|Molecular formula||T 2 (molecular form)|
|External identifiers / databases|
25.05 K (−248.1 ° C)
215.98 h Pa (at the melting point)
|Hazard and safety information|
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .|
Tritium (from ancient Greek τρίτος trítos 'the third'), also 3 H , superheavy hydrogen or superheavy hydrogen is an isotope of hydrogen that occurs naturally in traces . Its atomic nucleus , also known as the Triton , consists of one proton and two neutrons . Tritium is a radioactive beta emitter . It decays into stable helium ( 3 He) with a half-life of 12.32 years .
Due to the great importance of the hydrogen isotopes and because the masses differ greatly, the isotopes have not only been given their own names and the isotopes deuterium and tritium also have their own element symbols: for 2 H you can also write D, and T for 3 H. In this one Case, H then specifically stands for 1 H.
With other elements the mass ratio between the isotopes is far lower (it is still greatest with the isotope pair 3 He and 4 He, namely 1: 1.327). Therefore there are no specific names and symbols for them.
The discoverers are Ernest Rutherford , Mark Oliphant and Paul Harteck (1934) who produced it from deuterium . The evidence of a magneto-optical effect (Allison effect) by Wendell Latimer in 1933 faded into the background after this effect fell into disrepute in the same year. It was first isolated in 1939 by Luis W. Alvarez and Robert Cornog, who also recognized its radioactivity. Willard Libby realized in the 1950s that tritium could be used for radiometric dating of water and wine.
Tritium arises naturally mainly in the stratosphere . Fast protons of cosmic rays partly form tritium directly through spallation , but mainly secondary neutrons, which form tritium in reactions with nitrogen in the stratosphere and upper troposphere:
Tritium initially forms HT (tritium hydrogen), diffuses (if formed in the stratosphere) to the tropopause , oxidizes in the troposphere with a time constant of 6.5 years through photochemical reactions to HTO (T-containing water) and then rains out comparatively quickly. A steady state is established from formation and radioactive decay, as a result of which there are constantly around 3.5 kg of tritium from natural production in the biosphere , 99% of which is in near-surface layers of the oceans.
Nuclear fission by-product
In reactors moderated with heavy water (see e.g. CANDU ), tritium occurs as an unavoidable by-product in an amount of around 1 kg per 5 GWa (gigawatt-years) - that is around 150 peta-joules of electrical energy generated. Extraction from the cooling water is complex, since isotope separation is required for this .
Tritium also forms in the coolant of the primary circuit of many pressurized water reactors , as a certain amount of boric acid is added to the water to control the reactivity in the reactor core. The desired reaction is that boron-10 absorbs a neutron, and then immediately decays into an alpha particle and lithium-7. An undesirable side reaction, however, is that boron-11 absorbs a neutron and then breaks down into tritium and beryllium-9.
Tritium is also a less common by-product in the fission of fissile atomic nuclei such as 235 U, 239 Pu or 233 U and is formed with a frequency of approximately 1 tritium nucleus per 10 4 fission. The tritium is formed in 7% of ternary decays, i.e. when the starting nuclide is split into three instead of two fragments. As a rule, this tritium remains in the fuel elements together with the other fission products. However , it can be released in the event of a core meltdown or the reprocessing of nuclear fuel.
Production from lithium
Tritium can be made by reacting 6 Li with neutrons:
In hydrogen bombs , the tritium required for fusion is mainly produced in this way during the explosion of the bomb. The neutrons required for this initially come from the fission of uranium or plutonium , later also from the fusion reaction itself.
For nuclear fusion reactors , the production of tritium from lithium is planned in the same way: Initially, especially for the operation of research reactors, tritium is to be produced in nuclear reactors by means of the neutron flux . Later the necessary tritium will be produced in the blanket of the nuclear fusion reactors.
While there is no neutron next to the proton in the atomic nucleus of the hydrogen atom ( 1 H) and one neutron in deuterium ( 2 H or D), there are two in tritium. In contrast to 1 H and 2 H, the tritium nucleus is unstable and decays into the helium isotope 3 He ( beta-minus decay ) with a half-life of 12.32 years with the emission of an electron and an antineutrino :
During this decay an energy of 18.6 keV is released:
The electron receives an average of 5.7 keV of kinetic energy. Compared to other beta emitters, the radiation is very soft. In water it stops after a few micrometers; it cannot penetrate the upper layers of the skin either. The radioactivity of tritium is therefore particularly dangerous if swallowed or inhaled .
The symbol is 3 H; for the sake of simplicity in the formula notation, T is also often used.
Tritium oxide (super-heavy water) T 2 O has a boiling point of 101.51 ° C and a melting point of 4.48 ° C.
In tritium gas light sources (long-life lamps), gaseous tritium is used together with a phosphor in sealed borosilicate glass tubes. The beta radiation of the tritium stimulates the fluorescent coating on the inside of the glass tube to give a weak glow ( fluorescence ). These "cold lights" have a theoretical lifespan of several decades and are available in different colors.
The aforementioned tritium gas light sources are also used as a light source on watch dials and hands of certain watch models. In compasses , as used by the US Army in the 1980s, 120 mCi tritium was used in the color to mark the cardinal points. Luminous paint containing tritium was also applied to the sights of weapons.
Ionization smoke detectors sometimes work with a tritium gas ampoule as an ionizer. However, there are health risks associated with the production and storage of larger quantities due to the radioactivity. Therefore, it is through phosphorescent light sources, such as. B. Superluminova replaced. 241 Am (americium) can be used in smoke alarms instead of tritium .
A 1: 1 mixture of deuterium and tritium (DT) has the most favorable properties as a fuel for the fusion energy : a high energy yield, a relatively large cross-section, the smallest possible Coulomb force to be overcome (only one electric charge per atom) and therefore a comparative one low fusion temperature. In fusion reactors it is about 100 million Kelvin, compared to 400 million Kelvin in the next most suitable deuterium-deuterium reaction. Therefore, only a DT mixture can be considered for future fusion power plants . For their continuous operation, however, sufficient quantities of tritium could only be produced by incubating from lithium-6 in the reactor itself. The first experiments in which DT fusions were detected have so far taken place at the Joint European Torus (JET) test facilities in Culham, England and at the Tokamak Fusion Test Reactor (TFTR) in Princeton. Large-scale DT experiments are planned for the second experimental phase of the ITER project . Systems for researching the fundamentals of a fusion power plant such as the tokamak ASDEX Upgrade in Garching or the stellarator Wendelstein 7-X in Greifswald, on the other hand, only use deuterium or hydrogen plasmas, because at first it was and is all about adding a stable plasma produce. This means that there is access to the system and the measuring devices immediately after each experiment and the radiation protection effort can be kept lower (this is also necessary with a deuterium plasma, because numerous deuterium-deuterium fusions take place in it even at 100 million Kelvin).
Tritium is also an integral part of certain nuclear weapons . Just a few grams of a gaseous deuterium-tritium mixture in nuclear fission weapons can increase their explosive effect by a factor of 2, also known as “boosting”. Tritium is even essential for neutron bombs to function; up to 20 grams of tritium per warhead are required here. In hydrogen bombs, tritium is only used as a booster and to adjust the explosive power in the fission stage , while lithium deuteride is used in the fusion stage , from which tritium is only formed under neutron bombardment.
Because of its relatively short half-life of 12.3 years, tritium is used to determine the age of near-surface groundwater or to study hydrological flow conditions. The starting point for the calculations is the entry of tritium into the groundwater in the 1950s and early 1960s. The cause of the entry was the numerous atomic tests in the atmosphere, which released considerable amounts of tritium.
The beta decay of tritium produces non-radioactive helium-3 . Due to its extreme rarity in natural helium sources, this is currently the least expensive source of helium-3. It is required in basic research.
The chemical hazards emanating from tritium are identical to those of hydrogen, but are comparatively negligible compared to the radioactive hazards as gaseous beta emitters, which also require completely different handling regulations. The labeling for hydrogen according to Annex VI of Regulation (EC) No. 1272/2008 (CLP) , which only deals with the dangers emanating from chemistry, would have a rather trivial effect and was therefore left out, especially since tritium is only technically qualified for it Laboratories and only handled in small quantities.
Tritium is not highly radio-toxic , but can be stored and converted in the body in the form of water. A French-Belgian study from 2008 comes to the conclusion that its radiological effects have so far been underestimated. B. store it in the DNA (genetic material), which can be problematic, especially during pregnancy.
Tritium is detected, among other things, on the basis of the effects of radioactivity using liquid scintillation counters or open ionization chambers . Mass spectrometers can also be used for detection.
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- The hazards emanating from radioactivity do not belong to the properties to be classified according to the GHS labeling. With regard to other hazards, this substance has either not yet been classified or a reliable and citable source has not yet been found.
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