Tritium

A radioactive isotope of hydrogen

Tritium is a beta-emitting radioactive isotope of hydrogen. Its nucleus consists of one proton and two neutrons, making it three times as heavy as a hydrogen nucleus (with its one proton) and 1,5 time as heavy as deuterium (which contains one proton and only one neutron).

Tritium would no longer exist in our environment if cosmic radiation produces it in the atmosphere in very small amounts. The half-life of the unstable tritium nucleus is of 12.3 years, which is very short on the radioactive time-scale. This comparatively fast disappearance means that very little tritium can accumulate in any place.

Its short half-life has also led to its classification as a highly radioactive element. This high activity, fortunately, is attenuated by some of the other properties of the decay processes. The average energy of the emitted electron tends to be very low – 5.7 keV as opposed to the several hundred keV in most beta decays. In addition to the low energy electron, tritium emits no gamma ray.

Tritium, like hydrogen, is particularly mobile. It can combine with oxygen to form tritiated water and therefore has the ability to get easily into the human body thanks to the water cycle. Once inside the body, tritium can lead to internal exposure though the element is eliminated very quickly. Its biological half-life of 10 days is far shorter than the 12.3 years of its radioactive half-life. Only one tritium nucleus out of 650 will decay while still inside the human body. Because of the low emission energy, the beta electron trajectory will not exceed a few microns inside the body.

Tritium therefore has a particularly low radiotoxicity (dose factor). The World Health Organization, WHO) considers that the limit of acceptability of water containing tritium is 10,000 becquerels per liter. This limit is protective. One should drink 2 liters of such water everyday a day for a year to be exposed to a dose of 0.1 mSv per year equivalent to two weeks of natural radioactivity in France.

In February 2016, during the unloading operation of nuclear fuel unloading, tritium contaminated water above normal levels spread under the Indian Point nuclear power plant near New York. According to the Nuclear Regularity Commission – the US safety authority – this leak did not pose a threat to the environment. Indeed, tritium the tritium radoactive toxicity particularly little small. Once spilled into the Hudson, the radioactive water was so diluted that tritium became virtually undetectable. The same goes for tritiated effluents from the La Hague plant in France.

In biology, tritium is often used to mark hydrogen and thus in the study of metabolism. This has allowed us to narrow down the length of the biological half-life inside the human body to within 6 and 9 days.

In daily life, tritium has replaced radium in making dials of watches and navigational instruments luminescent.

Tritium can be found among the radioactive waste generated by reprocessing plants and military facilities, as they can be produced by ternary fission reactions (which are comparatively rare) occuring in the nuclear fuel of reactor cores.

The fusion reaction of deuterium and tritium is the thermonuclear reaction which releases the most energy. This reaction was used by the United States and the Soviet Union in the 1950s and 1960s to test thermonuclear bombs or H-bombs, much more powerful and devastating than atomic bombs based on fission. These tests were the cause of a significant pollution near the nuclear test sites.

These fabrication of these thermonuclear weapons require the production of sufficient quantities of tritium. Since natural tritium is extremely rare, this military tritium is obtained by bombarding lithium with neutrons. In addition, tritium from military facilities generates tritium waste, problematic more because of the tritium mobility than its very low radioactive toxicity.

The nuclear fusion of deuterium and tritium will be exploited in reactors based on nuclear fusion. It remains to be proved that such reactors can be designed and built. This is the goal of the research carried out on a worldwide scale with the ITER project.