Nuclear fission is the splitting of an atom's nucleus to release heat. Nuclear power stations use the fission of uranium-235 to heat water. Fossil fuel power stations burn their fuel (coal, oil or gas) to heat their water.
Dangerous radioactive waste is sealed in glass-like blocks and buried in rocks.
Fission is the splitting of a large nucleus. When this happens energy is released in the form of heat. Nuclear reactors use the heat released by the fission of uranium. If 1kg of uranium-235 undergoes nuclear fission it can release about the same amount of energy as 2 million kg of coal.
The fission of uranium can set up a chain reaction that will keep on releasing energy as long as there are uranium nuclei present. If this chain reaction is allowed to get out of control, energy is released very quickly and the result is a nuclear bomb. Nuclear power stations are designed so that this cannot happen.
Uranium-235 is a naturally occurring element which has a special property. When a neutron is absorbed by the nucleus, the new heavier nucleus becomes unstable. It is unable to hold itself together so splits into two smaller nuclei and gives out three neutrons and a burst of energy.