Uranium 232

Uranium-232 (232
92U
140, 232
U
, U-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 of ²³²U's decay products (i.e, ²⁰⁸Tl) 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, 1.9 year) ²²⁴Ra (α, 3.6 day, 0.24 MeV) (at this point, the decay chain is identical to that of ²³²Th) ²²⁰Rn (α, 55 s, 0.54 MeV) ²¹⁶Po (α, 0.15 s) ²¹²Pb (β−, 10.64 h) ²¹²Bi (α, 61 m, 0.78 MeV) ²⁰⁸Tl (β−, 3 m, 2.6 MeV) (35.94% branching ratio) ²⁰⁸Pb (stable)

This makes manual handling in a glove box with only light shielding (as commonly done with plutonium) too hazardous, (except possibly in a short period immediately following chemical separation of the uranium from thorium-228, radium-224, radon-220, and polonium) 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 75 barns (b), resonance integral 380 b) as well as for neutron capture (thermal 73 b, resonance integral 280 b).