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Types of Fission Bombs
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Fission Bombs or Atomic Bombs

Below is a description of two different types of fission bombs.  A gun triggered fission bomb and a implosion triggered fission bomb.

The famous bomb called the "Fat Man" was the second atomic bomb dropped on Nagasaki after Hiroshima.  This is an example of a implosion triggered fission bomb.  Though the amount of energy generated by the bomb was greater than "Little Boy" used at Hiroshima but caused less damage due to the geographic structure of the city. It is estimated that approximately 70,000 people died by the end of the year because of the bombing.

Gun-Triggered Fission Bomb
This type of Atomic bomb uses the simplest way to bring the subcritical masses together.   It uses a gun to shoot one subcritical mass into another subcritical mass, which makes the required supercritical mass for the explosion.  Some U-235 is made into a sphere around the neutron generator at the far end of the long tube.  A small bullet of U-235 is then place at the other end of the long tube with explosives behind it that will fire it into the sphere.  A barometric-pressure sensor inside the bomb determines the appropriate altitude for detonation and triggers the following sequence of events:

  1. The bullet is shot down the tube from the force of the explosives
  2. The bullet travels down the tube and collides with the sphere and generator, which starts the fission reaction
  3. The fission reaction begins
  4. The bomb explodes

Click on remove shell button below to see a demonstration of "Little Boy".  Click on the detonate button to watch it explode.

This bomb had a  14.5-kiloton yield (equal to 14,500 tons of dynamite) with an efficiency of about 1.5 percent.  An example of this type of bomb is "Little Boy" dropped on Hiroshima.


Implosion-Triggered Fission Bomb (Fat Man)
During the Manhattan Project it was soon noticed that by imploding the subcritical masses into a sphere would be a good way to make a supercritical mass. The problem with this idea was how to control and direct the shock wave uniformly across the sphere. But the Manhattan Project team solved the problem. The implosion device had many subcritical masses of plutonium-239 enveloped by high explosives within a sphere of uranium-238 (tamper). When the bomb was detonated, this is what happened:

explosives fire, shockwave created
A sphere of Plutonium is created from the shockwave
A small piece of beryllium/polonium is struck by the plutonium chunks in the middle
Fission begins
Bomb goes boom

Click on remove shell button below to see a demonstration of "Fat Man".  Click on the detonate button to watch it explode.

The Fat Man was dropped on Nagasaki, Japan and was the final straw that lead to the surrender of Japan during World War II.  The Fat Man was more powerful than the bomb dropped on Hiroshima, yet it did less damage because of the location and the terrain at Nagasaki.  The Fat Man was an Implosion Fission Bomb, which means that the bomb imploded or (collapsed) on itself. 

Fat Man had a  23-kiloton yield with an efficiency of 17 percent. It explodes in less than a fraction of a second. The fission usually occurred in 560 billionths of a second.

Fission Quiz



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