Active nuclear power plants
The heart of a nuclear-rocket engine, of course, is the reactor that converts nuclear energy into heat. The fuel of the Active nuclear power plants consists of a special kind of isotope of uranium, called uranium 235. When properly bombarded with neutrons the uranium nuclei break up of fission into a pair of fragments and emit more neutrons in the process, thus keeping the reaction Active nuclear power plants going.
The fission process releases energy and the excess energy is carried away by the neutrons and by gamma rays. Since all of the fragments and most of the neutrons and gamma rays are stopped within the reactor, the energy that is released by U 235 fission will heat the reactor.
For making active nuclear power plants thermally efficient the reactor`s temperature must be as high as possible.
The melting point of uranium, 2070 degrees F, sets a theoretical limit. Graphite, which withstands much higher temperatures is a very good material for reactors moderator. So all present e[perimental reactors for nuclear rocket engines are made of U 235 metal powder placed in graphite.
A cold gas, the hydrogen, enters several hundred narrow passages drilled through the graphite uranium reactor core and is heated almost to the white hot operating temperature. On coming from the passages, the hot gas expands through a nozzle in which it attains supersonic speed. The exhaust speed of the Active nuclear power plants can probably reach 23000 to 30000 feet per second, which is twice as much as from a rocket engine using chemical combustion of hydrogen and oxygen.