Nuclear Weapons design
Alternative methods for creating a
supercritical mass in a nuclear weapon.
In the technically less sophisticated "gun-type" method used
in the Hiroshima bomb (left), a subcritical projectile
of HEU is propelled towards a subcritical target of
HEU. This assembly process is relatively slow. For
plutonium, the faster "implosion" method used
in the Nagasaki bomb is required. This involves
compression of a mass of fissile material. Much less
material is needed for the implosion method because
the fissile material is compressed beyond its
normal metallic density. For an increase in density
by a factor of two, the critical mass is reduced to one
quarter of its normal-density value.
From IPFM, Global Fissile Material Report 2011
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