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Nuclear Fission
Nuclear fission occurs when a heavy nucleus splits into more stable, medium-mass nuclei. It releases an extreme amount of energy during the process. Fission can occur spontaneously or when nuclei are bombarded by neutrons.
Fission releases a very large amount of energy; in fact, the atomic bombs used by the US to end World War II were powered by nuclear fission. Nuclear fission is also the main type of nuclear reaction used to produce nuclear power.
See a larger version of this image of nuclear fission here.
During the nuclear fission process, as the nuclei are bombarded by neutrons, they release more neutrons, creating a chain reaction. A chain reaction is a series of repeated fission reactions caused by the release of additional neutrons in each reaction. The material that starts the chain reaction is also one of the products and can start another reaction.
See a larger version of this image of a chain reaction here.
Nuclear Fusion
Nuclear fusion occurs when low-mass nuclei combine to form a heavier, more stable nucleus. Fusion reactions release more energy than fission reactions. In order for fusion to occur, the nuclei must have enough kinetic energy and get close enough to overcome the natural repulsion of the overall positive charges. As the temperature of the particles increase, the kinetic energy also increases. Once the temperatures reach in the millions of degrees Celsius range, the nuclei will have great enough kinetic energy for nuclear fusion to occur
See a larger version of this image of nuclear fusion here.
This nuclear fusion occurs in the cores of stars, and, since the Sun is a star, nuclear fusion occurs in the Sun as well.
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