Tritium Totally Explained

Everything about Tritium totally explained

Tritium (symbol or ) is a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of protium (the most abundant hydrogen isotope) contains no neutrons and one proton.

Decay

While Tritium has several different experimentally-determined values of its half-life, the NIST recommends 4500±8 days (approximately 12.32 years). It decays into helium-3 by the reaction

   All atomic nuclei, being composed of protons and neutrons, repel one another because of their positive charge. However, if the atoms have a high enough temperature and pressure (as is the case in the core of the Sun, for example), then their random motions can overcome such electrical repulsion (called the Coulomb force), and they can come close enough for the strong nuclear force to take effect, fusing them into heavier atoms. Since tritium has the same charge as ordinary hydrogen, it experiences the same electrostatic repulsive force (see Coulomb's law). However, due to tritium's supply of neutrons which are carried into reactions and feel the attractive strong force once delivered, tritium can more easily fuse with other light atoms. The same is also true, albeit to a lesser extent, of deuterium, and that's why brown dwarfs (so-called failed stars) can't burn hydrogen, but do indeed burn deuterium. Before the onset of atmospheric nuclear weapons tests, the global equilibrium tritium inventory was estimated at about 80 megacuries (MCi).
   Like hydrogen, tritium is difficult to confine; rubber, plastic, and some kinds of steel are all somewhat permeable. This has raised concerns that if tritium is used in quantity, in particular for fusion reactors, it may contribute to radioactive contamination, although its short half-life should prevent any significant accumulation in the atmosphere.
   Atmospheric nuclear testing (prior to the Partial Test Ban Treaty) proved unexpectedly useful to oceanographers, as the sharp spike in surface tritium levels could be used over the years to measure the rate at which the lower and upper ocean levels mixed.

Regulatory limits

The legal limits for tritium in drinking water can vary. The U.S. limit is calculated to yield a dose of 4 mrem (or 40 microsieverts in SI units) per year.

Canada 7,000 Bq/L.
United States 740 Bq/L or 20,000 pCi/L (Safe Drinking Water Act)
World Health Organization 10,000 Bq/L.
European Union 'investigative' limit of 100* Bq/L.

Usage

Self-powered lighting
The emitted electrons from small amounts of tritium cause phosphors to glow so as to make self-powered lighting devices called trasers which are now used in watches and exit signs. It is also used in certain countries to make glowing keychains, and compasses. This take the place of radium, which can cause bone cancer and has been banned in most countries for decades.
   The aforementioned IEER report claims that the commercial demand for tritium is 400 grams per year.
Nuclear weapons
Tritium is widely used in nuclear weapons for boosting a fission bomb or the fission primary of a thermonuclear weapon. Before detonation, a few grams of tritium-deuterium gas are injected into the hollow "pit" of fissile plutonium or uranium. The early stages of the fission chain reaction supply enough heat and compression to start DT fusion, then both fission and fusion proceed in parallel, the fission assisting the fusion by continuing heating and compression, and the fusion assisting the fission with highly energetic (14.1 MeV) neutrons. As the fission fuel depletes and also explodes outward, it falls below the density needed to stay critical by itself, but the fusion neutrons make the fission process progress faster and continue longer than it would without boosting. Increased yield comes overwhelmingly from the increase in fission; the energy released by the fusion itself is much smaller because the amount of fusion fuel is much smaller.
   Besides increased yield (for the same amount of fission fuel with vs. without boosting) and the possibility of variable yield (by varying the amount of fusion fuel), possibly even more important advantages are allowing the weapon (or primary of a weapon) to have a smaller amount of fissile material (eliminating the risk of predetonation by nearby nuclear explosions) and more relaxed requirements for implosion, allowing a smaller implosion system.
   Because the tritium in the warhead is continuously decaying, it's necessary to replenish it periodically. The estimated quantity needed is 4 grams per warhead. To maintain constant inventory, 0.22 grams per warhead per year must be produced.
   As tritium quickly decays and is difficult to contain, the much larger secondary charge of a thermonuclear weapon instead uses lithium deuteride as its fusion fuel; during detonation, neutrons split lithium-6 into helium-4 and tritium; the tritium then fuses with deuterium, producing more neutrons. As this process requires a higher temperature for ignition, and produces fewer and less energetic neutrons (only - fusion and splitting are net neutron producers), isn't used for boosting, only for secondaries.

Controlled nuclear fusion
Tritium is an important fuel for controlled nuclear fusion in both magnetic confinement and inertial confinement fusion reactor designs. The experimental fusion reactor ITER and the National Ignition Facility (NIF) will use Deuterium-Tritium (-) fuel. The - reaction is favored since it has the largest fusion cross-section (~ 5 barns peak) and reaches this maximum cross-section at the lowest energy (~65 keV center-of-mass) of any potential fusion fuel.
Small arms sights
Tritium is used to make the sights of some small arms illuminate at night. Most night sights are used on semi-automatic handguns. The reticule on the SA80's optical SUSAT sight (Sight Unit Small Arms Trilux) contains a small amount of tritium for the same effect as an example of tritium use on a rifle sight.
Analytical chemistry
Tritium is sometimes used as a radiolabel. It has the advantage that hydrogen appears in almost all organic chemicals making it easy to find a place to put tritium on the molecule under investigation. It has the disadvantage of producing a comparatively weak signal.
History
Tritium was first predicted in the late 1920s by Walter Russell, using his "spiral" periodic table, then produced in 1934 from deuterium, another isotope of hydrogen, by Ernest Rutherford, working with Mark Oliphant and Paul Harteck. Rutherford was unable to isolate the tritium, a job that was left to Luis Alvarez and Robert Cornog, who correctly deduced that the substance was radioactive. Willard F. Libby discovered that tritium could be used for dating water, and therefore wine.

Further Information
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