To better understand the decay rate for various radioactive isotopes, natural and man-made, the engineers at M.I.T. wrote an nice overview. See my *NEWS* page for their website, or check the article on their blog here:
“The chart below lists various important fission products along with their yields – the frequency at which they are produced from fission. For example, 6.3% of fission events (on average) will produce xenon-135 (after the highly unstable fission products rapidly decay). The half-life is a general time scale for how long the listed radioactive fission product will exist before decaying to a more stable fission product. Note that cesium and iodine, which were detected near the Fukushima site, are by far the most frequently occurring radioactive fission product elements.
|6.3%||iodine-135 / xenon-135||7 hours|
|6.3%||zirconium-93||1.5 million years|
|6.1%||molybdenum-99 / technetium-99**||200,000 years|
|0.7%||iodine-129||15 million years|
|0.2%||palladium-107||7 million years|
*Cs-133 is stable but has a high fission yield, but it will then produce Cs-134 from absorbing neutrons in the reactor and Cs-134 is radioactive with a ~2 year half-life.
**Half-life reported in the table is for Tc-99. Mo-99 has a half-life of ~66 hours, which then decays to Tc-99m (metastable form of Tc-99) with a half-life of ~6 hours. The Tc-99m then decays to the Tc-99 with the 200,000 year half-life reported in the table.
Note that longer half-lives do not necessarily mean more danger. Some fission products have extremely long half-lives but emit very little radiation at any given time, so they are not dangerous. Other fission products emit huge amounts of radiation but exist for such a short period of time that they are not dangerous. How harmful a given fission product is to humans is a complicated function of half-life, radiation intensity, and various human biology factors.” [end quote]