Investigation of the Environmental Fate of Tritium in the Atmosphere

INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE
2.0 SOURCES AND CHEMICAL FORMS OF TRITIUM
This section outlines the sources and chemical forms of tritium in the atmosphere.
Both natural and anthropogenic sources of tritium are described.
2.1 Chemical Forms
Tritium is a rare but natural isotope of hydrogen (H), and is the only natural hydrogen isotope
that is radioactive. Whereas the common hydrogen nucleus contains a single proton, the
nucleus of a tritium atom also contains two neutrons (ANL, 2005). Thus, the mass of a
tritium atom is 3 times that of an ordinary hydrogen atom. The tritium atom is sometimes
designated T to distinguish it from the common lighter isotope.
Notwithstanding the difference in mass, tritium can be found in the same chemical forms
as hydrogen. The most important forms, from the perspective of atmospheric behaviour of
tritium, are tritiated hydrogen gas (HT) and tritiated water (HTO). These tritiated forms
behave chemically like hydrogen gas (H2) and water (H2O). When tritium is incorporated
into hydrocarbon molecules within an organism, it is referred to as organically bound tritium
(OBT). Since OBT is not involved in atmospheric processes, it is not considered further in
this review. Tritiated methane (CH3T) accounts for a small amount of atmospheric tritium.
2.2 Natural Abundance
Hydrogen is the most abundant element in the universe, comprising approximately 90%
of the luminous universe by weight. Ordinary hydrogen ( 1 H) accounts for greater than
99.985% of all naturally-occurring hydrogen, whereas deuterium ( 2 H) comprises
approximately 0.015%. By comparison, tritium ( 3 H) represents only approximately
10 -16 percent of hydrogen naturally occurring (Gross et al., 1951).
Natural atmospheric hydrogen has been estimated to contain approximately 4 x 10 -15
tritium atoms per hydrogen atom, whereas hydrogen in natural surface waters contains
approximately 10 -18 tritium atoms per hydrogen atom, according to early surveys (Bibron,
1963; Faltings and Harteck, 1950; Fireman and Rowland, 1961; Grosse et al., 1951,1954;
Harteck, 1954; Harteck and Faltings, 1950). On this basis, the ‘tritium unit’ (TU) was
defined as 1 tritium atom per 10 18 atoms of hydrogen (Gross et al., 1951). Today, tritium
levels are more commonly reported in activity units of Bq per litre of water, where 1 TU
is equivalent to 0.118 Bq/L (e.g., ICRU, 1963).
Tritium levels present in the atmosphere (T/H ratio) are approximately 10 3 to 10 4 times
higher than in precipitated rainwater, although hydrogen gas quantities in the atmosphere
are 10 4 -fold lower than the mean quantity of water vapour (Begemann, 1962; Bibron, 1963;
Grosse et al., 1954; Harteck, 1954; Harteck and Faltings, 1950). Global levels correspond
to approximately 0.12% of natural tritium in the form of hydrogen gas, approximately 0.1%
as water vapour and the remaining 99.78% in the hydrosphere, predominantly in the oceans
(Begemann, 1962; Jacobs, 1968). Of the total natural tritium inventory, almost 99% has been
estimated to occur in the oceans, with less than 1% occurring in the atmosphere and in the
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