How Nuclear Power Plants Work

What is Nuclear Energy?

Although nuclear power plants are the second largest source of America's electricity after coal, their operation is often a puzzle. Some people have misconceptions about these plants. Some worry that a nuclear power plant may explode. Others fear that the electricity might be radioactive. And some think that these plants produce power through nuclear explosions. None of this is true.

In fact, power plants that generate electricity from nuclear energy are not so very different from other kinds of power plants. At all power plants (except hydroelectric plants), high-pressure steam "blows" the propeller-like blades of a turbine, which spins the shaft of a huge generator. Inside the generator, coils of wire and magnetic fields interact to create electricity.

The heat needed to boil water into steam is produced in one of two ways: by burning coal, oil or gas - the fossil fuels - in a furnace or by splitting certain atoms of uranium in a nuclear power plant.

Nothing is burned or exploded in a nuclear power plant. Rather, the uranium fuel - many tons of it - generates heat through a process called fission. The uranium is formed into ceramic pellets about the size of the end of your finger. These pellets are inserted into long, vertical tubes within the reactor.

As certain atoms in these pellets are struck by atomic particles, they split - or fission - to release particles of their own. These particles - called neutrons - strike other atoms, splitting them. This sequence of one fission triggering others, and those triggering still more, is called a chain reaction. When the atoms split, they also release heat.

The nuclear reaction is controlled by rods inserted among the tubes holding the fuel. These control rods are made of a material that absorbs neutrons and prevents them from hitting atoms which can fission. So the nuclear reaction can be speeded up or slowed down by varying the number of control rods withdrawn and how much they are withdrawn.

About nuclear power plants

Commercial nuclear power plants in the U.S. are pressurized water reactors or boiling water reactors. Both types operate on basically the same principles. Both are cooled by ordinary water.

The coolant - water - is the main link in the chain that converts fission energy to electrical energy.

In boiling water reactors, made by General Electric Company, the water boils to steam directly in the reactor vessel. In pressurized water reactors, made by Babcock & Wilcox Company, Combustion Engineering, Inc., and Westinghouse Electric Corporation, the reactor water is pressurized to prevent it from boiling. Instead, the hot water is pumped to a steam generator, where its heat is transferred to a separate supply of water. This water boils into steam.

At both types of plants, the steam is used to make electricity just as it is at a power plant burning fossil fuel - by spinning the turbine that drives an electric generator. The end product - electricity - is the same.

Both reactor types use essentially the same fuel - a solid material containing two, kinds, or isotopes, of uranium atoms. One isotope - U-235 - makes up less than one percent of natural uranium but fissions readily. The other isotope - U-238 - makes up most of natural uranium but is practically non-fissionable.

Through a process known as "enrichment," the concentration of U-235 in the uranium is increased to three to four percent. Among other benefits, enrichment allows the reactor to be smaller than it would be if fueled with natural uranium.

The concentration of U-235 is so low that a bomb-like nuclear explosion is impossible.

Built-in safety

Several natural features help ensure the nuclear plant's safe operation. The uranium fuel is formed into ceramic pellets, which resist the effects of high temperature and corrosion during operation. Also the concentration of fissionable U-235 in the fuel pellets is very low. Because the U-235 is diluted with other, non-fissionable, materials, the chain reaction tends to slow down as it gets hotter - another natural safety feature.

The pellets are stacked end-to-end in 12-foot long tubes made of a special metal. They are made of zirconium alloy, which resists heat, radiation and corrosion. The tubes are precisely arranged as assemblies within the reactor, with spaces between them for the control rods. Water flowing up through the assemblies removes the heat of the chain reaction.

The water also serves as a "moderator." It slows down the atomic particles (neutrons) and increases the probability of fission. Rather than "clobbering" the nucleus, the neutron must combine with it momentarily. If the neutrons are moving too quickly, fission is unlikely to occur.

The "moderating" effect of the water adds another safety feature. Any loss of water slows the chain reaction.

Replacing and storing fuel

Certain changes take place in the fuel pellets during their time in the nuclear power plant. In addition to causing the fissioning of the U-235 fuel, some neutrons strike the U-238 and turn it into plutonium, another fissionable element. Some of the plutonium itself undergoes fission, adding to the production of heat in the reactor.

Most of the fragments of fission-the pieces left over after the atom has split-are radioactive. During the life of the fuel, these radioactive fragments collect within the pellets. The fuel remains in the reactor for three to four years before these trapped fission fragments begin to reduce the efficiency of the chain reaction - like ashes smothering a fire. Between one-fourth and one-third of the fuel rods are replaced every 12-18 months.

The fuel removed from a nuclear power plant is stored underwater at the power plant site in large concrete vaults lined with stainless steel. Eventually, this used fuel will be transported to a federal government facility for disposal deep underground.

US Council on Energy Awareness (8/93)

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