Uncontrolled Nuclear Fission: The A-Bomb
From Thermal-FluidsPedia
Uncontrolled Nuclear Fission: The A-Bomb As we discussed before, the difference between a nuclear explosion and a nuclear reactor is in the ability to control the chain reaction in the fission process. Unlike a nuclear reactor, where the rate of neutron production must be carefully controlled, there is no such effort in a nuclear bomb.
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Critical Mass
A small mass of pure fissile material, such as uranium-235 or plutonium-239, would not sustain a chain reaction. Too many neutrons leak out through the large empty volume surrounding nuclei. A large enough mass of these materials is needed to ensure that enough neutrons are generated to compensate for the loss through the void. This mass is dependent upon the size, shape, and purity of the isotopes being used.
The minimum amount of fissile material (of a given shape) required for maintaining a chain reaction is known as the critical mass. In an atomic bomb, a mass of fissile material greater than the critical mass must be assembled and held together for about a millionth of a second to permit the chain reaction to propagate and the bomb to explode. Plutonium has a very small critical mass, whereas uranium must be enriched substantially before it can serve as a weapons-grade material. Naturally occurring uranium has only 0.7% of the fissile uranium-235 isotope and must be enriched to a minimum 90% before it qualifies as weapons-grade. For this reason, plutonium is favored over highly enriched uranium (HEU). Building weapons with HEU is, however, easier, as no reprocessing facility to separate plutonium from spent reactor fuel is needed. The critical mass necessary to construct a bomb using Pu-239 is only the size of a baseball. Construction of a plutonium bomb is more difficult, however, as a byproduct of plutonium processing operation, Plutonium-240 is highly unstable and cause predetonation which reduces the effectiveness of the bomb.
It is far easier for countries to develop secret weapons using enriched uranium fuel; the necessary facilities can be disguised as ordinary chemical plants, and they do not produce signatures that can be readily identified. Plutonium, on the other hand, requires waste-reprocessing facilities that, unlike the uranium enrichment plants, cannot be easily hidden.
Weapon Design
To make detonation possible, the mass of the fissile material must become critical. This is accomplished by bringing two or more subcritical masses together. The simplest mechanism for assembling a supercritical mass is to shoot one piece of the material against another in a gun tube. A heavy material, called a tamper, surrounds the fissile mass. The tamper (also called neutron reflector) reduces the number of neutrons that can escape and prevents the bomb’s premature explosion (Figure 1a). An alternative method (called implosion method) is to detonate a conventional explosive that surrounds the fissionable material. The rapid increase in pressure compresses the fissile mass, preventing neutrons from escaping through the void and reducing the critical mass necessary to initiate the fission reaction (Figure 1b).The implosion device is very difficult to build, as the implosion has to be highly symmetrical. The Little Boy was a gun-type; the Fat Man was of the implosive design.
In addition to the A-bomb, which is a result of fission reactions, radioactive materials can be used in conventional bombs and warheads. Two examples are dirty bombs and depleted uranium warheads.
Dirty Bombs
The term “dirty bomb” usually refers to any device that generates a significant amount of radioactive waste without actually undergoing a fission reaction. These may include conventional weapons which, upon explosion, spread radioactive, biological, or toxic materials and may be delivered as an aerosol or simply by wrapping the materials around dynamite. These weapons do not require weapons-grade materials and even the relatively common materials used in radiological medical equipment would be enough to cause catastrophic results and an extensive loss of life.
Depleted Uranium
Depleted uranium (DU) is the byproduct of the reprocessing of spent nuclear fuel that has been enriched for use in nuclear reactors or weapons-- essentially pure uranium-238. Much like natural uranium, depleted uranium is both toxic and radioactive and, because of its long half-life (4.5 billion years), it is practically indestructible. DU is 1.7 times denser than lead and, when turned into metal and used to make shells, it penetrates heavy steel and concrete with relative ease. The search for more effective weapons has led to the development of armor-piercing shells made of depleted uranium. These weapons were used for the first time in the 1991 Gulf War after the Iraqi invasion of Kuwait. Since then, they have been used extensively in other regional conflicts in areas including the Balkans (1994, 1999), Afghanistan (2001), and again in Iraq (2003). The use of this material in ammunitions has remained highly controversial (Figure 2) (1). When a DU round hits its target, as much as 70 percent of the projectile burns on impact and forms an aerosol of radioactive particles. The uranium dust can then be spread by the wind and may become part of the food chain. Studies by the United Nations have concluded that DU is at least partially responsible for the increase in the rate of cancer among Iraqi children and in the number of spontaneous abortions among pregnant Iraqi women (2). DU may even be linked to the Gulf War Syndrome that is affecting US veterans of the 1991 Gulf War (3, 4).
References
(1) Catalinotto ,J. Metal of Dishonor-Depleted Uranium : How the Pentagon Radiates Soldiers & Civilians with DU Weapons, 2nd. Ed., Independent Publishers Group, Chicago, 1995.
(2) Fifty-sixth Genral Assembly, Uniteed Nation Press Release GA/SHC/3639, 2001.
(3) Peterson, S., “Remains of toxic bullets litter Iraq,” Christian Science Monitor, May 15, 2003.
(4) Hastings, D., “Is an Armament Sickening U.S. Soldiers?,” Associated Press, August 12, 2006.
(5) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005