Nuclear fusion
A reaction in which light atomic nuclei join to form a heavier one, releasing a large amount of energy. It is what makes the Sun and the stars shine.
Atomic nuclei carry positive charge and repel one another. At very high temperatures, though, they can collide hard enough to stick together and become a different nucleus. The product is slightly lighter than the nuclei that went in, and the missing mass comes out as energy. In 1920 Arthur Eddington argued that the Sun shines by turning hydrogen into helium[1]. In 1939 Hans Bethe worked out which reactions actually keep going inside stars[2].
Using fusion to generate electricity on Earth means making a plasma hotter than 100 million °C and holding it for long enough. It is the reverse of fission, the splitting of heavy nuclei such as uranium that powers today's nuclear plants. Its attractions are that the deuterium fuel can be taken from seawater and that the reaction simply stops if conditions slip. No machine has yet steadily produced more energy than was put in.
Related terms
- D-T fusionThe fusion of deuterium (D) and tritium (T) into a helium-4 nucleus and a neutron. Almost every fusion reactor now under development is built around this reaction.
- PlasmaA gas in which electrons have been stripped from their atoms, so that electrons and nuclei move around separately. Often called the fourth state of matter, after solid, liquid and gas.
- Lawson criterionThe condition that a plasma's density, confinement time and temperature must meet for a fusion reactor to produce net energy, derived by John Lawson in Britain in 1955.