Fission




Fission

Nuclear fission is a process in which the nucleus of an atom splits, usually into two daughter nuclei, with the transformation of tremendous levels of nuclear energy into heat and light.

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.

Long before the internal construction of the atom was well understood in terms of protons, neutrons, electrons, nuclear transformations that resulted in observable radioactivity were observed as early as 1896 by Henri French physicist Henri Becquerel (1852–1908). The fission reaction was first articualted 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=mc 2 . 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 the 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 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 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 l942, 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 l938 Nobel Prize winner in physics. Fermi emigrated to the United States to escape 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 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 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 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 U.S. 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 possibility of a terrorist group or a dictator hostile to Western democracies 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, intelligence agencies released evidence 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 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 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. 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 l00% 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 ten years, much of their radioactivity has decayed. Only those materials with a long radioactive lifetime remain, and eventually they must be stored in a suitable underground depository.

There are vehement arguments for and against nuclear power. As with other forms of electricity production, 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 poser plant in 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 potential global warming. Shipments of fuel are minimal and so the hazards of coal transportation and oil spills are avoided.

█ FURTHER READING:

BOOKS:

Cottingham, W. Noel and Derek A. Greenwood. An Introduction to the Standard Model of Particle Physics. New York: Cambrige Univeristy Press, 1999.

Sagan, Scott D. and Kenneth N. Waltz. The Spread of Nuclear Weapons: A Debate Renewed, Second Edition. New York: W W Norton & Co., 2003.

Whiting, Jim. Otto Hahn and the Story of Nuclear Fission. Childs. MD: Mitchell Lane Publishers, Inc. 2003.

PERIODICALS:

Ladika, Susan. "Tracing the Shadowy Origins of Nuclear Contraband." Science no. 5522 (2001): 1634.

ELECTRONIC:

United States Department of Energy. "Guide to the Nuclear Wallchart: Energy From Nuclear Science" (August 2000) < http://www-nsd.lbl.gov/abc/wallchart/chapters/14/0.html > (March 20, 2003).

SEE ALSO

Fusion
Heavy Water Technology
Manhattan Project
Nuclear Detection Devices
Nuclear Emergency Support Team, United States
Nuclear Power Plants, Security
Nuclear Reactors
Nuclear Regulatory Commission (NRC), United States
Nuclear Spectroscopy
Nuclear Weapons
Oak Ridge National Laboratory (ORNL)
Weapon-Grade Plutonium and Uranium, Tracking




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