UNCLASSIFIED
The faliout program aad the Task Force prediction unit functioned
independently. It «as not feesibie ior the two to employ the same technique because the post shot variability of the winds aloft were especially
critical in ship location probiems of the fallout program. This problem
will be discussed in detail later.
oe
1.20
of Hti0 min, however fairly reliable data are. known for the optical
cload dimensions as functions of yield to H#10 win. The ultimate cloud
diameter can be extrapolated from low yield curves and some qualitative data, Figures l ana 2 present values of the cloud dimensions from
past tests. Tie source mudel was asnumed cylindrical baving, for a
given yield, these dimensions. lts stem diameter was taken as 10 percent of mushroom diameter.
.
Objective
This report describes a technique for forecasting fallout employed
at a recent weapons test operation. The results obtained in the field are
iscussed as examples of the reliability of the techniques, Although the
technique was designed for analysis of land surface detonations where
the fallout is particulate, its application to water suriace detonations is
2.1.2
2
Fe ea ee
considered,
FORECASTING TECHNIQUE
The forecasting technique uses mary ideas from faliout model work,
Several simplifications, as well as a plotting device, have been developed
to the end that the time involved has been reduced greatly without sacrificing accuracy. In general, an initial source of activity is defined ,
describing the “stabilized nuclear cloud by appropriate spatial and size
distributions of radioactive particles. These pacsticies are tracked to
the earth's surface by considering their falling speeds and effects of the
winds existing aloft.
2.1
Basic Considerations
In some cases the input parameters for the forecasiing technique
were obtained from‘“ezpons test measurements.
In others, where data
were lacking, the parameters were derived from theory.
2.4.1
Source Model
The optica! or visible dimensions of the initial cloud froma
nuclear detonation have been documented in past weapons tests. Available data describe such parameters as keight to base of mushroom,
height to top of mushroom, and mushroom diameter as functians of time,
Vertical rise stabilizes in approximately 6 min post detonation, This
time is independent of yield nowever, the expaasion of the mushroom
diameter, particularly for the megaton devices, continues for perhaps
30 min, Available diameter measurements have not been made in excess
-2-
Activity Distribation in Source Model
The greater pazt of the activity was assumed to be concentrated
-in the lower third of the musaroom. The lower two-thirds of the stem
was iguored; the remaincer of the stem and upper two-thirds of the clood
were weighted lightly. This description (Fig. 3) of the activity distribction within the cloud apreared most reasonable in the “ight of available
data and logical thearetical considerations. The acti.aty was concentrated nearer the axis of symmetry of the cload than at ifs outer edges.
2.1.3
Particle Size Distribction in Source Model
AU particle sizes were assumed at all elevations within the
cloud except the lower two-thirds of the stem. However, to cdtain
agreement with past fallout sneasurements and with the optical diameter
of the mushroom, it was necessary to fractiozate the particle size
distribution radially within the cloud. QO:therwise the computed fallour
area about ground zere suuld be too large. The fractiovation was
specified as follows: particles of 1060 microns in diameter and larger
were restricted to the inner 10 percent of the mushroom radius or
approximately the stem racius; those from 56010 i000 microns in
diameter were limited to the inner 50 percent of the cloud radius. Since
the relation of activity to particle size is some direct funczion of the particle
diameter,
this fractionation tends to concentrate the activity about the
axis of symmetry of the cloud.
2.1.4
Particle Falling Speeds or Settling Rates
Computstioas of the terminal velocities of the particles were
based on aerodynamic ccnsiderations for a still atmosphere having
tempesature and density distributions typical of the Marshall Islands
atmosphere in the spring moaths.
Experimentz2 data from past tests at Exiwetok Atoll indicated
that the particles were irreguiar in shape and had a mean densify of
2.36 g/cy cm.
© Dnpoblshed data from a teornt weapons test,
=3-
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