Uranium from natural sources is enriched by isotope separation, and plutonium is produced in a suitable nuclear reactor.
The critical mass for any material is the smallest amount needed for a sustained nuclear chain reaction.
Bare-sphere critical masses at normal density of some actinides are listed in the accompanying table.
[20] While U-233 would thus seem ideal for weaponization, a significant obstacle to that goal is the co-production of trace amounts of uranium-232 due to side-reactions.
U-232 hazards, a result of its highly radioactive decay products such as thallium-208, are significant even at 5 parts per million.
Implosion nuclear weapons require U-232 levels below 50 PPM (above which the U-233 is considered "low grade"; cf.
Pu-239 is produced artificially in nuclear reactors when a neutron is absorbed by U-238, forming U-239, which then decays in a rapid two-step process into Pu-239.
Power stations such as the obsolete British Magnox and French UNGG reactors, which were designed to produce either electricity or weapons material, were operated at low power levels with frequent fuel changes using online refuelling to produce weapons-grade plutonium.
[26] Weapons made with reactor-grade plutonium would require special cooling to keep them in storage and ready for use.
The content of Pu-239 in material used for the 1962 test was not disclosed, but has been inferred to have been at least 85%, much higher than typical spent fuel from currently operating reactors.
If the period of irradiation has been sufficiently short, this spent fuel could be reprocessed to produce weapons grade plutonium.