Perspectivity of Nuclear Reaction

FISSION AND FUSION

Any process that involves a change in the nucleus of an atom is called a nuclear reaction. Nuclear reactions include fission, in which a nucleus splits into two or more nuclei, and fusion, in which one or more nuclei combine.

Stable nuclei can be made unstable

When a nucleus is bombarded with energetic particles, it may capture a particle, such as a neutron. As a result, the nucleus will no longer be stable and will disintegrate. For example, protons can be released when alpha particles collide with nitrogen atoms, as follows:

According to this expression, an alpha particle (4He) strikes a nitrogen nucleus ( 14N) and produces an unknown product nucleus (X) and a proton (1H). By balancing atomic numbers and mass numbers, we can conclude that the unknown product has a mass number of 17 and an atomic number of 8. Because the element with an atomic number of 8 is oxygen, the product can be written symbolically as 170, and the reaction can be written as follows:

This nuclear reaction starts with two stable isotopes—helium and nitrogen— that form an unstable intermediate nucleus (18F). The intermediate nucleus then disintegrates into two different stable isotopes, hydrogen and oxygen. This reaction, which was the first nuclear reaction to be observed, was detected by Rutherford in 1919.

Heavy nuclei can undergo nuclear fission

Nuclear fission occurs when a heavy nucleus splits into two lighter nuclei. For fission to occur naturally, the nucleus must release energy. This means that the nucleons in the daughter nuclei must be more tightly bound and therefore have less mass than the nucleons in the parent nucleus. This decrease in mass per nucleon appears as released energy when fission occurs, often in forms such as photons or kinetic energy of the fission products. Because fission produces lighter nuclei, the binding energy per nucleon must increase with decreasing atomic number. Figure 25-8 shows that this is possible only for atoms in which A > 58. Thus, fission occurs naturally only for heavy atoms.

One example of this process is the fission of uranium-235. First, the nucleus is bombarded with neutrons. When the nucleus absorbs a neutron, it becomes unstable and decays. The fission of 235U can be represented as follows:

The isotope 236U* is an intermediate state that lasts only for about 10 -12 s before splitting into X and Y. Many combinations of X and Y are possible. In the fission of uranium, about 90 different daughter nuclei can be formed. The process also results in the production of about two or three neutrons per fission event. A typical reaction of this type is as follows:

To estimate the energy released in a typical fission process, note that the binding energy per nucleon is about 7.6 MeV for heavy nuclei (those having a mass number of approximately 240) and about 8.5 MeV for nuclei of intermediate mass (see Figure 25-8 ). The amount of energy released in a fission event is the difference in these binding energies (8.5 MeV - 7.6 MeV, or about 0.9 MeV per nucleon). Assuming a total of 240 nucleons, this is about 220 MeV. This is a very large amount of energy relative to the energy released in typical chemical reactions. For example, the energy released in burning one molecule of the octane used in gasoline engines is about one hundred-millionth the energy released in a single fission event.

Neutrons released in fission can trigger a chain reaction

When 235U undergoes fission, an average of about 2.5 neutrons are emitted per event. The released neutrons can be cap- captured by other nuclei, making these nuclei unstable. This triggers additional fission events, which lead to the possibility of a chain reaction, as shown in Figure 25-9. Calculations show that if the chain reaction is not controlled—that is, if it does not proceed slowly—it could result in the release of an enormous amount of energy and a violent explosion. If the energy in 1 kg of 235U were released, it would equal the energy released by the detonation of about 20 000 tons of TNT. This is the principle behind the first nuclear bomb.The first nuclear fission bomb, often called the atomic bomb, was tested in New Mexico in 1945.

A nuclear reactor is a system designed to maintain a controlled, self- sustained chain reaction. Such a system was first achieved with uranium as the fuel in 1942 by Enrico Fermi, at the University of Chicago. Primarily, it is the uranium-235 isotope that releases energy through nuclear fission. Uranium from ore typically contains only about 0.7 percent of 235U, with the remaining 99.3 percent being the 238U isotope. Because uranium-238 tends to absorb neutrons instead of undergoing fission, reactor fuels must be processed to increase the proportion of 235U so the reaction can sustain itself. This process is called enrichment. At this time, all nuclear reactors operate through fission. One difficulty associated with fission reactors is the safe disposal of radioactive materials when the core is replaced. Transportation of reactor fuel and reactor wastes poses safety risks. As with all energy sources, the risks must be weighed against the benefits and the availability of the energy source.

Light nuclei can undergo nuclear fusion

Nuclear fusion, the opposite of nuclear fission, occurs when two light nuclei combine to form a heavier nucleus. As with fission, the product of a fusion event must have a greater binding energy than the original nuclei for energy to be released in the reaction. Because fusion reactions produce heavier nuclei, the binding energy per nucleon must increase as atomic number increases. As shown in Figure 25-8 , this is possible only for atoms with A less than 58. Hence, fusion occurs naturally only for light atoms. One example of this process is the fusion reactions that occur in stars. All stars generate energy through fusion. About 90 percent of the stars, including our sun, fuse hydrogen and possibly helium. Some other stars fuse helium or other heavier elements. The proton-proton cycle is a series of three nuclear- fusion reactions that are believed to be stages in the liberation of energy in our sun and other stars rich in hydrogen. In the proton-proton cycle, four protons combine to form an alpha particle and two positrons, releasing 25 MeV of energy in the process. The first two steps in this cycle are as follows:

This is followed by either of the following processes:

The released energy is carried primarily by gamma rays, positrons, and neutrinos. These energy-liberating fusion reactions are called thermonuclear fusion reactions. The hydrogen (fusion) bomb, first detonated in 1952, is an example of an uncontrolled thermonuclear fusion reaction.

Fusion reactors are being developed

The enormous amount of energy released in fusion reactions suggests the possibility of harnessing this energy for useful purposes on Earth. Efforts are under way to create controlled thermonuclear reactions in the form of a fusion reactor. Because of the ready availability of its fuel source—water—controlled fusion is often called the ultimate energy source.

For example, if deuterium (2H) were used as the fuel, 0.16 g of deuterium could be extracted from just 1 L of water at a cost of about one cent. Such rates would make the fuel costs of even an inefficient reactor almost insignificant. An additional advantage of fusion reactors is that few radioactive byproducts are formed. The proton-proton cycle shows that the end product of the fusion of hydrogen nuclei is safe, nonradioactive helium. Unfortunately, a thermonuclear reactor that can deliver a net power output for an extended time is not yet a reality. Many difficulties must be resolved before a successful device is constructed. For example, the energy released in a gas undergoing nuclear fusion depends on the number of fusion reactions that can occur in a given amount of time. This varies with the density of the gas because collisions are more frequent in a denser gas. It also depends on the amount of time the gas is confined. In addition, the Coulomb repulsion force between two charged nuclei must be overcome before they can fuse. The fundamental challenge is to give the nuclei enough kinetic energy to overcome this repulsive force. This can be accomplished by heating the fuel to extremely high temperatures (about 108 K, or about 10 times greater than the interior temperature of the sun). Such high temperatures are difficult and expensive to obtain in a laboratory or a power plant.

Perspectivity

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