Uranium-232

Uranium-232 (****) is an isotope of uranium. It has a half-life of 68.9 years and is a side product in the thorium cycle. It has been cited as an obstacle to nuclear proliferation using 233U as the fissile material, because the intense gamma radiation emitted by 208Tl (a daughter of 232U, produced relatively quickly) makes the 233U contaminated with it more difficult to handle.

Production of 233U (through the neutron irradiation of 232Th) invariably produces small amounts of 232U as an impurity, because of parasitic (n,2n) reactions on uranium-233 itself, or on protactinium-233, or on thorium-232:

:232Th (n,γ) 233Th (β−) 233Pa (β−) 233U (n,2n) 232U :232Th (n,γ) 233Th (β−) 233Pa (n,2n) 232Pa (β−) 232U :232Th (n,2n) 231Th (β−) 231Pa (n,γ) 232Pa (β−) 232U

Another channel involves neutron capture reaction on small amounts of thorium-230, which is a tiny fraction of natural thorium present due to the decay of uranium-238:

:230Th (n,γ) 231Th (β−) 231Pa (n,γ) 232Pa (β−) 232U

The decay chain of 232U quickly yields strong gamma radiation emitters:

:232U (α, 68.9 years) :228Th (α, 1.9125 years) (after this, the decay chain is identical to that of 232Th; thorium-232 is nevertheless much less dangerous because its much longer half-life, 14 billion years or 200 million times that of uranium-232, means the build-up of daughters is that much less for equal mass) :224Ra (α, 3.632 days) :220Rn (α, 55.6 s) :216Po (α, 0.144 s) :212Pb (β−, 10.627 h) :212Bi (α, 60.55 min, 0.78 MeV), with 35.94% branching ratio to :208Tl (β−, 3.053 min), 99.75% chance to emit 2.6 MeV gamma ray :208Pb (stable)

This makes manual handling in a glove box with only light shielding (as commonly done with plutonium) too hazardous, except in a period short compared to the Th-228 half-life just after chemical separation of the uranium, and instead requiring remote manipulation for fuel fabrication.

Unusually for an isotope with even mass number, 232U has a significant neutron absorption cross section for fission (thermal neutrons , resonance integral ) as well as for neutron capture (thermal , resonance integral ). This makes it a fissile isotope, though using it alone in a reactor or bomb is not reasonable.

neptunium-232 (β+) protactinium-232 (β−)