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The nuclei of some isotopes
(different types of atoms of the same element) of heavy elements such as uranium and plutonium can split when they absorb a neutron. This splitting is known as nuclear fission. Nuclear fission releases a great amount of energy in the form of heat and radiation. It also expels neutrons that can be absorbed by other nuclei, which may then fission and give off more energy and neutrons, and so on. This process is known as a fission chain reaction.
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Nuclear fission diagram |
Unlike in nuclear explosions, where the chain reaction happens at an extremely rapid and uncontrolled pace, in nuclear reactors, the fission chain reaction takes place slowly and in a controlled manner. The resulting energy can, therefore, be used in a nuclear power reactor to generate electricity or in a nuclear research reactor to create radioactive materials needed for medicine or industry. Isotopes capable of fission are called
fissionable isotopes. The most common fissionable isotope is uranium-238 (U-238). Large quantities of U-238 exist in nature, but U-238 does not fission very easily. For this reason, while U-238 can be used to fuel some types of nuclear reactors, it cannot be used to create a nuclear explosion. Other fissionable isotopes, such as uranium-235 (U-235) or plutonium-239 (Pu-239), fission much more easily. These isotopes are called
fissile
isotopes.
Because they fission easily, fissile isotopes are very useful for producing energy in nuclear reactors; however, they can also be used to create a nuclear explosion. All fissile isotopes are extremely rare, and most do not exist in nature in appreciable amounts. (Natural uranium consists of 99 percent U-238 but less than 0.7 percent U-235, while Pu-239 is not found in nature.) Highly complex industrial processes are needed to increase the concentrations of U-235 and to produce and separate Pu-239, activities that probably are beyond the capabilities of terrorist organizations.
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