Nuclear Fission
Nuclear Fission
From uranium fission to chain reaction
The Manhattan Project, 1942–1945
Nuclear reactors for electric power production
The controversy about nuclear power plants
Nuclear fission when the nucleus of an atom splits, usually into two pieces. This reaction was discovered when a target of uranium was bombarded by neutrons. Fission fragments were shown to fly apart with a large release of energy. The fission reaction was the basis of the atomic bomb, which was developed by the United States during World War II. After the war, controlled energy release from fission was applied to the development of nuclear reactors. Reactors are utilized for production of electricity at nuclear power plants, for propulsion of ships and submarines, and for the creation of radioactive isotopes used in medicine and industry.
History
The fission reaction was discovered in l938 by two German scientists, Otto Hahn (1879–1968) and Fritz Strassmann (1902–1980). They had been doing a series of experiments in which they used neutrons to bombard various elements. If they bombarded copper, for example, a radioactive form of copper was produced. Other elements became radioactive in the same way. When uranium was bombarded with neutrons, however, an entirely different reaction seemed to occur. The uranium nucleus apparently underwent a major disruption.
The evidence for this supposed process came from chemical analysis. Hahn and Strassmann published a scientific paper showing that small amounts of barium (element 56) were produced when uranium (element 92) was bombarded with neutrons. It was very puzzling to them how a single neutron could transform element 92 into element 56.
Lise Meitner (1878–1968), a long–time colleague of Hahn who had left Germany due to Nazi persecution, suggested a helpful model for such a reaction. One can visualize the uranium nucleus to be like a liquid drop containing protons and neutrons. When an extra neutron enters, the drop begins to vibrate. If the vibration is violent enough, the drop can break into two pieces. Meitner named this process “fission” because it is similar to the process of cell division in biology. It takes only a small amount of energy to start the vibration which leads to a major breakup.
Scientists in the United States and elsewhere quickly confirmed the idea of uranium fission, using other experimental procedures. For example, a cloud chamber is a device in which vapor trails of moving nuclear particles can be seen and photographed. In one experiment, a thin sheet of uranium was placed inside a cloud chamber. When it was irradiated by neutrons, photographs showed a pair of tracks going in opposite directions from a common starting point in the uranium. Clearly, a nucleus had been photographed in the act of fission.
Another experimental procedure used a Geiger counter, which is a small, cylindrical tube that produces electrical pulses when a radioactive particle passes through it. For this experiment, the inside of a modified Geiger tube was lined with a thin layer of uranium. When a neutron source was brought near it, large voltage pulses were observed, much larger than from ordinary radioactivity. When the neutron source was taken away, the large pulses stopped. A Geiger tube without the uranium lining did not generate large pulses. Evidently, the large pulses were due to uranium fission fragments. The size of the pulses showed that the fragments had a very large amount of energy.
To understand the high energy released in uranium fission, scientists made some theoretical calculations based on Albert Einstein’s famous equation E =mc 2. The Einstein equation says that massm can be converted to energy E, and the conversion factor is a huge number, c, which is the velocity of light squared. One can calculate that the total mass of the fission products remaining at the end of the reaction is slightly less than the mass of the uranium atom plus neutron at the start. This decrease of mass, described in the correct units and multiplied by c, accounts numerically for why the fission fragments are so energetic.
From uranium fission to chain reaction
Through fission, neutrons of low energy can trigger off a very large energy release. With the imminent threat of war in l939, a number of scientists began to consider the possibility that a new and very powerful “atomic bomb” could be built from uranium. Also, they speculated that uranium perhaps could be harnessed to replace coal or oil as a fuel for industrial power plants.
Nuclear reactions in general are much more powerful than chemical reactions. A chemical change such as
burning coal or even exploding TNT affects only the outer electrons of an atom. A nuclear process, on the other hand, causes changes among the protons and neutrons inside the nucleus. The energy of attraction between protons and neutrons is about a million times greater than the chemical binding energy between atoms. Therefore, a single fission bomb, using nuclear energy, might destroy a whole city. Alternatively, nuclear electric power plants theoretically could run for a whole year on just a few tons of fuel.
In order to release a substantial amount of energy, many millions of uranium nuclei must split apart. The fission process itself provides a mechanism for creating a so–called chain reaction. In addition to the two main fragments, each fission event produces two or three extra neutrons. Some of these can enter nearby uranium nuclei and cause them in turn to fission, releasing more neutrons, which cause more fissions, and so forth. In a bomb explosion, neutrons have 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. 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 Enrico Fermi (1901–1954), an Italian physicist and the 1938 Nobel Prize winner in physics. He had emigrated to the United States to escape from Benito Mussolini’s fascism. Fermi’s reactor design had three main components: lumps of uranium (the fuel), blocks of carbon (the moderator, which slows down the neutrons), 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 carbon blocks was about 10 ft (3 m) 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 200 watts, but it was enough to verify the basic principle of reactor operation. The power level of the chain reaction could be varied by moving the control rods in or out.
The Manhattan Project, 1942–1945
General Leslie R. Groves was put in charge of the project to convert the chain reaction experiment into a usable military weapon. Three major laboratories were built under wartime conditions of urgency and secrecy. Oak Ridge, Tennessee, became the site for purifying and separating uranium into bomb–grade material. At Hanford, Washington, four large reactors were built to produce another possible bomb material, plutonium. At Los Alamos, New Mexico, the actual work of bomb design was started in l943 under the leadership of the physicist J. Robert Oppenheimer (1904–1967).
The element uranium is a mixture of two isotopes, uranium–235 and uranium–238. Both isotopes have 92 protons in the nucleus, but uranium–238 has three additional neutrons. Both isotopes have 92 orbital electrons to balance the 92 protons, so their chemical properties are identical. When uranium is bombarded with neutrons, the two isotopes have differing nuclear reactions. A high percentage of the uranium–235 nuclei undergo fission, as described previously. The uranium–238, on the other hand, simply absorbs a neutron and is converted to the next heavier isotope, uranium–239. It is not possible to build a bomb out of natural uranium. The reason is that the chain reaction would be halted by uranium–238 because it removes neutrons without reproducing any new ones.
The fissionable isotope, uranium–235, constitutes only about l% of natural uranium, while the nonfissionable neutron absorber, uranium–238, makes up the other 99%. To produce bomb–grade, fissionable uranium–235, it was necessary to build a large isotope separation facility. Since the plant would require much electricity, the site was chosen to be in the region of the Tennessee Valley Authority (TVA). The technology of large–scale isotope separation involved solving many difficult, unprecedented problems. By early l945, the Oak Ridge Laboratory was able to produce kilogram amounts of uranium–235 purified to better than 95%.
An alternate possible fuel for a fission bomb is plutonium–239. Plutonium, which results from radioactive decay of uranium–239, is extremely rare in nature. Fermi’s chain–reaction experiment had shown that uranium–239 can be made in a reactor. However, to produce several hundred kilograms of plutonium required a large increase from the power level of Fermi’s original experiment. Plutonium production reactors were constructed at Hanford, Washington, located near the Columbia River to provide needed cooling water. A difficult technical problem was how to separate plutonium from the highly radioactive fuel rods after irradiation. This was accomplished by means of remote handling apparatus that was manipulated by technicians working behind thick protective glass windows.
With uranium–235 separation started at Oak Ridge and plutonium–239 production under way at Hanford, a third laboratory was set up at Los Alamos, New Mexico, to work on bomb design. In order to create an explosion, many nuclei would have to fission almost simultaneously. The key concept was to bring together several pieces of fissionable material into a so–called critical mass. In one design, two pieces of uranium–235 were shot toward each other from opposite ends of a cylindrical tube. A second design used a spherical shell of plutonium–239, to be detonated by an “implosion” toward the center of the sphere.
The first atomic bomb was tested at an isolated desert location in New Mexico on July l6, l945. On August 8, a single atomic bomb destroyed the city of Hiroshima with about 140,000 deaths, most immediate and some lingering. On August 9, a second bomb was dropped on Nagasaki, causing about half as many deaths. Japan surrendered unconditionally a few days later.
The decision to use the atomic bomb has been vigorously debated over the years. Some claim that it brought a quick end to the war and avoided many casualties that a land invasion of Japan would have cost. However, many historians have come to doubt these claims, noting that Japan was already essentially defeated, its air and sea forces being almost entirely destroyed. Declassified documents have shown that the Japanese leadership had already been seeking terms of surrender for some months through diplomatic channels, asking only that the Emperor’s personal safety be guaranteed. (The Emperor was viewed as a divine figure.) The U.S. leadership, however, insisted on unconditional surrender. Many U.S. military leaders objected beforehand to the atomic bombings, horrified by the prospect of deliberately destroying innocent civilians in order to terrorize distant political and military leaders. Nor were Hiroshima and Nagasaki significant military targets; in fact, they were deliberately spared all U.S. conventional bombing beforehand in order that they might show the effects of the atomic bombs clearly. The statement that the atomic bombings saved half a million or a million U.S. lives, often made by Allied leaders and others after the war, is not, historians have shown, based on the casualty estimates actually made by the U.S. military during planning for an autumn invasion of the main Japanese island, Honshu; Allied death forecasts were more on the order of 20,000. Most historians who specialize in this topic agree that a desire to impress the Soviet Union with the almost cosmic power at the United States’ disposal was a contributing motive to the dropping of the atomic bombs, and that the war could have been ended rapidly without either an invasion or the atomic bombings if the U.S. had offered guarantees for the Emperor’s safety. After receiving Japan’s unconditional surrender, the U.S. did grant the Japanese Emperor immunity from war–crimes prosecution. It should be noted that some historians continue to defend the traditional view that the dropping of the atomic bombs was necessary to end the war promptly and so prevent a far larger slaughter.
The horror of mass annihilation in a nuclear war is made vivid by the images of destruction at Hiroshima. The possibility of a ruthless dictator or a terrorist group getting nuclear weapons—or of their use, again, by the United States or some other non-dictatorial state—is a continuing threat to world peace.
Nuclear reactors for electric power production
The first nuclear reactor designed for producing electricity was put into operation in 1957 at Shippingsport, Pennsylvania. From l960 to l990, more than 100 nuclear power plants were built in the United States. These plants now generate about 20% of the nation’s electric power. World–wide, there are over 400 nuclear power stations.
The most common reactor type is the pressurized water reactor (abbreviated PWR). The system operates like a coal–burning power plant, except that the firebox of the coal plant is replaced by a reactor. Nuclear energy from uranium is released in the two fission fragments. The fuel rod becomes very hot because of the cumulative energy of many fissioning nuclei. A typical reactor core contains hundreds of these fuel rods. Water is circulated through the core to remove the heat. The hot water is prevented from boiling by keeping the system under pressure.
The pressurized hot water goes to a heat exchanger where steam is produced. The steam then goes to a turbine, which has a series of fan blades that rotate rapidly when hit by the steam. The turbine is connected to the rotor of an electric generator. Its output goes to cross–country transmission lines that supply the electrical users in the region. The steam that made the turbine rotate is condensed back into water and is recycled to the heat exchanger. This method of generating electricity was developed for coal plants and is known to be very reliable.
Safety features at a nuclear power plant include automatic shutdown of the fission process by insertion of control rods, emergency water cooling for the core in case of pipeline breakage, and a concrete containment shell. It is impossible for a reactor to have a nuclear explosion because the fuel enrichment in a reactor is intentionally limited to about 3% uranium–235, while almost l00% pure uranium–235 is required for a bomb. The worst accident at a PWR would be a steam explosion, which could contaminate the inside of the containment shell.
The active ingredient in the reactor core consists of several tons of uranium. As the reactor is operated, the uranium content gradually decreases because of fission, and the radioactive waste products (the fission fragments) build up. After about a year of operation, the reactor must be shut down for refueling. The old fuel rods are pulled out and replaced. These fuel rods, which are very radioactive, are stored under water at the power plant site. After five to ten years, much of their radioactivity has decayed. Only those materials with a long radioactive lifetime remain, and eventually they will be stored in a suitable underground depository.
The controversy about nuclear power plants
All methods of producing electricity have serious environmental, social, and political impacts, and nuclear power is no exception. There are vehement arguments both for and against nuclear power. The main objections to nuclear power plants are the possibility of catastrophic accidents at plants or related facilities that lead to the release of radiation, the unresolved problem of nuclear waste storage, and the possibility of plutonium diversion for weapons production by a terrorist group. As of 2006, an ongoing crisis was centered on the question of whether Iran’s nuclear power program was purely peaceful or was intended to enable to the production of a nuclear weapon. The United Nations Atomic Energy Agency stated that there was no evidence of a weapons program, but the U.S. and others pointed to Iran’s efforts to enrich uranium—also a necessary step in peaceful fuel production—as indicative of weapons ambitions.
As for nuclear waste storage, proponents of nuclear power argue that waste will eventually be processed into a safe form, or remain safely buried in stable geological formations for hundreds of thousands of years. Opponents argue that schemes for reprocessing are unproven or prohibitively expensive, and that it is not possible to realistically guarantee containment, even deep underground, over such long periods of time. Waste may also be diverted for terrorist use; it is intrinsically toxic and long–lived, and could be dispersed by a conventional explosive rather than a nuclear weapon. Nuclear power plants themselves might be targeted by terrorists (security was tightened at all U.S. plants after the attacks of September 11, 2001) or by the enemy in a time of war.
The main advantage of nuclear power plants is that they do not cause large amounts of atmospheric pollution. No smokestacks are needed because no fuel is being combusted. France initiated a large–scale nuclear program after the Arab oil embargo in l973 and has been able to reduce its acid rain and carbon dioxide emissions by more than 40%. (Its nuclear waste is accumulating steadily, however, and efforts to reprocess it at plants in Germany have met with intense popular opposition and actual blockade.) Germany and Sweden have both committed to phasing out their own nuclear programs (though political see–sawing has repeatedly cast doubt on these commitments).
On a related note, nuclear power plants do not contribute directly to global warming because they do not emit carbon dioxide, a feature that has caused a few prominent environmentalists to renounce their longstanding opposition to nuclear power. On the other hand, opponents of nuclear power argue that nuclear power is so expensive—despite promises in the 1950s from U.S. officials that it would produce electricity “too cheap to meter”—that buying it in effect puts carbon dioxide into the atmosphere, because the same money could have been spent far more effectively on other technologies for reducing carbon dioxide emissions, such as wind power or end–use energy efficiency. By analogy, caviar is a nutritious food, but it is so expensive that if one has a limited budget then buying caviar reduces the amount of food in the larder rather than increasing it. Proponents of nuclear power note that shipments of nuclear fuel are minimal compared to other fuel–dependent energy sources, so the hazards of oil spills are avoided—although the shipments of fuel that do occur are tempting targets for terrorists.
KEY TERMS
Chain reaction —An on–going process in which a neutron produced from fission enters a nearby nucleus, causes it to fission, and releases additional neutrons to continue the process.
Critical mass —The minimum amount of fissionable uranium or plutonium that is necessary to maintain a chain reaction.
Geiger counter —A small, cylindrical tube used to detect radioactivity, giving an output of voltage pulses when radiation passes through it.
Isotopes —Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.
Plutonium —A man–made element that is created from uranium–238 by neutron bombardment and can be used as a material for fission energy.
Radioactive waste —The radioactive fragments produced by fission, which accumulate in the fuel rods of a nuclear reactor and eventually must be removed.
As of late 2006, no new orders for a nuclear power plant had been made in the United States since 1973. However, thanks to heavy government subsidies for nuclear power offered under the Energy Policy Act of 2005, some experts expected a few new orders for U.S. nuclear plants to be placed in 2007 or 2008.
Resources
BOOKS
Dahl, Per F. From Transmutation to Nuclear Fission, 1932–1939. Washington, DC: Institute for Physics, 2002.
Leventhal, Paul, et al. Nuclear Power and the Spread of Nuclear Weapons: Can We Have One without the Other? Washington, DC: Potomac Books, 2002.
Rhodes, Richard. The Making of the Atomic Bomb. New York: Touchstone, 1995.
Richardson, Hazel, and Scoular Anderson. How to Split the Atom. Franklin Watts, 2001.
Smith, Brice. Insurmountable Risks: The Dangers of Using Nuclear Power to Combat Global Climate Change. Muskegon, MI: Rdr Books, 2006.
OTHER
Union of Concerned Scientists. “Nuclear Safety.” September 7, 2005. <http://www.ucsusa.org/clean_energy/nuclear_safety/> (accessed November 13, 2006).
Hans G. Graetzer