Fission Bombs

To bring the subcritical masses together into a supercritical mass, two techniques are used:

• Gun-triggered
• Implosion

Neutrons are introduced by making a neutron generator. This generator is a small pellet of polonium and beryllium, separated by foil within the fissionable fuel core. In this generator:

1. The foil is broken when the subcritical masses come together and polonium spontaneously emits alpha particles.
2. These alpha particles then collide with beryllium-9 to produce beryllium-8 and free neutrons.
3. The neutrons then initiate fission.

Finally, the fission reaction is confined within a dense material called a tamper, which is usually made of uranium-238. The tamper gets heated and expanded by the fission core. This expansion of the tamper exerts pressure back on the fission core and slows the core's expansion. The tamper also reflects neutrons back into the fission core, increasing the efficiency of the fission reaction.

Gun-triggered Fission Bomb
The simplest way to bring the subcritical masses together is to make a gun that fires one mass into the other. A sphere of U-235 is made around the neutron generator and a small bullet of U-235 is removed. The bullet is placed at the one end of a long tube with explosives behind it, while the sphere is placed at the other end. A barometric-pressure sensor determines the appropriate altitude for detonation and triggers the following sequence of events:

1. The explosives fire and propel the bullet down the barrel.
2. The bullet strikes the sphere and generator, initiating the fission reaction.
3. The fission reaction begins.
4. The bomb explodes.

Little Boy was this type of bomb and had a 14.5-kiloton yield (equal to 14,500 tons of TNT) with an efficiency of about 1.5 percent. That is, 1.5 percent of the material was fissioned before the explosion carried the material away.

Implosion-Triggered Fission Bomb
Early in the Manhattan Project, the secret U.S. program to develop the atomic bomb, scientists working on the project recognized that compressing the subcritical masses together into a sphere by implosion might be a good way to make a supercritical mass. There were several problems with this idea, particularly how to control and direct the shock wave uniformly across the sphere. But the Manhattan Project team solved the problems. The implosion device consisted of a sphere of uranium-235 (tamper) and a plutonium-239 core surrounded by high explosives. When the bomb was detonated, this is what happened:

• The explosives fired, creating a shock wave.
• The shock wave compressed the core.
• The fission reaction began.
• The bomb exploded.

Fat Man was this type of bomb and had a 23-kiloton yield with an efficiency of 17 percent. These bombs exploded in fractions of a second. The fission usually occurred in 560 billionths of a second.

Modern Implosion-Triggered Design
In a later modification of the implosion-triggered design, here is what happens:

• The explosives fire, creating a shock wave.
• The shock wave propels the plutonium pieces together into a sphere.
• The plutonium pieces strike a pellet of beryllium/polonium at the center.
• The fission reaction begins.
• The bomb explodes.

In the next section we'll look at the inner workings of fusion bombs.­