Investigation of the Environmental Fate of Tritium in the Atmosphere

INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE
contribution of HT to total tritium levels (Harteck, 1954). The tritium concentration in
atmospheric hydrogen is expected to remain relatively stable, whereas localized variability
in rainwater tritium levels may occur (Harteck, 1954; Libby, 1953).
In thermonuclear clouds, tritiated hydrogen is produced in the stratosphere by radiative
dissociation of water under oxidizing conditions (Martell, 1963). Tritiated methane is formed
through tropospheric exchange of methane with the tritiated hydrogen that has moved down
from the stratosphere. The tritium specific activity of atmospheric methane is several orders
of magnitude higher than that of rainfall, but somewhat less than that of atmospheric
hydrogen and stratospheric water vapour (Harteck, 1954).
Following production in the upper atmosphere by cosmic radiation, tritium atoms have a high
kinetic energy and are readily oxidized (Jacobs, 1968). At lower than atmospheric pressures,
this likely predominantly occurs through a three-body collision with oxygen (O2) to initially
form a relatively stable radical, HO2 (Harbeck, 1954). This radical can then react with ozone
(O3) following repeated photochemical decomposition reactions of TO2 to form HTO, as
opposed to HT, in the atmosphere. Once tritium has been incorporated into water molecules,
HTO then falls to the surface of the earth as rain and snow, entering the natural hydrological
cycle in this way (Figure 3.1; Stamoulis et al., 2005; UN, ILO and WHO, 1983).
A second feasible initial reaction would involve the collision of tritium with an H2 molecule,
with a resultant isotopic exchange to form HT. Small amounts of tritium, representing less
than 1% of the amount produced per day, may also be produced through the initiation of a
reaction between tritium and oxygen by the beta radiation from tritium decay (Dorfman and
Hemmer, 1954; Yang and Gevantman, 1960).
Transfer of tritiated water vapour from the stratosphere to the troposphere occurs with a halftime
of approximately a year, after which it is deposited onto the earth’s surface as rainfall
and through molecular exchange with a half-time on the order of 10 days. Deposited HTO
is then transported in the environment as part of the hydrological cycle (Figure 3.1; Jacobs,
1968).
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