Investigation of the Environmental Fate of Tritium in the Atmosphere

INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE
atmosphere, partitions easily to soil and plants, and imparts a larger radiological dose in
the body. As noted by Spencer and Vereecken-Sheehan (1994), HTO is usually the more
abundant chemical form in plumes arising from nuclear facilities.
3.3.2. Characteristics of Tritium Releases at Nuclear Facilities
In addition to the type of tritium released, the atmospheric dispersion of contaminants
depends on many facility-specific factors such as number of release stacks, stack height,
exhaust velocity, gas exit temperature, air moisture (humidity), size and location of adjacent
buildings, local atmospheric turbulence and wind conditions, and local topography. It is
important to understand whether stack emissions are continuous or intermittent and whether
the amounts released are constant or variable. Since these factors vary on a facility-by
facility basis it is difficult to generalize about their relative importance.
The specific release characteristics are more important when considering short-term releases
and trying to predict air concentrations for this period. They can be particularly important for
receptors located in the near-field. However, they are less important when considering longterm
annual average concentrations, which are primarily of interest in determining facility
impacts associated with tritium releases to air. A further discussion of how facility specific
factors can affect the atmospheric dispersion of tritium is provided in the following section.
3.3.3 Dispersion of Tritium Plumes
3.3.3.1 Normal Dispersion Processes
Tritium plumes undergo similar dispersion processes to other types of contaminant plumes.
Releases of HT or HTO disperse in similar fashion. The plume rises from the stack until it is
bent over by the wind, after which it travels downwind, spreading horizontally and vertically
as it goes. The final rise of the plume depends on atmospheric conditions and on the
characteristics of the release (stack diameter, exit velocity, temperature). Generally, the
rise is highest under low-wind conditions and lowest under strong winds.
The horizontal and vertical spread or dispersion of the plume depends on atmospheric
conditions. The dispersion tends to be greatest under calm, clear, daytime conditions
and least under calm, clear, nighttime conditions. The former is referred to as unstable or
convective conditions and the latter is known as stable or inversion conditions. Moderate
and high wind conditions result in an intermediate level of dispersion. This condition,
which is known as neutral stability, is the most common and tends to have a dominant
effect on long-term average plume concentrations.
The lateral profile of concentration within a dispersing plume approximates a symmetric,
bell-shaped curve, which is known as the Gaussian distribution. The Gaussian distribution
is the basis of the various numerical dispersion models used to predict the dispersion of
contaminant plumes. The calculation of the spread rate for the Gaussian distribution, which
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