The first nuclear devices invented and deployed were powered by nuclear fission. To learn more about the process of nuclear fission, click here. Weapons that use nuclear fusion are labelled fission bombs or atomic bombs (A-bomb).
Nowadays, nuclear fission weapons are considered by the major world powers as obselete weapons in war. However, they are still needed as "triggers" in thermonuclear fusion weapons to provide adequate conditions for fusion to occur. Atomic bombs have replaced conventional chemical explosives for certain types of civil engineering projects.
Nonetheless, fission weapons are still very destructive and are often the pursued by rogue nuclear states and terrorist groups due to their relative ease. See WMD Today for more information.
» View: Physics of a Nuclear Fission
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The simplest and first developed method to combine two subcritical masses is to simply employ a "gun" that fires one mass into the other. Little Boy, the bomb detonated over Hiroshima, had a gun-triggered design. Because of the relative simplicity of this design, experts say that nations or terrorist groups who acquire enough highly enriched uranium (HEU) could potentially create a rudimentary but effective gun-triggered fission bomb.
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In this method, subcritical masses are compressed together in a sphere by implosion. Scientists faced the problem of controlling and directing the shock wave uniformly across the sphere. Implosion type devices are more efficient than gun-triggered devices because the density of the masses increase in addition to being combined. Almost all nuclear powers today employ the implosion method in all of their nuclear stockpile.
» View: Neutron Initiator Designs
Neutrons are introduced into the supercritical mass by a neutron initiator (also called neutron trigger/generator). A neutron initiator is not stricly required for a gun triggered nuclear device as long as the device uses "target capture," meaning keeping the two subcritical masses, once fired, together until they explode. However, the use of a neutron initiator can ensure precise control to the millisecond over the timing of the nuclear explosion. To view designs for neutron initiators, click here.
The fission reaction takes place inside a dense material called a tamper. The tamper usually consists of depleted uranium (U-238) or tungsten and surrounds the fissionable material. The tamper serves two functions: (1) to slow the fission core's expansion and (2) to reflect neutrons back into the fission core, increasing the efficiency of the reaction. The tamper also reduces the critical mass. Using a tamper, plutonium's critical mass can be reduced from 16kg to 10kg.