Fission and Fusion

Unstable Nucleus

A nucleus become unstable, or radioactive, when the strong nuclear force can no longer overcome the repulsive electric forces among protons. While the strong nuclear force does not increase with the size of the nucleus, the electric forces do. There is, therefore, a point beyond which all elements are radioactive. All nuclei with more than 83 protons are radioactive.

Nuclear Energy Concerns

Another concern about nuclear power is that the operators of the plant could lose control of the reactor. For instance, if the reactor's cooling system failed, then a meltdown might occur. During a meltdown, the core of the reactor melts and radioactive material may be released. If the structure that houses the reactor is not secure, then the environment can become contaminated. In 1986, one of the reactors at the nuclear power station in Chernobyl, Ukraine, overheated during an experiment. A partial meltdown resulted, and large amounts of radioactive material were released into the atmosphere.

Fusion

Another type of nuclear energy that can release huge amounts of energy is fusion. Fusion is a process in which the nuclei of two atoms combine to form a larger nucleus. As in fission, during fusion, a small fraction of the reactant mass is converted into energy. On any day or night, you can detect the energy released by fusion reactions occurring far away from Earth. The sun and other stars are powered by the fusion of hydrogen into helium. Inside the sun, an estimated 600 million tons of hydrogen undergo fusion each second. About 4 million tons of this matter is converted into energy.

The Effect of Size on Nuclear Forces

Electric forces in atomic nuclei depend on the number of protons. The greater the number of protons in the nucleus, the greater is the electric force that repels those protons. So in larger nuclei, the repulsive electric force is stronger than in smaller nuclei.

Fusion Part 2

Fusion may someday provide an efficient and clean source of electricity. Scientists envision fusion reactors fueled by two hydrogen isotopes, deuterium (hydrogen-2) and tritium (hydrogen-3). The fusion of deuterium and tritium produces helium, neutrons, and energy. 2/1H+3/1H-->4/2He+1/0n+energy Scientists face two main problems in designing a fusion reactor. They need to achieve the high temperatures required to start the reaction, and they must contain the plasma.

Fission

In 1938, two German chemists, Otto Hahn and Fritz Strassman, performed a series of important transmutation experiments. By bombarding uranium-235 with high-energy neutrons, Hahn and Strassman hoped to produce more massive elements. Instead, their experiments produced isotops of a smaller element, barium. Unable to explain their data, Hahn and Strassman turned to a colleague for help. In 1939, Lise Meitner and Otto Frisch, another physicist, offered a groundbreaking explanation for the experiments. The uranium-235 nuclei had bee broken into smaller fragments. Hahn and Strassman had demonstrated nuclear fission. Fission is the splitting of an atomic nucleus into to smaller parts.

Nuclear Fission Energy

In nuclear fission, tremendous amounts of energy can be produced from very small amounts of mass. For example, the nuclear energy released by the fission of 1 kilogram of uranium-235 is equivalent to the chemical energy produced by burning more than 17,000 kilograms of coal.

Critical Mass

In order to sustain a chain reactioin, each nucleus that is split must produce, on average, one neutron that causes the fission of another nucleus. This condition corresponds to a specific mass of fissionable material, known as a critical mass. A critical mass is the smallest possible mass of a fissionable material that can sustain a chain reaction.

Converting Mass Into Energy

In the nuclear equation, the mass numbers on the left equal the mass numbers on the right. Yet when the fission of uranium-235 is carried out, about 0.1 percent of the mass of the reactants is lost during the reaction. This "lost" mass is converted into energy. In 1905, more than 30 years before the discovery of fission, physicist Albert Einstein had introduced the mass-energy equation. It describes how mass and energy are related. Mass-Energy Equation: E=mc^2 In the mass-energy equation, E represents energy, m represents mass, and c represents the speed of light (3.0*10^8 m/s). The conversion of a small amount of mass releases an enormous amount of energy. Likewise, a large amount of energy can be converted into a small amount of mass. The explosion of the first atomic bomb in 1945 offered a powerful demonstration of the mass-energy equation. The bomb contained 5 kilograms of plutonium-239. Fission of the plutonium produced an explosion that was equivalent to 18,600 tons of TNT.

Plasma

Matter within the sun and other stars exists as plasma. Plasma is a state of matter in which atoms have been stripped of their electrons. You can think of plasma as a gas containing two kinds of particles--nuclei and electrons. Although fusion occurs at millions of degrees Celsius, plasma can exist at much lower temperatures. Scientists estimate that more than 99 percent of matter in the universe is plasma.

Chain Reaction

Nuclear fission can follow a pattern in which one reaction leads to a series of others. During the fission of uranium-234, each reactant nucleus splits into two smaller nuclei and releases two or three neutrons. If one of these neutrons is absorbed by another uranium-235 nucleus, another fission can result, releasing more neutrons. In a chain reaction, neutrons released during the splitting of an initial nucleus trigger a series of nuclear fissions.

Describe the process of nuclear fusion.

Nuclear fission is the process of combining two hydrogen atoms to create a helium atom.

Describe the process of nuclear fission.

Nuclear fission is the process of splitting the nucleus of an atom to release the atomic energy.

Why is nuclear fusion preferred over nuclear fission?

Nuclear fusion is preffered over nuclear fission because the by-product of nuclear fusion is harmless to living things.

Alternative Energy Source

One alternative energy source that is widely used today is nuclear energy. Nuclear energy is the energy released by nuclear reactions. Shortly after the discovery of radioactivity, scientists realized that atomic nuclei contained vast amounts of energy. By the late 1930s, scientists discovered that transmutations involved more than just the conversion of one element into another--they also involved the conversion of mass into energy.

Modified Conservation Law

Recall how the law of conservation of mass applied to chemical reactions. In nuclear reactions, however, the energies involved are much larger. To account for the conversion of mass into energy, a modified conservation law is used. According to the law of conservation of mass and energy, the total amount of mass and energy remains constant.

The Effect of Size on Nuclear Forces Part 2

The effect of size on the strong nuclear force is more complicated. On one hand, the more protons and neutrons there are in a nucleus, the more possibilities there are for strong nuclear force attractions. However, as the size of the nucleus increases, the average distance between protons and neutrons increases. Because the strong nuclear force only acts over short ranges, the possibility of many attractions is never realized in a large nucleus. As a result, the strong nuclear force felt by one proton or neutron in a large nucleus is about the same as a in a small nucleus.

Why do nuclear power plants choose uranium as their nuclear energy source?

The nuclei of uranium atoms split easily.

Speed of a Chain Reaction

The speed of a chain reaction can vary. In an uncontrolled chain reaction, all of the released neutrons are free to cause other fissions, resulting in a gast, intense release of energy. Nuclear weapons are designed to produce uncontrolled chain reactions. In a controlled chain reaction, some of the neutrons are absorbed by nonfissionable materials, resulting in only one new fission for each splitting of an atom. The heat from controlled chain reactions can be used to generate electrical energy. Unfortuantely, another product of controlled chain reactions is radioactive waste.

Strong Nuclear Force

The strong nuclear force is the attractive force that binds protons and neutrons together in the nucleus. Because the strong nuclear force does not depend on charge, it acts among protons, among neutrons, and among protons and neutrons. Over very short distances, the strong nuclear force is much greater than the electric forces among protons. For example, at distances as short as the width of a proton, the strong nuclear force is more than 100 ties greater than the electric force that repels protons. However, the strong nuclear force quickly weakens as protons and neutrons get farther apart. Figure 15 summarizes the forces acting on protons and neutrons in the nucleus.

Nuclear Energy from Fission

Today, nuclear power plants generate about 20 percent of the electricity in the United States. In a nuclear power plant, controlled fission of uranium-235 occurs in a vessel called a fission reactor. Unlike power plants that burn fossil fuels, nuclear power plants do not emit air pollutants such as oxides of sulfur and nitrogen. However, nuclear power plants have their own safety and environment issues. For example workers in nuclear power plants need to wear protective clothing to reduce their exposure to nuclear radiation. In addition, the fission of uranium-235 produces many radioactive isotopes with half-lives of hundreds or thousands of years. The radioactive waste must be isolated and stored so that it cannot harm people or contaminate the environment while it decays.