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Tritium
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Tritium (symbol or , 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 one proton and no neutrons.
e Tritium has several different experimentally-determined values of its half-life, the NIST recommends 4,500±8 days (approximately 12.33 years).
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Encyclopedia
Tritium (symbol or , 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 one proton and no neutrons.
Decay
While Tritium has several different experimentally-determined values of its half-life, the NIST recommends 4,500±8 days (approximately 12.33 years). It decays into helium-3 by the reaction
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and releases 18.6 keV of energy in the process. 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, ingested, or—if it is in water molecules, as with tritiated water—absorbed through pores in the skin. Its low energy also creates difficulty detecting 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 (greater than 4MeV) interacts with atmospheric nitrogen:
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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.
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, which is more than one-quarter as great as the energy that fusion of the produced triton with a deuteron can later produce.
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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.
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High-energy neutrons irradiating boron-10 will also occasionally produce tritium. The more common result of boron-10 neutron capture is and a single alpha particle.
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The reactions requiring high neutron energies are not attractive production methods.
Helium-3
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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Fission
Tritium is occasionally a direct product of nuclear fission, with a yield of about 0.01% (one per 10,000 fissions). 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 (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.
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.
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.
Properties
Tritium has an atomic mass of 3.0160492. It is a gas (2 or 2) at standard temperature and pressure. It combines with oxygen to form a liquid called tritiated water, 2, or partially tritiated water, .
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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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.
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.
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
Self-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, exit signs, and a variety of other devices. 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.
Firearms Night Sights
The radioactive decay of tritium is used in firearms night sights in much the same way as the clock hands discussed above. The electrons emitted by the radioactive decay of the tritium cause phosphor to glow, thus providing a long lasting (several years) and non battery powered firearms sight which is visible in dim lighting conditions. The tritium glow is not noticeable in bright conditions such as during daylight however. As a result, some manufacturers have started to integrate fiber optic sights with tritium vials to provide bright, high-contrast firearms sights in both bright and dim conditions.
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. 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 - fusion and splitting are net neutron producers), 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 (-) fuel. The - reaction is favored since it has the largest fusion cross-section (about 5 barns peak) and reaches this maximum cross-section at the lowest energy (about 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.
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.
Use as an Oceanic Transient Tracer
Aside from chlorofluorocarbons, tritium can act as a transient tracer and has the ability to “outline” the biological, chemical, and physical paths (along with climate change) throughout the world oceans because of its evolving distribution. Tritium can thus be used as a tool to examine ocean circulation and ventilation and, for oceanographic and atmospheric science interests, is usually measured in Tritium Units where 1 TU is defined as the ratio of 1 tritium atom to 1018 hydrogen atoms. As noted earlier, nuclear weapons testing, primarily in the high-latitude regions of the Northern Hemisphere, throughout the late 1950’s and early 1960’s introduced large amounts of tritium into the atmosphere, especially the stratosphere. Before these nuclear tests, there were only about 3 to 4 kilograms of tritium on the Earth’s surface; but these amounts rose by 2 or 3 orders of magnitude during the post-test period.
Water samples taken must typically undergo the following procedure (generally-speaking) and significant testing before the tritium can officially and successfully be utilized a tracer:
- Desalting via vacuum distillation;
- Electrolysis and volume reduction to affect enrichment of the tritium;
- Reduction of the electrolyzed sample to hydrogen in a super-heated furnace;
- Tritium labeling by catalytic hydrogenation of tank ethylene; and
- Gas-proportional counting of tritiated ethane
In an attempt to examine the downward transport of tritium into the ocean via the use of a cloud model, it is necessary and customary to use the following model structure:
1) anelastic continuity equation;
2) momentum equation – includes pressure gradient term, Newtonian damping term, buoyancy term, and turbulent mixing terms;
3) thermodynamic energy equation;
4) conservation of water vapor;
5) bulk cloud physics – includes the Kessler parameterization (conservation equations for cloud water and rainwater); and
6) Tritium budget equations – includes tritium for water vapor, cloud water, and rainwater; rate of change of tritium concentration as a function of decay rate
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 hydrologic cycle as well as the ventilation and formation of water masses in the North Atlantic Ocean. In fact, bomb-tritium data were utilized 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. Most of the bomb tritiated water (HTO) throughout the atmosphere can enter the ocean through the following processes: a) precipitation, b) vapor exchange, and c) river runoff – these processes make HTO a great tracer for time-scales up to a few decades. Using the data from these processes for the year 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 to 20 year time-scales.
Also evident in the Atlantic Ocean is the tritium profile near Bermuda between the late 1960’s and late 1980’s. There is a downward propagation of the tritium maximum from the surface (1960’s) to 400 meters (1980’s), which corresponds to a deepening rate of approximately 18 meters per year. There are also tritium increases at 1,500 meters depth in the late 1970’s and 2,500 meters in the middle of the 1980’s, 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.
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, the Fremantle Bay, the Bay of Bengal, the Penang Bay, and the Strait of Malacca. Results indicated that the tritium concentration in surface seawater was highest at the Fremantle Bay (approximately 0.40 Bq/liter), which could be accredited to the mixing of runoff of freshwater from nearby lands due to large amounts found in costal waters. Typically, lower concentrations were found between 35 and 45 degrees South latitude and near the equator. Results also indicated that (in general) tritium has decreased over the years (up to 1997) due to the physical decay of bomb tritium in the Indian Ocean. As for water vapor, the tritium concentration was approximately one order of magnitude greater than surface seawater concentrations (ranging from 0.46 to 1.15 Bq/liter). Therefore, the water vapor tritium is not affected by the surface seawater concentration; thus, the high tritium concentrations in the vapor were concluded to be a direct consequence of the downward movement of natural tritium from the stratosphere to the troposphere (therefore, the ocean air showed a dependence on latitudinal change)
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 (isopycnals) in the upper ocean. Some of these maxima even correlate well with salinity extrema. In order to obtain the structure for ocean circulation, the tritium concentrations were mapped on 3 surfaces of constant potential density (23.90, 26.02, and 26.81). Results indicated that the tritium was well-mixed (at 6 to 7 TU) on the 26.81 isopycnal in the subarctic cyclonic gyre and there appeared to be a slow exchange of tritium (relative to shallower isopycnals) between this gyre and the anticyclonic gyre to the south; also, the tritium on the 23.90 and 26.02 surfaces appeared to be exchanged at a slower rate between the central gyre of the North Pacific and the equatorial regions.
The depth penetration of bomb tritium can be separated into 3 distinct layers. Layer 1 is the shallowest layer and includes the deepest, ventilated layer in winter; it has received tritium via radioactive fallout and lost some due to advection and/or vertical diffusion and contains approximately 28 % of the total amount of tritium. Layer 2 is below the first layer but above the 26.81 isopycnal and is no longer part of the mixed layer. Its 2 sources are diffusion downward from the mixed layer and lateral expansions outcropping strata (poleward); it contains about 58 % of the total tritium. Layer 3 is representative of waters that are deeper than the outcrop isopycnal and can only receive tritium via vertical diffusion; it contains the remaining 14 % of the total tritium.
Mississippi River System
The impacts of the nuclear fallout was even felt in the United States throughout the Mississippi River System. Tritium concentrations can be used to understand the residence times of continental hydrologic systems (as opposed to the usual oceanic hydrologic systems) which include surface waters such as lakes, streams, and rivers. Studying these systems can also provide societies and municipals with information for agricultural purposes and overall river water quality.
In a 2004 study, several rivers were taken into account during the examination of tritium concentrations (starting in the 1960’s) throughout the Mississippi River Basin: Ohio River (largest input to the Mississippi River flow), Missouri River, and Arkansas River. The largest tritium concentrations were found in 1963 at all the sampled locations throughout these rivers and correlate well with the peak concentrations in precipitation due to the nuclear bomb tests in 1962. The overall highest concentrations occurred in the Missouri River (1963) and were greater than 1,200 TU while the lowest concentrations were found in the Arkansas River (never greater than 850 TU and less than 10 TU in the mid-1980’s).
Several processes can be identified using the tritium data from the rivers: direct runoff and outflow of water from groundwater reservoirs. Using these processes, it becomes possible to model the response of the river basins to the transient tritium tracer. Two of the most common models are the following:
- Piston-flow approach – tritium signal appears immediately; and
- Well-mixed reservoir approach – outflow concentration depends upon the residence time of the basin water
Unfortunately, both models fail to reproduce the tritium in river waters; thus, a two-member mixing model was developed that consists of 2 components: a prompt-flow component (recent precipitation – “piston”) and a component where waters reside in the basin for longer than 1 year (“well-mixed reservoir”). Therefore, the basin tritium concentration becomes a function of the residence times within the basin, sinks (radioactive decay) or sources of tritium, and the input function.
For the Ohio River, the tritium data indicated that about 40% of the flow was composed of precipitation with residence times of less than 1 year (in the Ohio basin) and older waters consisted of residence times of about 10 years. Thus, the short residence times (less than 1 year) corresponded to the “prompt-flow” component of the two-member mixing model. As for the Missouri River, results indicated that residence times were approximately 4 years with the prompt-flow component being around 10% (these results are due to the series of dams in the area of the Missouri River).
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. And current fluxes through the Mississippi River are about 1 to 2 grams per year as opposed to the pre-bomb period fluxes of roughly 0.4 grams per year.
History
Tritium was first predicted in the late 1920s by Walter Russell, using his "spiral" periodic table, 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.
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