We generally consider nuclear fission as the "splitting" of atoms. In this process, a large atomic nucleus splits into two smaller nuclei. The smaller nuclei are called fission fragments, and they are radioactive. Making nuclear energy, which we do in nuclear reactors, results in the production of large quantities of highly radioactive fission products. These byproducts require that we cool the spent fuel for long periods, and also that we store it for centuries so that the radioactive materials have a chance to decay and become less hazardous.