Transient and Load following Characteristics of a Fully Integrated Single-cell Thermionic Fuel Element

Practical application of thermionic space nuclear power systems necessitates the investigation of such systems responses to changes in operation and design parameters as well as load-following characteristics. The principle building block in these systems is the thermionic fuel element (TFE). A Thermionic Transient Analysis Model (TUAM) is developed to simulates transient and steady-state operations of a fully integrated, single-cell TFE. TTTAM is used to investigate the responses of the TFE to a step input in reactivity, a change in Cs pressure and/or in the size of the interelectrode gap, a change in the coolant temperature, and a change in load demand. The effects of these parameters on load electric power, emitter temperature, overall conversion efficiency, and load-following characteristics of the TFE are determined. Results show that although nuclear reactors having negative temperature reactivity coefficients are always loadfollowing, TFEs are only partially load-following. However, for a given load electric power need there are several combinations of fission powers and load resistances. Results also show that for TFEs having a large interelectrode gap, it is desirable to conserve Cs by lowering its vapor pressure at the begmning-of-life, since increasing the Cs pressure insignificantly affects the load electric power. However, should fuel swelling, after operating the reactor for an extended period of time, reduces the width of the interelectrode gap, both the conversion efficiency and the load electric power will decrease. In this case, the load electric power could be restored by increasing the fission power, and only partially by changing the coolant temperature and/or increasing the Cs vapor pressure.