Tritium

Infobox isotope
background = #ffcccc
text_color =
isotope_name = Hydrogen-3


alternate_names = tritium, triton
mass_number = 3
symbol = H
num_neutrons = 2
num_protons = 1
abundance = trace
halflife = 4500±8 days
decay_mode1 = Beta emission
decay_energy1 = 0.018590
decay_product = Helium-3
decay_symbol =He
decay_mass =3
mass = 3.0160492
spin = 1/2+
excess_energy = 14949.794
error1 = 0.001
binding_energy = 8481.821
error2 = 0.004

Tritium (pronEng|ˈtɹɪt.i.əm, symbol Element|Tritium or SimpleNuclide|Hydrogen|3, also known as Hydrogen-3) 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) [ [http://nvl.nist.gov/pub/nistpubs/jres/105/4/j54luc2.pdf Comprehensive Review and Critical Evaluation of the Half-Life of Tritium] , National Institute of Standards and Technology] . It decays into helium-3 by the reaction

:

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 [ [http://www.ieer.org/reports/tritium.html#(11) IEER Tritium Report ] ] .

: Tritium is occasionally a direct product of nuclear fission, with a yield of about 0.01% (one per 10000 fissions). [ [http://www.ead.anl.gov/pub/doc/tritium.pdf Tritium (Hydrogen-3)] , Human Health Fact Sheet, Argonne National Laboratory, August 2005] [cite journal|author=Serot, O.; Wagemans, C.; Heyse, J.|title=New Results on Helium and Tritium Gas Production From Ternary Fission|journal=INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY. AIP Conference Proceedings|volume=769|pages=857–860|year=2005|url=http://adsabs.harvard.edu/abs/2005AIPC..769..857S|doi=10.1063/1.1945141] 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. Tritium is also produced in heavy water-moderated reactors when deuterium captures a neutron. This reaction has a very small cross section (which is why heavy water is such 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. [ [http://www.nuclearfaq.ca/cnf_sectionD.htm#x5 The Canadian Nuclear FAQ - Section D: Safety and Liability ] ]

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. [ [http://www.ieer.org/reports/tritium.html#(11) Tritium: The environmental, health, budgetary, and strategic effects of the Department of Energy's decision to produce tritium] , Hisham ZerriffiJanuary, 1996]

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 at SRS starting in November 2006. [http://www.srs.gov/general/news/factsheets/tef.pdf]

Properties

Tritium has an atomic mass of 3.0160492. It is a gas (Element|Tritium₂ or SimpleNuclide|Hydrogen|3₂) at standard temperature and pressure. It combines with oxygen to form a liquid called tritiated water, Element|Tritium₂Element|link|Oxygen, or partially tritiated water, Element|TritiumElement|link|HydrogenElement|link|Oxygen.

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:

:

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 they do indeed 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.

Health risks

Tritium is relatively similar to hydrogen, which makes that it binds 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-containg foodstuffs. As tritium is 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. [ [http://www.greenpeace.org/raw/content/canada/en/documents-and-links/publications/tritium-hazard-report-pollu.pdf Tritium Hazard Report: Pollution and Radiation Risk from Canadian Nuclear Facilities] , I. Fairlie, 2007 June] [ [http://www.nuclearfaq.ca/ReviewofGreenpeacereport_Final.pdf Review of the Greenpeace report: "Tritium Hazard Report: Pollution and Radiation Risk from Canadian Nuclear Facilities"] , R.V. Osborne, 2007 August] [http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/tritium-radiation-fs.html] [http://www.dep.state.pa.us/brp/Radiation_Control_Division/Tritium.htm]

Regulatory limits

The legal limits for tritium in drinking water can vary. Some figures are given below.

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

The U.S. limit is calculated to yield a dose of 4 mrem (or 40 microsieverts in SI units) per year.

Usage

elf-powered lighting

The emitted electrons from small amounts of tritium cause phosphors to glow so as to make self-powered lighting devices called betalights, which are now used in watches and exit signs. It is also used in certain countries to make glowing keychains, and compasses. This takes 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 is necessary to replenish it periodically. The estimated quantity needed is 4 grams per warhead. [ [http://www.ieer.org/reports/tritium.html IEER Tritium Report ] ] 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 Element|link|Deuterium-Element|link|Deuterium fusion and SimpleNuclide|link|Lithium|7 splitting are net neutron producers), Element|link|LithiumElement|link|Deuterium is not 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 (Element|link|Deuterium-Element|Tritium) fuel. The Element|Deuterium-Element|Tritium 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.

The Tritium Systems Test Assembly (TSTA) was a facility at Los Alamos National Laboratory dedicated to the development and demonstration of technologies required for fusion-relevant Deuterium-Tritium processing.

mall 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 tableFact|date=August 2007, 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.

References

External links

* [http://atom.kaeri.re.kr/ Nuclear Data Evaluation Lab]
* [http://alsos.wlu.edu/qsearch.aspx?browse=science/Tritium Annotated bibliography for tritium from the Alsos Digital Library]
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+tritium,+radioactive NLM Hazardous Substances Databank – Tritium, Radioactive]
* [http://books.google.com/books?id=sYKYZaxg0RUC Tritium on Ice: The Dangerous New Alliance of Nuclear Weapons and Nuclear Power by Kenneth D. Bergeron]

Isotope|element=Hydrogen
lighter=Hydrogen-2
heavier=Hydrogen-4
before=Hydrogen-4
after=Helium-3