(b)
Residual Nuclear Radiation.
Radiation that lasts after the first minute is residual nuclear
radiation. It consists primarily of fallout and neutron-induced
radiation. The primary hazard of residual radiation results from the
creation of fallout.
Fallout results when the fireball draws material from the earth,
vaporizes it, combines it with radioactive material, and condenses it
to particles which then fall back to earth. The larger particles fall
back immediately in the vicinity of ground zero. Winds carry the
smaller particles until they gradually settle to the earth's surface.
The contaminated area created by fallout may be very small or may
extend over many thousands of square kilometers, and the dose rate may
vary from a militarily significant level to an extremely dangerous one
for all personnel not taking necessary defensive measures. Fallout of
radioactive material from an airburst is not of military significance
unless rain or snow falls through the radioactive cloud and brings the
material to earth.
Neutrons from the detonation will cause induced radiation in the soil
around ground zero. Except for very high airbursts, neutron-induced
radiation in the area of ground zero will be of concern to both
mounted and dismounted personnel who must enter or cross the area. As
units pass through such an area, they must conduct radiological
monitoring to detect hazardous levels of radiation that they must
avoid if possible. Induced radiation is present in a surface burst
but will become residual radiation from fallout. The fallout produced
by a surface burst is by far its most dangerous effect in that it can
cover thousands of square kilometers with high levels of
If the fireball of a subsurface burst breaks through the surface, it
will produce fallout. Thermal radiation will not present a
significant hazard; the soil will almost completely absorb it. Blast
effects will also reduce significantly, but shock waves passing
through the ground or water will extend for a limited distance. The
range of the nuclear radiation will be considerably less than that
from either of the other two types of bursts, because the soil will
absorb much of the radiation. However, extremely hazardous residual
radiation will occur in and around any crater. If the fireball does
not break the surface, shock waves will pass through the ground and
cause craters because of settling.
A secondary hazard that may arise is the neutron-induced radioactivity
on the earth's surface in the immediate vicinity of ground zero. The
intensity and extent of the induced radiation field depends on the
type of soil in the area around ground zero, the height of burst, and
the type and yield of the weapon. The
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