Fission Products
2 min read
When a 235U nucleus absorbs a neutron and undergoes fission, it usually splits asymmetrically into two lighter nuclei known as fission fragments. One fragment typically has a mass number around 90—100 and the other around 130—145. Fission can produce many different combinations of fragments, resulting in hundreds of different radioactive fission-product nuclides. Important fission products include isotopes of iodine, cesium, strontium, xenon, krypton, barium, zirconium and many other elements. Most primary fission fragments are radioactive and undergo a series of beta decays towards more stable nuclei.
Fission product yield distributions as a function of mass number for thermal-neutron-induced fission of 235U, 239Pu and 233U.
Free Downloads › Images
238U can capture a neutron to form 239U, which undergoes two successive beta decays to form fissile 239Pu. Some of this plutonium subsequently undergoes fission in the reactor and contributes significantly to energy production, producing another range of radioactive fission products.
A tonne of spent light water reactor fuel typically contains around 30—40 kg of fission products, depending mainly on the fuel burnup.
Neutron capture also produces heavier isotopes and transuranic elements in reactor fuel. Successive neutron captures and radioactive decays can form isotopes of plutonium, neptunium, americium and curium. For example, neutron capture can ultimately produce 241Pu, which undergoes beta decay to 241Am.
Schematic comparison of the composition of fresh and spent nuclear fuel.
Free Downloads › Images
After several years in a reactor, nuclear fuel contains a complex mixture of uranium, plutonium, other actinides and numerous fission products. The exact composition of spent fuel depends on its initial enrichment, burnup and irradiation history. Typical spent light water reactor fuel consists of about 95—96% uranium, around 1% plutonium, approximately 3—4% fission products and a smaller fraction of minor actinides such as neptunium, americium and curium.
Fission and the subsequent radioactive decay of fission products produce hundreds of different radionuclides, many of which have very short half-lives.
