In physics, nuclear transmutation is the conversion of one chemical element or an isotope into another. In nuclear engineering, also nuclear reactors cause artificial transmutation by exposing elements to neutrons produced by fission. In nuclear reactors the most important phenomenon is the transmutation of fertile isotopes (e.g. uranium 238) into fissile isotopes (e.g. plutonium 239). The process of the transmutation of fertile materials to fissile materials is referred to as fuel breeding.
See also: Nuclear Breeding
Fertile materials are not capable of undergoing fission reaction after absorbing thermal (slow or low energy) neutrons and these materials are not capable of sustaining a nuclear fission chain reaction. There are two basic fertile materials: 238U and 232Th.
232Th is the predominant isotope of natural thorium. If this fertile material is loaded in the nuclear reactor, the nuclei of 232Th absorb a neutron and become nuclei of 233Th. The half-life of 233Th is approximately 21.8 minutes. 233Th decays (negative beta decay) to 233Pa (protactinium), whose half-life is 26.97 days. 233Pa decays (negative beta decay) to 233U, that is very good fissile material. On the other hand proposed reactor designs must attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.
Neutron capture may also be used to create fissile 239Pu from 238U, which is the dominant constituent of naturally occurring uranium (99.28%). Absorption of a neutron in the 238U nucleus yields 239U. The half-life of 239U is approximately 23.5 minutes. 239U decays (negative beta decay) to 239Np (neptunium), whose half-life is 2.36 days. 239Np decays (negative beta decay) to 239Pu.
Transmutation of transuranium elements (i.e. the chemical elements with atomic numbers greater than 92 ) such as the isotopes of plutonium has the potential to help solve some problems posed by the management of radioactive waste by reducing the proportion of long-lived isotopes it contains.
Scheme of the production of californium-252 from uranium-238 by neutron irradiation.