The radioactivity in the fission product mixture is initially mostly caused by short lived isotopes such as I and Ba; after about four months Ce, Zr/ Nb and Sr take the largest share, while after about two or three years the largest share is taken by Ce/ Pr, Ru/ Rh and Pm. Later Sr and Cs are the main radioisotopes, being succeeded by Tc. In the case of a release of radioactivity from a power reactor or used fuel, only some elements are released; as a result, the isotopic signature of the radioactivity is very different from an open air nuclear detonation, where all the fission products are dispersed.