Fission and fusion are both natural atomic processes that release incredible amounts of energy, but in many ways they are opposites. When splitting, a single, mostly heavy atomic nucleus is split, while during fusion, two or more light atoms have to be combined.
Atoms are made up of protons and neutrons bound together in a central nucleus. Radioactive elements like uranium can contain dozens of these particles in their atomic hearts.
Fission occurs when heavy elements such as uranium spontaneously decay, causing their nuclei to be split. Each of the resulting halves has slightly less mass than the original nucleus, and the missing mass is converted into energy.
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Physicists Lise Meitner and Otto Frisch discovered the fundamentals of nuclear fission after receiving a private letter from nuclear chemist Otto Hahn in December 1938. Hahn’s experiments showed that uranium atoms bombarded with neutrons would split, and Meitner and Frisch used the new science of quantum mechanics to explain why this happened.
All three scientists soon realized the dire effects of their discovery, which took place in the shadow of World War II. A single fission could release a relatively small amount of energy, but many concurrent fission reactions had the potential to be quite destructive if used to develop something like an atomic bomb.
Nuclear fission for energy and weapons
When a uranium atom naturally splits, it releases a neutron that swirls around. When this neutron hits other nearby uranium atoms, they too split, creating a cascading chain reaction. In 1951, according to the US Department of Energy, engineers built the first power plant that used the process of nuclear fission to generate energy.
This process is carefully controlled in a nuclear power plant. During the cracking process, heat is released that brings water to a boil and generates steam that drives a turbine.
But in an atomic bomb, the cascading chain reaction gets out of control, with the splitting happening faster and faster. This releases an enormous amount of energy in a short time, which creates the devastating explosion of the bomb.
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Why fusion doesn’t generate energy yet
The plasma core of the International Thermonuclear Experimental Reactor is half finished. This is the tokamak complex, which when completed will house plasma that is ten times hotter than the sun. (Image credit: ITER)
Fusion as a human energy source, on the other hand, is not yet fully developed. In nuclear fusion, two nuclei of a light element such as hydrogen must overcome their natural electromagnetic repulsion and fuse into a single, heavier nucleus.
The resulting structure is slightly less massive than the two original nuclei, and just like nuclear fission, this missing mass is converted into energy. Generating enough energy to knock atoms together until they stick together is not easy, however, and generally requires the extreme environment of a star belly.
Engineers have long dreamed of carrying out sustainable fusion reactions here on earth. Fusion energy would generate less nuclear waste than nuclear fission and use relatively common light elements such as hydrogen – and not less frequently uranium – as fuel, according to the International Atomic Energy Agency.
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But creating and sustaining fusion is difficult. As part of the International Thermonuclear Experimental Reactor (ITER), an international experiment to test the feasibility of using sustained nuclear fusion for power generation has built a magnet as high as a four-story building and 280,000 times stronger than the Earth’s magnetic field.
But ITER, a scientific partnership of 35 countries, has suffered numerous delays during its construction and is not expected to produce more electricity than it consumes until the 2030s.
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