Abstract: A nuclear reactor (10) includes a vessel (12) containing a primary liquid, a core (14) comprising nuclear fuel and arranged in the internal volume of the vessel (12), at least one primary pump generating a main primary flow (56) of primary liquid in the vessel (12), at least one control member (16) for controlling the reactivity of the core (14), at least one movement mechanism (18) for moving the control member (16), arranged in the internal volume of the vessel (12) and linked to the control member (16), and a pressurizer (20) situated in a top portion of the vessel (12). The movement mechanism (18) comprises an electrical actuator and a transmission mechanism. The electrical actuator is completely immersed in the primary fluid and situated outside the main primary flow (56).

Abstract: A nuclear propulsion reactor. A reactor vessel is provided with an annular first core and a cylindrical second core that is radially encompassed by the first core. Nuclear fuel elements in the first core provide first stage heating of propellant as they direct the propellant axially through the first core. The second core, which contains fissionable material in a highly refractory form, is in fluid communication with the first core for receiving the heated propellant. Fission reactions in the second core driven by leakage neutrons from the first core provide second stage heating of the propellant as it passes therethrough. The second core directs the coolant to a propellant nozzle for providing propulsive thrust.