A thermionic energy-conversion diode that uses a liquid metal as the electron collector is evaluated. Liquids, such as cesium, rubidium, or potassium and their alloys, are separated from the hot emitter by the vapor film produced in film boiling. Such a vapor film would maintain electrical separation if the emitter became warped because of radiation damage, thus greatly improving the reliability of the in-core thermionic diode. An experiment showed that stable film boiling occurs for ranges of emitter temperatures and cesium and potassium vapor pressures that are appropriate for thermionic diodes. The typical thermionic series-connected fuel-element geometry can be used in a film-boiling liquid-metal design to produce power in the zero g field of space. The improved tolerance to radiation damage and to emitter evaporation requires more precise control of collector temperatures, a modified startup and shutdown schedule to avoid nucleate boiling, and further development of a new thermal divider to exclude nucleate boiling at the edges of the collector surfaces.