The isotope responsible for most of the gamma exposure in fuel reprocessing plants (and the Chernobyl site in 2005) is caesium-137. Iodine-129 is a major radioactive isotope released from reprocessing plants. In nuclear reactors both caesium-137 and strontium-90 are found in locations away from the fuel because they're formed by the beta decay of noble gases (xenon-137, with a 3.8-minute half-life, and krypton-90, with a 32-second half-life) which enable them to be deposited away from the fuel, e.g. on control rods.