Gun-Assembled Nuclear Weapon

The first nuclear weapon which was used in war was a gun-type bomb which was called
"Little Boy". It was dropped on Hiroshima. The gun assembly is very simple in its concept and
design. The fissile material is divided into two subcritical pieces. One of these (the projectile)
is propelled to the other, which is called the target, as a result of the pressure of propellant
combustion gases in a gun barrel. Artillery technology is very well developed, thus there are
no great technical problems with regard to design or manufacture of such assembly systems.

The basic "singular-gun" design contains a singular projectile which is fired to a singular
target (above diagram) limits the performance of the device. It is possible to greatly improve
this process with a "double-gun" design: here two projectiles fire at each other. Gun devices are advantageous with regard to the strong military need for design conservatism (for example,
the Americans did not have nuclear tests on the gun design before military use, not like more
complicated designs), or if the inherent characteristics of this design are not a problem - i.e. the
devices are long and heavy (earth penetrator weapons, for example).

Gun Designs

If the objective is to assemble a supercritical mass of fissile material, then the theoretical
higher limit of the quantity of fissile material in a gun design is a little bit under two critical
masses. This is because it is necessary for the projectile and target to remain subcritical
before the device operates.

Criticality not only depends on mass, but also on the geometry of the fissile material. As a result
of modifying the geometry of the target and projectile, it is possible to greatly reduce their
criticality (until the point of system assembly). An example of this can be to hollow out the target
(to greatly reduce its criticality) and to fire the projectile to the target cavity. A more optimum
geometry thus assembles than the initial configuration could have allowed. This methodology
can allow the use of three times the bare sphere critical mass of fissile material in a
gun design.

Use of a neutron moderator (a medium which slows down neutrons and increases the
probability of a fission occurring) can also increase the allowable mass of fissile material. Use
of boron in the target and/or projectile can moderate neutrons and thus raise the critical
mass.

Little Boy

It was reported that "Little Boy" used approximately 64 kg of uranium and achieved a yield
of 13-18 kT. The explosive efficiency of Little Boy was thus approximately half of the
efficiency of "Fat Man" (a plutonium-based implosion device).

Assembly

It is necessary for the combination of choice of propellant and length/thickness of the gun
barrel to give sufficient acceleration to the projectile before insertion. Because it is desirable
to minimize weight and length of the weapon, insertion velocities are limited to velocities
which are below one km per second.

Because it is probable that the target and projectile will be close to a critical mass before
assembly, it is probable that a critical configuration will be obtained before the complete
insertion of the projectile in the target, or possibly before the projectile reaches the target.
The probability of this occurring increases at the same time as the mass of the target and
projectile increases. As soon as a critical mass assembles, there is a chance of pre-detonation.
Thus it is desirable to have as a high insertion velocity as possible to minimize the risk of a
"fizzle".

Because of the inherent pre-detonation risk during assembly, the choice of fissile material is
limited. Plutonium undergoes spontaneous fission and thus is a neutron source itself - this
greatly raises the risk of pre-detonation. This is not the case for uranium, and so it becomes
the preferred material. However, utilizing natural or depleted uranium for the tamper
around the target can provide significant background neutrons. Thus it is necessary to avoid
them. Implosion devices do not suffer in the same manner, because the implosion timescale
is a lot shorter than the assembly time for a gun device.

Neutron Initiation

When it is assembled, the supercritical configuration needs neutron initiation to begin the
fission chain reaction. It is possible to achieve this with internal neutron initiators or external
neutron initiators. Reliance on background neutrons is acceptable with regard to gun designs
because of the relatively slow assembly time (in comparison with implosion devices), and it is
possible to improve the reliability of this method if the projectile can be made to rest in the
target without recoil - this increases the supercritical window for neutron initiation.