The fission product mixture contains significant amounts of molybdenum. Molybdenum-99 is of enormous interest to nuclear medicine as the parent nuclide to Tc but its short half life means it'll usually be decayed long before the spent fuel is reprocessed. Mo can be produced both by fission followed by immediate reprocessing (usually only done in small scale research reactors) or in particle accelerators. As Molybdenum-100 only decays extremely slowly via double beta decay (half life longer than the age of the universe) the molybdenum content of spent fuel will be essentially stable after a few days have passed to allow the Molybdenum-99 to decay.