Investigation of the Environmental Fate of Tritium in the Atmosphere

INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE
Condensation of gaseous HTO may occur at or very near the stack as hot gases cool and lose
moisture, particularly in winter. Water collected from stack walls and roofs near the point of
release may contain high concentrations of tritium. Runoff of such water may have a
significant effect on local soil porewater and groundwater near the stack.
3.3.3.4 Stack and Building Wakes
Depending on the height and size of stack, the basic pattern of dispersion may be altered
by aerodynamic effects of adjacent buildings and the stack itself. Turbulent zones (known
as wakes) occur on the leeward sides of these structures as the wind flows around them.
A tritium plume can, at least under some wind conditions, be drawn downward into the
wakes, bringing the plume closer to the ground. This effect, known as downwash, can be
significant for both short-term and long-term plume concentrations. If some of the tritium is
present in the form of airborne particles or droplets, these particles will undergo gravitational
settling which may also bring the plume downward somewhat. This will result in greater
contaminant concentrations in the near field.
The model outlined in N288.1 includes downwash effects. It uses a simplified approach
based on a method developed by Huber (1984), in which the effective release height (Hik)
is reduced by an amount that depends on the inside stack diameter and exit velocity, and
the height of adjacent buildings, and the plume spread is enhanced by an amount that is
calculated based on the cross-sectional area of adjacent buildings. Relevant stack and
building parameters are shown in Table 3.5. If exit velocity exceeds 1.5 times wind speed,
downwash at the tip of the stack will not occur. However, downwash in the lee of the
building may still occur, depending on stack height and building dimensions. If the stack
height adjusted for stack tip downwash exceeds 2.5 times the height of adjacent buildings,
plume entrainment by buildings will not occur. Full entrainment may be reasonably assumed
when stack-height is less than the height of adjacent buildings, which is typical of many
Canadian reactor sites.
It is often not obvious how to choose the cross-sectional area, since releases from most
nuclear sites are affected by several different buildings. Newer generation dispersion models
have replaced this approach with a somewhat more sophisticated approach (the so-called
PRIME algorithm).
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