Uranium-233

Uranium-233, ²³³U

An ampoule containing solidified pieces of a FLiBe and uranium-233 tetrafluoride mixture
General
Symbol	233U
Names	uranium-233, 233U, U-233
Protons (Z)	92
Neutrons (N)	141
Nuclide data
Half-life (t1/2)	160,000 years[1]
Isotope mass	233.039 Da
Parent isotopes	237Pu (α) 233Np (β+) 233Pa (β−)
Decay products	229Th
Isotopes of uranium Complete table of nuclides

Uranium-233 (²³³U or U-233) is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel.^[2] It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years.

Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of only 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur, to maintain the neutron economy (if it misses the ²³³U window, the next fissile target is ²³⁵U, meaning a total of 4 neutrons needed to trigger fission).

²³³U usually fissions on neutron absorption, but sometimes retains the neutron, becoming uranium-234. For both thermal neutrons and fast neutrons, the capture-to-fission ratio of uranium-233 is smaller than those of the other two major fissile fuels, uranium-235 and plutonium-239.^[3]