Nuclear fission happens when a neutron hits a uranium-235 atom and is absorbed by its nucleus. The new heavier nucleus becomes unstable and splits into two smaller nuclei.
This gives out a burst of energy (as heat) and two or three extra neutrons. These go on to hit, and split, other uranium nuclei, resulting in a chain reaction. The reaction keeps going, making more and more heat.
Nuclear power stations are engineered to control this chain reaction, enabling the amount of heat/energy to be regulated.
Clean energy
Unlike burning fossil fuels, nuclear fission releases absolutely no CO2, or any other greenhouse gases. CO2 is released though during construction (as with all power sources), by the mining and transportation of uranium, and in decommissioning power stations.
