10.1 Characteristics of a Nuclear Explosion
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10.1.2 Thermal Effects
The great majority of the energy of the fission is in the form of the kinetic energy of
the fission products; these fission products slow down and stop rapidly, depositing
their energy in the air in the immediate vicinity of the weapon and heating that air to
high temperatures. Initially the air is hot enough to emit x-ray energy radiation; these
x-rays are absorbed and are re-emitted as lower-energy photons which are absorbed
and emitted once again at somewhat lower energies, reflecting the cooling of the
fireball as it expands. When the photons drop to a wavelength to which the air is
transparent the thermal radiation escapes the fireball and exposes the surrounding
areas.
Thermal radiation travels at the speed of light and will reach its target long before
the blast wave arrives. This is clearly seen in some videos on nuclear weapons effects
from the 1950s in which a puff of smoke is seen as the thermal pulse burns the paint
and outermost layer of wood in a frame house built for testing weapons effects,
followed by the arrival of the blast wave that tears the building apart [2].
According to Eden [6], who wrote about the thermal effects of nuclear weapons,
an energy flux of about 10 cal cm
−2 is sufficient to ignite many flammable materials.
This energy can be in the form of infrared, visible light, ultraviolet, or x-ray radiation;
as the photons are absorbed they deposit their energy and it is this energy that raises the
temperature of the material absorbing them. With sufficiently high energy deposition
the temperature will rise high enough that the material will ignite to start a fire; in
an urban environment there will be many of these small fires ignited in apartments,
offices, stores, and any other location in which flammable material is within the line
of sight of the fireball.
As these fires grow, they will begin to merge and when enough small fires coalesce
and are consolidated into a single conflagration the resulting mass fire is nearly
impossible to extinguish, in part fanned by the winds formed by the hot air rising
away from the fires. A mass fire can produce temperatures high enough to warp and
even partially melt steel train tracks and close to 100% of combustible materials will
burn. Hiroshima and Nagasaki suffered mass fires following their nuclear bombings,
as did Dresden, Tokyo, and other cities attacked with non-nuclear incendiary devices
during this same war.
10.1.3 Radiation Effects
Gamma and neutron radiation are released during fission and, from a device with
a yield of about 10 kt, they are fatal to a distance of about 1 km from the site of
the detonation. Gamma radiation is absorbed by whatever happens to be in its path;
neutron radiation will eventually be absorbed as well, but this process can cause the
material absorbing the neutrons to become radioactive as well. The neutron radiation
93
10.1.2 Thermal Effects
The great majority of the energy of the fission is in the form of the kinetic energy of
the fission products; these fission products slow down and stop rapidly, depositing
their energy in the air in the immediate vicinity of the weapon and heating that air to
high temperatures. Initially the air is hot enough to emit x-ray energy radiation; these
x-rays are absorbed and are re-emitted as lower-energy photons which are absorbed
and emitted once again at somewhat lower energies, reflecting the cooling of the
fireball as it expands. When the photons drop to a wavelength to which the air is
transparent the thermal radiation escapes the fireball and exposes the surrounding
areas.
Thermal radiation travels at the speed of light and will reach its target long before
the blast wave arrives. This is clearly seen in some videos on nuclear weapons effects
from the 1950s in which a puff of smoke is seen as the thermal pulse burns the paint
and outermost layer of wood in a frame house built for testing weapons effects,
followed by the arrival of the blast wave that tears the building apart [2].
According to Eden [6], who wrote about the thermal effects of nuclear weapons,
an energy flux of about 10 cal cm
−2 is sufficient to ignite many flammable materials.
This energy can be in the form of infrared, visible light, ultraviolet, or x-ray radiation;
as the photons are absorbed they deposit their energy and it is this energy that raises the
temperature of the material absorbing them. With sufficiently high energy deposition
the temperature will rise high enough that the material will ignite to start a fire; in
an urban environment there will be many of these small fires ignited in apartments,
offices, stores, and any other location in which flammable material is within the line
of sight of the fireball.
As these fires grow, they will begin to merge and when enough small fires coalesce
and are consolidated into a single conflagration the resulting mass fire is nearly
impossible to extinguish, in part fanned by the winds formed by the hot air rising
away from the fires. A mass fire can produce temperatures high enough to warp and
even partially melt steel train tracks and close to 100% of combustible materials will
burn. Hiroshima and Nagasaki suffered mass fires following their nuclear bombings,
as did Dresden, Tokyo, and other cities attacked with non-nuclear incendiary devices
during this same war.
10.1.3 Radiation Effects
Gamma and neutron radiation are released during fission and, from a device with
a yield of about 10 kt, they are fatal to a distance of about 1 km from the site of
the detonation. Gamma radiation is absorbed by whatever happens to be in its path;
neutron radiation will eventually be absorbed as well, but this process can cause the
material absorbing the neutrons to become radioactive as well. The neutron radiation