Nuclear explosions give combined immediate and delayed destructive effects. It is possible
to create immediate effects (blast, thermal radiation, ionizing radiation) and great
destruction after seconds or minutes of a nuclear detonation. The delayed effects, for
example radioactive fallout, cause damage during an extended period which lasts from
hours to hundreds of years. They can cause damage in locations which are very distant from
the site of the detonation. This section discuss these two types of effect.
With regard to the three damage-causing effects, the distribution of energy which becomes
released in the first minute after detonation is:
|Low Yield (<100 kt)||High Yield (> 1 Mt)|
The three classes of prompt or immediate effects are:
Their relative importance can be different due to the magnitude of the nuclear explosion
and all three effects can be great causes of injury. With regard to a low explosive yield, the
three effects are more or less equal. All categories are capable of causing fatal injuries up to
about 1000 meters away.
In essence the fraction of a bomb's yield which is emitted as thermal radiation, blast, and
ionizing radiation is reasonably constant for all yields. However, the way the different forms
of energy interact with air and targets greatly differs.
Essentially air is transparent to thermal radiation. The thermal radiation affects exposed
surfaces, and produces damage as a result of rapid heating. The size of the area affected by
thermal radiation is directly proportionate to the size of the bomb. And the destructive
radius increases in relation to the square root of the yield. The area which is damaged as a
result of thermal radiation increases almost linearly with the yield.
The blast wave puts energy in the material through which it passes - this includes air. When
the blast wave passes through solid material, the energy which is left behind causes damage.
When it passes through air it only becomes weaker. The more matter the energy crosses
through, the smaller becomes the effect. Thus blast effects scale with the inverse cube law
which relates radius to volume.
The intensity of nuclear radiation decreases with thermal radiation as it is similar to the
inverse square law. However nuclear radiation is also absorbed by the air through which it
travels. This causes the intensity to reduce with a much quicker rate.
In the attack at Hiroshima it was possible to see casualties - including fatalities - as a result
of all three causes. The more common serious injury consisted of burns, including those
which were caused by the fire storm: more than half of all the people who died during the
first day died of burns. Blast and burn injuries affected the majority survivors. People who
were sufficiently close to suffer significant radiation illness were clearly in the lethal effects
radius to suffer from blast and flash burns. As a result only approximately one third of
injured survivors demonstrated any radiation illness. Many of these people were sheltered
from burns and blasts . Thus they escaped the main effects. Despite that situation, most of
the victims who suffered radiation illness also had blast injuries or burns in addition.
Immediate radiation injury becomes insignificant because of yields which have hundreds of
kilotons or more . Dangerous radiation levels exist only so close to the explosion that it is
impossible to survive the blast. On the other hand, it is possible to cause fatal burns far
beyond the range of substantial blast damage. There the blast effect is limited - it can break
windows and cause superficial cuts.
A convenient method to estimate short-term fatalities as a result of all causes from a nuclear
attack is to count as a fatality everyone inside the 5 psi blast overpressure contour which is
around the hypocenter. In fact, large numbers of people who are located in the contour
survive and large numbers beyond the contour die. However there is a pre-supposition that
these two groups will be approximately equal with regard to size. This principle does not
include at all any possible effects of fallout.
The main delayed effect consists of the creation of huge amounts of radioactive material
which has long lifetimes, produced from fission reactions. Potentially, an important
secondary source is neutron activation.
When atoms undergo fission an abundant mixture of different isotopes are produced. These
isotopes differ greatly with regard to stability. Some of them are completely stable and
others undergo radioactive decay with very short half-lifes. It is possible that the decaying
isotopes may themselves form stable or unstable “daughter isotopes”. Thus the mixture
quickly becomes even more complex.
Short-lived isotopes release their decay energy quickly and create intense radiation which
also declines quickly. Long-lived isotopes release energy during long periods of time and
create radiation which is a lot less intense but which is more persistent. Thus initially fission
products have a very high level of radiation which declines quickly. However at the same
time as the intensity of radiation reduces, the rate of decline reduces also.
These radioactive products are most hazardous when they settle to the ground in the form
of “fallout”. The rate with which fallout settles depends very strongly on the altitude of the
explosion and, to a lesser extent, on the size of the explosion.
In an air-burst explosion, when there is no contact with the ground, the vaporized
radioactive products become sufficiently cool to condense and solidify, they do this to form
microscopic particles. Mostly these particles lift high into the atmosphere as a result of the
rising fireball, although great amounts become deposited in the lower atmosphere as a
result of mixing which occurs due to convective circulation in the fireball. The larger the
explosion, the higher and faster the fallout rises, and the smaller becomes the portion which
deposits in the lower atmosphere. With regard to lower yield nuclear explosions, the fireball
does not rise above the troposphere where there is precipitation. Thus all of this fallout
comes to the ground by weather processes. With regard to higher yield nuclear explosions,
the fireball rises so high that it enters the stratosphere. The stratosphere is dry, and no
weather processes exist there to bring fallout down quickly. Small fallout particles will
descend during months or years. Such long-delayed fallout loses most of its hazard before it
comes down, and distributes globally.