Soon after the development of nuclear weapons in the middle of the 20th century, we found ways to transform this new, potent source of energy into electricity. Nuclear energy is released when the bonds holding the protons and neutrons making up the nucleus of an atom break, a process called nuclear fission (Figure 9.15). Nuclear energy is also released when the nuclei of two atoms fuse together under high temperature and pressure, a process called nuclear fusion. Nuclear fusion provides the fuel for stars, including our own Sun, and is the basis for one type of atomic weapon.

Nuclear bonds hold much more energy than chemical bonds. Therefore, the amount of energy released during nuclear fission is much greater than that released during the combustion of fossil fuels. For example, the energy content of a gram of uranium-235, the most common nuclear fuel, is approximately 3 million times the energy content of a gram of coal.

Uranium, the fuel used in today’s nuclear power plants, is a nonrenewable resource. Uranium ore is mined from the ground and processed to produce a powdery, concentrated form known as yellowcake. The isotope used for fission, uranium-235 (U-235), makes up only 0.71% of the uranium in nature. Through a process known as enrichment, uranium-235 is separated from less valuable uranium-238. The highly radioactive enriched uranium can be used as nuclear fuel, whereas the depleted uranium is typically placed in storage.

Approximately 97% of the known reserves of uranium have been found in just 15 countries (Figure 9.16). The largest known reserves, nearly one-third of the world total, occur in Australia. The United States, with 100 commercial reactors, is the largest producer of nuclear power in the world, and nuclear energy accounts for 19% of the electricity produced in the country.

In contrast to nuclear fission, fusion doesn’t require uranium or other rare, radioactive materials, but rather a seemingly limitless fuel: hydrogen, which can be extracted from water (Figure 9.15). Presently, there are no commercial fusion reactors for power generation, but we will discuss its potential in the Solutions section of this chapter.

In nuclear power plants, the heat energy released during the fission of uranium can be harnessed to generate electricity. This heat produces steam that drives a turbine connected to an electrical generator (Figure 9.17), similar to what we saw in the coal-fired power plant. However, using nuclear energy safely requires a number of special precautions.

Secure operation of a nuclear power station depends on controlling the speed of neutrons released during nuclear fission. Uranium-235 is formed into small pellets that are contained in tubes called fuel rods. In the most commonly used nuclear reactors, the fuel rods are submerged in pressurized water, which acts as a moderator by reducing the speed at which neutrons travel (see Figure 9.17). By slowing down the neutrons released during fission, the moderator increases their chance of triggering fission of other uranium-235 atoms, which releases more energy and yet more neutrons in a chain reaction.

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The increasing number of fission reactions is an example of positive feedback (see Chapter 2, page 48). If uncontrolled, a chain reaction could generate so much heat that it would lead to a meltdown of a nuclear reactor. The chain reaction in nuclear reactors, and thus the amount of heat generated, must therefore be regulated using control rods, which are made of substances that absorb neutrons. Because they absorb neutrons, control rods can be inserted into a reactor core to slow the rate of nuclear fission or to shut down a reactor completely, either in an emergency or for repairs.

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The pressurized water circulating through the reactor is heated by the energy released during fission reactions. This heat produces the steam, which spins the turbine attached to a generator. Cooling water circulated through the turbine chamber cools the steam, condensing it back to liquid water. This water is returned to the steam generator, where it is again converted to steam. Notice that the three streams of water in the nuclear power plant—pressurized water in the reactor, water in the steam chamber, and cooling water in the condenser—do not mix. For safety, the nuclear reactor and steam generator are enclosed in a containment structure with steel and concrete walls generally 1 to 2 meters (3 to 6 feet) thick.