There were 269 functional nuclear centrals in 2001: 143 in the European Union, 120 in North America, 4 in Latin America and 2 in Africa.

The most common reactor nucleus is that functioning with under water pressure. Inside the reactor occurs the atomic fission, which releases huge heat amounts. The container is made from a special steel of 20-25 cm thickness. Some bars ascend and descend in order to control the reaction and the heat.

They measure the neutrons' flux, the refrigerator pressure and the temperature. The fuel utilized consists of radioactive minerals: plutonium 239 or enriched uranium 235. The fuel elements are metal bars of 4 m length with square faces which are submerged in a liquid. In the primary circuit, the heat generated in the nucleus is absorbed. Even if most common water is used, gas also can be employed.

The water sucks the heat and at the same time cools the nucleus. The cooling circuit has a "U" shape, to return cooled water to the nucleus. The heated water enters another cycle where electricity is produced. A turbine moves under the action of the water vapors and operates an electricity generator of 25,000 volts. This electric current reaches a transformer which magnifies the voltage to 300,000 volts in order to be distributed.

The water vapors generated in the nucleus turn back to a liquid stage passing through refrigeration towers and a condenser. The water is pumped again to the heat exchanger (the "U" shaped circuit), and the process starts again. The central needs to have cold water (maximum 18.5 C) to guarantee this phase. The nuclear fission of the atoms generates three types of waste: the alpha particles (helium nuclei made of two protons and two neutrons), the beta particles (electrons produced by the neutrons decomposition) and gamma radiations, the most dangerous, having a lifespan to 1,000 years.