The Importance of Uranium in Nuclear Weapons

Little Boy was powered by Uranium and Fat Man used plutonium. The two designs fundamentally do the same thing, which is take a massive radioactive material that takes a chain reaction in order to cause an explosion. So there's a small conventional explosion to create a much bigger explosion - exactly the same idea behind how guns work. And you need to combine those masses quickly enough for it to overcome the fission of the atoms.