Fission and fusion are different types of nuclear reactions in which energy is released from the high-powered bonds between particles in the atomic nucleus. The atomic nucleus is most stable when binding energies between particles are strongest. This occurs with iron and nickel. For lighter atomic nuclei, energy can be extracted by combining these nuclei together, a process known as nuclear fusion. For nuclei heavier than those of iron or nickel, energy can be extracted by splitting them apart in a process called nuclear fission.
Because the binding force in the atomic nucleus contains enormous energy, nuclear reactions can provide tons of power, in principle. Practical considerations make the exploitation of nuclear power more difficult than something as simple as starting a fire, however. For fission, highly purified feedstock, usually uranium or plutonium isotopes, must be used. Isotopes are favored because their instability makes them easier to break apart. The purification of these isotopes is extremely expensive and requires multimillion-dollar centrifuges.
In fusion, an extremely high threshold energy must be reached to combine atomic nuclei, and the temperature required is in the millions of degrees. In nature, the only place where this occurs is in the core of a star. Superheated plasma and the focusing of laser power are two methods to achieve this threshold energy. Because the matter that serves as the medium of fusion must be so hot, it must be isolated from surrounding matter using powerful magnetic fields or inertial containment, which is the principle behind the Tokamak reactor. Still, fusion requires so much energy that no one has yet built a reactor that produces more than it consumes.
The downsides to fission power include both radioactive byproducts and its association with nuclear weapons and meltdowns. In the last decade or so, nuclear physicists have developed safer ways of building reactors, including methods for recycling the radioactive byproducts. These advances have caused the US government to begin advocating the construction of nuclear reactors again.
By Michael Anissimov