The high short-term radioactivity of spent nuclear fuel is primarily from fission products with short half-life. The radioactivity in the fission product mixture is mostly due to short-lived isotopes such as I and Ba, after about four months Ce, Zr/ Nb and Sr constitute the largest contributors, while after about two or three years the largest share is taken by Ce/ Pr, Ru/ Rh and Pm. Note that 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.