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Whilst the commonly cited figure at Wikipedia is 64 Kg (141 lbs), the actual figure is 56 kilograms of 80% enriched Uranium 235, as cited in "The Last Mission, The Secret History of World War Two's Final Mission," published 2002 by Jim Smith and Malcolm McConnell giving the story of B-29 missions in the final days of WW2.

In contrast with:

http://en.wikipedia.org/wiki/Little_boy

The Quantity used was to ensure achieving natural critical mass, which falls in a range from 52 kilograms to 64 kilograms.

Various factors affect what determines natural critical mass such as:

Warhead temperature (cooler the better)

Density of fissile material

Geometry of the Warhead (spherical is smallest mass)

Isotope Purity

Neutron Flux & Uniformity

Use of Reflectors (ie containment of stray neutrons)

Of the 56 kilogram warhead of Little Boy which exploded over Hiroshima, only 780 grams or so exploded at the very heart of the Uranium sphere, or about 1-2% of the mass. That is because Neutron flux only attained criticality at the very heart of the sphere and not at the extremities where the Uranium 235 was wasted. When you increase efficiency of the Neutron Flux you require less fissile material. This is the key to shrinking a nuclear warhead.

In terms of the Hiroshima bomb however, 640 kilograms of un-enriched Uranium Oxide needed to be enriched to 80% 235U for a single weapon.

Uranium 235 is almost never used for nuclear weapons now. Pakistan still uses Uranium for it's warheads. Almost everywhere else now Plutonium is favoured for it's fail safe features.

Plutonium is harvested in a nuclear reactor by the radioactive bombardment of fuel which is enriched to 20% Uranium 235. The remaining Uranium 238 is converted to Neptunium 239 and then decays to Plutonium 239. The Plutonium is harvested by chemical separation from nuclear waste.

Only about 40 kilograms of un-enriched Uranium Oxide is required to obtain enough Plutonium via a heavy water nuclear reactor for a Plutonium weapon.

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