A fast, uranium carbide (UC) fueled in-core thermionic space craft nuclear reactor, both for electricity generation and nuclear thermal propulsion, is presented. Being a very compact critical assembly, the reactor has an extremely high neutron leakage. Thus, the reactor control can be performed, elegantly, through the manipulation of the radial neutron leakage with the help of the rotating reflector drums containing natural B4C neutron absorbers, either in form of segments or strips. Calculations with natural B4C segments imbedded in rotating beryllium drums in the radial reflector have shown that reflector drums with 100 % natural B4C in form of strips (drum diameter = 13.5 cm, strip width = 5 mm) at the outer periphery of the radial reflector of 16 cm thickness would make possible reactivity changes of ?keff,max = 10.7 % without a significant distortion of the heat generation during all phases of the space mission. A reduction of the B4C in the strips to 20 and 10 % would still allow a reactivity change of ?keff,max = 8.4 and 7.7 %, respectively, amply sufficient for an effective control of a fast reactor during all phases of the space mission. By a nuclear thermal thrust around F = 5000 N and a specific impulse of 670 sec -1 at an hydrogen exit temperature around 1900 °K, the maximum temperature in the drums rises to 1023°K, with 100 % natural B4C content in the strips, far below the melting point of beryllium. The maximum drum temperature is depressed to 663 and 519 °K, with 20 and 10 % natural B4C content in the strips, respectively.
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