Sorry for the over-detailed paragraphs and complex wording. Estimates say that only about 20% of the 13.6 pound P-239 core underwent fission, throwing the other 11 pounds away, scattered in the nearby areas. Conventional explosive, surrounded by Uranium-238, were placed around the plutonium core, and when they exploded, they sent a shock wave of energy and force, which was reflected by the U-238 by its neutron reflecting properties, to the aforementioned core, causing it to undergo nuclear fission. The material that fueled it was Plutonium-239. The bomb that was used on Nagasaki, 'Fat Man,' was an imploding-type bomb that was more powerful, as well as complex. Once the 'bullet' has struck the spike at the front, the atoms split, causing an explosion, along with a mushroom cloud. Basically, a small explosive in the back launches a hollow uranium 'bullet' to the front of the bomb, where a target cylinder or spike of sorts is waiting for it. The first bomb, which was used on Hiroshima, 'Little Boy,' was a Uranium-235 gun-type fission weapon.

This process is called 'nuclear fission.' There are two types of bombs that were dropped. It can be achieved by bringing an element to 'critical mass,' which means that it's so dense that it can't be packed any tighter. When aforementioned atom is split, massive amounts of energy are released. The way the bomb works is this: inside of the bomb, an atom is split.

Atomic bomb on japan