Many light elements and their isotopes can undergo fusion reactions, but when considering nuclear fusion as a source of energy and electricity, some types of fuel are much more suitable than others.

The first criterion is whether suitable conditions can be achieved for such a reaction. The core of the Sun has a temperature of about 15 million kelvin, while many other fusion reactions require much higher temperatures.

The energy yield of a reaction is also important. The more energy released, the more attractive the reaction is as an energy source. Another important factor is how likely the reaction is to occur when suitable particles collide at a given energy. This probability is characterised by the reaction cross-section, which varies greatly between different fusion reactions. All else being equal, a higher cross-section is advantageous.

Last but not least are the products of the reaction. Neutrons cannot be confined by magnetic fields because they have no electric charge. When they interact with reactor materials, they cause radiation damage and can make the materials radioactive. Charged reaction products, such as alpha particles, remain affected by electromagnetic fields and may potentially allow direct conversion of their energy into electricity.

Considering all of these conditions, here are several thermonuclear fusion reactions that have been identified as promising for energy production:

keV means kiloelectronvolt. One electronvolt is the energy gained by an electron when it moves through an electric potential difference of one volt. In plasma physics it is often used to express temperature, where 1 keV equals 11.6 million kelvin.

Q is the reaction energy yield.

Hydrogen (H)

Hydrogen is a very abundant element on Earth and throughout the Universe. It can easily be obtained from water by electrolysis. However, using ordinary hydrogen for terrestrial fusion through the proton–proton reaction that powers the Sun is impractical. In the Sun, this reaction proceeds at a core temperature of about 15 million kelvin, but its reaction rate is extremely low. To obtain usable fusion power on Earth, much faster reactions are required.

Deuterium (D, or ²H)

Deuterium can be extracted from seawater.

Deuterium is an isotope of hydrogen containing one proton and one neutron in its nucleus. It is a naturally occurring stable isotope and accounts for approximately 0.0156 percent of hydrogen atoms — in other words, about one in 6,400 hydrogen atoms is deuterium. Deuterium was discovered in 1931 by the American chemist Harold C. Urey. It can be extracted from water using well-established isotope-separation processes.

Heavy water can, for example, be concentrated using the Girdler sulfide process, followed by electrolysis to obtain deuterium. Distillation methods can also be used. However, the deuterium–deuterium fusion reaction is less favourable than the deuterium–tritium reaction, which has a higher reaction rate at lower plasma temperatures.

Tritium (T, or ³H)

Tritium is rare on Earth.

Tritium is a radioactive isotope of hydrogen containing one proton and two neutrons in its nucleus. It was first detected by Ernest Rutherford, Mark Oliphant and Paul Harteck in 1934. Its half-life is about 12.3 years. Naturally occurring tritium is extremely rare on Earth. Small amounts are formed in the upper atmosphere through interactions with cosmic rays. Tritium is also produced artificially in nuclear reactors.

Tritium for future D–T fusion power plants is expected to be produced from lithium in a breeding blanket using neutrons generated by the fusion reaction. A disadvantage of D–T fusion is the production of highly energetic neutrons. The reactor chamber is therefore exposed to intense neutron fluxes, causing activation and radiation damage, including embrittlement of structural materials.

Lithium

Lithium extraction from brine in Argentina.

Lithium is a light metal that can be cut with a knife. It is highly reactive with water and air, so it does not occur freely in nature, only in compounds. Lithium was discovered in 1817 by Johan August Arfwedson. It is present in seawater and in minerals of the Earth’s crust, although not in high concentrations. The total lithium content of the world’s oceans is estimated at about 200 billion tonnes.

Lithium is currently produced mainly from mineral ores and continental brines; its extraction from seawater is not practised on an industrial scale. Pure lithium can be obtained by electrolysis of molten lithium salts.

Fusion reactions that require higher temperatures but produce no neutrons in their primary reaction include deuterium-helium-3 and proton–boron-11 fusion. Their products are charged particles, although secondary reactions can still produce some neutrons. If D–T fusion is successfully mastered, such advanced fuels may become another challenge for future fusion research.

Helium-3

Helium-3 could potentially be extracted from lunar regolith.

Helium-3 is a stable isotope of helium with two protons and one neutron. It was first isolated by Luis Alvarez and Robert Cornog in 1939. Helium-3 is rare on Earth; small amounts occur in the atmosphere and in gases originating from the Earth’s mantle.

Helium-3 is present in lunar regolith, where it has accumulated through implantation by the solar wind, although its concentration is very low. It is also present in the atmospheres of the giant planets. Using such sources for fusion fuel would require extraterrestrial extraction and processing.

Boron

Boron.

In its crystalline form, boron is a brittle, dark, lustrous metalloid; in its amorphous form, it is a brown powder. It constitutes about 0.001 percent by weight of the Earth’s crust and occurs mainly in borate minerals such as borax, ulexite and colemanite. Boron minerals are mined commercially in several countries.

Boron was first isolated in 1808 by the French chemists Joseph-Louis Gay-Lussac and Louis-Jacques Thénard.