# Tritium

Hydrogen-3
Image:Hydrogen-3.png
General
Name, symbol tritium, triton, 3H
Neutrons 2
Protons 1
Nuclide Data
Natural abundance trace
Half-life 12.32 years
Decay products helium-3
Isotope mass 3.0160492 u
Spin 1/2+
Excess energy 14949.794 ± 0.001 keV
Binding energy 8481.821 ± 0.004 keV
Decay mode Decay energy
Beta emission 0.018590 MeV

Tritium (symbol T or 3H) is a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called triton) contains 1 proton and two neutrons, whereas the nucleus of protium (the most abundant hydrogen isotope) contains no neutrons. Its atomic weight is 3.0160492. It is a gas (T2 or 3H2) at standard temperature and pressure. Tritium combines with oxygen to form a liquid called tritiated water T2O or partially tritiated THO.

Tritium is radioactive with a half-life of 12.32 years. It decays into helium-3 by the reaction

${}^3_1\hbox{T}\to{}^3_2\hbox{He}^++\hbox{e}^-+\overline{\nu}_{\hbox{e}}$

releasing 18.6 keV of energy. The electron has an average kinetic energy of 6.5 keV, while the remaining energy is carried off undetectably by the 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.

## Sources

Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. In the most important reaction for natural tritium production, a fast neutron interacts with atmospheric nitrogen:

${}^{14}_7\hbox{N}+{}^1\hbox{n}\to{}^{12}_6\hbox{C}+{}^3_1\hbox{T}$

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. Industrially, tritium is produced in nuclear reactors by neutron activation of lithium-6.

${}^6_3\hbox{Li}+{}^1\hbox{n}\to{}^4_2\hbox{He}+{}^3_1\hbox{T}$

Tritium is also produced in heavy water-moderated reactors when deuterium captures a neutron; however, this reaction has a much smaller cross section and is only a useful tritium source for a reactor with a very high neutron flux. It can also be produced from boron-10 through neutron capture.

## Properties

Tritium figures prominently in studies of nuclear fusion due to its favorable reaction cross section and the high energy yield of 17.6 MeV for its reaction with deuterium:

${}^3_1\hbox{T}+{}^2_1\hbox{D}\to{}^4_2\hbox{He}+\hbox{n}$

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 (as is the case in the core of the Sun, for example), 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. Since tritium has the same charge as ordinary hydrogen, it experiences the same electrostatic repulsive force (see Coulomb's law). However, due to tritium's supply of neutrons which are carried into reactions and feel the attractive strong force once delivered, tritium can more easily fuse with other light atoms. The same is also true, albeit to a lesser extent, of deuterium, and that is why brown dwarfs (so-called failed stars) can not burn hydrogen, but do indeed burn deuterium.

Image:Trtium.jpg
Radioluminescent 1.2 Curie 4" x .2" Tritium vials are simply tritium gas filled thin glass vials whose inner surfaces are coated with a phosphor. The "gaseous tritium light source" vial shown here is 1.5 years old.

Before the onset of atmospheric nuclear weapons tests, the global equilibrium tritium inventory was estimated at about 80 megacuries (MCi).

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 any significant 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 at which the lower and upper ocean levels mixed.

## Regulatory limits

The legal limits for tritium in drinking water can vary. The U.S. limit is calculated to yield a dose of 4 mrem per year.

• Canada 7,000 Bq / l.
• United States 740 Bq / l or 20,000 pCi / l (Safe Drinking Water Act)
• World Health Organization 1,000 Bq / l.

## 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 trasers which are now used in watches and exit signs. It is also used in certain countries to make glowing keychains, and compasses. In recent years, the same process has been used to make self-illuminating gun sights for firearms. These take the place of radium, which can cause bone cancer, and so has been banned in most countries for decades.

### Isotopic tracing

Tritium can be used as an isotopic tracer, e.g. tritiated thymidine is used in cell proliferation assays. The molecule, a nucleoside, is incorporated into the DNA of cells as they are replicated during cell division. The extent of cell proliferation may then be determined by liquid scintillation counting.

Natural tritium, one of the environmental isotopes, is used for tracing and dating water, and water-containing substances such as wine.

### Nuclear weapons

Tritium is used in nuclear weapons to obtain higher yields, either through boosting of fission, or through thermonuclear fusion. However, as tritium quickly decays and is difficult to contain, many thermonuclear weapons contain lithium instead, since the high neutron fluxes will produce tritium from the lithium when the bomb detonates. Injection of a variable amount of tritium into the fission core pit before initiation is one of the techniques to achieve variable yield. Increased yields from tritium injection is due to increased fission efficiency from the high flux of neutrons produced by the fusion of tritium. Tritium injection can double the yield of a fission bomb for the same amount of plutonium; however comparatively little energy is produced by the fusion of the tritium per se, so such boosted weapons are not conventional two-stage thermonuclear weapons ("hydrogen bombs"). See nuclear weapon design.

### 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 (D-T) fuel. The D-T 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.

## 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, who correctly deduced that the substance was radioactive. Willard F. Libby discovered that tritium could be used for dating water, and therefore wine.

## Pop Culture

In the Tom Clancy novel The Sum of All Fears, tritium was used to try to increase the explosive power of a nuclear bomb.

Featured in the film Spider-Man 2, tritium fueled the experimental fusion reaction of Dr. Otto Octavius.