Nuclear power produces energy that is converted into electrical energy in nuclear power stations. Nuclear fuel does not burn. Instead a process called nuclear fission takes place.
During nuclear fission a neutron is fired at a Uranium atom. The neutron is absorbed, which makes the atom extremely unstable so it splits into two smaller atoms, releasing more neutrons and a huge amount of energy at the same time.
The neutrons that are released can go on to collide with other uranium atoms causing more fission and more neutrons to be released. This is called a chain reaction.
The new atoms that are formed are radioactive.
The amount of energy released during nuclear fission is much larger than the energy released when substances react chemically. For instance, 1 kg of uranium undergoing fission can release the same energy as 10 000kg of coal burning!
Uranium is not the only element that can be used in nuclear power. Plutonium is an alternative fuel.
Note: Don't confuse this with nuclear fusion, which is what happens in stars. Two hydrogen atoms are pushed together to fuse and make a helium atom. This also releases massive amounts of energy!