I would think that neutrons can break up any nucleus apart. Why not just use aluminium or iron. Is it because of E=mc^2 ? Greater mass equals greater energy? Would a bomb made of another material be less radioactive?
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It has nothing to do with E=mc^2. Iron is the most stable atomic nucleus because reasons. Elements lighter than iron release energy during fusion, and elements heavier than iron release energy during fission. Uranium is used because it’s far enough away from iron to release a lot of energy during fission, but also close enough to iron to be naturally generated. Uranium-235 also has the somewhat unique quality that each fission event will, on average, produce more than one fission event. This means that if you get enough U-235 together and just one of them fissions, you get a chain reaction of fission reactions that we call a nuclear bomb.
Plutonium-239 is also used in nuclear bombs. Plutonium doesn’t naturally occur on Earth because its half-life is too short. It has to be made here by human processes. Luckily, the aforementioned uranium bomb is a perfect source of Pu-239. Natural uranium ore is roughly 99.3% Uranium-238 and 0.7% Uranium-235. In a nuclear bomb, both types are used in a roughly 1-to-5 ratio. When one of those U-238 atoms is struck by a neutron from a U-235 fission event, it absorbs it and turns into U-239, which quickly beta-decays into Neptunium-239, which beta-decays again into Plutonium-239.
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