While tritium has several different experimentally-determined values of its half-life, NIST recommends 4,500±8 days (approximately 12.33 years).[1] It decays into helium-3 by the beta decay:
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31T → 32He + e− + νe
and releases 18.6 keV of energy in the process. The electron's kinetic energy varies, with an average of 5.7 keV, while the remaining energy is carried off by the almost-undetectable electron antineutrino. Beta radiation has more inherent power than alpha.[dubious ][citation needed] Beta particles from tritium can penetrate only 6 mm of air and are incapable of passing through the dead layer of human skin.[2]
Tritium is dangerous if inhaled, ingested, if combined with oxygen in tritiated water molecules, absorbed through pores in the skin leading to cell damage and increased chance of cancer.
The low energy of tritium's radiation makes it difficult to detect tritium-labelled compounds except by using liquid scintillation counting.
Production
Cosmic rays
Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. In the most important reaction for natural tritium production, a fast neutron (which must have energy greater than 4 MeV[3]) interacts with atmospheric nitrogen:
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147N + n → 126C + 31T
Because of tritium's relatively short half-life, tritium produced in this manner does not accumulate over geological timescales, and thus it occurs only in negligible quantities in nature.
Lithium
Tritium is produced in nuclear reactors by neutron activation of lithium-6. This is possible with neutrons of any energy, and is an exothermic reaction yielding 4.8 MeV. In comparison, the fusion of deuterium with tritium releases about 17.6 MeV of energy.
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63Li + n → 42He ( 2.05 MeV ) + 31T ( 2.75 MeV )
High-energy neutrons can also produce tritium from lithium-7 in an endothermic reaction, consuming 2.466 MeV. This was discovered when the 1954 Castle Bravo nuclear test produced an unexpectedly high yield.[4]
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73Li + n → 42He + 31T + n
High-energy neutrons irradiating boron-10 will also occasionally produce tritium.[5] The more common result of boron-10 neutron capture is 7Li and a single alpha particle.[6]
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105B + n → 2 42He + 31T
The reactions requiring high neutron energies are not attractive production methods.
Helium-3
Tritium's decay product helium-3 has a very small cross section for the (n,p) reaction with thermal neutrons and is rapidly converted back to tritium in a nuclear reactor.
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32He + n → 11H + 31T
Fission
Tritium is occasionally a direct product of nuclear fission, with a yield of about 0.01% (one per 10,000 fissions).[7][8] This means that tritium release or recovery needs to be considered in nuclear reprocessing even in ordinary spent nuclear fuel where tritium production was not a goal.
Deuterium
Tritium is also produced in heavy water-moderated reactors when deuterium captures a neutron. This reaction has a very small cross section (Making heavy water a good neutron moderator) and relatively little tritium is produced; nevertheless, cleaning tritium from the moderator may be desirable after several years to reduce the risk of escape to the environment. Ontario Power Generation's Tritium Removal Facility can process up to 2.5 thousand tonnes (2,500 Mg) of heavy water a year, producing about 2.5 kg of tritium.[9]
Deuterium's absorption cross section for thermal neutrons is .52 millibarns, while oxygen-16's is .19 millibarns and oxygen-17's is .24 barn. 17O makes up .038% of natural oxygen, which has an overall absorption cross section of .28 millibarns. Therefore in D2O with natural oxygen, 21% of neutron captures are on oxygen, a proportion that may rise further as 17O accumulates from neutron capture on 16O. Also, 17O emits an alpha particle on capture, producing radioactive carbon-14.
Production history
According to IEER's 1996 report about the United States Department of Energy, only 225 kg of tritium has been produced in the US since 1955. Since it is continuously decaying into helium-3, the stockpile was approximately 75 kg at the time of the report.[4]
Tritium for American nuclear weapons was produced in special heavy water reactors at the Savannah River Site until their shutdown in 1988; with the Strategic Arms Reduction Treaty after the end of the Cold War, existing supplies were sufficient for the new, smaller number of nuclear weapons for some time. Production was resumed with irradiation of lithium-containing rods (replacing the usual boron-containing control rods) at the commercial Watts Bar Nuclear Generating Station in 2003-2005 followed by extraction of tritium from the rods at the new Tritium Extraction Facility[10] at SRS starting in November 2006.[11]
Properties
Tritium has an atomic mass of 3.0160492. It is a gas (T2 or 3H2) at standard temperature and pressure. It combines with oxygen to form a liquid called tritiated water, T2O, or partially tritiated water, THO.
Tritium figures prominently in studies of nuclear fusion because of its favorable reaction cross section and the large amount of energy (17.6 MeV) produced through its reaction with deuterium:
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31T + 21D → 42He + n
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 (for example, in the core of the Sun), 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.
The tritium nucleus, containing one proton and two neutrons, has the same charge as the nucleus of ordinary hydrogen, and it experiences the same electrostatic repulsive force when brought close to another atomic nucleus. However, the neutrons in the tritium nucleus increase the attractive strong nuclear force when brought close enough to another atomic nucleus. As a result, tritium can more easily fuse with other light atoms, compared with the ability of ordinary hydrogen to do so.
The same is true, albeit to a lesser extent, of deuterium. This is why brown dwarfs (so-called failed stars) cannot burn hydrogen, but do burn deuterium.
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 significant long-term 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 of mixing of the lower and upper ocean levels.
[edit] Health risks
Tritium is relatively similar to hydrogen, which makes it bind to OH as Tritiated water (HTO), and that it can make organic bonds (OBT) easily. The HTO and the OBT are easily ingested by drinking, through organic or water-containing foodstuffs. As tritium is not a strong beta emitter, it is not dangerous externally, but it is a radiation hazard when inhaled, ingested via food, water, or absorbed through the skin.[12][13][14][15]
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