Uranium-232

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Uranium-232 (²³²
U
) is an isotope of uranium. It has a half-life of around 69 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:^[1]

²³²U (α, 68.9 years)

²²⁸Th (α, 1.9 year)

²²⁴Ra (α, 3.6 day, 0.24 MeV) (from this point onwards, the decay chain is identical to that of ²³²Th; thorium-232 is nevertheless much less dangerous because its extremely long half-life of about 14-15 billion years means that not as much of its dangerous daughters builds up)

²²⁰Rn (α, 55 s, 0.54 MeV)

²¹⁶Po (α, 0.15 s)

²¹²Pb (β⁻, 10.64 h)

²¹²Bi (α, 61 min, 0.78 MeV)

²⁰⁸Tl (β⁻, 3 min, 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 its decay products) 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).