D-T fusion
The 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.
Besides ordinary hydrogen there are two heavier forms: deuterium (D), with one neutron, and tritium (T), with two. When they fuse they produce a helium-4 nucleus and a neutron, releasing 17.6 MeV per reaction. The neutron carries about four fifths of that (14.1 MeV) and the helium nucleus the rest (3.5 MeV).
D-T is chosen because, compared with other fuel combinations, it reacts at lower temperatures and is far more reactive at any given temperature. Even so, it is most reactive only above about 100 million °C[1].
It has two weaknesses. The first is the tritium: it is radioactive (half-life about 12 years) and almost absent in nature, so a reactor has to breed its own by hitting lithium with neutrons. The second is the fast neutron: being uncharged, it cannot be held by a magnetic field, and it strikes the reactor walls, damaging them and making them radioactive.
The magnetic-confinement record for D-T is 16.1 MW of fusion power, set by JET in the UK in 1997[2].
Related terms
- 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.
- IgnitionThe state in which a plasma keeps burning on the heat of its own fusion products — the helium nuclei — without any outside heating.
- Q valueFusion energy released divided by the energy put in from outside to heat the plasma. Q = 1 is breakeven; Q becoming infinite corresponds to ignition.