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Even after acquiring fissile material—itself a very difficult task—terrorists hoping to possess a nuclear explosive would still face a considerable challenge in constructing a workable nuclear device.
There are essentially two designs for fission weapons, the "gun-type" design and the implosion design. Each has distinct advantages and disadvantages that are discussed in the following section. Both designs are within the technical reach of most states, and some technically advanced terrorist groups may also have the ability to build crude or improvised versions of a gun-type nuclear explosive device. Yields would vary according to the amount of fissile material used and the sophistication of the design, which, in turn, would depend on the scientific and technical capabilities of the bomb manufacturing team assembled by the terrorist organization. In any case, the explosive power of a nuclear weapon designed and built by terrorists would probably be measured in the tens of kilotons at most.
It is important to realize that there is a great difference between military nuclear weapons, most likely beyond the capability of terrorists to manufacture, and the crude nuclear explosives that terrorists might be able to produce. Military weapons require precise, predictable explosive yields and very reliable delivery systems. In contrast, terrorist nuclear bombs need only produce an uncertain yield of a few kilotons and could be delivered by vehicles as common as trucks or cargo ships.
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Hiroshima bomb diagram
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The simplest and easiest type of nuclear weapon to design and manufacture is a "gun-type" fission weapon, because it does not require sophisticated explosive or electronic components. Gun-type weapons relying on uranium must use uranium that contains a high proportion of U-235. This category of uranium is known as highly enriched uranium (HEU), defined as uranium enriched to more than 20 percent U-235. Most weapons use uranium enriched to more than 80 percent U-235. Uranium enriched to 90 percent and above is considered "weapons-grade." A gun-type weapon uses chemical explosives to "shoot" one
subcritical mass of HEU into another at high speed, much as a bullet is shot from a gun. When the two masses collide, they form a
supercritical mass, which produces a nuclear explosion.
Gun-type designs need more fissile material than implosion devices, but the quantity required is still quite small in absolute terms. The exact amount of HEU depends on the level of enrichment of the uranium used in the weapon, the explosive yield desired, and the technical sophistication of the bomb design. The greater the enrichment, the larger the proportion of U-235, and therefore the less HEU needed to ignite the explosive chain reaction.
It is impossible to achieve a large nuclear explosion by using plutonium in a gun-type device. Nonetheless, a plutonium gun-type bomb could release as much energy as a few tons of TNT, which could conceivably cause many casualties. Moreover, this kind of bomb would release large amounts of plutonium and other radioactive materials, thereby making it a potent radiation dispersal device (RDD), or "dirty bomb."
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Nagasaki bomb diagram
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An implosion-type fission weapon is more sophisticated than a gun-type design. An implosion weapon uses a complex arrangement of explosives to rapidly compress one or more spheres of fissile material into a critical mass. Implosion-type weapons are more difficult to design and build than gun-type weapons, because they often require advanced explosive components and sophisticated fusing systems.
The first nuclear weapon exploded, at the "Trinity" test near Alamogordo, New Mexico on July 16, 1945, was an implosion-type weapon. The "Fat Man" bomb dropped on Nagasaki on August 9, 1945, was also an implosion weapon, with an explosive power of about 21 kt. (In contrast, the United States considered the gun-type design of the Hiroshima bomb so reliable that
it did not require testing before the weapon was used.)
Implosion weapons can use either plutonium or HEU to create nuclear explosions with yields in excess of 10 kt. Also, they typically require much less fissile material than gun-type weapons, because they use the fissile material available in the core more efficiently. According to the
International Atomic Energy Agency (IAEA), a first-generation implosion-type weapon would require about 25 kg (55 lbs) of HEU or 8 kg (18 lbs) of plutonium, but more sophisticated, advanced designs could require less fissile material. Other sources estimate lower thresholds of 12-15 kg of HEU and 4 kg of plutonium. Even the first-generation Nagasaki implosion bomb used only 6 kg (13.6 lbs) of plutonium. This means that implosion weapons can be relatively small and light in weight. Such smaller weapons would be highly attractive targets for terrorists seeking nuclear weapons.
Some experts question whether a terrorist organization, even if it obtained the necessary fissile material, could construct an implosion device because of its inherent complexity. At the very least, the task would be considerably more difficult than building a gun-type device. For this reason, it is far more likely that a terrorist organization could succeed in constructing a nuclear explosive having a 10 kt yield from weapons-grade uranium than from plutonium. As HEU is likely to be far more attractive to terrorists seeking fissile material than plutonium, it should be the focus of the most immediate protection efforts.
As noted earlier, advanced types of nuclear weapons include boosted fission and thermonuclear weapons. Because boosted fission and thermonuclear weapons are much more difficult to design and manufacture than fission weapons, it is extremely unlikely that terrorists would be able to build them without active assistance from a nuclear-weapon state.
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