Nuclear Fission

History

From uranium fission to chain reaction

The Manhattan Project, 1942–1945

Nuclear reactors for electric power production

The controversy about nuclear power plants

Resources

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

NUCLEAR FISSION

Nuclear fission is a process in which a heavy nucleus—usually one with a nucleon number of two hundred or more—separates into two nuclei. Usually the division liberates neutrons and electromagnetic radiation and releases a substantial amount of energy. The discovery of nuclear fission is credited to Otto Hahn and Fritz Strassman. In the process of bombarding uranium with neutrons in the late 1930s, they detected several nuclear products of significantly smaller mass than uranium, one of which was identified as 137Ba. The theorectical underpinnings that exist to this day for nuclear fission were proposed by Lise Meitner and Otto Frisch. Shortly after Hahn and Strassman's discovery.

Some heavy nuclei will fission spontaneously. Others can be induced to fission through interaction with a neutron. In both spontaneous nuclear fission and induced nuclear fission the pool of neutrons and protons is conserved. For example, the nucleus ²⁵²Cf (Californium) fissions spontaneously. The 98 protons and 154 neutrons in the nucleus of ²⁵²Cf are reconfigured into other nuclei. Usually a few neutrons are released in the process. Pictorially, a typical spontaneous fission of ²⁵²Cf producing two nuclei and three neutrons is shown in Figure 1.

The pictorial depiction of the spontaneous fission of ²⁵²Cf can be summarized as an equation: The nucleus ²³⁵U (uranium) does not fission spontaneously, but it can be induced to fission through interaction with a neutron. Pictorially, a typical neutron-induced fission of ²³⁵U producing two nuclei and three neutrons is depicted in Figure 2.

The pictorial depiction of the neutron-induced fission of ²³⁵U can be summarized as an equation: Visually, nuclear fission is simply a rearrangement of neutrons and protons, and would seem to be possible for any nucleus. However, energy conservation principles limit significantly the number of possibilities for either spontaneous nuclear fission or induced nuclear fission. Total energy is conserved in a nuclear fission reaction as it is in all nuclear reactions. This means that the total energy before the fission equals the total energy of the fission products. For example, in the induced nuclear fission reaction discussed earlier, the combined energy of the neutron and ²³⁵U nucleus equals the combined energy of the ¹⁴⁴Ba and ⁸⁵Kr nuclei and the three neutrons. The energy accounting includes kinetic energy as well as any potential energy associated with the nuclei and the neutrons. The energy associated with mass through the Einstein relation E = mc² is particularly relevant. The interaction of the neutron and the ²³⁵U nucleus takes place when both are essentially at rest and, therefore, have negligible kinetic energy. Yet the reaction products—¹⁴⁴Ba, ⁸⁵Kr, and three neutrons—have significant kinetic energy. The kinetic energy of the products has its origin in the conversion of potential energy associated with the ²³⁵U nucleus and the interacting neutron. The gain in kinetic energy is accompanied by a loss of mass in the interacting nuclei (neutron and ²³⁵U nucleus). The kinetic energy of the reaction products (¹⁴⁴Ba, ⁸⁵Kr and 3 neutrons) is determined from E = mc², where m is the mass difference between the interacting nuclei and reaction products. The following example illustrates this principle. In atomic mass units the masses of the

nuclei involved in the illustrative nuclear fission reaction are:

Adding the left and the right columns yields 236.052588 u and 235.8665677 u, respectively. The reaction products have 0.1860 u less mass than the interacting nuclei. One atomic mass unit has an energy equivalent of 931.50 million electron volts (MeV). Therefore, 0.1860 u has an energy equivalent of 173.3 MeV. The energy acquired by the reaction products totals 173.3 MeV. This nuclear fission energy release is huge compared to that in a chemical reaction.

A typical chemical reaction such as that in burning a fossil fuel releases about 10 eV of energy. A release of 170 MeV in one nuclear fission reaction liberates the energy equivalent of 17 million chemical reactions in the burning of a fossil fuel.

A nuclear fission reaction will not occur unless the following occur: (1) the total mass of the reaction products is less than the total mass of the interacting nuclei, and (2) the sum of the neutrons and the sum of the protons in the interacting particles equals the sum of the neutrons and the sum of the protons in the products of the fission.

Any combination of reaction products consistent with these conservation principles is possible. For example, in the neutron-induced nuclear fission of ²³⁵U it is possible to produce ¹⁴⁰Xe, ⁹⁴Sr, two neutrons, and 185 MeV of energy. The most likely reaction products are close in atomic number to xenon (Xe) and strontium (Sr), but the possibilities number in the hundreds.

Describing a neutron-induced nuclear fission reaction such as does not convey the likelihood of it actually happening. When a neutron encounters the ²³⁵U nucleus, several nuclear events other than fission may occur. For example, the neutron may bounce off, changing the course of its path much like a marble bouncing off a bowling ball. Or the 235U nucleus may capture the neutron and disintegrate in a way that does not involve nuclear fission. Whatever the event, there is only some chance that the event occurs when the neutron and the nucleus interact, and this probability depends strongly on the energy of the neutron. The fissioning of ²³⁵U by a neutron is likely only for low-energy neutrons. Low in this sense means that the energy is of the order of the energy of a molecule in the air in a room.

A drop of water is a collection of molecules bound together by electric forces. Perturbing the drop in some way often causes the drop to separate into two smaller drops. A heavy nucleus such as 235U has 92 protons and 143 neutrons and has features of a liquid drop that are useful for visualizing the dynamics of nuclear fission. The strong nuclear force that tends to bind the protons and the neutrons together competes with electric forces between protons that tend to cause the nucleus to break apart. Competition between these forces produces collective oscillations that are analogous to oscillations of a liquid drop. The energy added to the nucleus when it captures a neutron enhances the oscillations, and on some occasions the nucleus separates into two parts. The model is useful for visualizing the nuclear fission mechanism as well as for doing quantitative calculations involving the probability aspects of the process.

NUCLEAR FISSION CHAIN REACTION

A chain reaction results when some event triggers a sequence of identical events. A chain reaction on a crowded highway develops when a car runs into the rear of another car, the struck car collides with a car in front of it, and so on. An expanding chain reaction develops if a car hits two cars, each struck car moves on to strike two cars, and so on. A chain reaction is central to the operation of a nuclear fission reactor and a nuclear fission bomb. It begins with a neutron-inducing fission in a nucleus. Neutrons liberated in the fission in turn induce fission in other nuclei, and so on (see Figure 3). Each fission releases energy, and unless there is some purposeful control, the energy release proceeds uncontrolled. Controlled or uncontrolled, the chain reaction does not propagate unless (1) the fuel is properly arranged and (2) there is sufficient likelihood that a neutron induces fission.

This is tantamount to saying that a chain reaction of cars does not propagate unless (1) the cars are arranged properly and (2) there is a sufficient chance that a car will make other collisions.

NUCLEAR FUEL

Commercially, there are only two nuclear species that will function in a self-sustaining nuclear fission chain reaction; 235U and 239Pu (plutonium). Uranium occurs naturally in two predominant forms, ²³⁵U and ²³⁸U. The bulk, 99.3 percent, of natural uranium is ²³⁸U. Even though ²³⁸U differs from ²³⁵U by having only three additional neutrons, the likelihood of neutron-induced fission of 238U is so low that it cannot be used in a self-sustaining chain reaction. Thus the great bulk of natural uranium is not of a form that can be used in self-sustaining chain reactions. 239Pu is not found naturally in appreciable amounts. Rather, ²³⁹Pu is made through nuclear transmutations. The process begins by having 238U capture a neutron to form 239U. 239U is unstable and undergoes radioactive decay, with the end product being ²³⁹Pu.

NUCLEAR FISSION REACTOR PRINCIPLES

The fuel in a nuclear fission reactor is generally 235U atoms arranged appropriately in a reactor vessel. Neutrons instigate fission of nuclei of ²³⁵U atoms and liberate energy. The energy output may be controlled either by regulating the fuel and/or adjusting the neutron supply. Generally, the energy output is controlled by regulating the number of neutrons. Cylindrical control rods made of materials that readily absorb neutrons are moved up or down in strategically arranged spaces in the fuel assembly. Water circulating around the nuclear fuel removes heat produced by nuclear fission reactions and reduces moderates) the speed of neutrons liberated by fission. The main components of a nuclear fission reactor are shown in Figure 4.

Moderator

Neutrons produced in nuclear fission reactions are very energetic and are referred to as fast neutrons. To optimize the chain reaction, the speed of the reaction neutrons must be reduced. Water molecules surrounding the fuel help slow the neutrons through collisions with atoms. A proton is an optimum target for a neutron. Hydrogen atoms in water are a good source of protons because each water molecule has two hydrogen atoms and each atom has a single proton in its nucleus. A neutron produced in a nuclear fission reaction bounces around in a pool of water, losing energy with each collision with a proton. After its energy is reduced sufficiently, it may react with a ²³⁵U nucleus to induce fission. Reducing the energy is referred to as moderation and the material used in the moderation is called a moderator. An object colliding head-on with a stationary object loses the greatest amount of kinetic energy when the masses of the two objects are the same. This is apparent when a billiard ball collides head-on with another billiard ball at rest. A hydrogen atom has roughly the same mass as a neutron. Accordingly, a neutron loses virtually all its kinetic energy in a head-on collision with a hydrogen atom. Because a water molecule (H₂O) has two hydrogen atoms, water is an efficient moderator for neutrons produced in nuclear fission reactions. Water is the moderator in nearly all reactors.

Control Rods

A nuclear chain reaction can be controlled by inserting materials within the fuel that deprive the fuel of neutrons. These materials are usually rod-shaped and are referred to as control rods. Control rods are made of materials that are more likely than the fuel to absorb neutrons. Boron and cadmium are favorite materials for control rods. To decrease the energy output, the control rods are inserted closer to the fuel. To increase the energy output, the control rods are pulled away from the fuel. Boric acid (H₃BO₃), a boron-containing liquid that strongly absorbs neutrons, is often included in the water moderator to provide further control of the energy output of the reactor.

Fuel

Uranium is a metal that is found naturally as a constituent of chemical compounds in minerals such as pitchblende. Uranium ore is mined much like coal: Open pits are used to mine shallow deposits, deeper deposits require shaft mining. Commercial ores yield 3 to 5 lb of uranium compounds per ton of ore. A material called yellow-cake is produced that is rich in the uranium compound U₃O₈. About 7 of every 1,000 uranium atoms in the yellow cake are of the variety ²³⁵U.

Some nuclear fission reactors are designed to use natural uranium having 0.7 percent 235U and 99.3 percent 238U. CANDU reactors, manufactured in Canada, are examples. These reactors do not use ordinary water for the moderator. Most nuclear fission reactors use ordinary water for a moderator which requires that the fuel be about 3 percent ²³⁵U and about 97 percent ²³⁸U. Achieving this enrichment requires that the solid uranium compounds in the yellow cake be converted to gaseous uranium hexafluoride (UF₆). Following enrichment, gaseous UF₆ is converted to solid uranium oxide (UO₂) for fabrication of fuel elements for a nuclear reactor.

Uranium oxide fuel is formed into solid pellets about ⅜ in. in diameter and ½ in. long. Fuel pellets are stacked in tubes about 12 ft long. Neutrons must be able to penetrate the tube walls, called cladding, to interact with the uranium, and the walls must withstand the temperature required for heating water for a steam turbine. Stainless steel or a material called zircaloy is used for the cladding. Some 200 tubes are packed into a fuel bundle. About 175 appropriately arranged fuel bundles form the reactor core, where the energy is produced. A cylindrical steel vessel about 20 ft in diameter and about 40 ft high houses the core. Energy and neutrons are liberated from nuclear fission reactions within the fuel pellets. About 83 percent of the energy is in kinetic energy of the fission fragments. Fission fragments convert their kinetic energy to heat in the process of coming to rest in the fuel pellets. Heat is conducted from the pellets by water circulating around the fuel cladding. Water has two very important roles: conducting heat from the reactor core and moderating neutron speeds.

Energy Production

There are about 100 tons of uranium in the fuel bundles. With the control rods in their lowest position, there is no chain reaction, no energy produced, and no neutrons liberated from nuclear fission reactions. Energy production starts by raising the control rods and inserting a source of neutrons. Nuclear fission reactions begin producing energy and neutrons. Neutrons escape from the fuel elements into the water, where they lose speed and change direction with each collision with a proton. After losing sufficient speed, a neutron may reenter a fuel element and induce a ²³⁵U nucleus to fission to help foster the chain reaction. If on the average one neutron from a nuclear fission reaction goes on to produce another nuclear fission reaction, the chain reaction is self-sustaining, and energy is produced at a steady rate. Raising the control rods allows more nuclear fission reactions and more energy production. Higher, but steady, energy production occurs at the new control rod position. Control of the chain reaction depends on

• the arrangement of the fuel elements
• the quality of the moderator
• the quantity of ²³⁵U
• the neutron energy required for a high probability for inducing nuclear fission
• the material surrounding the reactor core that is used to minimize the escape of neutrons.

Although protons are very efficient neutron moderators, they also efficiently capture neutrons to form bound proton-neutron pairs called deuterons. Reactors using ordinary water for the moderator compensate for neutron capture by using fuel enriched to about 3 percent ²³⁵U.

Usually atoms resulting from nuclear fission are radioactive. There are also radioactive atoms produced from neutron capture by both 235U and 238U. Both types of radioactive atoms remain in the nuclear fuel. It is these radioactive atoms that comprise the nuclear wastes that require disposal in an environmentally acceptable manner.

Boiling-Water Reactors and Pressurized-Water Reactors

Current light-water reactors have two basic designs. One is termed a boiling-water reactor (BWR), the other a pressurized-water reactor (PWR). The principles of the PWR design is illustrated in Figure 5. Water comes to a boil, producing steam inside the containment vessel of a boiling-water reactor. Water is kept under pressure and does not boil inside the containment vessel of a pressurized-water reactor. Steam for the turbine in a nuclear power plant using a pressurized water reactor comes from water brought to a boil in a steam generator garnering heat from water circulating around the reactor core. In a boiling-water reactor, steam is produced in the reactor vessel. In a pressurized-water reactor, steam comes from a system separated from the reactor vessel. Pressure and temperature in a boiling-water reactor are about 1,000 lb per sq in. and 545°F (285°C). These numbers rise to about 2,250 lb per sq in. and 600°F (316°C) in a pressurized-water reactor. Roughly 70 percent of the reactors in the United States are of the PWR type. Essentially, the safety aspects of PWR and BWR reactors are equivalent.

At the beginning of the twenty-first century, 439 nuclear power reactors in 31 countries were generating 352 billion watts of electric power. More than 16 percent of the world's electricity was produced by these electric power plants. On average the power output of one of these plants is roughly 800 megawatts and is comparable to the power output of a large coal-burning electric power plant. Either one of these plants could supply the electricity needs of about a quarter of a million homes. The electric power outputs of solar and wind systems, for example, are usually factors of ten to a hundred times less than that of a large nuclear-fueled or coal-burning electric power plant.

REACTOR CONTROL

The products of nuclear fission reactions are radioactive and disintegrate according to their own time scales. Often disintegration leads to other radioactive products. A few of these secondary products emit neutrons that add to the pool of neutrons produced by nuclear fission. Very importantly, neutrons from nuclear fission occur before those from radioactive decay. The neutrons from nuclear fission are termed prompt. Those from radioactive decay are termed delayed. A nuclear bomb must function on only prompt neutrons and in so doing requires nearly 100 percent pure 235U (or 239Pu) fuel. Although reactor fuel is enriched above its natural 0.7 percent abundance, it is still some thirty times less concentrated than the fuel needed for bombs. A light-water nuclear reactor cannot achieve a self-sustaining condition using only prompt neutrons; delayed neutrons are needed as well. Control rods are slammed into position if a critical excursion from normal operation is sensed. The time involved in delayed neutron emission is crucial for success of the control operation.

A nuclear reactor and its auxiliaries involve a complex assembly of pumps, pipes, and valves for circulating water and steam. Minor malfunctions of the components are inevitable, and routine maintenance is inescapable. Distinct from the routine malfunctions is the possibility of loss of water in the main cooling of the reactor core. This bears the name "loss of cooling accident." Possible causes of such an accident include reactor operator errors, faulty construction, improper maintenance, natural disaster, and sabotage. A loss of cooling terminates the nuclear fission chain reaction because of the need for a moderator. However, radioactivity in the reactor core continues to produce heat at a level of 5 to 7 percent of its rated power. This percentage may seem low, but 5 percent of 3 billion watts of thermal power from normal operation amount to 150 million watts of thermal power that remains after the chain reaction stops. Many of the radioactive products are short-lived, and the power level drops accordingly. Still, there is sufficient heat to begin melting the core in about thirty seconds. Unless checked, this molten radioactive mass could melt its way through the bottom of the containment facilities. Emergency core-cooling systems must operate within this thirty-second time period. Although safety measures make the chance of an accident releasing radioactivity unlikely, it cannot be made zero.

THE NUCLEAR FISSION BREEDER REACTOR

Contemporary nuclear fission reactors are fueled with either ²³⁵U or ²³⁹Pu. In a nuclear fission reactor fueled with ²³⁵U, some ²³⁸U in the fuel is converted to ²³⁹Pu through nuclear transmutations and becomes part of the fuel inventory. However, the reactor is not designed to optimize the production of 239Pu. A nuclear breeder reactor is designed to both produce energy and convert 238U to 239Pu for fuel. The conversion process is termed breeding.

The fission of ²³⁵U or ²³⁹Pu liberates, on average, two to three neutrons. One neutron is required to sustain the nuclear fission chain reaction. In a nuclear breeder reactor, the extra neutrons are used to induce nuclear reactions that lead to the production of ²³⁹Pu. The sequence begins by arranging for a neutron to be captured by a ²³⁸U nucleus to form ²³⁹U. 239U is unstable and disintegrates producing a variety of neptunium, 239Np. 239Np is unstable and decays, forming 239Pu. The breeding principle requires both fissionable fuel (²³⁵U or 239Pu) and a breeding material (²³⁸U). Both resources are depleted as time passes.

The chance of inducing nuclear fission of 235U is optimum for slow neutrons. Neutron capture by ²³⁸U is significant only for energetic (or fast) neutrons. Accordingly, the nuclear fission reactions must be initiated by fast neutrons even though it is more difficult. Water cannot be used as a coolant, as in a light-water nuclear reactor, because of its moderating effect on neutrons. The alternative to water is usually liquid sodium. These reactors are termed liquid metal (for cooling) fast (for energetic neutrons) nuclear breeder reactors.

Nuclear Breeder Reactor Projects

An experimental nuclear breeder reactor has been operating at the Argonne National Laboratory near Idaho Falls, Idaho, since 1963. However, widespread commercial utilization of nuclear breeder reactors has not materialized. A commercial nuclear breeder reactor electric power plant was built in Monroe County, Michigan, in 1963, but was permanently terminated in 1966 after a blockage in a sodium cooling line produced a partial meltdown of its core. Plans were announced in February 1972 for a major nuclear fission breeder electric power plant to be built on the Clinch River in eastern Tennessee and planned for completion in 1980, but the project ultimately was not funded by the U.S. Congress. France and Japan have constructed major nuclear breeder reactor electric power plants, but at the turn of the century all had been closed. Reprocessing nuclear fuel to recover 239Pu is necessity in nuclear fission breeder technology. This is a major stumbling block because of concerns for proliferation of 239Pu for making nuclear bombs.

Joseph Priest

See also: Electric Power, Generation of; Environmental Problems and Energy Use; Explosives and Propellants; Meitner, Lise; Military Energy Use, Historical Aspects of; Molecular Energy; Nuclear Energy; Nuclear Energy, Historical Evolution of the Use of; Nuclear Fission Fuel; Nuclear Fusion; Nuclear Waste.

BIBLIOGRAPHY

Bodansky, D. (1996). Nuclear Energy: Principles, Practices, and Prospects. Woodbury, NY: AIP Press.

Clark, R. W. (1980). The Greatest Power on Earth: The Story of Nuclear Fission. London: Sidgewick & Jackson.

El Wakil, W. W. (1984). Powerplant Technology. New York: McGraw-Hill.

Forsberg, C. W., and Weinberg, A. (1990). "Advanced Reactors, Passive Safety, and Acceptance of Nuclear Energy." Annual Review of Energy 15: 133–152.

Golay, M. W., and Todreas, N. E. (1990). "Advanced Light Water Reactors." Scientific American262(4):82–89.

Graetzer, H. G., comp. (1981). The Discovery of Nuclear Fission. New York: Arno Press.

Hafele, W. (1990). "Energy from Nuclear Power." Scientific American 263(8):136–143.

Krane, K. S. (1987). Introductory Nuclear Physics. New York: John Wiley & Sons.

Priest, J. (2000). "Nuclear Energy." In Energy: Principles, Problems, Alternatives, 5th ed., ed. J. Priest. Dubuque, IA: Kendall/Hunt.

Ruzic, D. N. (1998). "Light-Water Reactors and Their Advances." Urbana, IL: University of Illinois. <http://starfire.ne.uiuc.edu/ne201/course/topics/light_water_reactors/> August 28, 2000.

Nuclear fission

Nuclear fission is a process in which the nucleus of an atom splits, usually into two daughter nuclei. Spontaneous fission of uranium and other elements in Earth's interior provides an internal source of heat that drives plate tectonics . Fission tracks in mineral crystals , a result of spontaneous fission of uranium, can be used in radiometric dating of rock and rock layers.

Long before the internal construction of the atom was well understood in terms of protons, neutrons, and electrons, nuclear transformations that resulted in observable radioactivity were observed as early as 1896 by French physicist Henri Becquerel (1852–1908). In 1905, British physicist Ernst Rutherford (1871–1937) and American physicist Bertram Borden Boltwood (1870–1927) first used radioactive decay measurements to date minerals .

The fission 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 first 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.

The fission reaction was first articulated by two German scientists, Otto Hahn (1879–1968) and Fritz Strassmann (1902–1980). In 1938, Hahn and Strassmann conducted a series of experiments in which they used neutrons to bombard various elements. Bombardment of copper, for example, produced a radioactive form of copper. Other elements became radioactive in the same way. When uranium was bombarded with neutrons, however, an entirely different reaction occurred. The uranium nucleus apparently underwent a major disruption. Accordingly, the initial evidence for the fission 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. Hahn and Strassmann questioned 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. Moreover, it takes only a relatively small amount of energy to initiate nuclear instability.

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 German-American physicist Albert Einstein's (1879–1955) famous equation E=mc². The Einstein equation states that mass (m ) can be converted into energy (E ) (and, conversely that energy can create mass). The conversion factor becomes c, 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, multiplied by c, shows numerically why the fission fragments are so energetic.

Through fission, neutrons of low energy can trigger off 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 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 causes more fission, 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 (e.g., neutron absorbing control rods).

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. Fermi 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). Fermi and other scientists constructed the first nuclear reactor pile 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.

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 1943 under the leadership of the physicist J. Robert Oppenheimer (1904–1967).

The fissionable uranium isotope, uranium-235, constitutes only about l% of natural uranium, while the non-fissionable 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 1945, 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 does not exist in nature but results from radioactive decay of uranium-239. Fermi's chain reaction experiment had shown that uranium-239 could 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 16, 1945. President Truman then issued an ultimatum to Japan that a powerful new weapon could soon be used against them. On August 8, a single atomic bomb destroyed the city of Hiroshima with over 80,000 casualties. On August 11, a second bomb was dropped on Nagasaki with a similar result. Japan surrendered three days later to end WWII.

The decision to use the atomic bomb has been vigorously debated over the years. It ended the war and avoided many casualties that a land invasion of Japan would have cost. However, the civilians who were killed by the bomb and the survivors who developed radiation sickness left an unforget-table legacy of fear. 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 obtaining nuclear weapons is a continuing threat to world peace. In late 2001, in the aftermath of the terrorist attacks on the World Trade Center in New York, evidence became public of terrorist attempts to acquire weapons-grade uranium and the other technology related to bomb production.

The first nuclear reactor designed for producing electricity was put into operation in 1957 at Shippingsport, Pennsylvania. From 1960 to 1990, more than 100 nuclear power plants were built in the United States. These plants now generate about 20% of the nation's electric power. Worldwide, 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 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 (i.e., creating superheated steam).

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 with electricity. The steam that made the turbine rotate is condensed back into water and is recycled to the heat exchanger.

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 100% pure uranium-235 is required for a bomb. Regardless, nuclear power plants remain potential targets for terrorists who would seek to cause massive and lethal release of radioactivity by compromising the containment shell.

The fuel 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 10 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 under-ground depository.

There are vehement arguments for and against nuclear power. As with other forms of producing electricity, nuclear power generation can have serious and unintended environmental impacts. The main objections to nuclear power plants are the fear of possible 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 becomes particularly emotional because leakage from a waste depository could contaminate ground water. Opponents of nuclear power often cite accidents at the Three Mile Island nuclear power plant in the United States and the massive leak at the Chernobyl nuclear plant in the USSR (now the Ukraine) as evidence that engineering or technical failures can have long lasting and devastating environmental and public health consequences.

The main advantage of nuclear power plants is that they do not cause atmospheric pollution . No smokestacks are needed because nothing is being burned. France initiated a large-scale nuclear program after the Arab oil embargo in 1973 and has been able to reduce its acid rain and carbon dioxide emissions by more than 40%. Nuclear power plants do not contribute to global warming . Shipments of fuel are minimal so the hazards of coal transportation and oil spills are avoided.

Environmentalists remain divided in their opinions of nuclear power. It is widely viewed as a hazardous technology but there is growing concern about atmospheric pollution and dwindling fossil fuel reserves that may make increased usage of nuclear power more widespread.

See also Atom; Atomic mass and weight; Atomic number; Atomic theory; Chemical elements; Chemistry; Dating methods; Energy transformations; Radioactive waste storage (geological considerations); Radioactivity

Nuclear fission

Nuclear fission is a process in which 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=mc2. The Einstein equation says that mass m can be converted ito 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, multiplied by c, shows numerically 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 socalled 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 non-fissionable 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 does not exist in nature but results from radioactive decay of uranium-239. 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. President Truman then issued an ultimatum to Japan that a powerful new weapon could soon be used against them. On August 8, a single atomic bomb destroyed the city of Hiroshima with over 80,000 casualties. On August 11, a second bomb was dropped on Nagasaki with a similar result. The Japanese leaders surrendered three days later.

The decision to use the atomic bomb has been vigorously debated over the years. It brought a quick end to the war and avoided many casualties that a land invasion of Japan would have cost. However, the civilians who were killed by the bomb and the survivors who developed radiation sickness left an unforgettable legacy of fear. 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 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 fuel 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

There are vehement arguments for and against nuclear power. The various advantages and problems should be thoroughly aired so that the general public can evaluate for itself whether the benefits outweigh the risks. Additional electric power plants will be required in the future to supply a growing world population that desires a higher standard of living.

All methods of producing electricity have serious environmental impacts. The main objections to nuclear power plants are the fear of possible 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 becomes particularly emotional because leakage from a waste depository could contaminate ground water. Chemical dump sites have leaked in the past, so there is distrust of all hazardous wastes .

The main advantage of nuclear power plants is that they do not cause atmospheric pollution . No smokestacks are needed because nothing is being burned. 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%. Nuclear power plants do not contribute to the global warming problem. Shipments of fuel are minimal so the hazards of coal transportation and oil spills are avoided.

Environmentalists are divided in their opinions of nuclear power. It is widely viewed as a hazardous technology but there is growing concern about atmospheric pollution making nuclear power more acceptable.

Resources

books

Carlisle, Rodney P. Encyclopedia of the Atomic Age. New York: Facts on File, 2001.

Dahl, Per F. From Transmutation to Nuclear Fission, 1932-1939. Washington, DC: Institute for Physics, 2002.

Graetzer, Hans G., and David L. Anderson. The Discovery ofNuclear Fission: A Documentary History. New York: Van Nostrand Reinhold, 1971. Reprint: Arno Press, 1981.

Graetzer, Hans G., and Larry M. Browning. The Atomic Bomb:An Annotated Bibliography. Pasadena: Salem Press, l992.

Kaku, Michio, and Jennifer Trainer. Nuclear Power, BothSides: The Best Arguments For and Against the Most Controversial Technology. New York: W.W. Norton, l982.

Murray, Raymond L. Nuclear Energy. 3rd ed. New York: Pergamon Press, l988.

Rhodes, Richard. The Making of the Atomic Bomb. New York: Touchstone, 1995.

Richardson, Hazel, and Scoular Anderson. How to Split theAtom. Franklin Watts, 2001.

Wagner, Henry N., and Linda E. Ketchum. Living with Radiation: The Risk, the Promise. Baltimore: The Johns Hopkins University Press, 1989.

Wolfson, Richard. Nuclear Choices: A Citizen's Guide to Nuclear Technology. Cambridge, MA: MIT Press, 1991.

periodicals

Hafele, Wolf. "Energy from Nuclear Power," Scientific American 263: 136-144, September, l990.

organizations

World Nuclear Association [cited February 2003] <http://www.world-nuclear.org/info/inf66.htm>.

other

The United Nations. "Nuclear Non-Proliferation Treaty." [cited March 2003] <http://www.iaea.org/worldatom/Documents/Legal/npttext.shtml>.

Hans G. Graetzer

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.

Nuclear fission

Nuclear fission is a process in which the nucleus of a heavy atom is broken apart into two or more smaller nuclei. The reaction was first discovered in the late 1930s when a target of uranium metal was bombarded with neutrons. Uranium nuclei broke into two smaller nuclei of roughly equal size with the emission of very large amounts of energy. Some scientists immediately recognized the potential of the nuclear fission reaction for the production of bombs and other types of weapons as well as for the generation of power for peacetime uses.

History

The fission reaction was discovered accidentally in 1938 by two German physicists, Otto Hahn (1879–1968) and Fritz Strassmann (1902–1980). Hahn and Strassmann had been doing a series of experiments in which they used neutrons to bombard various elements. When they bombarded copper, for example, a radioactive form of copper was produced. Other elements became radioactive in the same way.

Their work with uranium, however, produced entirely different results. In fact, the results were so unexpected that Hahn and Strassmann were unable to offer a satisfactory explanation for what they observed. That explanation was provided, instead, by German physicist Lise Meitner (1878–1968) and her nephew Otto Frisch (1904–1979). Meitner was a longtime colleague of Hahn who had left Germany due to anti-Jewish persecution.

In most nuclear reactions, an atom changes from a stable form to a radioactive form, or it changes to a slightly heavier or a slightly lighter atom. Copper (element number 29), for example, might change from a stable form to a radioactive form or to zinc (element number 30) or nickel (element number 28). Such reactions were already familiar to nuclear scientists.

What Hahn and Strassmann had seen—and what they had failed to recognize—was a much more dramatic nuclear change. An atom of uranium (element number 92), when struck by a neutron, broke into two much smaller elements such as krypton (element number 36) and barium (element number 56). The reaction was given the name nuclear fission because of its similarity to the process by which a cell breaks into two parts during the process of cellular fission.

Putting nuclear fission to work

In every nuclear fission, three kinds of products are formed. The first product consists of the smaller nuclei produced during fission. These nuclei, like krypton and barium in the example mentioned above, are called fission products. Fission products are of interest for many reasons, one of which is that they are always radioactive. That is, any time a fission reaction takes place, radioactive materials are formed as by-products of the reaction.

Words to Know

Chain reaction: A reaction in which a substance needed to initiate a reaction is also produced as the result of that reaction.

Fission products: The isotopes formed as the result of a nuclear fission reaction.

Fission weapon: A bomb or other type of military weapon whose power is derived from a nuclear fission reaction.

Isotopes: Two or more forms of an element that have the same chemical properties but that differ in mass because of differences in the number of neutrons in their nuclei.

Manhattan Project: A research project of the United States government created to develop and produce the world's first atomic bomb.

Mass: A measure of the amount of matter in a body.

Neutron: A subatomic particle with a mass about equal to that of a hydrogen atom but with no electric charge.

Nuclear reactor: Any device for controlling the release of nuclear power so that it can be used for constructive purposes.

Radioactivity: The property possessed by some elements of spontaneously emitting energy in the form of particles or waves by disintegration of their atomic nuclei.

Radioactive isotope: An isotope that spontaneously breaks down into another isotope with the release of some form of radiation.

Subatomic particle: Basic unit of matter and energy (proton, neutron, electron, neutrino, and positron) smaller than an atom.

The second product of a fission reaction is energy. A tiny amount of matter in the original uranium atom is changed into energy. In the early 1900s, German-born American physicist Albert Einstein (1879–1955) had showed how matter and energy can be considered two forms of the same phenomenon. The mathematical equation that represents this relationship, E = mc², has become one of the most famous scientific formulas in the world. The formula says that the amount of energy (E) that can be obtained from a certain amount of matter (m) can be found by multiplying that amount of matter by the square of the speed of light (c²). The square of the speed of light is a very large number, equal to about 9 × 10²⁰ meters per second, or 900,000,000,000,000,000,000 meters per second. Thus, if even a very small amount of matter is converted to energy, the amount of energy obtained is very large. It is this availability of huge amounts of energy that originally made the fission reaction so interesting to both scientists and nonscientists.

The third product formed in any fission reaction is neutrons. The significance of this point can be seen if you recall that a fission reaction is initiated when a neutron strikes a uranium nucleus or other large nucleus. Thus, the particle needed to originate a fission reaction is also produced as a result of the reaction.

Chain reactions. Imagine a chunk of uranium metal consisting of trillions upon trillions of uranium atoms. Then imagine that a single neutron is fired into the chunk of uranium, as shown in the accompanying figure of a nuclear chain reaction. If that neutron strikes a uranium nucleus, it can cause a fission reaction in which two fission products and two neutrons are formed. Each of these two neutrons, in turn, has the potential for causing the fission of two other uranium nuclei. Two neutrons produced in each of those two reactions can then cause fission in four uranium nuclei. And so on.

In actual practice, this series of reactions, called a chain reaction, takes place very rapidly. Millions of fission reactions can occur in much less than a second. Since energy is produced during each reaction, the total amount of energy produced throughout the whole chunk of uranium metal is very large indeed.

The first atomic bomb

Perhaps you can see why some scientists immediately saw fission as a way of making very powerful bombs. All you have to do is to find a large enough chunk of uranium metal, bombard the uranium with neutrons, and get out of the way. Fission reactions occur trillions of times over again in a short period of time, huge amounts of energy are released, and the uranium blows apart, destroying everything in its path. Pictures of actual atomic bomb blasts vividly illustrate the power of fission reactions.

But the pathway from the Hahn/Strassmann/Meitner/Frisch discovery to an actual bomb was a long and difficult one. A great many technical problems had to be solved in order to produce a bomb that worked on the principle of nuclear fission. One of the most difficult of those problems involved the separation of uranium-238 from uranium-235.

Naturally occurring uranium consists of two isotopes: uranium-238 and uranium-235. Isotopes are two forms of the same element that have the same chemical properties but different masses. The difference between these two isotopes of uranium is that uranium-235 nuclei will undergo nuclear fission, but those of uranium-238 will not. That problem is compounded by the fact that uranium-238 is much more abundant in nature than is uranium-235. For every 1,000 atoms of uranium found in Earth's crust, 993 are atoms of uranium-238 and only 7 are atoms of uranium-235. One of the biggest problems in making fission weapons a reality, then, was finding a way to separate uranium-235 (which could be used to make bombs) from uranium-238 (which could not, and thus just got in the way).

The Manhattan Project. A year into World War II (1939–45), a number of scientists had come to the conclusion that the United States would have to try building a fission bomb. They believed that Nazi Germany would soon be able to do so, and the free world could not survive unless it, too, developed fission weapons technology.

Thus, in 1942, President Franklin D. Roosevelt authorized the creation of one of the largest and most secret research operations ever devised. The project was given the code name Manhattan Engineering District, and its task was to build the world's first fission (atomic) bomb. That story is a long and fascinating one, a testimony to the technological miracles that can be produced under the pressures of war. The project reached its goal on July 16, 1945, in a remote part of the New Mexico desert, where the first atomic bomb was tested. Less than a month later, the first fission bomb was actually used in war. It was dropped on the Japanese city of Hiroshima, destroying the city and killing over 80,000 people. Three days later, a second bomb was dropped on Nagasaki, with similar results. For all the horror they caused, the bombs seemed to have achieved their objective. The Japanese leaders appealed for peace only three days after the Nagasaki event. (Critics, however, charge that the end of the war was in sight and that the Japanese would have surrendered without the use of a devastating nuclear weapon.)

Nuclear fission in peacetime

The world first learned about the power of nuclear fission in the form of terribly destructive weapons, the atomic bombs. But scientists had long known that the same energy released in a nuclear weapon could be harnessed for peacetime uses. The task is considerably more difficult, however. In a nuclear weapon, a chain reaction is initiated—energy is produced and released directly to the environment. In a nuclear power reactor, however, some means must be used to control the energy produced in the chain reaction.

The control of nuclear fission energy was actually achieved before the production of the first atomic bomb. In 1942, a Manhattan Project research team under the direction of Italian physicist Enrico Fermi (1901–1954) designed and built the first nuclear reactor. A nuclear reactor is a device for obtaining the controlled release of nuclear energy. The reactor had actually been built as a research instrument to learn more about nuclear fission (as a step in building the atomic bomb).

After the war, the principles of Fermi's nuclear reactor were used to construct the world's first nuclear power plants. These plants use the

energy released by nuclear fission to heat water in boilers. The steam that is produced is then used to operate turbines and electrical generators. The first of these nuclear power plants was constructed in Shippingport, Pennsylvania, in 1957. In the following three decades, over 100 more nuclear power plants were built in every part of the United States, and at least as many more were constructed throughout the world.

By the dawn of the 1990s, however, progress in nuclear power production had essentially come to a stop in the United States. Questions about the safety of nuclear power plants had not been answered to the satisfaction of most Americans, and, as a result, no new nuclear plants have been built in the United States since the mid-1980s.

Despite these concerns, nuclear power plants continue to supply a good portion of the nation's electricity. Since 1976, nuclear electrical generation has more than tripled. At the beginning of the twenty-first century, 104 commercial nuclear power reactors in 31 states accounted for about 22 percent of the total electricity generated in the country. Combined, coal and nuclear sources produce 78 percent of the nation's electricity.

[See also Nuclear fusion; Nuclear power; Nuclear weapons ]