Tritium

• Main characteristics

• Origin of tritium

• Tritium and the environment

• Tritium and Man

• Tritium and fusion

Main characteristics

	Hydrogen	Deuterium	Tritium
Symbol	H	D	T
Discovery	alchemist	RT Bridge (1931)	Rutherford (1934)
Abundance	H=90% of atoms and 75% of mass	1 D for 6000 H in water 30mg/l	1T for 1017 H , it must therefore be manufactured (n+Li-->He+T)
Properties	Lightest atom	Captures neutrons 1000 times less than H	Radioactive : T-->3He+e- (5.7 kev) period=12.3 years biological period =10j
Usage	Fuel …	Heavy water reactor (moderator)	H bomb Paint : heliports Biology:markers Various : dating wine, tracking rivers...

Tritium is a hydrogen and as such, it possesses the same physio-chemical properties. It has, like hydrogen, the property of high permeation through a large number of materials, since it is an atom that is small in relation to the typical crystalline meshes in steels, for example. It has the chemical properties of hydrogen and mostly reacts in the same way. The isotopic effect (mass 3 instead of 1) comes relatively rarely into play.

Tritium is radioactive. It decomposes through beta emission (one electron) to give helium 3. The energy of the electron emitted is on average 5,7 keV. This therefore means radiation that is only slightly penetrating, stopped by 5 millimetres of air or by 5 µm of water. The radioactive period is 12,35 years. Thus 5,6 % disappears per year. After 125 years, there is less than a thousandth part left (a millionth after 250 years).

Tritium is a radioisotope which was used extensively right from the start in molecular biology research. As a hydrogen it made it possible to mark a large number of molecules, of which DNA is one example.

Origin of tritium

• Natural origin : Essentially the two following reactions in the atmosphere
  ¹⁴N+n --> 3⁴He+T
  ¹⁴N+n --> 3 C + T
  The neutrons are themselves produced by the impact of cosmic rays on the atmosphere. There is also telluric origin in much smaller quantity. Radioactive bodies like Uranium and Thorium produce neutrons, which bombard the traces of ⁶Li present in natural rocks. Globally, all these phenomena produce 0,2 kg of tritium per year. An inventory in the natural environment amounts to 3 to 4 kgs in the whole atmosphere.
  

• Sources linked to human activity
  - Production and (in the past) nuclear weapons testing. Manufacturing (tritium breeding reactors) generates outputs of around 0,04 kg / year. Atmospheric tests, for their part, have produced 650 kgs of T (2 kgs per mega-tonne of TNT) essentially between 1952 and 1963. Today, there is less than 100 kgs of this tritium left.
  - Electricity generating reactors release 0,02 kgs / year into the atmosphere.
  - Traces are also found in waste from institutions using tritium as a biological tracker.

Tritium and the environment

• Tritium is very easily disseminated into the environment, as it takes the place of the hydrogen atom, itself omnipresent in water, organic matter and so on. Tritium does not build up in any biological component. In particular, there is no bioaccumulation in the various food chains.

• The ending of tritium depends greatly on the chemical form that it takes. The four following exists : 

  • The Hydrogen-tritium (HT) form
    This first form is not assimilated by plants and only slightly assimilated by animals and humans. HT only oxidizes very slowly in the air (thus does not become HTO) with a lifetime longer than 10 years. On the other hand, HT is oxidized very fast and almost completely by bacteria in the interface between the soil and the atmosphere. HT is only oxidized to a small extent by the bacteria in human lungs.

  • Tritium water (tritium takes the place of one or two water hydrogens)
    Tritium water is easily and completely assimilated. It spreads throughout the whole living body (cellular water...). It is in any case responsible for at least 80 % of the doses received in the event of various kinds of exposure. Contamination channels are obviously inhaling, ingestion and skin transmission.

  • Organic tritium in ‘’exchangeable" position
    This is the tritium present in organic molecules where it has taken the place of hydrogen in the typical radicals (-OH, -SH, = NH, and so on). Organic tritium in exchangeable position in fact, gets into equilibrium with the tritium contained in cellular water and undergoes the same developments.

  • Organic tritium in ‘’non-exchangeable‘’ position. This is the tritium directly linked to carbon : T C. Organic tritium in non-exchangeable position remains by definition more durably present than HTO. It ‘’marks the environment’’ durably. In a typical contamination situation 97 % of the tritium eliminates itself in a few days and only 2 to 3 % eliminates itself with biological half-life reaching up to hundreds of days. The role played by this fraction remains low (from the point of view of radioprotection).

• All of the phenomena that have just been discussed (physical dispersion in the air or in water, biochemical transformation -oxidization etc -, progressive elimination) have been modelled methodically and well validated from an experimental point of view.

Tritium and Man

• The risks : 
  Like hydrogen, tritium does not present any chemical toxicity (in contrast with uranium for example). We are therefore in the presence of radio-toxicity due to its beta emission. The low energy of the beta ray means that there is no external contamination. Contamination, if it occurs, is always internal and the means of absorption are inhaling, ingestion and skin absorption. The hazards linked to tritium gas are more than 10,000 times lower than those linked to tritium in the form of tritium water. In case of emission of a mixture of HT and HTO (practically always the case) contamination will be direct by HTO. On the other hand, for HT, the mechanism will first of all be oxidization by soil-atmosphere interface bacteria then a re-emission of tritium water into the atmosphere.
  Quantitatively, we may say that: (H. METIVIER, The Tritium, IPSN). "The effective dose per ingestion unit, for tritium, is nevertheless one of the lowest among all the radio nucleids ". Let us remember that " lowest " here means by several orders of magnitude. The rationof the derived limit of concentration in the air (LDCA) is 10 million between tritium and plutonium.

• The remedies
  Tritium appears in the human body essentially as tritium water. It disappears naturally after a period of around 10 days. The very feeble quantities of tritium organically linked disappear after longer periods and the CIPR (International Commission for Radiological Protection) recommends taking into account a period of 40 days. The replacement of water may be accelerated so as to eliminate the tritium more rapidly. This may be done in the following three ways :
       - Massive ingestion of water
       - Administration of diuretics
       - Possible peritoneal dialysis, which reduces T residence time by a factor of 2 or 3.

• Prevention
  Protection obviously follows the conventional rules of safety relative to installations handling radioactive products. In addition to these rules further considerations are specific to tritium. The systems are multi-barrier systems with a suitable choice of materials limiting to a minimum the effects of permeation. Tritium being a light gas, ventilation of the premises (always in depression) is carried out from bottom to top in contrast with laboratories where transuranians are handled. Two other precautions are taken due to the properties of tritium. Contact with oxygen is reduced to a minimum through various inerte means and efforts are made to trap the tritium water by permanent circulation through molecular sieves.

Tritium and Fusion

Fusion has given considerable attention to all aspects of the use of tritium from production up to waste. For more than 20 years and in a very open international context, the production conditions have been examined in ways that are different to those used today in the manufacture of weapons. The basis of safe production well adapted to the Fusion environment are today well known (for further information : tritium breeding blankets).

Several installations have been built worldwide to study all the aspects of the " tritium process ", i.e. the aspects independent from the fusion plasma. The most outstanding are the tritium Systems Test Assembly (TSTA) of the Los Alamos National Laboratory (LANL, or Division Fusion du LANL, ), in the United States, calibrated to reactor conditions (mass, flow etc), TLK in Europe (Karlsruhe) and TPL in Japan which, in addition, benefits from support from small university laboratories.

Two large Tokamaks have operated with tritium,  JET in Europe (first DT pulse) and TFTR in the United States. Finally, an efficient  complete installation project was developed within the framework of ITER.

All this work has obviously gone hand in hand with safety studies. In particular we must bear in mind the considerable effort made to comprehend the mechanisms of the tritium impact their modelling. Significant experiments – France’s tritium experiment of 1988 in particular – have backed up and validated these models.