The Reactor That Eats Its Own Fuel express
An ordinary nuclear reactor is like a boiler where fuel burns evenly throughout the volume. Here the picture is different: a fission wave, a narrow zone where atoms split and release heat, slowly creeps along a cylindrical rod of uranium carbide. Ahead of the wave, the fuel is untouched; behind, it is already spent. So fire runs along a candle wick, leaving wax behind.
The main trick lies in the neutron spectrum. In ordinary fast reactors, neutrons are very energetic and quickly damage walls and mechanisms. Here the spectrum is softened: the peak falls at 20–50 thousand electronvolts (for comparison, the energy of a neutron in this range is hundreds of times lower than in a typical fast reactor). This reduces radiation damage to materials by more than an order of magnitude. Additionally, the fuel slowly moves relative to the channel walls via a hydraulic system, so a given point on the wall is not irradiated all the time.
Such a reactor is a prototype of a single-channel device with cylindrical symmetry. It requires no complex refueling systems and allows much fuller use of uranium than ordinary thermal-neutron reactors. If a pilot model confirms the calculations, humanity will get an energy source that itself "finishes off" its fuel, leaving less waste.
The fission wave in such a reactor moves at a speed of a few centimeters per year — slower than fingernails grow.
The idea of a self-sustaining fission wave resembles the fictional "atomic fire" from Larry Niven's novel Ringworld, but here it is real and under control.