What were the first reactors like?
In Chicago, on 2nd,December 1942. a research team led by Enrico Fermi (1901-1954) and Leó Szilárd succeeded in starting the first controlled chain reaction in a small reactor built by them, called the nuclear pile.
The first reactors were used for preparing plutonium, the basic component of atomic weapons. In the middle 1950s the Soviet Union as well as the Western countries, began their researches at a quick pace, both for military and other purposes. The 20th  December  1951 was the first time men had ever generated current in an atomic power station. This happened inside the Experimental Breed Reactor I. (EBR-1), in Ancor, Idaho County.
      The first nuclear power station equipped with an atomic reactor started its “career” in Obninsk, Russia on 27th June. Now there are more than 400 nuclear reactors around the world, all in all with 300,000 MW current efficiency. The majority of reactors obtain the heat from fission, with the use of uranium (rarely plutonium or thorium)      

The Fermi pile
  The Fermi nuclear reactor –pile on its earlier name - consisted of graphite bricks and uranium pieces. The amount of the substances was raised until the mass of the uranium – the fuel – was big enough to sustain the chain reaction. They used graphite moderators to slow down the impacting neutrons, these retarded the nuclei more efficiently. As the neutrons taking part in the reaction clash into the moderator’s nuclei, they lose energy. This process is similar to a billiard ball hitting another, stationary one. These are called thermal neutrons, because their energy, as they are slowing down, is almost the same as the heat-energy of the surrounding atoms and molecules. Among the uranium and graphite bricks, cadmium regulating rods are placed to control reaction speed by absorbing a part of the neutrons. In a Fermi pile, the uranium consists of 0.7% uranium-238 and 99.3% uranium-235. With the absorption of a neutron, the uranium-238 becomes uranium-239 which cannot split, but sends out gamma and beta radiations, and two of its neutrons discompose. Plutonium-239 arises from the reaction, which is used as nuclear fuel (it was discovered in 1941)

How does uranium become electricity?
As the fission reaction takes place only in relatively big and unstable atoms, just a few substances can be used as nuclear fuel.
Uranium is one of the densest elements which can be found in nature. Each and every atom is “on the edge” of fissure.
Fissionable substances
Uranium-235 is the most frequently used fissionable substance; its nucleus consists of 92 protons and 143 neutrons. The chain reaction stops if the mass of the fissile substance is too small. It only becomes self-supporting if the mass of the nuclear fuel exceeds a certain amount. This amount is called the critical mass, a value typical to the given fissuring substance. In the case of uranium-235 this is around 50 kilograms.
Uranium-238 is the most frequent uranium isotope that can be found in nature, but if it clashes into a neutron, it doesn’t fissure as easily as uranium-235. However, it can be converted into a new element, plutonium-239 isotope. Therefore, if a reactor is designed to work with natural uranium, there’s the risk of uranium-238 isotopes absorbing too many neutrons before they could clash into the uranium-235 isotopes and start a chain reaction. If this happened, the reaction would never start.
There are two possibilities for solving this problem. One of them is called the enrichment process, when -in the fuel- they raise the proportion of uranium-235 atoms, compared to the other elements. This operation happens before the preparation of the fuel, usually in a machine which spins around its axis, called the centrifugal. Here the uranium-235 isotope, due to the outward force, detaches itself from the 238 isotope. The other method is to slow down the neutrons inside the reactor with the use of some kind of moderator. This raises the chances of starting a chain reaction.