fissionproduct

A fission product is a nuclide produced by the fission of a heavy nucleus, typically in a nuclear reactor or a nuclear weapon. When a heavy nucleus such as uranium-235 or plutonium-239 absorbs a neutron and fissions, it splits into two lighter fragments, known as fission fragments, along with one or more prompt neutrons. The fragments cover a broad range of mass numbers, roughly from about 60 to 160, and the exact products depend on the fissioning isotope and the neutron energy.

Most fission products are radioactive and decay through beta emission, often accompanied by gamma radiation. Their half-lives span from milliseconds to years, and they decay in chains to stable or long-lived nuclides. A subset of fission products releases delayed neutrons during decay, which play a critical role in the control of nuclear reactors.

Fission products are produced with varying yields per fission, with individual nuclides typically formed at fractions of a percent to a few percent per fission. Although any one nuclide may be short-lived, the aggregate inventory contributes to decay heat and radiological dose after reactor shutdown or fuel discharge. Noble-gas products, such as krypton-85 and xenon-133, can diffuse from fuel assemblies.

Common and notable fission products include strontium-90, cesium-137, iodine-131, ruthenium-106, and technetium-99m (a daughter product of molybdenum-99) used in medical imaging. The relative importance of different products depends on applications and safety considerations, such as reactor operation, waste management, and environmental monitoring.

Because fission products govern decay heat and radiological consequences of fission events, they are central to reactor safety, spent-fuel storage, and nuclear accident analyses. They are also used in medicine, industry, and environmental studies as tracers and sources of gamma or beta radiation.