Uranium-232

Uranium-232 (²³²
U) 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:^[3]

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 75 barns (b), resonance integral 380 b) as well as for neutron capture (thermal 73 b, resonance integral 280 b). This makes it a fissile isotope, though using it alone in a reactor or bomb is not reasonable.