However, Carson Mark, former chief of nuclear-weapons design at Los Alamos, estimated that Iraq could have built a nuclear bomb using about 12.3 kilograms of HEU, or half the official SQ.12 Thomas Cochran, senior scientist at the Natural Resources Defense Council (NRDC), stated that "a low-technology bomb in the 10- to 20-kiloton range would require about 10 kilograms of highly enriched uranium."13
A research team at the University of California (Santa Cruz) found that three kilograms, in fact, would be sufficient to produce a nuclear bomb. By means of computer modeling of a simple fission weapon design, they found a nuclear yield equivalent to more than 100 tons of high explosives could be achieved with only one kilogram of HEU and "a yield half that of the Hiroshima bomb" with five kilograms.14 The authors concluded that "one can make an atomic weapon with much less nuclear material than generally thought," and they proposed "that the use of HEU should be eliminated in applications in order to prevent the spread of small-scale atomic weapons."15
That tracks with a technical article I read quite a while ago which put it at "about 22 lbs".