Oceans are an almost inexhaustible source of deuterium for future fusion power.

Deuterium and Tritium

Thermonuclear fusion needs fuel. The deuterium–tritium reaction is considered the most suitable for the first generation of fusion power plants because it provides a high reaction rate at comparatively low fusion temperatures. Both deuterium and tritium are isotopes of hydrogen. Deuterium is stable and contains one proton and one neutron, while tritium is radioactive and contains one proton and two neutrons.

The most common hydrogen isotope, containing a single proton and no neutrons, is called protium. Deuterium occurs naturally in water and is readily available on a very large scale. Seawater contains about 33 grams of deuterium per cubic metre, making the global resource effectively inexhaustible for fusion-energy purposes. Tritium, however, is much more difficult to obtain.

Tritium has a half-life of 12.3 years and occurs naturally only in trace quantities. Small amounts are produced by cosmic rays in the upper atmosphere and remain in the environment from past atmospheric nuclear tests. Tritium is also produced as a by-product in heavy-water reactors such as CANDU units, but the available global inventory is only of the order of tens of kilograms and is far too small to support commercial fusion power.

A fusion power plant would consume tritium on a much larger scale: roughly 70 kg per gigawatt of fusion thermal power during one full-power year. Large DEMO-scale reactors could therefore require around 100 kg or more of tritium annually.

Transmutation from Lithium

The solution is to breed tritium from lithium inside the reactor. In a deuterium–tritium reaction reaction, an alpha particle and a high-energy neutron are produced. The electrically neutral neutron escapes the magnetic confinement and enters the surrounding breeding blanket, where it can interact with a lithium-6 nucleus.

The principal breeding reaction is ⁶Li + n → ⁴He + T + 4.8 MeV. A breeding blanket contains lithium-bearing material and, in many concepts, a neutron multiplier that increases the number of neutrons available for tritium production.

Neutron multipliers such as beryllium-containing materials or lead can increase the number of neutrons available for breeding. Blanket concepts use either liquid lithium–lead or solid lithium-containing ceramic breeders. ITER will test four different Test Blanket Module concepts using water or helium cooling and both liquid-metal and ceramic breeder technologies.

Tritium Breeders

In a power reactor, the breeding blanket will operate as part of a continuous fuel cycle. Tritium produced in the lithium-containing material will be extracted during operation, purified and returned to the plasma fuel system. Different DEMO concepts use either ceramic breeder materials or liquid lithium–lead systems. The blanket structures themselves will gradually be damaged and activated by neutron irradiation and will therefore require remote replacement after a defined service life.

In solid ceramic breeder concepts, a purge gas carries released tritium to extraction and purification systems; liquid breeder concepts use different recovery methods. The recovered tritium is then recycled into the reactor fuel cycle together with deuterium. Replaced blanket modules will require remote handling, interim storage and, where possible, later recycling rather than being treated like spent nuclear fuel.

Fuelling

Fuelling will be continuous. Gas injection can be used for the initial plasma fill and for density control, but in a large burning plasma it is inefficient at delivering fuel deep into the core because the gas is ionised near the plasma edge. Cryogenic deuterium–tritium pellets can penetrate much deeper before being fully ionised and are therefore expected to play a major role in core fuelling. ITER, for example, is developing approximately 5 mm pellets injected from the high-field side at speeds of up to about 300 m/s.

Unburned deuterium and tritium will be continuously pumped from the plasma exhaust, separated from helium and impurities, and recycled back into the fuelling system.