Uranium 232 is not naturally-occurring isotope of uranium. It is a man-made isotope and is a side product in the thorium fuel cycle and also this isotope is a decay product of 236Pu in the uranium fuel. 232U is produced from 235U and 232Th via two of the reaction chains shown below. The formation of this isotope in both reactions results from specific (n,2n) reactions in which an incoming neutron knocks two neutrons out of a target nucleus. 232U can also be produced by two successive single radiative captures of neutron starting with naturally-occurring isotope 230Th. 230Th is a decay product of 234U, which is in turn a decay product of 238U.
It is unusual, but 232U is a fissile isotope and it is therefore capable of undergoing fission reaction after absorbing thermal neutron. This feature plays no significant role in nuclear reactors, because the amount of 232U is negligible in terms of sustaining a nuclear fission chain reaction. 232U has a significant fission cross-section (75 barns for thermal neutrons) as well as radiative capture cross-section (73 barns for thermal neutrons). Therefore 232U also belongs to the group of fertile isotopes. Radiative capture of a neutron leads to the formation of fissile 233U.
The isotope 232U has another very important feature. 232U has a relatively short half-life of 68.9 years, and therefore the specific activity of 232U is much higher than specific activity of the isotope 238U. In addition the decay chain of 232U produces very penetrating gamma rays. The most important gamma emitter, accounting for about 85 percent of the total dose from 232U after 2 years, is thallium 208, that emits gamma rays of 2.6 MeV which are very energetic and highly penetrating. These intense radiations make handling of fissile 233U or reprocessed uranium contaminated with 232U far more dangerous than conventional fuels.
Reference: Kang, J.; Von Hippel, F. N. (2001). “U‐232 and the proliferation‐resistance of U‐233 in spent fuel”. Science & Global Security
See also: Neutron Cross-section
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