6
THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD
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METEOROLOGY-~FALLOUT AND WEATHERING
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with time and location through a very large
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sizes as far as radioactivity is concerned are
easily moved by moderate winds.
Smaller
particles may be eroded by a process known as
saltation, in which the impact of larger, erodible,
particles jars them loose and allows the wind
range.
The movement of various particles by the
action of wind is shown in Figure 6 (from a
paper by G. R. Hilst of the Hanford Atomic
Products Operation). The table shows that
to carry them away.
the particles most easily moved by wind are
those in the range of 50-800 microns in diameter. A wind of only about 10 knots (measured
at the meter level) is sufficient to pick up
particles in the 60 to 130-mieron level (A. Sund-
The particles whictt are lifted by the wind
will, of course, settle elsewhere. However, in
general, (he importance of this dispersal process
is the diluting of the higher radietion intensities. Furthermore, particles initially or sub-
so it is apparent that. the important particle
most apt to remain.
borg, Geagrafiska Annaler NRXVIL (1955) ).
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sequently settling inte crevices will be the ones
Radiation from these
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'W.8. HAND COMPUTATION
(WINDS PREDICATED AT 2)
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STATUTE MILES.
Frours 2.—-Sample Fallout Computation— Predicted Winds.
WEATHERING
Tae radioactive particles deposited on the
ground may be transported or modified in their
into the ground—carrying some particles with
it. Once the air space in the soil is filled with
water, most of the additional rainfall will run
off along the surface andinto streams—-carrying
along more of the radioactive particles.
To this must be addedthe action of raindrops
effect on man by the action of wind and precipitation. Three types of weathering can be
imagined:
1, Particles can be washed away.
2. Particles can be blown away.
3. Particles can be covered.
In the first case, particles embedded in rain-
in dislodging the particles by their striking
force. With light winds or on level ground this
the initial part of even heavyrain tends to soak
tremendous variability in rainfall can differ
water or snow melt can be washed into the
ground or carried away by runoff. Light rainor
WB. HAND COMPUTATION
{SHOT- TIME WINDS)
is unimportantbutin strong winds or on slopes
with as little as 10 percent grade, there will be
significant transport,
This qualitative picture combined with the
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STATUTE MILES
Fioure 3.—Sample Fallout Computation—Observed Winds at Test Site.