Investigation of the Environmental Fate of Tritium in the Atmosphere

INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE
The decay and ingrowth term Dk reduces to a simple decay term for tritium, since there are
no radioactive progeny of interest. Thus, Dk=exp(-λ • (x/uk)), where λ is the decay constant
of 1.79 x 10 -9 s -1 . The travel time to distance x is x/uk. Since receptors of interest are
generally within a few km, at typical wind speeds of about 2 m/s, tritium will reside for
approximately 30 min in the area of interest.
With modern day computational power, more detailed models that determine long-term
concentrations by performing dispersion calculations for each hour of the meteorological
record are now readily available, such as in AERMOD. As previously mentioned, AERMOD
also provides more refined approaches for predicting dispersion under unstable and stable
atmospheric conditions. However, since long-term exposure tends to be dominated by
neutral rather than stable or unstable conditions, these refinements have only a modest
effect on the prediction of long-term concentrations.
The available evidence from monitoring at Canadian nuclear facilities (Chouhan and Davis,
2001) shows that the sector-averaged Gaussian model works reasonably well, predicting
annual average tritium within a factor of 2, with a slightly high bias on average.
Atmospheric processes that may influence tritium dispersion in defined situations are
discussed in the following sections. They are illustrated in Figure 3.2.
3.3.3.2 Effects of Topography
When a dispersing plume encounters a hill, it deflects up and over the hill, or around it,
depending on the atmospheric conditions. The deflection is only partial, and as the plume
passes over the hill, its height above ground is lower than it would be in the absence of the
hill, which results in higher ground level concentrations. Simple Gaussian approaches do
not explicitly represent dispersion around hills and need to be used with caution where
significant hills are present between source and receptor.
This might be an important consideration for facilities located in more complex surroundings
like Chalk River or unimportant for facilities located in relatively flat surroundings like
Gentilly. The latest regulatory air quality models provide a better capability to handle
complex surrounding conditions, which can be an important consideration for some facilities.
Where there is significant variation in surface roughness surrounding the source, the use of
an average uniform roughness, may over or underestimate concentration at some locations,
depending on the actual terrain. Considering the range in terrain roughness around Canadian
nuclear facilities, the N288.1 Guideline recommends a lower value, which will be generally
conservative with one exception. For receptors across water, vertical dispersion is overestimated
by assuming a roughness value for land surfaces.
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