Nuclear fission can be maintaned as a controlled cjiain reaction in a nuclear reactor to produce energy. Fermi had found from experiment that slower moving neutrons were more effective in causing nuclearfission. But most neutrons produced by the splitting of the nuclei are quite fast. A way was required to slow them down. It was found that certain materials slow down the neutrons. Graphite. a pure form of carbon, is one such material. Such materials are called moderators. There was still the problem of controlling the chain reaction so created, that is, to stop the , reaction or allow it to proceed at will. Materials which absorb neutrons would serve to control the reaction. The neutrons absorbed by such materials would be removed and would no longer split atomic nuclei. And the reaction would be controlled. The material usually chosen as an absorber of neutrons, is cadmium or boron steel. In a nuclear reactor . rod-like containers of Uranium-235 are inserted in holes made in a huge block of graphite. The graphite block slows down neutrons to enhance the chain reaction. Control rods of cadmium are also inserted into the graphite block. When pushed out, they absorb fewer neutrons and the reaction is speeded up. The problem, then, is to remove the heat and use it to generate electrical energy. This is achieved by circulating water, or liquid sodium to absorb the heat generated in the graphite block. This heat may generate steam, which can turn a turbine (a wheel with slanting blades) anqhe connected electrical generator.
Today we have reactors capable of yielding power upto 500 Megawatts. Smaller reactors which give 1 to 5 Megawatt power are mostly used for reseasch work. The large ones are used for producing electricity, and driving submarines, or ships. From the uranium rods used in the reactor, another fissionable material like Plutonium 239 may be obtained.