Introduction to Reconsidering Nuclear Power
With growing numbers of people in the United States purchasing computers, televisions, appliances, and many other electricity-gobbling devices, the demand for electric power has been surging. Between 1991 and 2000, Americans' electricity usage jumped more than 30 percent. Meanwhile, power plants became extremely difficult and expensive to build, requiring exhaustive and time-consuming scientific studies to determine how the plant would affect the surrounding environment. As a result, relatively few new plants were constructed.
In the summer of 2000, residents of California experienced a severe electricity shortage caused by a complex combination of factors, including a shortage of power-generating capability, skyrocketing prices for the natural gas used to fuel power plants, and exploding demand for electricity. By March 2001, the shortage became so serious that the state's largest utility companies called for rolling blackouts—controlled, successive power outages in designated areas—to avoid even more widespread power outages. Although investigators learned in May 2002 that the crisis was caused in large part by questionable, and perhaps illegal, business practices by the Enron Corporation, an energy company in Houston, it still pointed up weaknesses in the power-generation system in the United States.
The crisis in California exposed the need for greater power-production capabilities throughout the country, forcing energy officials to begin an urgent effort to increase generating capacity. As they sought solutions to the problem, many energy experts said it was time to reconsider nuclear energy as an alternative to fossil fuels (oil, coal, and natural gas).
One of the big advantages of nuclear power plants is that they do not generate greenhouse gases—gases, such as carbon dioxide, that slow the radiation of heat away from the Earth, thereby raising the planet's surface temperature, and possibly contributing to global warming. Power plants that burn fossil fuels produce such gases in abundance, and they also contribute to air pollution. Although nuclear energy has its drawbacks, including the production of radioactive waste and the potential for dangerous accidents, it has been a part of the U.S. energy mix since the late 1950's. In 2002, more than 100 nuclear reactors generated about 20 percent of the nation's energy, second only to coal-fired plants. Plants that burn coal or other fossil fuels produce about 70 percent of U.S. electricity. Of the remaining 10 percent, nearly all is produced by hydroelectric power, electricity generated at dams. Various other energy sources, including windmills, solar-energy, geothermal energy (Earth's internal heat), and waste-burning collectively generate about 1 percent.
In May 2001, U.S. President George W. Bush unveiled his national energy policy, which called for an expanded role for nuclear energy. One of the reasons for the reviving interest in nuclear energy is that it is a very cost-effective way to generate electricity. Nuclear reactions release millions of times more energy than the chemical reactions involved in the burning of fossil fuels. For example, a piece of uranium fuel weighing just 0.04 ounce (1 gram) can release as much energy as the burning of about 2.3 metric tons (2.5 tons) of coal. Moreover, the cost of nuclear power is relatively stable while the prices of fossil fuels are always in flux. To make nuclear energy an even more attractive source of energy, nuclear engineers have been working to develop simpler, safer nuclear power plants that are easier to maintain than the plants operating in 2002.
Splitting Atoms to Produce Electricity
In most nuclear power plants, the uranium fuel is compressed into small pellets, which are stacked inside long, thin metal tubes called fuel rods about 3.7 meters (12 feet) long. Between 64 and 280 fuel rods are then bundled together into groups called fuel assemblies. The fuel assemblies are arranged inside the core (central part) of the reactor. The core is encased in a sturdy container called the reactor vessel, which has steel walls about 13 to 20 centimeters (5 to 8 inches) thick. The vessel, in turn, is housed within a containment building, a reinforced concrete structure designed to prevent the escape of radioactivity in case of an accident.
Nuclear power plants generate energy through a process called nuclear fission, which involves splitting the nucleus (central part) of a large, heavy atom, such as uranium. When a neutron (nuclear particle with no electric charge) hits a uranium nucleus, it acts much like a billiard ball hitting a group of other balls, causing the nucleus to split into two roughly equal parts. The fissioning of one uranium atom releases some energy plus two or more neutrons, which can then go on to split other uranium atoms. When sufficient numbers of uranium atoms are close enough to one another, a chain reaction occurs. In a chain reaction, neutrons freed by fissioned uranium nuclei go on to split additional uranium nuclei, releasing more energy and more neutrons until the reaction is self-sustaining (continues with no outside assistance). In order to generate electricity, a nuclear reactor must sustain a controlled chain reaction.
Nuclear reactors are outfitted with control rods, safety devices made of neutron-absorbing materials such as boron carbide. When the rods are inserted into the reactor core to absorb neutrons, fewer neutrons are then available to initiate fission reactions. This action slows or stops the self-sustaining reaction and prevents the core from overheating.
Nuclear reactors contain a liquid or a gas, called a coolant, that performs the critical function of absorbing and carrying away the heat that is released by the fission reactions. In most reactors, the coolant is water, which surrounds the reactor core and becomes heated by the fission reactions. The heated water is continually piped out of the core to produce steam that is used to drive large, heavy wheels called turbines. The turbines are hooked up to electric generators, which spin to generate electricity.
Types of Reactors
The two most common types of reactors are boiling-water reactors and pressurized-water reactors. These reactors are often called "light-water reactors" to distinguish them from reactors that use heavy water (deuterium oxide, a special type of water in which the hydrogen nuclei contain a neutron as well as a proton) as the coolant.
In a boiling-water reactor, the water surrounding the reactor core boils, creating steam directly in the reactor vessel. This slightly radioactive steam is sent through pipes to a turbine, which turns electric generators, creating electricity. The steam coming out of the turbine then cools and condenses into liquid water, which is returned to the reactor vessel to be used again.
In a pressurized-water reactor, which is the most common type of nuclear reactor, the water is under extremely high pressure, about 160 kilograms per square centimeter (2,000 pounds per square inch). That pressure enables the water to be heated to about 325 degrees C (620 degrees F), well above its normal boiling temperature of 100 degrees C (212 degrees F). The pressurized and slightly radioactive water from the reactor goes to a steam generator that contains a large number of small tubes through which the hot, pressurized water flows. Water from a separate system flows over these tubes to absorb this heat and begins to boil, creating steam that is used to turn a turbine and generate electricity.
Once the steam passes through the turbines, it is cooled and condenses back to water. Some nuclear power plants use tall structures called cooling towers to release the heat of condensation into the air. Many plants that burn fossil fuels also have cooling towers. Many people mistakenly think that the clouds of water vapor emitted by cooling towers are radioactive. In a nuclear power plant, the water that is used to condense the steam never comes into contact with the reactor core, and so it is not radioactive.
In 2002, there were more than 400 nuclear reactors in operation worldwide. Most of these reactors were conventional light-water reactors, and some were of a type known as a breeder reactor. A breeder reactor produces another fissionable material, plutonium, as it operates. The plutonium can then be used as fuel for other nuclear reactors. Breeder reactors are often cooled with liquid metals such as sodium.
Early Research In Nuclear Fission
While fossil fuels have been used to generate electricity since the late 1800's, nuclear energy is a more recent development. The main discovery that prompted scientists to investigate nuclear power was the discovery of the neutron in 1932. This led to many experiments by Enrico Fermi, an Italian-born physicist, and many others. These experiments led to the discovery of nuclear fission. Then, in December 1942, a group of researchers led by Fermi created the first controlled, self-sustaining fission reaction as part of the U.S. effort to develop an atomic bomb. Fermi and his colleagues assembled blocks of graphite and embedded spheres of uranium into what they called an “atomic pile" at the University of Chicago.
The first well-known use of atomic power was the atomic bomb, which exploits the power released during a very fast fission reaction to release tremendous amounts of energy. The United States dropped two atomic bombs on Japan to end World War II (1939-1945). After the war, however, scientists made rapid progress in developing peaceful uses for nuclear energy, including in nuclear power plants.
Nuclear Power Goes Commercial
The world's first nuclear reactor, the Experimental Breeder Reactor I, went into operation in the United States on Dec. 20, 1951. In 1957, the Shippingport Atomic Power Station near Pittsburgh, Pennsylvania, began operating under the control of the U.S. Department of Energy. The Shippingport station was a small demonstration plant that supplied electricity to Pittsburgh. The first large-scale commercial reactor in the United States was the Yankee atomic power plant in Rowe, Massachusetts, which entered service in 1960. The electrical output of all power plants is measured in megawatts (Mw). One megawatt, or 1 million watts, is enough electricity to power up to 1,000 homes.
The development of nuclear power plants has progressed in four stages, or generations. Generation I plants, such as the Shippingport and Yankee stations, typically generated from 185 to 600 Mw. Generation II plants, built mostly in the 1980's, were larger and generated at least 1,150 Mw, enough electricity to power about 1 million homes. Generation III plants were developed as part of a cooperative effort between government agencies and private industry to improve safety and economy in the nuclear industry. As of 2002, the only Generation III plants in existence were all in Japan. Beyond Generation III, Generation IV plants were being designed to be even more safe and economical than traditional nuclear power plants.
Despite the fact that nuclear energy provided a large amount of electricity without creating air pollution or generating greenhouse gases, the nuclear-power industry began falling into disfavor in the late 1970's as public confidence in it waned. People became increasingly concerned about two issues: nuclear waste disposal and the possibility of reactor accidents.
The Problem of Radioactive Waste
There are two main types of radioactive waste—high-level and low-level. High-level waste includes used fuel assemblies that have been removed from the reactor core and waste from facilities that reuse nuclear fuel. Low-level waste is composed of items that are only slightly radioactive, such as items that are used to filter the coolant water in the reactor.
Over time, fuel assemblies in reactors lose most of their ability to produce heat. The remaining fuel in them, however, can continue to emit dangerous radiation for as long as 10,000 years. This poses a serious environmental problem, because it is difficult to guarantee that a burial site for such waste will not experience a major earthquake, flood, or other natural disaster over thousands of years. The U.S. nuclear power industry creates about 2,000 metric tons (2,200 tons) of high-level radioactive waste each year and, as of 2002, there were about 44,000 metric tons (48,000 tons) of it at various sites around the United States. Most of the waste was being held in specially designed storage pools at each nuclear plant.
To address the problem of radioactive waste, the U.S. Congress in 1987 selected Yucca Mountain, a natural ridge about 145 kilometers (90 miles) northwest of Las Vegas, Nevada, as a possible permanent storage site for high-level waste. The Department of Energy and several other laboratories and federal agencies spent more than 10 years studying Yucca Mountain to determine whether it would be a safe place to store the radioactive waste and concluded that it was. Yucca Mountain was chosen as the site for permanent storage of high-level nuclear waste because it is in a very dry environment. In addition to these advantages, the Yucca Mountain site is on government-owned land where nuclear tests have been conducted for decades, making the land unsuitable for other purposes. Although many Nevada residents and environmental groups opposed the plan, arguing that the site is not as dry or as geologically stable as most scientists claim, President Bush announced his support of it in February 2002. The site was scheduled to begin accepting waste by 2010 if all the appropriate approvals were obtained.
The second category of waste, low-level waste, consists of contaminated tools, rags, and clothing used in reactor operations, medicine, or research. It emits only a small amount of radioactivity, and that usually fades after a few months or tens of years. Still, by law, such waste must be disposed of in a way that will keep it isolated from the environment for 100 to 500 years, depending on its level of radioactivity. This waste is routinely disposed of in shallow burial sites in several locations around the country. In 2002, there were commercial low-level waste disposal sites in Barnwell, South Carolina; Richland, Washington; and Clive, Utah.
The Specter of Nuclear Accidents
Although waste disposal was a major concern, most people were also concerned about the possibility of reactor accidents. Despite the stringent safety procedures in place at nuclear plants, serious accidents can happen.
U.S. public confidence in nuclear power hit a low point on March 28, 1979, when the worst nuclear accident in the United States occurred at the Three Mile Island nuclear plant near Harrisburg, Pennsylvania. A safety system failed, causing the coolant level in the reactor to drop and preventing the reactor core from being adequately cooled. The reactor then overheated, and the fuel rods melted. Although the containment structure prevented the escape of radioactive material into the environment, engineers at the plant later vented some radioactive gases that were building up within the structure. No one was killed or injured in the accident, but more than 100,000 people had to seek emergency shelter for several days. News coverage of the event frightened the public, and people began to have serious doubts about nuclear energy.
After the accident at Three Mile Island, nuclear power officials began to reassess the safety of nuclear plants and temporarily suspended the granting of licenses for new nuclear facilities. Industry officials and the Nuclear Regulatory Commission (NRC), the government agency charged with regulating nuclear power, instituted a program to improve the monitoring of nuclear plants. They also upgraded nuclear plant emergency plans. Still, the accident at Three Mile Island cast a shadow on the future of nuclear energy. In fact, as of 2002, no new nuclear power plants had been ordered in the United States since the accident, though many that were under construction had been completed with modifications ordered by the government.
Public fears about nuclear power were magnified in April 1986, when the worst nuclear disaster in history occurred at the Chernobyl complex in the Ukraine, then part of the Soviet Union. Operators of one of the complex's four reactors, which were of a type known as RBMK's (short for the Russian words “graphite-moderated, boiling water cooled, channel-type”), were running tests that Soviet officials later said were unauthorized. With the reactor running at low power, operators raised the control rods to speed up the reaction. The operators apparently pulled the control rods out too far, causing the reactor to heat up extremely rapidly. The heat caused a steam explosion that blew off the concrete lid of the reactor. The explosion resulted in a meltdown—the destruction of a reactor caused by the heat of the fission reactions—causing a fire in the plant that burned for several days. A huge cloud of radioactive dust and gas escaped into the air because, unlike nuclear reactors in the United States, the Chernobyl reactor was not enclosed by a containment structure. There are no RBMK reactors in the United States. Most of them are located in republics of the former Soviet Union.
Thirty-one plant workers and firefighters were reportedly killed in the accident and more than 200 others were sickened by radiation. The immense cloud of radioactive material spread over much of Europe. No one was injured immediately after the radioactive cloud was released, but radioactivity contaminated the soil in a 30-kilometer (19-mile) radius around Chernobyl. Radiation emitted during the accident was also blamed for health problems experienced later by residents living near the plant, including abnormally high rates of thyroid cancer, especially among children. Because RBMK reactors operated only in the former Soviet Union, the accident at Chernobyl did not have a significant impact on reactor technologies.
Looking Toward Next-generation Reactors
Although, as of 2002, only Generation I and II plants had been built in the United States, the more advanced Generation III plants had been certified for construction by the Nuclear Regulatory Commission, the government agency charged with regulating the nuclear power industry. Generation I and II reactors were almost exclusively boiling-water and pressurized-water reactors. Generation III designs were also mainly of these types, employing advanced reactor technologies. Designs for Generation IV nuclear plants focused on more innovative technologies that experts said would require further development.
One of the main weaknesses of the Generation I and II plants has been their safety systems, which usually include complex “active” systems to pump water into the reactor core in case of an accident. These systems depend on many pumps and valves that have to operate perfectly during an accident. These systems typically have backup safety systems to assure their safety, but such safeguards are expensive, and they raised the cost of building nuclear plants. In the late 1980's, government and nuclear industry engineers began working together to develop Generation III nuclear reactors, which have simpler emergency systems and are easier to operate. Because the accidents at Three Mile Island and Chernobyl were caused mainly by human error, Generation III plants were being designed to minimize the possibility of human error.
A Generation III Plant: the AP-600
One of the more innovative Generation III designs, the AP-600, was developed by Westinghouse Electric Company of Pittsburgh. The AP-600 produces 600 megawatts of electricity, about half as much as most commercial reactors operating in 2002. It is also much simpler than most existing reactors, using passive safety systems that are easier to operate. For example, the AP-600 features large tanks of water in containers directly above the reactor vessel. If the core began to overheat, the water would be released directly onto the core to cool it. This gravity-fed design avoids the need for the complex arrangements of pipes and valves required in most existing reactors. The AP-600 uses much less piping and electric cable and fewer valves and pumps than most Generation II reactors. In addition, because its design is based on standard modules (building components that are the same for every plant), the AP-600 would be much easier and less expensive to build. Although no AP-600 or other Generation III plant had been sold in the United States as of 2002, a number of U.S. utility companies were considering buying them.
Planned Features of Generation IV Reactors
Generation IV plants, which many experts believe will be in operation after about 2020, are significant departures from traditional nuclear plants. Some of these more advanced concepts involve very high temperatures, requiring the use of helium gas or liquid metals as coolants. The main emphasis of Generation IV technologies is to employ “natural” safety features, such as coating fuel pellets with silicon carbide, that would not permit the reactor core to melt, regardless of the circumstances. These plants would also make use of advanced construction techniques that would make them less expensive to build and, therefore, economically competitive with fossil fuel plants. Generation IV plants are also expected to set new standards for safety, efficiency, long operating life, and reduction of radioactive waste.
One promising Generation IV design is the fast-spectrum reactor, which would use neutrons of a higher energy than those used in light-water reactors. Fast-spectrum reactors would use gases such as carbon dioxide or helium or liquid metal, such as sodium, lead, or lead bismuth, as a coolant, since water would slow down the fast neutrons. Fast-spectrum reactors would be so efficient that they could actually use radioactive waste from other reactors as fuel, therefore eliminating one of the major drawbacks of nuclear energy. In addition, these reactors, like breeder reactors, could create plutonium fuel that could be used in other nuclear plants. Fast-spectrum plants, however, would be expensive to build and operate because of the challenges of dealing with toxic liquid heavy metals in a safe manner. However, their ability to reduce waste and produce a virtually inexhaustible fuel supply might make them very attractive alternatives.
"Pebble-bed" Reactors
One of the most radical Generation IV designs is called the high-temperature-gas pebble-bed modular reactor, or simply “pebble-bed.” The pebble-bed was designed in Germany and was being developed in 2002 in South Africa and the United States. The reactor takes a novel approach to both the fuel and the coolant it uses. Instead of long fuel rods, a pebble-bed design calls for tiny beads of uranium about the size of poppy seeds encased in several layers of graphite and silicon carbide to slow the neutrons from the reactions. About 10,000 of these tiny particles would be packed into a tennis-ball-sized sphere coated with graphite. Some 330,000 of these spheres would be packed together inside a metal reactor vessel surrounded by blocks of graphite to slow down the neutrons so that they could more efficiently create a sustained fission reaction.
Pebble-bed reactors would use helium gas as a coolant, enabling the plants to operate at temperatures of about 900 degrees C (1,650 degrees F). Helium of that temperature could drive a turbine much more efficiently than steam. The gas would be pumped through the reactor core, where it would absorb the heat generated by the fission reactions occurring inside the pebbles. One major advantage of the pebble-bed design is that the small amount of power produced by each pebble and the large volume of heat-absorbing graphite would make it impossible for the reactor to reach temperatures hot enough to melt the core. In addition, in the case of a severe accident at a pebble-bed reactor, very little radioactive material would be released into the environment. This essentially eliminates the need for using emergency core-cooling systems.
Nuclear power advocates maintain that nuclear energy offers an enormous potential for large-scale electricity production without polluting the atmosphere. Most of the nuclear plants operating in 2002, they say, have performed exceptionally well during nearly 40 years of operation, and engineers have learned a great deal about how to build even safer plants. Advanced new designs for nuclear plants should provide further assurances to the public that nuclear power can be a safe, reliable, and environmentally friendly method of generating electricity. While there are still many problems to solve, the future of nuclear energy has never been brighter.
Terms and concepts
- Control rod: A rod made of a neutron-absorbing material that is inserted in the core of a nuclear reactor to slow down fission reactions and prevent overheating.
- Fast-spectrum reactor: A type of nuclear reactor that exploits high-energy neutrons, rather than the low-energy neutrons used in traditional reactors, and usually uses liquid metal as a coolant.
- Fuel rod: A long, thin metal container that holds the uranium fuel pellets used to generate heat in nuclear reactors.
- Megawatt: A unit of measurement used to represent the output of power plants. A megawatt is 1 million watts, or enough power to supply electricity to about 1,000 homes.
- Neutron: An electrically neutral particle in the nucleus (central part) of an atom.
- Nuclear fission: A type of nuclear reaction in which a neutron strikes the nucleus of a heavy atom, such as uranium, and splits it into two parts, releasing energy and additional neutrons.
- Pebble-bed reactor: A type of nuclear reactor that generates fission reactions within tennis-ball-sized spheres packed with tiny grains of uranium encased in graphite and silicon to produce heat that is used to generate electricity.
- Radioactive waste: A by-product of nuclear power plants that emits radiation and is harmful to living things.
- Reactor core: The heart of a nuclear reactor, where the fuel rods generate the heat that is used to create electricity.