What were the atomic bombs filled with in world war 2?

Answer 1

"Little Boy" was the bomb dropped first, on Hiroshima. It was a Uranium bomb, the nuclear material being Uranium-235. The bomb was not "filled" with it - there was only about 140 pounds of U-235. There was a big mechanism inside the 10 foot long bomb. A sphere of the U-235 was fired down a "gun barrel" into the rest of the U-235 inside the bomb by conventional explosives. When the sphere impacted into the rest of the U-235 this compressed the U-235 and critical mass was achieved, initiating the nuclear explosion. The design was not very efficient. Of the 140 pounds of U-235 only about a pound and a half underwent fission, and the explosion was the result of the release of energy from .6 grams, or less than .02 ounces.

"Fat Man" was dropped three days later on Nagasaki. It was a Plutonium bomb, the nuclear material being Plutonium-239. It was exploded the way nuclear devices still are - it was an implosion bomb. An absolutely perfect sphere of sub-critical plutonium was placed inside a hollow sphere of high explosives. The high explosives were detonated exactly simultaneously, compressing the sphere of plutonium which brought it to critical mass, initiating the explosion. Again, the design was not terribly efficient. There were thirteen pounds of Plutonium in the bomb, less than three pounds fissioned, and the energy of less than one gram was released, causing the explosion.