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Tritium (), or hydrogen-3 (symbol T or H), is a rare and radioactive isotope of hydrogen with a half-life of 12.32 years. The tritium atomic nucleus (t, sometimes called a triton) contains one proton and two , whereas the nucleus of the common isotope hydrogen-1 ( protium) contains one proton and no neutrons, and that of non-radioactive hydrogen-2 ( deuterium) contains one proton and one neutron. Tritium is the heaviest particle-bound isotope of hydrogen. It is one of the few nuclides with a distinct name. The use of the name hydrogen-3, though more systematic, is much less common.
Naturally occurring tritium is extremely rare on Earth. The atmosphere has only trace amounts, formed by the interaction of its gases with . It can be produced artificially by irradiation of lithium or lithium-bearing ceramic pebbles in a nuclear reactor and is a low-abundance byproduct in normal operations of nuclear reactors.
Tritium is used as the energy source in radioluminescent lights for watches, night sights for firearms, numerous instruments and tools, and novelty items such as self-illuminating key chains. It is used in a medical and scientific setting as a radioactive tracer. Tritium is also used as a nuclear fusion fuel, along with more abundant deuterium, in tokamak reactors and is a vital component in . Tritium has also been used commercially in betavoltaic devices such as NanoTritium batteries.
Their experiment could not isolate tritium, which was first accomplished in 1939 by Luis Alvarez and Robert Cornog, who also realized tritium's radioactivity.
Willard Libby recognized in 1954 that tritium could be used for radiometric dating of water and wine.
releasing 18.6 Electronvolt 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 nearly undetectable electron antineutrino. from tritium can penetrate only about of air, and they are incapable of passing through the dead outermost layer of human skin. Because of their low energy compared to other beta particles, the amount of bremsstrahlung generated is also lower. The unusually low energy released in the tritium beta decay makes the decay (along with that of rhenium-187) useful for attempts at absolute neutrino mass measurement, none of which has yet succeeded.
The low energy of tritium's radiation makes it difficult to detect tritium-labeled compounds except by using liquid scintillation counting.
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.
The slowed neutrons from this reaction can still react with in the first, exothermic reaction; thus lithium can generate more tritium atoms than neutrons absorbed.
A more common result of boron-10 neutron capture is Li and a single alpha particle.
Especially in pressurized water reactors which only partially thermal neutron neutrons, the interaction between relatively fast neutrons and the boric acid added as a chemical shim produces small but non-negligible quantities of tritium.
typically produce of tritium per year, which is recovered at the Darlington Tritium Recovery Facility (DTRF) attached to the 3,512 MW Darlington Nuclear Generating Station in Ontario. The total production at DTRF between 1989 and 2011 was – with an activity of : an average of about per year.
Deuterium's absorption cross section for is about 0.52 millibarn, whereas that of oxygen-16 (O) is about 0.19 millibarn and that of oxygen-17 (O) is about 240 millibarns. While O is by far the most common isotope of oxygen in both natural oxygen and heavy water; depending on the method of isotope separation, heavy water may be slightly richer in O and O. Due to both neutron capture and (n,alpha particle) reactions (the latter of which produce C, an undesirable long-lived beta emitter, from O) they are net "neutron consumers" and are thus undesirable in a moderator of a natural uranium reactor which needs to keep neutron absorption outside the fuel as low as feasible. Some facilities that remove tritium also remove (or at least reduce the content of) O and O, which can – at least in principle – be used for isotope labeling.
India, which also has a large fleet of pressurized heavy water reactors (initially CANDU technology but since indigenized and further developed IPHWR technology), also removes at least some of the tritium produced in the moderator/coolant of its reactors but due to the dual use nature of tritium and the Indian nuclear bomb program, less information about this is publicly available than for Canada.
meaning it is produced in all reactors. The release or recovery of tritium needs to be considered in the operation of , especially in the reprocessing of nuclear fuel and storage of spent nuclear fuel. At some nuclear power plants it is discharged to the atmosphere.
Voloxidation is an optional additional step in nuclear reprocessing that removes volatile fission products (such as all isotopes of hydrogen) before an aqueous process begins. This would in principle enable economic recovery of the produced tritium, but even if the tritium is only disposed of and not used, it has the potential to reduce tritium contamination in the water used, reducing radioactivity released when the water is discharged since tritiated water cannot be removed from "ordinary" water except by isotope separation.
Given the specific activity of tritium at , one Becquerel is equivalent to roughly 2.8 mg.
on the status of tritium in tritiated water at Fukushima Daiichi nuclear plant, as part of considering options for final disposal of the stored contaminated cooling water. This identified that the March 2016 holding of tritium on-site was 760 Becquerel (equivalent to 2.1 g of tritium or 14 mL of pure tritiated water) in a total of 860,000 m of stored water. This report also identified the reducing concentration of tritium in the water extracted from the buildings etc. for storage, seeing a factor of ten decrease over the five years considered (2011–2016), 3.3 MBq/L to 0.3 MBq/L (after correction for the 5% annual decay of tritium).
According to a report by an expert panel considering the best approach to dealing with this issue, "Tritium could be separated theoretically, but there is no practical separation technology on an industrial scale. Accordingly, a controlled environmental release is said to be the best way to treat low-tritium-concentration water."
) interacts with atmospheric nitrogen:
Worldwide, the production of tritium from natural sources is about 4 megacuries (148 PBq) per year. The global equilibrium inventory of tritium created by natural sources remains approximately constant at 70 megacuries (2,590 PBq), as a balance between the fixed production rate and nuclear decay. These may be taken as 415 g and 7,250 g respectively.
of tritium was produced in the United States from 1955 to 1996. Since it continually decays into helium-3, the total amount remaining was about at the time of the report,and about as of 2023.27 years have passed since 1996, i.e. 2.25 half-lives, which reduce the 75kg of 1996 to 75/2^(2.25) ≈15.8 kg.
Tritium production was resumed with irradiation of rods containing lithium (replacing the usual containing boron, cadmium, or hafnium), at the reactors of the commercial Watts Bar Nuclear Plant from 2003 to 2005 followed by extraction of tritium from the rods at the Tritium Extraction Facility at the Savannah River Site beginning in November 2006.
Compared to hydrogen having its natural composition on Earth, tritium has a higher melting point (20.62 K vs. 13.99 K), a higher boiling point (25.04 K vs. 20.27 K), a higher critical temperature (40.59 K vs. 32.94 K) and a higher critical pressure (1.8317 MPa vs. 1.2858 MPa).
Tritium's specific activity is .
Tritium figures prominently in studies of nuclear fusion due to its favorable reaction cross section and the large amount of energy (17.6 MeV) produced through its reaction with deuterium:
All atomic nuclei have a positive charge from their , and therefore repel one another because like charges repel (Coulomb's law). 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 repulsion, and they can come close enough for the strong nuclear force to take effect, fusing them into heavier atoms.
A tritium nucleus (triton), containing one proton and two neutrons, has the same charge as any hydrogen nucleus, and it experiences the same electrostatic repulsion when close to another nucleus. However, the neutrons in the triton increase the attractive strong nuclear force when close enough to another nucleus. As a result, tritium can fuse more easily with other light atoms than ordinary hydrogen can.
The same is true, albeit to a lesser extent, of deuterium. This is why ("failed" stars) cannot fuse normal hydrogen, but they do fuse a small minority of deuterium nuclei.
Like the other isotopes of hydrogen, tritium is difficult to confine. Rubber, plastic, and some kinds of steel are all somewhat permeable. This has raised concerns that if tritium were used in large quantities, in particular for , it might contribute to radioactive contamination, though its short half-life should prevent significant long-term accumulation in the atmosphere.
The high levels of atmospheric nuclear weapons testing that took place prior to the enactment of the Partial Nuclear Test Ban Treaty proved to be unexpectedly useful to oceanographers. The high levels of tritium oxide introduced into upper layers of the oceans have been used in the years since then to measure the rate of mixing of the upper layers of the oceans with their lower levels.
Organisms can take up HO, as they would HO. Plants convert HO into organically bound tritium (OBT), and are consumed by animals. HO is retained in humans for around 12 days, with a small portion of it remaining in the body as OBT. Tritium can be passed along the food chain as one organism feeds on another, though the metabolism of OBT is less understood than that of HO. Tritium can incorporate to RNA and DNA molecules within organisms which can lead to somatic and genetic impacts. These can emerge in later generations.
HO has a short biological half-life in the human body of 7 to 14 days, which both reduces the total effects of single-incident ingestion and precludes long-term bioaccumulation of HO from the environment.
The US Nuclear Regulatory Commission states that in normal operation in 2003, 56 pressurized water reactors released of tritium (maximum: ; minimum: ; average: ) and 24 boiling water reactors released (maximum: ; minimum: 0 Ci; average: ), in liquid effluents. NRC: Frequently Asked Questions About Liquid Radioactive Releases "What are normal amounts of tritium released from nuclear power plants?" of tritium weigh about .
| + Tritium drinking water limits by country
!valign="bottom" | Country !valign="bottom" | Tritium limit (Bq/L) !valign="bottom" | Equivalent dose (microsievert/year) |
| Australia | 76,103 | 1,000 | |
| Japan | 60,000 | ||
| Finland | 30,000 | ||
| World Health Organization | 10,000 | ||
| Switzerland | 10,000 | ||
| Russia | 7,700 | ||
| Canada (Ontario) | 7,000 | ||
| United States | 740 | ||
| Norway | 100 | ||
| Germany | 100 | ||
The United States limit results in a dose of 4.0 millirems (or 40 micro in SI units) per year per EPA regulation 40CFR141, and is based on outdated dose calculation standards of National Bureau of Standards Handbook 69 circa 1963. Four millirem per year is about 1.3% of the average natural background radiation (~3 mSv). Updated dose calculation standards based on International Commission on Radiological Protection Report 30 and used in the NRC Regulation 10CFR20 results in a dose of 0.9 millirem (9 μSv) per year at 740 Bq/L (20 nCi/L).
or more.
The tritium in a warhead is continually undergoing radioactive decay, becoming unavailable for fusion. Also, its decay product, helium-3, absorbs neutrons. This can offset or reverse the intended effect of the tritium, which was to generate many free neutrons, if too much helium-3 has accumulated. Therefore, boosted bombs need fresh tritium periodically. The estimated quantity needed is per warhead. To maintain constant levels of tritium, about per warhead per year must be supplied to the bomb.
One mole of deuterium-tritium gas contains about of tritium and of deuterium. In comparison, the 20 moles of plutonium in a nuclear bomb consists of about of plutonium-239.
During the detonation of the primary fission bomb stage in a thermonuclear weapon (Teller–Ulam staging), the sparkplug, a cylinder of U/Pu at the center of the fusion stage(s), begins to fission in a chain reaction, from excess neutrons channeled from the primary. The neutrons released from the fission of the sparkplug split lithium-6 into tritium and helium-4, while lithium-7 is split into helium-4, tritium, and one neutron. As these reactions occur, the fusion stage is compressed by photons from the primary and fission of the U or U/U jacket surrounding the fusion stage. Therefore, the fusion stage breeds its own tritium as the device detonates. In the extreme heat and pressure of the explosion, some of the tritium is then forced into fusion with deuterium, and that reaction releases even more neutrons.
Since this fusion process requires an extremely high temperature for ignition, and it produces fewer and less energetic neutrons (only fission and fusion are net neutron producers), lithium deuteride is not used in boosted bombs, but rather for multi-stage hydrogen bombs.
The Tritium Systems Test Assembly (TSTA) was a facility at the Los Alamos National Laboratory dedicated to the development and demonstration of technologies required for fusion-relevant deuterium–tritium processing.
Bomb-tritium data were used from the Transient Tracers in the Ocean (TTO) program in order to quantify the replenishment and overturning rates for deep water located in the North Atlantic.
Bomb-tritium also enters the deep ocean around the Antarctic.
Using the data from these processes for 1981, the 1-TU isosurface lies between 500 and 1,000 meters deep in the subtropical regions and then extends to 1,500–2,000 meters south of the Gulf Stream due to recirculation and ventilation in the upper portion of the Atlantic Ocean. To the north, the isosurface deepens and reaches the floor of the abyssal plain which is directly related to the ventilation of the ocean floor over 10–20 year time-scales.
Also evident in the Atlantic Ocean is the tritium profile near Bermuda between the late 1960s and late 1980s. There is a downward propagation of the tritium maximum from the surface (1960s) to 400 meters (1980s), which corresponds to a deepening rate of about 18 meters per year. There are also tritium increases at 1,500 m depth in the late 1970s and 2,500 m in the middle of the 1980s, both of which correspond to cooling events in the deep water and associated deep water ventilation.
From a study in 1991, the tritium profile was used as a tool for studying the mixing and spreading of newly formed North Atlantic Deep Water (NADW), corresponding to tritium increases to 4 TU. This NADW tends to spill over sills that divide the Norwegian Sea from the North Atlantic Ocean and then flows to the west and equatorward in deep boundary currents. This process was explained via the large-scale tritium distribution in the deep North Atlantic between 1981 and 1983. The sub-polar gyre tends to be freshened (ventilated) by the NADW and is directly related to the high tritium values (>1.5 TU). Also evident was the decrease in tritium in the deep western boundary current by a factor of 10 from the Labrador Sea to the Tropics, which is indicative of loss to ocean interior due to turbulent mixing and recirculation.
In the North Pacific Ocean, the tritium (introduced as bomb tritium in the Northern Hemisphere) spread in three dimensions. There were subsurface maxima in the middle and low latitude regions, which is indicative of lateral mixing (advection) and diffusion processes along lines of constant potential density () in the upper ocean.
The depth penetration of bomb tritium can be separated into three distinct layers:
As for the mass flux of tritium through the main stem of the Mississippi River into the Gulf of Mexico, data indicated that approximately 780 grams of tritium has flowed out of the River and into the Gulf between 1961 and 1997, an average of 21.7 grams/yr and 7.7 PBq/yr. Current fluxes through the Mississippi River are 1 to 2 grams per year as opposed to the pre-bomb period fluxes of roughly 0.4 grams per year.
Tritium in hydrogen bomb secondaries
Since tritium undergoes radioactive decay, and is also difficult to confine physically, the much larger secondary charge of heavy hydrogen isotopes needed in a true hydrogen bomb uses solid lithium deuteride as its source of deuterium and tritium, producing the tritium in situ during secondary ignition.
Controlled nuclear fusion
Tritium is an important fuel for controlled nuclear fusion in both magnetic confinement and inertial confinement fusion reactor designs. The National Ignition Facility (NIF) uses deuterium–tritium fuel, and the experimental fusion reactor ITER will also do so. The deuterium–tritium reaction is favorable since it has the largest fusion cross section (about 5.0 barns) and it reaches this maximum cross section at the lowest energy (about 65 Electronvolt center-of-mass) of any potential fusion fuel. As tritium is very rare on earth, concepts for fusion reactors often include the breeding of tritium. During the operation of envisioned breeder fusion reactors, , often containing lithium as part of ceramic pebbles, are subjected to neutron fluxes to generate tritium to complete the fuel cycle.
Electrical power source
Tritium can be used in a betavoltaic device to create an atomic battery to generate electricity.
Use in electron tubes
Tritium is used in various electron tubes, such as the Zellweger ZE22/3 glow tube. These devices contain a small amount of tritium to ionize the fill gas, typically a noble gas like neon or argon. This ionization ensures reliable and consistent operation by providing a steady current when a high voltage is applied, enhancing the device's performance and stability. The tritium is sealed within a glass envelope with two electrodes, one of which is coated with the radioactive material to create an ion path between the electrodes.
Use as an oceanic transient tracer
Aside from chlorofluorocarbons, tritium can act as a transient tracer and can "outline" the biological, chemical, and physical paths throughout the world's oceans because of its evolving distribution.
(1993). 9780070397323, McGraw-Hill. ISBN 9780070397323
Estimated atmospheric levels at the height of weapons testing to approach 1,000 TU and pre-fallout levels of rainwater to be between 5 and 10 TU.
North Atlantic Ocean
While in the stratosphere (post-test period), the tritium interacted with and oxidized to water molecules and was present in much of the rapidly produced rainfall, making tritium a prognostic tool for studying the evolution and structure of the water cycle as well as the ventilation and formation of water masses in the North Atlantic.
These processes make HHO a good tracer for time scales of up to a few decades.
Pacific and Indian oceans
In a 1998 study, tritium concentrations in surface seawater and atmospheric water vapor (10 meters above the surface) were sampled at the following locations: the Sulu Sea, Fremantle Bay, the Bay of Bengal, Penang Bay, and the Strait of Malacca.
Mississippi River system
Trace amounts of radioactive materials from atomic weapons testing settled throughout the Mississippi River System. Tritium concentrations have been used to understand the of continental hydrologic systems such as lakes, streams, and rivers.
See also
Footnotes
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