Fission Bomb Triggers

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, the bomb dropped on Hiroshima, 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.

The second way to create a supercritical mass requires compressing the subcritical masses together into a sphere by implosion. Fat Man, the bomb dropped on Nagasaki, was one of these so-called implosion-triggered bombs. It wasn't easy to build. Early bomb designers faced several problems, particularly how to control and direct the shock wave uniformly across the sphere. Their solution was to create an implosion device consisting of a sphere of U-235 to act as the tamper and a plutonium-239 core surrounded by high explosives. When the bomb was detonated, it had a 23-kiloton yield with an efficiency of 17 percent. This is what happened:

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

Designers were able to improve the basic implosion-triggered design. In 1943, American physicist Edward Teller invented the concept of boosting. Boosting refers to a process whereby fusion reactions are used to create neutrons, which are then used to induce fission reactions at a higher rate. It took another eight years before the first test confirmed the validity of boosting, but once the proof came, it became a popular design. In the years that followed, almost 90 percent of nuclear bombs built in America used the boost design.

Of course, fusion reactions can be used as the primary source of energy in a nuclear weapon, too. In the next section, we'll look at the inner workings of fusion bombs.