What a coincidence. I am reading The York Times (Dec 11, 1996, page A16)
and find an article on plutonium. In this article they refer to "the 14.2
tons of Rocky Falls plutonium, enough to make about 3000 bombs". This makes
9 lb per bomb. If this is true then my 20 kg guess was too much. But who
knows what it really is and how it depends on the isotopic purity of Pu-239
(and on the technical know how). In any case 500 metric tons of bomb-grade
material, worldwide, is frightening.
Ludwik Kowalski