A thermionic transient analysis model is developed to simulate transient and steady-state operations of a fully integrated, single-cell thermionic fuel element (TFE). The responses of the TFE to a step input in reactivity and changes in the cesium pressure or in the size of the interelectrode gap, the coolant temperature, and the load demand are investigated. Also, the effects of these parameters on the 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 load following, TFEs are only partially load following. For TFEs having a large interelectrode gap, it is desirable to conserve cesium by lowering its vapor pressure at the beginning of life since increasing the cesium pressure insignificantly affects the load electric power. However, should fuel swelling reduce the width of the interelectrode gap (after operating the reactor for an extended period of time), 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 increasing the cesium vapor pressure.