The smaller atoms produced by nuclear fission are called fission products. Fission products may be of a wide variety of elements. Common fission products include xenon, krypton, iodine, cesium, and strontium.

Most fission products are highly radioactive and will undergo radioactive decay. Most decay quickly and will be gone within several days. Some, however, remain in the nuclear fuel for many years, and must be contained to prevent release into the environment.