4
THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD
METEOROLOGY~-FALLOUT AND WEATHERING
Since the air density decreases with altitude,
the amountof radioactivity in a given volume
of the cloud is much larger near the bottom
than at the top.
This appears to be borne out
by actual fallout data with the one additonal
fact that the peak activity seems to be located
on somewhatlarger particles near the bottom
of the mushroom cloud than near the top. In
general, both im Nevada and the Pacific, the
particle size with the greatest amount of
mushroom radioactivity is between 100 and
150” in diameter with a specific gravity of
about 2.5.
APPLICATIONS
Laboratories, the Rand Corporation, the University of California Radiation Laboratory, the
Naval Radiological Defense Laboratory, and
others.
The result of the preceding analysis yields a
model of a cloud from a specific explosion: A
given yield, fission-fusion ratio, and type of
burst. The best information, that from Nevada
tests, is limited largely to comparatively lowyield weapons tested on towers. A scaling
it is possible to provide estimates of the heights
of origin and the particle size in a given part
the upper left corner (from a paper by Dr.
lines for the CASTLE BRAVOevent, together
with the winds which carried the particles eastward. The remaining three figures are the
authors’ estimate of what the same isolines
would look like in different wind situations.
In the lower left is a typical winter case of
strong west winds, with an elongated fingerlike
configuration. In the lower right is a case of
light winds, changing from east to west. Note
the marked difference in patterns. The upper
right, a case of southerly low-level winds and
moderately strong upper westerly winds, shows
the stem fallout bulging northward somewhat
in comparison with the lower left-hand case.
of the radiation field. Ag indicated by particle
size measurements, meteorological predictions
do yield approximately the correct particle
sizes; but along with the activity on the pre-
of activity in the mushroom top and stem. The
EXAMPLES
that is, the radioactivity is mainly attached to
It may be of interest to consider a typical
prediction of fallout in the Nevada TestSite.
In Figure 2 the predicted fallout in milliroent-
The previous discussions and examples of
fallout have considered only the effects of gravity and wind. It should be pointed out that if
the airborne debris passes into an area of rain
as the solid lines. The thin dashed lines ere
the observed after-the-factfallout isolines in the
on the quantitative effects of precipitation on
of the fallout pattern, using a wind forecast
made 2 hours before shot time and a Weather
site is brought down byrain.
Models proposed by various groups studying
fallout have differed greatly in the proportions
particle sizes in the stem are relatively larger,
vide the details of each of the models of radio-
uniformly distributed in the vertical, From
first principles, one may argue that the thorough
turbulentmixing in the mushroom will meke the
amount of radioactivity per unit mass of air
uniform throughout this part of the cloud.
flavored by the Weather Bureau studies of
Nevadatests.
Not
The bulk of the radioactivity in the fallout
comes from the mushroom head. The ratio
of such mushroom to stem material in Nevada
bursts is roughly 3 to 1, but this distribution
seems quite variable even on shots of similar
yield, There probably is a smaller proportion
of stem material in bursts in the megaton range.
particles greater
activity created by different organizations.
Rather certain general results will be given,
structure on the fallout pattern is shown.
Gordon Dunning) is a set of idealized dosage
For one thing, it is comparatively
which is necessary in estimating the cumula~
tive dose from a dose-rate measurement. Also,
if there is fractionation in the nuclear. cloud
as a function of altitude or particle size, then
formula is required to refer the dosages to other
yields and heights of burst, but such scaling
velationships are not yet well understood.
It does not appear to be appropriate to pro-
Nevada, missed in the previous static wind
cases, is better accountedfor in this figure.
In Figure 5 the pronouncedeffect of the wind
posium.
easy to estimate the time of arrival of fallout,
practicallyall groups engaged in this procedure:
the shot time winds are used. A comparatively
smal) decrease in wind velocities has made the
fallout pattern shorter and wider than the H-2
forecast. Finally, as shown in Figure 4, a more
refined treatment of the wind has been attempted. The time and space changes of the
wind along the pathsofthefalling particles have
been incorporated. It is evident that the eastwardturning of the fallout pattern in northern
only the bearing of the fallout pattern, but also
the shape is controlled by winds. Shown in
features which may be of interest in this sym-
The Weather Bureau, the Los Alamos Scientific
Bureau model of radioactivity. Such forecasts
are used by the test management in making the
decision whether or not to fire. On Figure 3,
intensity field is predicted, there are other
Aside from demonstrating how afallout
FraurE L—WHypothetical Fallout Plot.
5
than
about 200
microns.
Further, the lower down in the stem, the larger
the particles appear to be.
The activity in the mushroom is also non-
dicted particle sizes, there is a disturbingly
large fraction of activity on particles too small
(even less than 5 microns) to have a significant
settling velocity.
gens per hour 12 hours #fter the burst is shown
same units.
This case shows the verification
SCAVENGING BY PRECIPITATION
or snow, a very different radiation pattern on
the ground mayresult. There is no good data
close-in fallout, but it has been observed that
most of the radioactivity remote from the test