Comment by vasama

Nuclear fuels such as U235 are radioactive but very slightly so, U235 specifically having a half life of roughly 700 million years. The various fission products produced by splitting these atoms in a reactor may have much shorter half lives and as such are more radioactive. In the operation of a reactor, some atoms inevitably absorb neutrons as well, which increases their mass number. Through subsequent beta decay whereby a neutron is converted into a proton, their atomic numbers may also increase, producing transuranic elements such as plutonium which account for some of the nastiest nuclear waste products.