Tritium is of interest for nuclear fusion because, along with deuterium, it is a basic fuel for the most readily achievable fusion reaction. Since tritium does not occur naturally in sufficient quantities and is radioactive, its production and management creates some special dynamical features of relevance.

13.1 Tritium Properties

An essential role of a tritium nucleus is as a reactant in the fusion reaction

As previously used, h again represents the helium-3 nucleus; the maximum (3′ energy is 18 keV with an average of ~5 keV. This property of nuclear instability is responsible for two important characteristics of tritium: it is naturally scarce and where it does exist, it is a radioactive hazard.

This natural scarcity of tritium, with the only known inventory being some 20 kg in the oceans and atmosphere where it is produced by reactions initiated by cosmic radiation-combined with the recognition that kilogram quantities of tritium may be required for a commercial central station fusion reactor-means that tritium will need to be bred on a substantial scale. As indicated in Sec. 1.4 and Ch. 13, tritium breeding can occur by incidental neutron capture in the (heavy) water of fission reactors or by neutron capture in lithium in the blanket of a fusion reactor. The feature that every d-t fusion reaction produces one neutron, Eq. (14.1), means that the breeding by neutron induced reactions of one triton for every one destroyed would only be possible if no neutrons escaped or were lost to parasitic reactions. Since some neutron losses are unavoidable, a means of neutron multiplication is also essential for the breeding process.

An indication of the radiation hazard associated with tritium is suggested by calculating the decay rate of, say, 1 kg of tritium. From the definition of nuclear activity, Act, we have

(14.3)

where A, is the decay constant, Eq.(14.2). The total number of tritons N,*

associated with a given mass of tritium M, is given by

M, — N*mt, (14.4)

Translating this quantity into Curies, knowing that 1 Ci = 3.7 x 1010 dps (= 3.7 x 1010 Bq), the activity of 1 kg of tritium is equal to 107 Ci. To place this quantity into context, we add that only about 10’3 Ci of tritium activity can be handled without special licensing provisions. Evidently then, extreme care needs to be exercised in the management of large quantities of tritium.

Tritium, being a hydrogen isotope, can be readily transported by gaseous, liquid, and solid carriers. Its extraction is possible by catalytic exchange and cryogenic distillation processes. Two tritium transport characteristics have been found useful. For the case of a local tritium density gradient VN, in a diffusion medium, Fick’s rule of diffusion provides for a tritium current Jt given by

J,=-DtVN, (14.6)

where D, is the tritium diffusion coefficient. Also, tritium permeation through a barrier containing a differential tritium density yields a tritium flux ф, well represented by

Ф,~К,

V /

Here N,,i and Nt,2 are the upstream and downstream tritium densities, Z is the barrier thickness, and K, is the tritium permeation constant. Parameters such as D, and K, are strongly material and temperature dependent, and, additionally, the permeation constant is a function of surface conditions. Low temperature, multiple wall barriers appear to be the most promising means of tritium containment.