In some ways, electricity generated using nuclear energy is no different than other forms of thermoelectric power (those that use heat to produce electricity). Just like power plants that run on oil or coal, nuclear plants use heat to boil water and produce steam, which is then used to generate electricity. The difference, really, is in where that heat comes from. Coal and oil plants create it by burning fossil fuels. In the thermonuclear production of electricity, heat is produced through a controlled nuclear reaction—usually a nuclear fission reaction.

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Nuclear fission reactions are those that result in the splitting of an atom. Specifically, these reactions involve a special type of atom known as a radioactive isotope. Some elements can exist in two or more forms; each form has the same number of protons and electrons but a different number of neutrons and, hence, a different atomic mass; these different versions of the atom are called isotopes. INFOGRAPHIC 22.1

Most isotopes are stable, meaning they do not spontaneously lose protons or neutrons. But some are radioactive: They emit subatomic particles and heat energy (radiation) in a process known as radioactive decay. Radioactive decay is measured in half-lives. An isotope’s radioactive half-life is the amount of time it takes for half of the radioactive material in question to decay to a new form. So after one half-life, 50% of the material will decay; in the next half-life, 50% of what’s left (or 25% of the original amount) will then decay. After 10 half-lives, just 0.1% of the original radioactive material is left. INFOGRAPHIC 22.2

Most nuclear reactors use uranium, which has several isotopes. Uranium-238 (U-238) is the most stable and is the most abundant form of uranium; it makes up roughly 99% of Earth’s total supply. But it’s U-235, the most reactive form, that is mined from Earth, processed into nuclear fuel (which, like all other mining work, involves both safety and environmental hazards), and packed into the fuel rods that are used in facilities like Fukushima. Estimates for supplies of uranium range from 80 to 200 years, but the worry that we would run out of nuclear fuel is less of an issue than it is with fossil fuels; other isotopes can also be used. INFOGRAPHIC 22.3

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A nuclear fission chain reaction begins when U-235 in the fuel rods is deliberately bombarded with neutrons. This bombardment makes the uranium nucleus unstable, causing it to split into a variety of two or more smaller atoms and releasing two or three additional neutrons in the process. These newly released neutrons then hit other U-235 atoms, causing them to split and release even more neutrons, and so on.

Unlike the type of nuclear reaction at work in a nuclear bomb, which uses much more radioactive material, setting off a massive chain reaction that is almost instantaneous, the reactions at nuclear power plants are highly controlled. Control rods—made of materials such as boron or graphite that absorb neutrons—are placed in the fuel rod assembly between the fuel rods to control the speed of the reaction. They can be added (to slow down or stop the reaction) or removed (to make it go faster). INFOGRAPHIC 22.4

Even controlled, this chain reaction releases a tremendous amount of heat—10 million times more than would be released by burning a comparable amount of coal or oil. The heat is used to boil water, which produces steam, which turns turbines that create electricity.

All types of thermoelectric power take a lot of water; that’s why power plants are sited near rivers and oceans. But at the moment, nuclear power requires the most—on average, around 2,500 liters per megawatt hour (MWh), compared with 1,900 liters per MWh for coal and 600 liters per MWh for natural gas. (A MWh is the production of 1 megawatt— 1 million watts—over an hour’s time.) That means a typical 1,000-MW nuclear reactor requires 2,500,000 liters of water per minute to flow through the cooling system. Some reactors use much more. For example, each of the two 845-MW reactors at the Calvert Cliffs nuclear power plant in Maryland requires 4,500,000 liters (that’s 1,200,000 gallons) of water per minute during operation. Though most of this water (more than 95%) is returned to the source (river or ocean), there are problems with the release of warmer-than-normal water back into the environment, as well as damage to aquatic life that gets trapped in or against intake filters.

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The reason for all that water is simple: With nuclear energy, water is needed not only to produce steam but also to keep spent fuel rods cool and to prevent the reactor from overheating. (Remember that heat is produced from radioactive decay of fission by-products, and it continues even after the reactor is shut down and fission stops; therefore, spent fuel needs constant cooling.) Without water to cool them, fuel rods can melt, releasing large amounts of radioactivity; the fuel rod metal casing can also get hot enough to react with steam in a way that produces highly explosive hydrogen gas.