Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release of heat energy (kinetic energy of the nuclei), and gamma rays. The two smaller nuclei are the fission products. (See also Fission products (by element)).

About 0.2% to 0.4% of fissions are ternary fissions, producing a third light nucleus such as helium-4 (90%) or tritium (7%).

The fission products themselves are often unstable and radioactive, due to being relatively neutron-rich for their atomic number, and many of them quickly undergo beta decay. This releases additional energy in the form of beta particles, antineutrinos, and gamma rays. Thus, fission events normally result in beta radiation and antineutrinos, even though these particles are not produced directly by the fission event itself.

Many of these isotopes have a very short half-life, and therefore give off huge amounts of radiation. For instance, strontium-90, strontium-89 and strontium-94 are all fission products, they are produced in similar quantities, and each nucleus decays by shooting off one beta particle (electron). But Sr-90 has a 30-year half-life, Sr-89 a 50.5-day half-life, and Sr-94 a 75-second half-life. When freshly created, Sr-89 will spray beta particles 10,600 times faster than Sr-90, and Sr-94 will do so 915 million times faster. It is these short-half-life isotopes that make spent fuel so dangerous, in addition to generating much heat, immediately after the reactor itself has been shut down. The good news is that the most dangerous fade quickly; after 50 days, Sr-94 has had 58,000 half-lives and is therefore 100% gone; Sr-89 is at half its original quantity, but Sr-90 is still 99.99% there. As there are hundreds of different isotopes created, the initial high radiation fades quickly, but never fades out completely.