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The fission reaction was discovered accidentally in 1938 by two German physicists, Otto Hahn and Fritz Strassmann. 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 and her nephew Otto Frisch. 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 having atomic number 29), for example, might change from a stable form to a radioactive form or to zinc (element having atomic number 30) or nickel (element having atomic 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 having atomic number 92), when struck by a neutron, broke into two much smaller elements such as krypton (element having atomic number 36) and barium (element having atomic 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.
Three kinds of products are formed during all nuclear fissions. The first product consists of the smaller nuclei produced during fission. These nuclei, like krypton and barium, 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. 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 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. 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 we 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.
When a single neutron is fired into the chunk of uranium metal consisting of trillions upon trillions of uranium atoms and 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.
We can see why some scientists immediately saw fission as a way of making very powerful bombs just by finding a large enough chunk of uranium metal to be bombarded the uranium nucleus with neutrons, and to 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. 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).
A year into World War II (1939–45), a number of scientists had believed that Nazi Germany would soon be able to build a fission bomb, and the free world could not survive unless it, too, developed fission weapons technology for which they thought that the United States should try to do so. Thus, in 1942, President Franklin D. Roosevelt authorized the creation of one of the largest and most secret research operations ever devised after receiving a letter written by the world renowned physicist, Einstein on behalf of many other scientists, mainly immigrants from Germany. 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, another big city of Japan, with similar results. For all the horror they caused, the bombs seemed to have achieved their objectives as 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.)
Though the world first learned about the power of nuclear fission in the form of terribly destructive weapons, the atomic bombs, 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 designed and built the first nuclear reactor 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 Shipping port, 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.
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Source by Dr.Badruddin Khan
