9.3 Types of Nuclear Weapons
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weapons designers construct a hollow sphere comprised of a critical mass of plutonium that is in a subcritical configuration. They then surround this sphere with explosive “lenses” that, when detonated, produce a spherically symmetric shock wave that
compresses the sphere into a critical geometry, causing a nuclear chain reaction and
a nuclear explosion.
The reason that this design is felt to be less likely to be used by a terrorist group
is that it requires a great deal more skill to develop. In fact, during the Manhattan
Project a great deal of research and testing was required to develop the explosive
lenses that would produce a symmetric implosion. The fact that there are so many
complex factors that must work nearly perfectly is why Zimmerman feels that a
terrorist group is unlikely to fabricate its own implosion-style device.
9.4 Immediate Physical Effects
All of the energy produced by a nuclear weapon will be produced and released during
the very short time, less than a microsecond, that the fission process will last. The
majority of the energy released will be in the form of [6]:
• The kinetic energy of the fission fragment (about 82.5%)
• Gamma radiation from fission (3.5%),
• The kinetic energy of fission neutrons (2.5%),
• The kinetic energy of beta particles from fission products (3.5%),
• Delayed gamma radiation from fission products (3%), and
• Neutrinos from fission products (5%).
The kinetic energy of the fission products will be deposited within the weapon
itself and within the rapidly expanding cloud of vapor formed during the detonation;
this will heat the materials to incandescence and much will be re-emitted as visible
and thermal radiation. The absorption of this energy by the air will cause it to expand
rapidly, forming a blast wave that will move outwards at high speeds. At the same
time, the visible light and thermal radiation escaping from the outer surface of the
expanding fireball (the volume of air heated to incandescence by the heat of the
explosion) will travel effectively instantly to be deposited in any objects in its way;
if those objects are flammable then this can cause them to burst into flame, even if
they are behind a window in a building’s interior. Thus, fires can be ignited in areas
distant from the explosion itself.
As noted above the air will initially heat up, causing it to expand rapidly enough to
form a blast wave. At the same time, the heated air will begin to rise, leaving a partial
vacuum at and near ground level; this will pull air in from all sides and will form
the “stem” of the mushroom cloud associated with nuclear explosions. Although the
explosion itself will be over within a second, the fires ignited by the pulse of thermal
radiation will continue heating the air which will rise, pulling still more air in at the
base from all directions. This air will help to fan the flames of the fires that were
89
weapons designers construct a hollow sphere comprised of a critical mass of plutonium that is in a subcritical configuration. They then surround this sphere with explosive “lenses” that, when detonated, produce a spherically symmetric shock wave that
compresses the sphere into a critical geometry, causing a nuclear chain reaction and
a nuclear explosion.
The reason that this design is felt to be less likely to be used by a terrorist group
is that it requires a great deal more skill to develop. In fact, during the Manhattan
Project a great deal of research and testing was required to develop the explosive
lenses that would produce a symmetric implosion. The fact that there are so many
complex factors that must work nearly perfectly is why Zimmerman feels that a
terrorist group is unlikely to fabricate its own implosion-style device.
9.4 Immediate Physical Effects
All of the energy produced by a nuclear weapon will be produced and released during
the very short time, less than a microsecond, that the fission process will last. The
majority of the energy released will be in the form of [6]:
• The kinetic energy of the fission fragment (about 82.5%)
• Gamma radiation from fission (3.5%),
• The kinetic energy of fission neutrons (2.5%),
• The kinetic energy of beta particles from fission products (3.5%),
• Delayed gamma radiation from fission products (3%), and
• Neutrinos from fission products (5%).
The kinetic energy of the fission products will be deposited within the weapon
itself and within the rapidly expanding cloud of vapor formed during the detonation;
this will heat the materials to incandescence and much will be re-emitted as visible
and thermal radiation. The absorption of this energy by the air will cause it to expand
rapidly, forming a blast wave that will move outwards at high speeds. At the same
time, the visible light and thermal radiation escaping from the outer surface of the
expanding fireball (the volume of air heated to incandescence by the heat of the
explosion) will travel effectively instantly to be deposited in any objects in its way;
if those objects are flammable then this can cause them to burst into flame, even if
they are behind a window in a building’s interior. Thus, fires can be ignited in areas
distant from the explosion itself.
As noted above the air will initially heat up, causing it to expand rapidly enough to
form a blast wave. At the same time, the heated air will begin to rise, leaving a partial
vacuum at and near ground level; this will pull air in from all sides and will form
the “stem” of the mushroom cloud associated with nuclear explosions. Although the
explosion itself will be over within a second, the fires ignited by the pulse of thermal
radiation will continue heating the air which will rise, pulling still more air in at the
base from all directions. This air will help to fan the flames of the fires that were