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from ground zero. For weapons larger than 10 KT, thermal radiation is the primary cause
of injury, as this extends over a greater distance than blast or radiation effects. Flashblindness
and retinal burns may occur out to 20 km during daytime and 50 km at night.
Fallout. Fallout is defined as radioactive material from a nuclear detonation that returns
to the ground after the explosion. A detonation close to ground level (i.e., surface burst)
will result in large quantities of earth and/or water being thermally vaporized and drawn
up into a radioactive cloud. Much of this material may be blasted into the atmosphere and
subsequently return to earth as fallout. This material becomes radioactive from either
neutron activation or from condensing together with various radioactive isotopes. In
highly contaminated areas, fallout can cause potentially lethal external radiation
exposure, as well as provide a serious internal hazard from the inhalation or ingestion of
radioactive materials, such as milk (see Figure 6.2).
A surface burst over the ground produces particles that range in size from submicron to
several millimeters in diameter. The finer particles rise into the stratosphere and may be
dispersed as worldwide fallout. The larger particles settle to earth within 24 hours as local
radioactive fallout. The heaviest fallout usually occurs in the first 4 hours. In contrast,
bursts over water are characterized by lighter and smaller particles, producing a smaller
volume of fallout that extends over a larger area. These particles consist mostly of sea
salts and water, which may produce a “cloud seeding” effect that results in areas of high
local fallout as radioactive materials are washed out of the air.
The primary method of fallout protection is initial sheltering, with rapid evacuation from
the contaminated area until the risk has been eliminated through decay and/or
remediation. In general, it is very difficult to accurately predict the rate of radioactive
decay for fallout. Thus, relevant decisions should be based on actual radiological survey
data. The 7:10 rule can be used to estimate residual radiation decay after all fallout is on
the ground and the dose rate is beginning to measurably decrease.
Radiological Dispersal Devices (Dirty Bombs)
A radiological dispersal device (RDD) is designed to spread radioactive material through
detonation of conventional explosives or other (non-nuclear) means. These “dirty bombs”
blast radioactive material into the area around the explosion, exposing people and
buildings. The purpose of a dirty bomb is to frighten people and make buildings or land
unusable for a long period of time.
An RDD is a weapon of “mass disruption” more than a weapon of mass destruction.
However, an RDD can produce external contamination, an exposure hazard, and a risk of
internal contamination (via inhalation, ingestion, or wounds) if basic safety and hygiene
precautions are not followed. An RDD also may pose a radiation injury risk in the event
that a strong source (e.g., Cs-137 or Co-60) is kept relatively concentrated. In this
situation, the RDD acts as a high-intensity sealed gamma source.
The principal use of an RDD is to cause fear and to disrupt infrastructure. Use of an RDD
also tends to generate panic and social and economic disruption from the physical and
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