Recent and old fission products...
Radioactive materials released by an atomic explosion differ from those of a reactor accident. During a nuclear explosion, fission has just occurred: fresh fragments are released. These fragments are extremely radioactive due to very short half-lives, such as these of the fragment cascade at 143 nucleons of the figure. During a reactor accident, most of fissionproducts are several months or years old. The radioelements left over come from cascades containing a long-lived radionuclide such as strontiun-90 with a 29 year half-life. The presence of shortlived radioisotopes like iodine-131, with a period of eight days, comes from recent fissions.
IN2P3