The Atomic Bomb http://www.atomicarchive.com/Fission/Images/chainreaction.jpg The nuclear fission creates a chain reaction where exponentially more fission can occur	http://www.atomicarchive.com/Fission/Images/littleboy.jpg This is a gun-type bomb. It fires a mass of Uranium 235 into another, creating critical mass. This is the mass for spontaneous nuclear fission to occur.	http://www.atomicarchive.com/Fission/Images/fatman.jpg A gun-type bomb would not work for Plutonium and so the implosion type bomb was created. Explosive charges compress a sphere of Plutonium so that a chain reaction can occur.