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.

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