Tritium is an isotope of hydrogen that possesses unique properties, making it a highly valued and difficult-to-acquire substance in scientific and industrial sectors. Because it is an unstable form of the most common element in the universe, it exists in extremely small quantities on Earth, requiring specialized and costly processes for human use.
Defining Tritium: The Basics of Hydrogen-3
Tritium, also known as hydrogen-3, is distinguished from the more common forms of hydrogen by the composition of its nucleus. While the most abundant hydrogen isotope, protium, contains a single proton and no neutrons, the tritium nucleus holds one proton and two neutrons, giving it an atomic mass of three. This makes tritium the heaviest particle-bound isotope of hydrogen and renders it radioactive.
The inherent instability of this isotope results in beta-minus decay, where one of the neutrons converts into a proton, emitting a low-energy electron. This transformation changes the element entirely, with tritium decaying into the stable isotope, helium-3. Tritium’s radioactivity is defined by a relatively short half-life of 12.32 years, meaning that half of any given amount converts into helium-3 after that period.
The low energy of the emitted beta particle is a specific characteristic of tritium’s decay, as the radiation is unable to penetrate the outer layer of human skin. Chemically, tritium behaves like ordinary hydrogen, allowing it to form compounds such as tritiated water, but its constant decay limits its long-term existence in nature.
Quantifying Scarcity: Tritium’s Natural Presence
The natural supply of tritium on Earth is negligible, a direct consequence of its slow, constant creation rate and its rapid radioactive decay. The primary mechanism for its natural production occurs high in the Earth’s atmosphere, where high-energy cosmic rays interact with nitrogen atoms. This continuous bombardment produces trace amounts of tritium, which then mix with the atmosphere and eventually enter the Earth’s water cycle.
Despite this constant generation, the 12.32-year half-life acts as a natural ceiling on the total standing stock, preventing large accumulation over geological time. The ratio of naturally occurring tritium to ordinary hydrogen is estimated to be approximately one part in $10^{18}$, illustrating its extreme scarcity.
The total inventory of natural tritium on the planet is estimated to be only a few kilograms, with a majority of that concentrated in the oceans. This small quantity contrasts sharply with the demands of modern science and industry, which necessitate a reliable, continuous supply of the isotope.
Industrial Production Methods
The immense gap between the small natural supply and industrial demand is bridged through highly controlled, energy-intensive manufacturing processes. The most common method involves neutron capture, which takes place inside specialized nuclear reactors. This process uses targets made from lithium-6, a relatively rare isotope of the element lithium.
When lithium-6 is bombarded with neutrons generated by the reactor’s fission chain reaction, the nucleus absorbs a neutron and transmutes, splitting into a tritium atom and a helium atom. This reaction yields tritium, which is then extracted, purified, and packaged for use. The United States, for example, produces tritium for defense purposes by irradiating lithium-containing rods in commercial light-water reactors.
Another source of tritium is as a byproduct in heavy water reactors, such as the CANDU designs used by countries like Canada, South Korea, and Romania. These reactors use heavy water, which contains deuterium, to moderate the fission process. During operation, neutrons can be captured by the deuterium, leading to the formation of tritium. The tritium produced must be managed and can be extracted to supplement the global supply.
Why Tritium Matters: Key Applications
The expense and difficulty of manufacturing tritium are justified by its unique properties, which are leveraged in highly specialized applications. The most prominent application is its role as a fuel component in nuclear fusion research, such as the International Thermonuclear Experimental Reactor (ITER). Tritium is fused with the more common isotope deuterium at extremely high temperatures to release immense amounts of energy, mimicking the process that powers the sun.
This deuterium-tritium reaction is currently the most accessible pathway for achieving sustainable fusion energy, making a reliable tritium supply a global concern for future power generation. Beyond fusion, tritium is also used in self-powered lighting devices known as radioluminescent sources. In these applications, the beta particles emitted by the decaying tritium excite a phosphorescent material, causing it to emit a continuous, low-level glow for years without external electricity. These devices are used for applications like firearm sights, exit signs, and watch illumination, where a persistent light source is necessary.