The importance of direct energy conversion to the long-term development of nuclear power (both fission and fusion) is discussed as an approach to alleviating waste heat problems, reducing overall system costs and enabling new uses such as nuclear space power propulsion. Various approaches that involve direct conversion of radiation energy are considered relative to fission reactors. The examples discussed involve the direct conversion of fission fragment energy to electricity, to thrust for propulsion, and to coherent laser light, illustrating the rich variety of potential conversion methods that can be envisioned.

Applications to future fusion systems are also discussed. Two approaches are considered: direct collection of fusion product energy, and extraction of plasma radiation energy.

Finally, thermal direct conversion is included as an important near term route to enhanced performance, e.g., in-core thermionic converters potentially offer enhanced operation of a SP100 type reactor for space applications.