A thermo-hydrodynamic-neutronic analysis is performed for a fast, uranium carbide (UC) fueled spacecraft nuclear in-core thermionic reactor. The thermo-hydrodynamic analysis shows that a hybrid thermionic spacecraft nuclear reactor can be designed for both electricity generation and nuclear thermal propulsion purposes. This reactor would deliver a thermal thrust ∼5000 N by a specific impulse of 670 s at a hydrogen exit temperature ∼1900K. During the nuclear thermal thrust phase, the electricity generation will drop, depending on the entry temperature of the hydrogen propellant. Fresh hydrogen can be preheated through nozzle cooling up to 1000 K or more before entering the reactor. The hydrogen pressure and velocity at reactor entry are selected p = 30 atm and ν = 200 m/s, respectively. The pressure drop along the reactor core height (= 35 cm) is calculated Δp = 8.59 atm. The neutronic analysis has been conducted in S8-P3 approximation with the help of one- and two-dimensional neutron transport codes ANISN and DORT, respectively. The calculations have shown that a UC fueled electricity generating single mode thermionic nuclear reactor can be designed to be extremely compact because of the high atomic density of the nuclear fuel (by 95 % sintering density), namely, with a core radius of 8.7 cm and core height of 25 cm, leading to power levels as low as 5 kW(electric) by an electrical output on an emitter surface of 1.243 W/cm2. A reactor control with boronated reflector drums at the outer periphery of the radial reflector of 16-cm thickness would make possible reactivity changes of Δkeff > 10%—amply sufficient for a fast reactor—without a significant distortion of the fission power profile during all phases of the space mission. The hybrid thermionic spacecraft nuclear reactor mode contains cooling channels in the nuclear fuel for the hydrogen propellant. This increases the critical reactor size because of the lower uranium atomic density in this design concept. Calculations have lead to a reactor with a core radius of 22 cm and core height of 35 cm leading to power levels ∼50 kW(electric) under the aforementioned thermionic conversion conditions.