A handful of uranium provides as much electricity as 70 tons of coal or 30 barrels of oil. A nuclear reactor to power a city of one million people consumes only 3 kg of uranium per day, making it the most concentrated energy source used by man.
Uranium is one of the densest natural elements and its atoms are very unstable. The nucleus of the atom only needs a small "push" to disintegrate, and when this happens it releases large amounts of energy, in the so-called nuclear fission.
This "push" can be received from neutrons, particles much smaller than atoms, which collide with the nucleus and disintegrate it. During this process at least two more neutrons are produced, which detach from the atom and then cause other fissures, so that these can continue indefinitely.
The energy of fission can be released little by little, and used to heat water. The water vapour is used to activate an electricity generator.
This is how a nuclear reactor works.
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