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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 this paper two recent analyses are reported which demonstrate advantages of a U233 fueled thermionic fuel element (TFE) compared to 93 w/o U235, and that application (mission) has broad latitude in how space power reactor systems could or should be optimized. A reference thermionic reactor system was selected to provide the basis for the fuel comparisons. Both oxide and metal fuel forms were compared. Of special interest was to estimate the efficiencies of the four fuel forms to produce electrical power. A figure of merit (FOM) was defined which is directly proportional to the electrical power produced per unit uranium mass. In a TFE the average electrical power produced is proportional to the emitter surface area (Esa), hence the ratio Esa/Mu was selected as the FOM. Results indicate that the choice of fuel type and form leads to wide variations in critical and system masses, FOM values, and system total power.