154
4 Complicating Factors
Murrah Federal Building in Oklahoma City in 1995 (Bernstein 2008). Thus, while an
efficient Trinity-like terrorist weapon based on purloined fuel rods is highly unlikely,
the issue of fissile-material security will remain a pressing one for years to come.
Figure 4.2 shows a photograph of the Trinity test device; Fig. 4.3 shows the
Nagasaki Fat Man bomb; the bulbous casing enclosed the implosion assembly within
and provided stable flight characteristics after release from its bomber.
To close this section, we ask: “How can one obtain an implosion?” After all,
explosions are normally seen to be outwardly-directed phenomena. In the Manhattan
Project, this was achieved by using an assembly of implosion lenses. The fundamental
idea is sketched in Fig. 4.4, which shows a single lens in cross-section; to extend the
idea to three dimensions, imagine a somewhat pyramidal-shaped block that would
fit comfortably on your lap. The block comprises two explosive castings that fit
together very precisely. The outer casting is of a fast-burning explosive known as
“Composition B” (or just Comp B), while the inner, lens-shaped one is a slowerburning material known as Baratol, a mixture of barium nitrate and TNT. A detonator
at the outer edge of the Comp B initiates an outward-expanding detonation wave.
When the detonation wave hits the Baratol, it too begins exploding. If the interface
between the two is of just the right shape, the two waves can be arranged to combine
as they progress along the interface in such a way as to create an inwardly-directed
converging wave in the Baratol; the dashed lines in Fig. 4.4 illustrate the right-toleft progression of the detonation. As sketched in Fig. P.5 of the Preamble, 32 such
“binary explosive” assemblies were fitted together to create an imploding sphere
inside the Trinity and Fat Man devices. Within the Baratol lenses resided another
spherical assembly of 32 blocks of Comp B, which are detonated by the Baratol to
achieve a high-speed symmetric crushing of tamper spheres that lay within them. A
very readable personal reminiscence of casting and machining implosion lenses was
published by Hull and Bianco (2005); for a more technical history, see Hoddeson
et al. (1993).
Fig. 4.2 The Trinity device
atop its test tower on July 15,
1945. Norris Bradbury
(1909–1997), who served as
Director of the Los Alamos
Laboratory from 1945 to
1970, stands to the right. The
spherical shape of this
implosion device is clearly
visible; the cables feeding
from the box halfway up the
device go to the
implosion-lens detonators
discussed in the text. Photo
courtesy Alan Carr, Los
Alamos National Laboratory
4 Complicating Factors
Murrah Federal Building in Oklahoma City in 1995 (Bernstein 2008). Thus, while an
efficient Trinity-like terrorist weapon based on purloined fuel rods is highly unlikely,
the issue of fissile-material security will remain a pressing one for years to come.
Figure 4.2 shows a photograph of the Trinity test device; Fig. 4.3 shows the
Nagasaki Fat Man bomb; the bulbous casing enclosed the implosion assembly within
and provided stable flight characteristics after release from its bomber.
To close this section, we ask: “How can one obtain an implosion?” After all,
explosions are normally seen to be outwardly-directed phenomena. In the Manhattan
Project, this was achieved by using an assembly of implosion lenses. The fundamental
idea is sketched in Fig. 4.4, which shows a single lens in cross-section; to extend the
idea to three dimensions, imagine a somewhat pyramidal-shaped block that would
fit comfortably on your lap. The block comprises two explosive castings that fit
together very precisely. The outer casting is of a fast-burning explosive known as
“Composition B” (or just Comp B), while the inner, lens-shaped one is a slowerburning material known as Baratol, a mixture of barium nitrate and TNT. A detonator
at the outer edge of the Comp B initiates an outward-expanding detonation wave.
When the detonation wave hits the Baratol, it too begins exploding. If the interface
between the two is of just the right shape, the two waves can be arranged to combine
as they progress along the interface in such a way as to create an inwardly-directed
converging wave in the Baratol; the dashed lines in Fig. 4.4 illustrate the right-toleft progression of the detonation. As sketched in Fig. P.5 of the Preamble, 32 such
“binary explosive” assemblies were fitted together to create an imploding sphere
inside the Trinity and Fat Man devices. Within the Baratol lenses resided another
spherical assembly of 32 blocks of Comp B, which are detonated by the Baratol to
achieve a high-speed symmetric crushing of tamper spheres that lay within them. A
very readable personal reminiscence of casting and machining implosion lenses was
published by Hull and Bianco (2005); for a more technical history, see Hoddeson
et al. (1993).
Fig. 4.2 The Trinity device
atop its test tower on July 15,
1945. Norris Bradbury
(1909–1997), who served as
Director of the Los Alamos
Laboratory from 1945 to
1970, stands to the right. The
spherical shape of this
implosion device is clearly
visible; the cables feeding
from the box halfway up the
device go to the
implosion-lens detonators
discussed in the text. Photo
courtesy Alan Carr, Los
Alamos National Laboratory