from the smaller yield (Reference 3). The curves from Reference 1 and from the Hardtack
model calculations are both based on experience gained from yields greater than 1 kt.
1.3 THEORY
3.1 Influence of Wind on Fallout Contamination. When a nuclear detonation occurs on
the ground, fission products become associated with particles of soil, which differ in size,
fall rate, and amountof activity collected. These particles are drawn up by the rising hot
gases that result from the detonation and are distributed throughout the cloud that is formed.
When equilibrium between the cloud and the surrounding atmosphere has been established,
particles are free to settle to the surface. During the settling period, horizontal motions
will be governed by the wind velocities experienced.
Thus, the wind structure from the
surface to the top of the cloud determines to a large extent the location and intensity of resulting surface contamination.
Since a single test can be made for only one wind condition, it is important to know what
will happen for other wind conditions, particularly those leading to extremes in militarily
significant contamination levels. If answers to these questions are to be established, a
model consisting of space-and-fail-rate distributions for the activity in the cloud must be
obtained.
1.3.2 Hodographs. Figure 1.7 shows an example of a convenient method of schematically
picturing the effect of a particular wind condition on fallout particles. The curved line,
ABCDEF, called a hodograph, represents the projected path of a balloon as it rises through
the wind structure at a constant rate of ascent. Since the balloon is continually rising, each
point of the hodograph is representative of its plan position at a specific altitude. Radiating
vectors are drawn through Altitudes A, B, C, D, E, and F. Point C, for example, is the
plan position of the balloon when it has reached Altitude C. Tite mear horizontal windspeed
from Altitude C to the surface is given by the distance OC divided by the elapsed time taken
by the balloon to reach Altitude C. Particles that descend from a position over zero at
Altitude C will land at Point C on the hodograph, provided the rate of descent is constant
and equal to the rise rate of the balloon.
Therefore, a hodograph is also the locus of final positions on the ground of particles that
descend through a given wind structure at a particular constant rate from points at different
altitudes directly over ground zero. Different constant rates of descent describe similarly
shaped hodographs of greater or lesser extent so that a straight line drawn from ground
zero through a specific altitude on one hodograph intersects all others at the same altitude.
Such a line is called a height line.
Under certain conditions, measured surface contamination levels may be used with the
measured hodograph to construct the space-and-fall-rate distribution necessary for a fall-
out model (Reference 3).
1.3.3 Surface Measurements and Fall Rate of the Activity.
fallout radiation I for a point on the surface is given by
In general, the intensity of
1 = ffath, dhdf
Where:
(1.1)
A = activity density per unit fall-rate interval in the cloud,
curies-hr/ft*
h = cloud height, feet
f = fall rate of the activity, ft/hr
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