P.2 Fat Man
In contrast to Little Boy, Fat Man was a spherical-geometry implosion weapon
wherein a subcritical core of
239 Pu was squeezed to critical density by explosives
configured to crush tamper shells which surrounded the core. Figure P.5 shows a
cross-sectional diagram of Fat Man. At the very center of the core was an 0.8-inch
diameter polonium-beryllium neutron initiator; this is discussed in Sect. 4.5.
Working outward from the center, the initiator was surrounded by the
239
Pu core
proper, of diameter of 3.62 inches. This was surrounded by a shell of depleted
uranium (DU; essentially pure
238
U) of outer diameter 8.75 inches, which was
surrounded by an aluminum shell of inner diameter 9 inches and outside diameter
18.5 inches; the two tamper shells were separated by a thin layer of plastic. Outside
the aluminum shell were the explosive charges which created the implosion.
Dimensions of these core components are summarized in Table P.2. Photographs
of the Trinity test implosion bomb and the Nagasaki Fat Man combat version can
be found in Sect. 4.2.
Table P.2 Data on core/tamper dimensions of Fat Man
Component
Inner
radius (in)
Outer
radius (in)
Mass
(kg)
239
Pu core
0.4
1.81
6.3
DU tamper shell
1.81
4.375
101
Al tamper shell
4.5
9.25
130
Fig. P.4 Little Boy test units. Little Boy was 126 inches long, 28 inches in diameter, and weighed
9,700 pounds when fully assembled. Photo courtesy Alan Carr, Los Alamos National Laboratory
xvi
Preamble