Thermionic converters for use in outer space may be heated by the sun,
by decaying radioisotopes, or by a fission reactor. Thermionic
converters can also be made into concentric cylindrical shells (Figure
8a) and wrapped around the uranium fuel elements in nuclear reactors.
The waste heat in this case would be carried out of the reactor to a
separate radiator[9] by a stream of liquid metal. Since thermionic
converters can operate at much higher temperatures than thermoelectric
couples or dynamic power plants, the radiator temperature, T_c, will
also be higher. Consequently, space power plants using thermionic
converters will have small radiators. Once thermionic converters are
developed which have high reliability and long life, they will provide
the basis for a new series of lighter, more efficient space power
plants.