an Esa/Mu 51% greater than that of the single cell with only an 8.4% increase in fuel mass. The dual-radial cell TFE reference reactor system mass is about 22% greater than that for the single cell reactor system. No effort was made to optimize the reactor system since the parameter to be optimized will depend on the mission the power system is to support. Since the emitter surface area of the dual-radial cell is nearly double that of the single cell, the total power is also nearly double. The value of Esa/Mu was also determined for the same dual-radial cell and reference system with U233 as the fuel.

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