Nuclear Fusion
There are two broad categories of nuclear reaction that generate energy:
- nuclear fission, whereby the nuclei of atoms such as uranium are split into lighter atoms. This is the reaction used to generate electricity in all the nuclear power plants that are currently in operation.




- nuclear fusion whereby two light atoms (e.g. deuterium and tritium, two hydrogen isotopes) join together to form a heavier nucleus. This is the reaction that will be used in the ITER reactor.

A fusion reaction is achieved by forcing together two nuclei in order to make them fuse. The difficulty to be overcome is the fact that both are positively charged and will naturally tend to repel each other. To achieve fusion, these nuclei have to be heated and kept at temperatures of over one hundred million degrees centigrade. At such high temperatures, the atoms are ionised* and form a plasma.

Ionised atom: an atom is made up of a nucleus, with a cloud of electrons orbiting around it. The nucleus itself is formed of protons and neutrons. Stable atoms have the same number of electrons as protons. We refer to an atom as ionised when it has gained or shed one or more electrons.

Plasma: Plasma is considered to be a fourth state of matter alongside solids, liquids and gases. In a plasma, the atoms are positively charged (ionised) as they lose their electrons due to the high temperature. The temperature of a plasma can vary from a few degrees to several billion degrees centigrade. Its density may be anything from one million times less than the density of air, to one million times greater. More than 99% of matter in the universe is found in the form of plasma. The sun and stars are super-heated, super-dense balls of plasma. In the uppermost layers of our atmosphere (ionosphere), the air is ionised by ultraviolet solar radiation and other cosmic radiation sources, cauing it to form plasma. Plasma is also found in neon strip lights, plasma welding torches and plasma television screens.