Tritium
31306
223249056
2008-07-03T06:45:07Z
Tedmund
6887330
Reverted edits by [[Special:Contributions/121.134.235.149|121.134.235.149]] to last version by 198.65.168.24 (using [[WP:HG|Huggle]])
{{Infobox isotope|
background = #ffcccc|
text_color = |
isotope_name = Hydrogen-3|
image = hydrogen-3.png|
alternate_names = tritium, triton|
mass_number = 3|
symbol = H|
num_neutrons = 2|
num_protons = 1|
abundance = [[Trace radioisotope|trace]]|
halflife = 4500±8 [[day]]s|
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}}''') is a radioactive [[isotope]] of [[hydrogen]]. The [[atomic nucleus|nucleus]] of tritium (sometimes called a '''triton''') contains one [[proton]] and two [[neutron]]s, whereas the nucleus of [[Hydrogen atom|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)<ref>[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</ref>. It decays into [[helium-3]] by the reaction
<!-- Autogenerated using Phykiformulae 0.11 by [[User:SkyLined]]
T -> He-3 + e + !ve
-->:{| border="0"
|- style="height:2em;"
|{{Nuclide|Tritium}} ||→ ||{{Nuclide|Link|helium|3}} ||+ ||{{SubatomicParticle|link=yes|Electron}} ||+ ||{{SubatomicParticle|link=yes|Electron Antineutrino}}
|}
releasing 18.6 [[keV]] of energy. The [[electron]] has an average kinetic energy of 5.7 keV, while the remaining energy is carried off by the nearly undetectable [[electron antineutrino]]. The low-energy [[beta radiation]] from tritium cannot penetrate human skin, so tritium is only dangerous if inhaled or ingested. Its low energy also creates difficulty detecting tritium labelled compounds except by using [[liquid scintillation counting]].
==Production==
Tritium occurs naturally due to [[cosmic ray]]s interacting with atmospheric gases. In the most important reaction for natural tritium production, a [[fast neutron]] (> 4[[MeV]] <ref>[http://www.fas.org/sgp/othergov/doe/lanl/lib-www/la-pubs/00320217.pdf An Evaluation of the Neutron and Gamma-ray Production Cross Sections for Nitrgoen], Los Alamos Scientific Laboratory</ref>) interacts with atmospheric [[nitrogen]]:
<!-- Autogenerated using Phykiformulae 0.11 by [[User:SkyLined]]
N-14 + n -> C-12 + T
-->:{| border="0"
|- style="height:2em;"
|{{Nuclide|Link|nitrogen|14}} ||+ ||{{SubatomicParticle|link=yes|Neutron}} ||→ ||{{Nuclide|Link|carbon|12}} ||+ ||{{Nuclide|Tritium}}
|}
Because of tritium's relatively short half-life, however, tritium produced in this manner does not accumulate over geological timescales, and its natural abundance is negligible.
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, which is more than one-quarter of the energy that fusion of the produced triton with a deuteron can later produce.
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Li-6 + n -> He-4 ( 2.05MeV ) + T ( 2.75MeV )
-->:{| border="0"
|- style="height:2em;"
|{{Nuclide|Link|lithium|6}} ||+ ||{{SubatomicParticle|link=yes|Neutron}} ||→ ||{{Nuclide|Link|helium|4}} ||( ||2.05 [[electron volt|MeV]] ||) ||+ ||{{Nuclide|Tritium}} ||( ||2.75 [[electron volt|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#High yield cause|Castle Bravo]] nuclear test produced an unexpectedly high yield<ref>[http://www.ieer.org/reports/tritium.html#(11) IEER Tritium Report<!-- Bot generated title -->]</ref>.
<!-- Autogenerated using Phykiformulae 0.11 by [[User:SkyLined]]
Li-7 + n -> He + T + n
-->:{| border="0"
|- style="height:2em;"
|{{Nuclide|Link|lithium|7}} ||+ ||{{SubatomicParticle|link=yes|Neutron}} ||→ ||{{Nuclide|Link|helium|4}} ||+ ||{{Nuclide|Tritium}} ||+ ||{{SubatomicParticle|link=yes|Neutron}}
|}
High-energy neutrons irradiating [[boron]]-10 will also occasionally produce tritium.<ref>http://meetings.lle.rochester.edu/Tritium/documents/3.ppt</ref> The more common result of boron-10 neutron capture is {{SimpleNuclide|Lithium|7}} and a single alpha particle.<ref>[http://nuclearweaponarchive.org/Nwfaq/Nfaq12.html Section 12.0 Useful Tables<!-- Bot generated title -->]</ref>
<!-- Autogenerated using Phykiformulae 0.11 by [[User:SkyLined]]
B-10 + n -> 2He + T
-->:{| border="0"
|- style="height:2em;"
|{{Nuclide|Link|boron|10}} ||+ ||{{SubatomicParticle|link=yes|Neutron}} ||→ ||2 {{Nuclide|Link|helium|4}} ||+ ||{{Nuclide|Tritium}}
|}
The reactions requiring high neutron energies are not attractive production methods.
Tritium's decay product [[helium-3]] has a very large 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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He-3 + n -> H + T
-->:{| border="0"
|- style="height:2em;"
|{{Nuclide|Link|helium|3}} ||+ ||{{SubatomicParticle|link=yes|Neutron}} ||→ ||{{Nuclide|Link|hydrogen|1}} ||+ ||{{Nuclide|Tritium}}
|}
Tritium is occasionally a direct product of [[nuclear fission]], with a yield of about 0.01% (one per 10000 fissions).<ref>[http://www.ead.anl.gov/pub/doc/tritium.pdf Tritium (Hydrogen-3)], Human Health Fact Sheet, Argonne National Laboratory, August 2005</ref><ref>{{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}}</ref> 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 (physics)|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. <ref>[http://www.nuclearfaq.ca/cnf_sectionD.htm#x5 The Canadian Nuclear FAQ - Section D: Safety and Liability<!-- Bot generated title -->]</ref>
According to [[IEER|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.<ref>[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 Zerriffi
January, 1996</ref>
Tritium for American [[nuclear weapon]]s was produced in special [[heavy water reactor]]s 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 rod]]s) 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.<ref>http://www.srs.gov/general/news/factsheets/tef.pdf</ref>
==Properties==
Tritium has an [[atomic mass]] of 3.0160492. It is a gas ({{Element|Tritium}}<sub>2</sub> or {{SimpleNuclide|Hydrogen|3}}<sub>2</sub>) at [[standard temperature and pressure]]. It combines with [[oxygen]] to form a liquid called [[tritiated water]], {{Element|Tritium}}<sub>2</sub>{{Element|link|Oxygen}}, or partially tritiated water, {{Element|Tritium}}{{Element|link|Hydrogen}}{{Element|link|Oxygen}}.
Tritium figures prominently in studies of [[nuclear fusion]] because of its favorable reaction [[Cross section (physics)|cross section]] and the large amount of energy (17.6 MeV) produced through its reaction with [[deuterium]]:
<!-- Autogenerated using Phykiformulae 0.11 by [[User:SkyLined]]
T + D -> He + n
-->:{| border="0"
|- style="height:2em;"
|{{Nuclide|Tritium}} ||+ ||{{Nuclide|Link|Deuterium}} ||→ ||{{Nuclide|Link|helium|4}} ||+ ||{{SubatomicParticle|link=yes|Neutron}}
|}
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's law| 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 dwarf]]s (so-called failed [[star]]s) cannot burn hydrogen, but they do indeed burn deuterium.
[[Image:Trtium.jpg|thumb|right|300px|[[Radioluminescent]] 1.2 Curie 4" x .2" Tritium vials are simply tritium gas-filled glass vials, the inner surfaces of which are coated with a [[phosphor]]. The "gaseous tritium light source" vial shown here is 1.5 years old.]]
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 reactor]]s, 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.
==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 pico[[Curie]] 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 [[Röntgen equivalent man|mrem]] (or 40 micro[[sievert]]s in [[SI units]]) per year.
==Usage==
===Self-powered lighting===
[[Image:Tritium-watch.jpg|thumb|right|A tritium illuminated watch face]]
The emitted electrons from small amounts of tritium cause [[phosphor]]s to glow so as to make [[self-powered lighting]] devices called [[traser]]s which are now used in [[watches]] and [[exit sign]]s. It is also used in certain countries to make [[Luminescence|glowing]] [[keychain]]s, 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 weapon]]s for [[boosted fission weapon|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.<ref>[http://www.ieer.org/reports/tritium.html IEER Tritium Report<!-- Bot generated title -->]</ref> 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|Lithium}}{{Element|link|Deuterium}} is not used for boosting, only for secondaries.
{{more|nuclear weapon design}}
===Controlled nuclear fusion===
Tritium is an important fuel for controlled [[nuclear fusion]] in both [[Magnetic fusion energy|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 [[Fusion power#The D-T fuel cycle|'''{{Element|Deuterium}}-{{Element|Tritium}}''' reaction]] is favored since it has the largest fusion [[Cross section (physics)|cross-section]] (~ 5 [[Barn (unit)|barns]] peak) and reaches this maximum cross-section at the lowest energy (~65 [[Electronvolt|keV]] center-of-mass) of any potential fusion fuel.
===Small arms sights===
Tritium is used to make the [[iron sights|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 ('''S'''ight '''U'''nit '''S'''mall '''A'''rms '''T'''rilux) 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==
{{Refimprove|date=August 2007}}
Tritium was first predicted in the late 1920s by [[Walter Russell]], using his "spiral" periodic table{{Fact|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 [[Radiometric dating|dating]] water, and therefore [[wine]].
==References==
{{reflist}}
==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]
{{Isotope|element=Hydrogen
|lighter=[[Hydrogen-2]]
|heavier=[[Hydrogen-4]]
|before=[[Hydrogen-4]]
|after=[[Helium-3]]
}}
[[Category:Isotopes of hydrogen]]
[[Category:Environmental isotopes]]
[[Category:Nuclear materials]]
[[Category:Radiochemistry]]
[[Category:Radioisotope fuels]]
[[Category:Nuclear fusion fuels]]
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