A nuclear explosion creates several kinds of radioactive atoms:

• Any atoms that get split during nuclear fission will be radioactive — some extremely radioactive, some less so. These "fission products" make up the most radioactive byproducts of a nuclear explosion and are what we care the most about from a health perspective.

• Any fuel atoms (plutonium or uranium) that don't get fissioned will still be around, and are still radioactive (but not that radioactive compared to the other things on here — they have relatively long half-lives).

• Some fuel atoms (plutonium or uranium) will absorb neutrons and not fission, becoming heavier atoms, which are also radioactive. This is not a huge issue, but I'm just pointing it out for completeness.

• Atoms in the bomb or the environment which absorb neutrons can become artificially radioactive. This is called neutron activation. Depending on the atom in question, it might be only a little radioactive, or very radioactive. The health consequences can vary depending on the elements in question.