9.3 Types of Nuclear Weapons
87
9.3 Types of Nuclear Weapons
There are two fundamental types of fission weapons (we will not discuss thermonuclear fusion weapons as their fabrication is likely beyond the capabilities of a terrorist
group), “gun-type” and implosion weapons. While both designs have been shown
to function as designed, the gun-type device is less complex and is easier to design
and build than an implosion weapon—it is simple enough that the US did not feel
it necessary to even test the gun-type device that was the first nuclear weapon used.
However, for reasons to be discussed below, plutonium cannot be used in a gun-type
nuclear weapon; an implosion design is required for bombs using plutonium as the
fuel.
9.3.1 Gun-Type Nuclear Weapons
In Zimmerman’s nuclear terrorism article in Foreign Affairs he described how a
terrorist group might construct a gun-type nuclear weapon. The reason for this is that
such a weapon is simple to design and to construct; the major difficulty in constructing
such a device lies with obtaining enough weapons-grade uranium to make it work.
This is not to understate the difficulty of making a working nuclear weapon, but the
fact is that a group that obtains enough highly enriched uranium can likely follow
relatively simple instructions to produce a working nuclear weapon. It must also be
noted that even a “fizzle” (a weapon that does not produce its designed yield) can
produce a blast equivalent to several hundred Oklahoma City-style weapons, enough
to destroy several city blocks and producing high levels of radioactive contamination.
In a gun-type device two pieces of weapons-grade uranium, each less than a critical
mass, are slammed together at high velocity using an explosive charge. In one design a
roughly cylindrical piece (the “bullet”) is propelled down a large-diameter gun barrel
into a cup-shaped target at the end of several rings of highly enriched uranium. While
the target, the bullet, and the rings are each less than a critical mass of uranium, when
assembled they achieve both critical mass and a critical geometry, a chain reaction
ensues with the concomitant production of energy, and the weapon explodes with a
nuclear yield [4, 8] (Fig. 9.2).
The reason Zimmerman feels a terrorist group is likely to use a gun-type device
is that they are conceptually simple and are relatively easy to construct compared
to an implosion-type weapon. The most significant drawback to gun-type devices is
that they cannot use
239 Pu, a more effective nuclear explosive, but this is felt to be
outweighed by the simplicity of their design.
87
9.3 Types of Nuclear Weapons
There are two fundamental types of fission weapons (we will not discuss thermonuclear fusion weapons as their fabrication is likely beyond the capabilities of a terrorist
group), “gun-type” and implosion weapons. While both designs have been shown
to function as designed, the gun-type device is less complex and is easier to design
and build than an implosion weapon—it is simple enough that the US did not feel
it necessary to even test the gun-type device that was the first nuclear weapon used.
However, for reasons to be discussed below, plutonium cannot be used in a gun-type
nuclear weapon; an implosion design is required for bombs using plutonium as the
fuel.
9.3.1 Gun-Type Nuclear Weapons
In Zimmerman’s nuclear terrorism article in Foreign Affairs he described how a
terrorist group might construct a gun-type nuclear weapon. The reason for this is that
such a weapon is simple to design and to construct; the major difficulty in constructing
such a device lies with obtaining enough weapons-grade uranium to make it work.
This is not to understate the difficulty of making a working nuclear weapon, but the
fact is that a group that obtains enough highly enriched uranium can likely follow
relatively simple instructions to produce a working nuclear weapon. It must also be
noted that even a “fizzle” (a weapon that does not produce its designed yield) can
produce a blast equivalent to several hundred Oklahoma City-style weapons, enough
to destroy several city blocks and producing high levels of radioactive contamination.
In a gun-type device two pieces of weapons-grade uranium, each less than a critical
mass, are slammed together at high velocity using an explosive charge. In one design a
roughly cylindrical piece (the “bullet”) is propelled down a large-diameter gun barrel
into a cup-shaped target at the end of several rings of highly enriched uranium. While
the target, the bullet, and the rings are each less than a critical mass of uranium, when
assembled they achieve both critical mass and a critical geometry, a chain reaction
ensues with the concomitant production of energy, and the weapon explodes with a
nuclear yield [4, 8] (Fig. 9.2).
The reason Zimmerman feels a terrorist group is likely to use a gun-type device
is that they are conceptually simple and are relatively easy to construct compared
to an implosion-type weapon. The most significant drawback to gun-type devices is
that they cannot use
239 Pu, a more effective nuclear explosive, but this is felt to be
outweighed by the simplicity of their design.