This invention relates to a reactor core for a nuclear reactor, particularly for a "fast" type nuclear reactor, provided with nuclear fuel, for direct conversion into electrical energy of released heat produced by nuclear fission. The heat-to-electricity conversion takes place by means of thermionic converters having an inner-positioned collector and an outer-positioned emitter. The nuclear fuel is subdivided into sub-critical discs whose reactivity is influenced by a change in their mutual distance from each other; and the thermionic converter is imbedded in these individual discs. Additional controls in the form of control rods may also be provided to change the reactivity of the reactor core.
