On December 2, 1942, in a squash court under the abandoned west stands of University of Chicago's Football Field, Italian physicist Enrico Fermi's "Chicago Pile," (CP-1) produced the first self-sustaining nuclear reaction. The Metallurgical Laboratory (Met Lab) in Chicago was part of the US government's Manhattan Project to produce an atomic weapon. Scientists from both coasts were brought together to develop chain-reacting "piles" for plutonium production, to devise methods for extracting plutonium from the irradiated uranium, and to design a weapon. In all, four methods of plutonium separation were considered, with the bismuth phosphate process displaying the most promise. In August 1942, scientists at the Met Lab isolated the first measurable amount of plutonium then in December started the first man-made reactor the size of a two car garage, consisting of control rods made of highly purified graphite, uranium, uranium oxide and cadmium. This CP-1 reactor produced only a few watts of energy but proved it was possible and controllable.
A nuclear reactor must control the reaction and reduce the neutron's speed and that is where moderators are used. Moderator molecules like deuterium(D2O), water, helium or carbon are low-mass atoms that do not undergo fission and decrease the flying neutrons energy by successive collisions. Also to control the reaction, control rods are moved in and out of the reactor to tune it and absorb neutrons without re-emitting them. These rods are commonly made with cadmium or boron. Installed all the way in the control rods can stop the chain reaction.
There are several fissionable elements but uranium is the fuel of choice in all commercial nuclear plants as of 2009. Uranium is 500 times more abundant than gold. There is 2-4 parts per million of uranium in most rocks. The concentration must be much larger to mine the raw ore. Even once the uranium yellow cake is leached from the ore over 99% of the uranium is the non-fissionable isotope uranium-235. Less than 1% is fissionable uranium-238. Most reactors require that the uranium must be enriched to near 4% U238 to be usable. The other Uranium, U235 can be made into a fissile material in a [[http://www.machine-history.com/Nuclear%20Breeder%20Reactor|Breeder Reactor]].
This basic production of energy is based on Einstein's law of relativity. E=mc² states energy and mass are equivalent and transmutable. The fission of U235 happens when it is struck by a slow neutron, it splits, which is fission, releasing two smaller atoms and two or three neutrons(products). The products from the nuclear reaction weigh less then the original atoms. The difference in weight is converted into energy.
In 2009 there were a total of 435 nuclear reactors operating (nine fewer than 2002) in 31 countries, with a total capacity of 370 gigawatts producing about 15% of the world's electricity consumption. France leads the developed world with 77% of their electric needs produced by nuclear energy. United States has the most production with 800 terawatts-hours. (800 billion kilowatt-hours).