Long-lived fission product
Long-lived fission products (LLFPs) are radioactive materials with a long half-life (more than 200,000 years) produced by nuclear fission of enriched uranium fueled reactors.
Nuclear fission produces fission products, as well as actinides from nuclear fuel nuclei that capture neutrons but fail to fission, and activation products from neutron activation of reactor or environmental materials.
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 short-lived isotopes such as I-131 and 140Ba, after about four months 141Ce, 95Zr/95Nb and 89Sr take the largest share, while after about two or three years the largest share is taken by 144Ce/144Pr, 106Ru/106Rh and 147Pm. 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.
After several years of cooling, most radioactivity is from the fission products caesium-137 and strontium-90, which are each produced in about 6% of fissions, and have half-lives of about 30 years. Other fission products with similar half-lives have much lower fission product yields, lower decay energy, and several (151Sm, 155Eu, 113mCd) are also quickly destroyed by neutron capture while still in the reactor, so are not responsible for more than a tiny fraction of the radiation production at any time. Therefore, in the period from several years to several hundred years after use, radioactivity of spent fuel can be modeled simply as exponential decay of the 137Cs and 90Sr. These are sometimes known as medium-lived fission products.
...
Wikipedia