Uranium-232

Uranium-232 (²³²
U
) is an isotope of uranium. It has a half-life of around 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.

Uranium-232, ²³²U

General
Symbol	^232U
Names	uranium-232, U-232
Protons	92
Neutrons	140
Nuclide data
Half-life	68.9 years
Parent isotopes	^236Pu (α) ^232Np (β^+) ^232Pa (β^−)
Decay products	^228Th
Isotopes of uranium Complete table of nuclides

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).

References

1. Griffin, H. C. Natural Radioactive Decay Chains, Chapter 13 of Handbook of Nuclear Chemistry, Second Edition, Springer 2011, ISBN 978-1-4419-0719-6