Nuclear Reactors: Design And Operation

Type of physical science: Nuclear physics

Field of study: Nuclear reactors

Nuclear reactors were developed during World War II for military applications; in 2011, they generated about 12.3 percent of the world's supply of electricity. Reactors emit no acid rain or greenhouse gases, but concern about safety and radioactive waste continues to cause controversy.

Overview

Nuclear reactors are used for four basic purposes: to generate electricity, to provide power for ship propulsion, to produce radioactive isotopes for medical or industrial applications, and to produce plutonium for military weapons. Although the military uses came first, civilian applications are now the major focus of reactor technology.

All reactors obtain their energy from a process called fission, in which a heavy nucleus, usually uranium, splits into two or more fragments. The fission reaction was discovered in 1938 by German scientists Otto Hahn and Lise Meitner, who had been performing experiments in which they bombarded various elements with neutrons in order to produce radioactive materials. Bombarding copper, for example, produced a radioactive form of copper. When they bombarded uranium with neutrons, however, an entirely different reaction took place: the uranium nucleus broke into two pieces, and a large amount of energy was released.

The fission process can be visualized by using a simple analogy. Imagine that the uranium nucleus is like a drop of liquid. When a neutron enters the nucleus, the drop begins to vibrate. If the vibration is violent enough, the drop can break into two pieces. Hahn and Meitner called this process fission because it is similar to the biological process of cell division, in which the cell nucleus splits in two.

In uranium fission, a small amount of energy emitted by the neutron can trigger a very large energy release. With the imminent threat of war in 1939, a number of scientists began to consider the possibility that a new and very powerful "atomic" bomb could be built. They thought that perhaps uranium could also be harnessed to replace coal or oil as a fuel for industrial power plants.

In order to release a large amount of energy, many millions of uranium nuclei must split apart. This can be accomplished because the fission process itself provides a mechanism for creating a chain reaction. In addition to the two main fragments of the nucleus, each fission reaction produces two or three extra neutrons. Some of these can enter nearby uranium nuclei and cause them in turn to undergo fission, releasing more neutrons, which cause more fission reactions, and so forth. In a bomb explosion, the number of neutrons has to increase very rapidly, in a fraction of a second. In a controlled reactor, however, the neutron population has to be kept in a steady state. Any excess neutrons must be removed by some type of absorber material.

In 1942, the first nuclear reactor with a self-sustaining chain reaction was built in the United States. The principal designer was Italian physicist Enrico Fermi, the 1938 Nobel Prize winner in physics, who had come to the United States in 1938 to escape from Benito Mussolini's fascism. Fermi's reactor design had three main components: lumps of uranium (the fuel), blocks of graphite (carbon moderator), and control rods made of cadmium (an excellent neutron absorber). The reactor was built at the University of Chicago. When the pile of uranium and graphite was about three meters high and the cadmium control rods were pulled out far enough, Geiger counters showed that a steady-state chain reaction had been successfully accomplished. The power output was only about two hundred watts, but it was enough to verify the basic principles of operation. The power level of the chain reaction could be varied by moving the control rods in or out.

After this pioneering experiment with uranium fission, a commitment was made to build an atomic bomb, under the secret code name Manhattan Project. Scientists were recruited to join the war effort. A plutonium production reactor with a power output of about two hundred million watts was designed—a millionfold step up from Fermi's experimental reactor. The reactor worked as designed and produced a supply of fissionable plutonium. Bombs were assembled and tested by Los Alamos Laboratory in New Mexico and were used against Japan in 1945.

After the war ended, nuclear-reactor technology was still classified in the United States as a military secret in the hope of preventing the Soviets from developing an atomic bomb. No civilian applications could be considered yet. In the early 1950s, however, the US Navy started building nuclear submarines, using reactors to replace diesel engines for propulsion. Under the leadership of Admiral Hyman George Rickover, the first nuclear-powered submarine, the Nautilus, was launched in 1954.

The first civilian use of a nuclear reactor was in an electric power plant at Shippingsport, Pennsylvania, in 1957, the year of the first International Conference on Peaceful Uses of Atomic Energy. Electricity usage was increasing rapidly, about 7 percent per year, so new generating facilities were needed. From 1960 to 1990, more than one hundred nuclear power plants were built in the United States; in 2012, these plants generated 19 percent of the nation's total electricity.

In the Western world, the most common reactor type is the pressurized water reactor, or PWR. Its operation is quite similar to that of coal-burning power plants, except that the firebox of the coal plant is replaced with a reactor core. Engineers have many years of experience using pressurized steam to generate electricity, so a high degree of reliability can be achieved.

Power generation in a PWR nuclear power plant begins with heat production in the reactor core. Nuclear energy is released by uranium fission in the form of two nuclear fragments, which fly apart. The whole fuel rod becomes hot because of the accumulated energy of many fissioning nuclei. A typical reactor core contains hundred of fuel rods. Water is circulated through the core to remove the heat. The water in the reactor core is prevented from boiling by keeping the system under pressure. The pressurized water goes to a heat exchanger, where heat is transferred to make steam. The steam in this secondary loop goes to a turbine, which has a series of fan blades that rotate when the steam hits them. The turbine is connected to the rotor of an electric generator. When the generator rotates, electric voltage is produced at its output terminals. The power output goes to cross-country transmission lines that feed the electrical users in the region. The steam that made the turbine rotate is condensed back into water and recycled through the heat exchanger.

Visually, the dominant feature of a nuclear power plant is usually a large cooling tower. This is where heat that was removed from the condensed steam is exhausted to the atmosphere. The cooling tower has nothing to do with the nuclear fission process, and any coal-burning power plant that generates steam needs the same kind of cooling tower. In some power plants, both nuclear and coal, the excess heat may be transferred to a nearby lake or river rather than to the atmosphere.

Safety features at a nuclear power plant include automatic shutdown of the fission process by control-rod insertion, emergency water cooling, and a concrete containment shell. A reactor core cannot have a nuclear explosion, because the fuel enrichment is only a few percent, and almost one hundred percent pure uranium 235 or plutonium must be used in a bomb. The worst accident that might take place in a PWR is a steam explosion, which could contaminate the inside of the containment shell.

The fuel in the reactor core consists of several tons of uranium. As the reactor is operated, the uranium content will gradually decrease because of fission, and the radioactive waste products (the fission fragments) will gradually increase. After a year or two of operation, the reactor must be shut down for refueling. Spent fuel rods are pulled out and replaced. The spent fuel, which is very radioactive, is stored under water at the power plant site. After five to ten years, most of the radioactivity has decayed. Only those materials with a long lifetime remain, and eventually they will be stored in a suitable underground depository.

Applications

During World War II, several reactors were built at Hanford, Washington, for one purpose: to produce enough plutonium for a nuclear weapon. Natural uranium cannot be used to construct a bomb, because only about 1 percent of it is the fissionable isotope uranium 235. The other 99 percent is uranium 238, which absorbs neutrons without fissioning. Uranium 238 plus neutrons, however, produces a new element, plutonium, which is fissionable. The energy obtained from uranium fission was of no interest in these early plutonium production reactors, so it was discarded as waste heat into the Columbia River.

In the nuclear submarine program during the 1950s and later, heat from the fission reactor was used to generate steam, which in turn provided power for propulsion. Nuclear submarines were able to remain submerged for several months without surfacing. The first voyage by a submarine beneath the arctic ice at the North Pole took place in 1957. Other Navy ships, such as modern large aircraft carriers, are powered by as many as four separate reactors.

Reactors for civilian electric-power production became possible after the technology was declassified. After the PWR, the boiling water reactor, or BWR, is the second most common type. In Canada, the favored reactor design, the CANDU (short for "Canada deuterium uranium") reactor, uses deuterium dioxide, also called heavy water, as a coolant with natural uranium fuel. In Europe and the former Soviet Union, a number of reactors use a graphite moderator with air cooling. The Chernobyl reactor, which burned and exploded in 1986, was a Soviet-designed graphite-moderated reactor called the RBMK, whose full name translates as "high-power channel-type reactor."

In France, a fast-breeder reactor named Superphénix operated from 1986 to 1996. Breeder reactors use the extra neutrons from fission to convert uranium 238 into fresh plutonium fuel. For every five uranium 235 fuel atoms that are used up, six new plutonium atoms are created. What is being consumed in the breeder is the nonfissionable uranium 238. The breeder provides a way to increase the fissionable fuel supply, which otherwise would be limited to the rare isotope of uranium 235. Superphénix was closed in 1997, officially due to the cost of its maintenance, following protests from antinuclear organizations. At the time, it was the last fast-breeder reactor operating in Europe.

The first generation III nuclear reactor began operation in 1996, at the Kashiwazaki-Kariwa Nuclear Power Plant in Japan. Generation III reactors represent an evolution in design from the previous generation, which includes reactor designs such as the PWR, the BWR, and the CANDU. The Kashiwazaki reactor was an advanced boiling water reactor (ABWR), which features numerous improvements on generation II BWR designs, including greater power output and superior safety measures. Other generation III reactors include the Advanced Pressurized Water Reactor (APWR) and the Enhanced CANDU 6 (EC6). Generation III+ refers to reactors that are more advanced than generation III but not advanced enough to be considered a new generation; these include the EU-ABWR, which is an improved ABWR that meets European Union safety standards, and the Advanced CANDU Reactor (ACR-1000), which uses light (that is, normal) water as a coolant in conjunction with a heavy-water moderator.

Before 1980, there were around 70 nuclear power plants in the United States, producing about 12 percent of the electricity for civilian use. Between 1980 and 1996, 51 more reactors were completed and brought up to power, while some were decommissioned, so that in 2012, 104 reactors supplied nearly 20 percent of US electricity. Between 1980 and 1990, no new orders for reactors were placed by electric utilities, so all the reactors that were completed in this decade had been started earlier. Electric utilities are reluctant to invest in new nuclear plants until questions of safety and waste disposal have been resolved and public confidence is restored.

The international view of nuclear power varies greatly from country to country. After the Arab oil embargo of 1973, France initiated a large-scale nuclear program to replace oil- and coal-burning power plants. Nearly forty years later, in 2012, fifty-eight reactors supplied 74.8 percent of France's electricity. At the same time, the country's atmospheric pollution caused by greenhouse gases has been significantly reduced, because reactors do not depend on combustion and do not emit such gaseous products. Other countries with a strong commitment to nuclear power for electricity production are Japan, South Korea, Hungary, Sweden, Great Britain, and Canada.

In addition to being used for military applications and electric power plants, nuclear reactors are used in research, where they produce radioactive isotopes for various industrial and medical uses. Cobalt 60 is the most widely used man-made radioactive material, manufactured by irradiating ordinary cobalt 59 with neutrons in a reactor. In the medical profession, cobalt 60 is used for cancer therapy. Radioactive cobalt is also used in the manufacture of medical products that must be sterile, such as bandages and face masks; it kills bacteria and microorganisms, replacing the toxic chemical gases that were used previously. Outside the medical field, cobalt 60 has been used in agriculture to control insect pests by means of the sterile male technique. For example, the Mediterranean fruit fly was virtually eliminated from Hawaii after millions of male flies that had been bred in cages and sterilized by cobalt 60 were released there.

Many ordinary elements can be made radioactive by means of irradiation with neutrons in a reactor. They can then be used as tracers in various applications. For example, radioactive phosphorus or potassium can be incorporated into fertilizers, and the uptake in plants of those substances can be studied by agronomists. Hundreds of different organic compounds containing radioactive hydrogen or carbon are available for biological research. Radioactive iron, copper, and zinc are used in manufacturing to measure tiny amounts of wear in machine parts.

In the space program, electric power for space flights can be supplied by chemical batteries or solar cells if the voyage is not too lengthy. Nuclear batteries are necessary, however, if the space flight lasts for several weeks and extends too far from the sun for solar energy to be effective. The radioactive fuel for these batteries is made in reactors. Some cardiac pacemakers use the same technology on a miniaturized scale, allowing their batteries to last for many years without replacement.

Context

Nuclear reactors are a controversial technology. Both their advantages and their problems should be clearly enumerated so that the public can evaluate for itself whether the benefits of nuclear reactors outweigh their risks.

Additional electric-power plants will be required to supply an increasing world population that desires a higher standard of living. Should the additional electricity be generated by coal, oil, water, nuclear, or solar energy? All methods of producing electricity have problems.

Burning coal and oil produces smog and greenhouse gases. Coal mining is hazardous and causes environmental problems. Oil has to be imported by industrial nations, making them dependent on foreign suppliers, and oil spills are an environmental hazard. Water power must be located at one of a limited number of possible dam sites, requiring long transmission lines to carry the electricity across the country, and it is unreliable in drought years. Occasional dam failures have caused major destruction.

Solar energy has two major problems: it is available only in the daytime, and it is not concentrated. To have solar electricity available for use at night requires an expensive storage system. It has often been stated that solar energy is free, but unfortunately the cost of collecting that free energy from a large area makes it much more expensive than any of the alternatives. Other alternate energy sources, such as geothermal, tidal, and methane gas, are possible at only a few geographical locations.

The main advantages of nuclear power plants are that they do not produce atmospheric pollution, do not contribute to global warming, require only a few truckloads of fuel per year, do not depend on foreign suppliers, and do not require long transmission lines. The accident at Three Mile Island in 1979 caused a partial meltdown of the core, but the radioactivity was retained in the containment building and has been largely cleaned up. In spite of adverse publicity by the media, proponents of nuclear power argue that the safety lessons learned at Three Mile Island make a repetition of that accident extremely unlikely.

The objections to nuclear power plants mainly are caused by the fear of possible future accidents, the unresolved problem of nuclear-waste storage, and the possibility of plutonium diversion for weapons production by a terrorist group. The issue of waste storage is of particular concern because leakage from a waste depository could contaminate groundwater. Chemical dump sites have leaked in the past, causing health problems, so there is distrust of all hazardous wastes, especially nuclear waste, with its long lifetime. The fear of nuclear weapons proliferation is felt most intensely in connection with countries such as Pakistan, Iran, and Iraq.

The Chernobyl accident in 1986 was an environmental disaster. The Soviets did not require a containment building, so the fire released large amounts of radioactivity. There were some immediate fatalities among firemen. It has been estimated by the International Atomic Energy Agency that the long-term effect of the radioactivity will eventually cause twenty-five thousand additional cancer deaths in Russia and eastern Europe.

In 2011, an earthquake off the coast of Japan resulted in a tsunami striking the Fukushima Daiichi Nuclear Power Plant in the eastern region of Japan's Fukushima Prefecture. Water flooded the facility, causing the emergency generators powering the coolant systems to fail and leading to chemical explosions that contaminated the area with radiation. However, the Tokyo Electric Power Company admitted that it had failed to take proper measures to prevent such a disaster due to fear of protests or lawsuits; had this not been the case, the incident might not have happened.

Environmentalists are divided in their views about nuclear power. On the one hand, the growing concern about global warming caused by burning coal or oil makes nuclear power more acceptable. On the other hand, many fear that nuclear waste and the possibility of future plant failures pose a different but equally serious risk. to protect the environment, people may have to get along with less electricity, using conservation and improved efficiency. The controversy about nuclear power plants should be viewed in a larger context, in which both the potential hazards and the need for electricity are valid concerns.

Principal terms

BREEDER REACTOR: a nuclear reactor that converts abundant, nonfissionable uranium 238 into plutonium fuel as a way to increase the total fuel supply

CHAIN REACTION: a process in which secondary neutrons produced by fission enter a nearby nucleus, causing it to fission with additional neutron production

CONTROL ROD: an efficient neutron absorber that can be moved into or out of the reactor core to start up, shut down, or regulate the power level

MODERATOR: a material, usually water, that slows down neutrons in the reactor core in order to increase the efficiency of the fission process

PLUTONIUM: an element not found in nature that can be manufactured from uranium to produce additional fuel for fission energy

RADIOACTIVE WASTE: the two fragments produced by the fission process, which accumulate in the fuel rods and eventually must be removed from the reactor

Bibliography

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Boiling-water reactor (BWR)

The Structure of the Atomic Nucleus

Radioactive Nuclear Decay and Nuclear Excited States

Reactor Fuels and Waste Products