Most of the hydrogen in the universe consists of a single proton orbited by a single electron and is also known as H-1 or protium. A small percentage of hydrogen atoms contain a neutron in the nucleus as well as the proton. This form of hydrogen is referred to as H-2, heavy hydrogen or deuterium. It is a stable atom like H-1. In the Earth's crust, for every six thousand four hundred and twenty H-1 atoms, there is a single H-2 atom. There is a third isotope of hydrogen called tritium that has two neutrons in the nucleus. This form of hydrogen is unstable and undergoes radioactive decay into stable helium-4 with a half life of twelve and a third years and emits a beta particle when it decays.
Tritium atoms can combine with oxygen to form tritiated water also call THO. Tritium is very rare in the natural environment and mostly occurs in the form of a few THO molecules mixed in with ordinary water.
Most tritium is formed from the collision of a high energy cosmic ray from space with nitrogen in Earth's atmosphere. The cosmic neutron combines with nitrogen-14 to yield an atom of carbon-12 and one atom of tritium or H-3. Small amounts of tritium are produced in reactors by interaction of deuterium or H-2 with lithium-6 and neutron absorption by deuterium. About one in ten thousand decays of U-233, U-235 and P-239 atoms produces tritium. Tritium is also produced in the explosion of nuclear weapons. Release of tritium from reactors must be below a threshold set by U.S government regulation.
Tritium was created and identified in 1934 by physicists Rutherford, Oliphant and Harteck shortly after the discovery of deuterium. Deuterium was bombarded with neutrons and, through absorption, tritium was produced.
Tritium was intentionally produced in a special reactor at Savannah River until it was shut down in 1988. Up to 1996, only about five hundred pounds of tritium were produced in the United States. In late 2006 a Tritium Extraction Facility was started up at Watts Bar Nuclear Generating Plant. The new facility recovers tritium from nuclear control rods containing lithium.
Tritium emits weak beta particles when it decays and the titrated water in which it usually enters the body is excreted in a short time. This makes tritium one of the least dangerous radioisotopes to human health.
Tritium is used in research dedicated to fusion reactors because of the large amount of energy released when it is mixed with deuterium in the reactor. This energy production also makes it useful for the triggering mechanisms in thermonuclear fusion weapons. It is also used in luminescent exit signs in buildings, in dials and gauges, in luminous paints and on the faces of wristwatches. Some tritium that has been detected in ground water has been traced back to landfills where people illegally disposed of old exit signs.