The question of direct transformation of nuclear energy into electricity by thermionic methods is discussed. A comparison is made of the properties of various materials, containing fissionable matter, and at the same time having good thermoemissive properties. Results are given of transformation experiments in the active zone of a reactor. The experiments have shown that in the course of operation, a transformer filled with inert gas is converted into a cesium plasma diode. This eliminates the necessity of introducing cesium vapor into the transformer. In addition, it becomes possible to pump out the gases during operation, which prolongs the useful life and improves the thermionic properties of the transformer.