Thermionic conversion for space power application
Efforts to develop thermionic conversion for space power application are discussed. Tungsten, niobium and Al2O3 were selected as the emitter, collect and insulator materials for the converter. Uranium carbides and uranium oxide were selected as candidates for the nuclear fuel. A total of 36 fueled thermionic converters and
fuel elements were life-tested during 1965 to 1972. These tests, supported by a dozen converter tests and several material irradiation tests, provided the base
of the in-core thermionic technology. Unfueled converters demonstrated a life of five years or more, while fueled converter fuel elements have been operated for one to one and one-half years. The major limiting factors for converter life and
performance were: component diffusion through cladding and emitter cracking for carbide-fueled converters, and emitter swelling for the oxide-fuel converters.
Various means for mitigating the fuel effects on converter life and performance were proposed but they were not thoroughly evaluated.