Nuclear fission involves splitting atomic nuclei, and is the process used in nuclear power stations. Fusion, as its name suggests, involves fusing nuclei and is the power source of the stars. While both fission and fusion release energy, the process and amount is very different. Fission exploits the instability of nuclei of heavy elements like uranium, which can be split using neutrons, producing fragments with a lower total mass.
The difference appears as energy -courtesy of E=mc2 – which is carried away by fast-moving neutrons. In contrast, fusion involves ramming together nuclei of light elements like hydrogen so violently they fuse together, producing fresh nuclei plus neutrons. Again, the lower mass of the fusion products is turned into energy via Einstein’s famous equation, but over 10 times the amount produced by fission for each gram of ‘fuel’.