Thomas B. Cochran, "Highly enriched uranium production for South African nuclear weapons," Science & Global Security, 4, no. 2, (1994): 161-176.
We estimate that South Africa produced 735 ± 53 kilograms of the equivalent of 90 percent highly enriched uranium (HEU). This amount, were it enriched to 80 to 90 percent, could be used to construct 12 Hiroshima-type fission bombs. The South African government maintains it constructed only six such devices, and never intended to construct more than seven. The excess HEU was apparently less enriched than that desired by South Africa for its weapons, but probably still weapons-usable. Implosion-type devices were apparently being researched at the time the nuclear weapons program was dismantled in 1989. Had this effort continued, eventually South Africa would have been able to construct four times as many weapons from the same amount of fissile material. Because of a 15.6 percent uncertainty in the tails assay, the two standard deviation uncertainty in the amount of U-235 in the HEU produced is 256 kilograms. It is in the interest of all parties to reduce this uncertainty.
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