25.3 Fission and Fusion of Atomic Nuclei 25.3 FOCUS. Guide for Reading INSTRUCT. Nuclear Fission. Section Resources.

Size: px

Start display at page:

Download "25.3 Fission and Fusion of Atomic Nuclei 25.3 FOCUS. Guide for Reading INSTRUCT. Nuclear Fission. Section Resources."

Paula Murphy
5 years ago
Views:

25.3 25.3 Fission and Fusion of Atomic Nuclei 1 FOCUS Objectives 25.3.1 Describe what happens in a nuclear chain reaction. 25.3.2 Explain the role of water in the storage of spent fuel rods. 25.3.3 Distinguish fission reactions from fusion reactions.

Reading Strategy Preview Before students read the section in detail, have them preview the section headings, visuals, and boldfaced material.

How do fission reactions and fusion reactions differ?

1 Fission and Fusion of Atomic Nuclei 1 FOCUS Objectives Describe what happens in a nuclear chain reaction Explain the role of water in the storage of spent fuel rods Distinguish fission reactions from fusion reactions. Guide for Reading Build Vocabulary Graphic Organizers Have students draw a concept map that correctly relates the vocabulary terms for this section. Reading Strategy Preview Before students read the section in detail, have them preview the section headings, visuals, and boldfaced material. 2 INSTRUCT Guide for Reading Key Concepts What happens in a nuclear chain reaction? Why are spent fuel rods from a nuclear reaction stored in water? How do fission reactions and fusion reactions differ? Vocabulary fission neutron moderation neutron absorption fusion Reading Strategy Identifying Details In the three paragraphs following the heading Nuclear Fission, the sentence describing a chain reaction states the main idea. List the details about energy changes in fission and chain reactions that are related to the main idea. The sun appears as a fiery ball in the sky so bright it should never be viewed with unprotected eyes. Although its surface temperature is about 5800 K, the sun is not actually burning. If the energy given off by the sun were the product of a combustion reaction, the sun would have burned out approximately 2000 years after it was formed, long before today. In this section, you will learn how energy is produced in the sun. Nuclear Fission When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the splitting of a nucleus into smaller fragments. Uranium- 235 and plutonium-239 are the only fissionable isotopes. Figure shows how a fissionable atom, such as uranium-235, breaks into two fragments of roughly the same size when struck by a slow-moving neutron. At the same time, more neutrons are released by the fission. These neutrons strike the nuclei of other uranium-235 atoms, continuing the fission by a chain reaction. In a chain reaction, some of the neutrons produced react with other fissionable atoms, producing more neutrons which react with still more fissionable atoms. Nuclear fission can release enormous amounts of energy. The fission of 1 kg of uranium-235, for example, yields an amount of energy equal to that generated in the explosion of 20,000 tons of dynamite. In an uncontrolled nuclear chain reaction, the total energy release takes only fractions of a second. Atomic bombs are devices that start uncontrolled nuclear chain reactions. Have students study the photograph and read the text that opens the section. Explain that the sun contains hydrogen nuclei. Ask, In which state of matter do these hydrogen nuclei exist? (plasma) Describe a hydrogen nucleus. (a proton) Explain that protons can combine to produce helium nuclei. Ask, How is the energy of the sun produced? (Energy is released during the formation of helium nuclei.) Nuclear Fission Discuss Ask, What is the general meaning of fission? (An object splits into smaller parts.) What is the general meaning of fusion? (Objects combine into a larger whole.) Explain that these terms have more precise meanings when applied to nuclear reactions but still include the concepts of fragmentation and merging. Figure In nuclear fission, a uranium-235 nucleus breaks into two smaller nuclei and releases neutrons. Predicting What happens when the released neutrons strike other uranium-235 nuclei? 810 Chapter 25 Section Resources Neutron U Uranium-235 (fissionable) Print Guided Reading and Study Workbook, Section 25.3 Core Teaching Resources, Section 25.3 Review Transparency, T U Uranium-236 (very unstable) Energy Krypton Kr Ba Barium-142 Technology Interactive Textbook with ChemASAP, Animation 30, Assessment Chapter 25

Containment shell Control rods Fuel rods Carbon moderator Reactor Cool coolant Fission can be controlled so energy is released more slowly. Nuclear reactors, such as the one illustrated in Figure 25.

A coolant fluid, usually liquid sodium or water, removes the heat from the reactor core. The heat is used to generate steam, which drives a turbine that in turn generates electricity.

2 Containment shell Control rods Fuel rods Carbon moderator Reactor Cool coolant Fission can be controlled so energy is released more slowly. Nuclear reactors, such as the one illustrated in Figure 25.11, use controlled fission to produce useful energy. In the controlled fission reaction within a nuclear reactor, much of the energy generated is in the form of heat. A coolant fluid, usually liquid sodium or water, removes the heat from the reactor core. The heat is used to generate steam, which drives a turbine that in turn generates electricity. The control of fission in a nuclear reactor involves two steps, neutron moderation and neutron absorption. Neutron Moderation Neutron moderation is a process that slows down neutrons so the reactor fuel (uranium-235 or plutonium-239) captures them to continue the chain reaction. Moderation is necessary because most of the neutrons produced move so fast that they will pass right through a nucleus without being absorbed. Water and carbon in the form of graphite are good moderators. Neutron Absorption To prevent the chain reaction from going too fast, some of the slowed neutrons must be trapped before they hit fissionable atoms. Neutron absorption is a process that decreases the number of slowmoving neutrons. Control rods, made of a material such as cadmium, are used to absorb neutrons. When the control rods extend almost all the way into the reactor core, they absorb many neutrons, and fission occurs slowly. As the rods are pulled out, they absorb fewer neutrons and the fission process speeds up. If the chain reaction were to go too fast, heat might be produced faster than the coolant could remove it. In this case, the reactor core would overheat, which could lead to mechanical failure and release of radioactive materials into the atmosphere. Ultimately, a meltdown of the reactor core might occur. Checkpoint Steam Hot coolant Pump Steam generator Pump Electrical output Condenser (steam from turbine is condensed) 38 o C Water source 27 o C Why must neutrons in a reactor be slowed down? Differentiated Instruction English Learners Have students write the definitions of fission and fusion in their notebooks in their native language and in English. Have them prepare a table, complete with sample reactions, describing the characteristics of fission and fusion processes in nuclear chemistry. Figure A nuclear reactor uses controlled fission to produce useful energy. The illustration shows the basic components of a nuclear reactor. Energy from the fission process heats the circulating coolant. The heated coolant is used to produce steam that turns a steam-driven turbine. The turbine drives a generator to produce electrical energy. Animation 30 Take a close look at a nuclear fission chain reaction. withchemasap Section 25.3 Fission and Fusion of Atomic Nuclei 811 Use Visuals L1 Figure Display the figure on an overhead projector and explain that uranium-235 does not spontaneously fission. Uranium-235 is called a fissionable material because its nucleus becomes unstable when it is struck by a neutron. Note that two additional neutrons (for a total of three) are produced by each fission event. Fission reactions also produce radioactive waste products such as krypton-91 and barium-142. Ask, Which types of radiation do these two products emit? (Barium-142 and krypton-91 both emit beta particles.) Have students draw a diagram showing how the neutrons from the single fission process in Figure can be used to induce subsequent fissions in other uranium-235 nuclei. The diagrams should illustrate the exponential growth of fission processes. Have students use their diagrams to discuss why fission reactors must be carefully monitored and controlled. TEACHER Demo Model a Chain Reaction Purpose Students observe a chain reaction. Materials wooden matchsticks Safety Keep all flammable materials away from the demonstration area. Have a fire extinguisher ready. Procedure Cut some wooden matchsticks in half and arrange the heads in branching chains on a noncombustible surface. Construct the chain so that the end of each matchstick is touching the heads of two other matchsticks, forming a Y. Ignite the first matchstick head. Expected Outcome The matches ignite in a chain reaction. Answers to... Figure They react and cause the production of more neutrons. Checkpoint so the reactor fuel can capture them to continue the chain reaction Nuclear Chemistry 811

3 Section 25.3 (continued) Nuclear Waste Discuss Have students compare and contrast nuclear fission and nuclear fusion. Ask students to describe the advantages and disadvantages of each type of process as a means to meet future energy needs. Ask, What types of technical hurdles remain to be solved in order to utilize the energy produced by nuclear fusion? (Accept any reasonable answer, such as containment of the reaction.) Relate Explain that the wastes produced in fission reactors contain isotopes with half-lives measured in the thousands or hundreds of thousands of years. Many proposals for storing or disposing of these wastes involve methods and materials that may be highly unsuitable. For example, the placement of wastes in thick drums that would then be sunk in the oceans one suggested method may contain the waste for only decades or a few centuries. The containers would begin to leak long before the contents were safe. The disposal problem has led some people to propose permanent abandonment of nuclear reactors as sources of energy. After students read the text, lead a discussion on the pros and cons of using nuclear energy from fission reactors. Figure Racks at the bottom of this pool contain spent fuel rods. The blue glow is from beta particles that the rods emit into the water. Nuclear Waste Fuel rods from nuclear power plants are one major source of nuclear waste. The fuel rods are made from a fissionable isotope, either uranium-235 or plutonium-239. The fuel rods are long and narrow typically 3 meters long with a 0.5-cm diameter. Three hundred fuel rods are bundled together to form an assembly, and one hundred assemblies are arranged to form the reactor core. During fission, the amount of fissionable isotope in each fuel rod decreases. Eventually there is no longer enough fuel in the rods to ensure that the output of the power station remains constant. The isotope-depleted, or spent, fuel rods must be removed and replaced with new fuel rods. Spent fuel rods are classified as high-level nuclear waste. They contain a mixture of highly radioactive isotopes, including both the fission products and what remains of the nuclear fuel. Some of these fission products have very short half-lives, on the order of fractions of seconds. Others have half-lives of hundreds or thousands of years. All nuclear power plants have holding tanks, or swimming pools, for spent fuel rods. Water cools the spent rods, and also acts as a radiation shield to reduce the radiation levels. The pools, like the one shown in Figure 25.12, are typically 12 meters deep, and are filled with water. Storage racks at the bottom of these pools are designed to hold the spent fuel assemblies. The rods continue to produce heat for years after their removal from the core. The assemblies of spent fuel rods may spend a decade or more in a holding tank. In the past, plant operators expected used fuel rods to be reprocessed to recover the remaining fissionable isotope, which would be recycled in the manufacture of new fuel rods. However, with large deposits of uranium ore available many in the United States it is less expensive to mine new fuel than to reprocess depleted fuel. At some nuclear plants, there is no space left in the storage pool. In order to keep these plants open, their fuel rods must be moved to off-site storage facilities. Facts and Figures Nuclear Waste and Nuclear Power Concerns about nuclear wastes are magnified by the timeframe during which the wastes will be hazardous. As of 1997, 440 nuclear power plants were in operation worldwide and about 30 countries got some of their electricity from nuclear power stations. In France, for example, 75% of the electricity is generated by nuclear power. By contrast, only about 20% of electricity generated in the United States comes from nuclear power plants. The DOE is responsible for cleaning up 130 nuclear sites and safely managing their waste. The sites include locations where uranium was milled, research labs, and former nuclear weapons production facilities. Have students research how the cleanups are progressing. 812 Chapter 25

4 1 1 4 H 2+1 e hydrogen nuclei helium nucleus positrons Nuclear Fusion 4 2 He The sun, directly and indirectly, is the source of most energy used on Earth. The energy released by the sun results from nuclear fusion. Fusion occurs when nuclei combine to produce a nucleus of greater mass. In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei. Figure shows that the reaction also produces two positrons. Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large nuclei split. However, fusion reactions occur only at very high temperatures in excess of 40,000,000 C. The use of controlled nuclear fusion as an energy source on Earth is appealing. The potential fuels are inexpensive and readily available. One reaction that scientists are studying is the combination of a deuterium (hydrogen-2) nucleus and a tritium (hydrogen-3) nucleus to form a helium nucleus. 2 1 H 3 1 H 4 2 He 1 0n energy The problems with fusion lie in achieving the high temperatures necessary to start the reaction and in containing the reaction once it has started. The high temperatures required to initiate fusion reactions have been achieved by using a fission bomb. Such a bomb is the triggering device used for setting off a hydrogen bomb, which is an uncontrolled-fusion device. Such a process is clearly of no use, however, as a controlled generator of power Section Assessment 15. Key Concept Explain what happens in a nuclear chain reaction. 16. Key Concept Why are spent fuel rods from a nuclear reaction stored in water? 17. Key Concept How are fusion reactions different from fission reactions? 18. What does nuclear moderation accomplish in a nuclear reactor? 19. What is the source of the radioactive nuclei present in spent fuel rods? 20. Assuming technical problems could be overcome, what are some advantages to using a fusion reactor to produce electricity? 0 Handbook Assessment 25.3 Test yourself on the concepts in Section withchemasap Figure In solar fusion, hydrogen nuclei fuse to produce helium nuclei. Interpreting Diagrams What other particles are produced by this reaction? Word Origins Fusion comes from the Latin word fusus meaning melt together. Fusion is the combination of two low-mass nuclei to form a nucleus of larger mass, accompanied by the release of a large amount of energy. In politics, what might be the goal of a fusionist? Heavy Water Reactors Read the paragraph on Heavy Water Reactors on page R39 of the Elements Handbook. Use what you have learned about nuclear reactors to explain what a neutron moderator does, and then explain the advantages of using heavy water instead of ordinary water as a neutron moderator. Nuclear Fusion Word Origins A fusionist might bring together different factions into one cohesive group. 3 ASSESS Evaluate Understanding Ask students to write nuclear equations describing fission and fusion. Ask, How might a meltdown of a nuclear reactor occur? (Rapid removal of the control rods could allow too many neutrons to react with fissionable nuclei.) Describe the conditions under which nuclear fusion will occur. (temperatures in excess of C, methods to confine and control the dense plasmas are needed) Reteach L1 Explain that both nuclear fission and nuclear fusion produce energy by the conversion of matter to energy. In fission, heavy nuclei are split into lighter nuclei. In fusion, light nuclei combine to form heavier nuclei. Fission reactions are relatively easy to control but produce radioactive wastes. Fusion reactions are difficult to initiate and control but produce little radioactive waste. Elements Handbook A neutron moderator slows neutrons so that reactor-fuel nuclei may capture them and sustain the chain reaction. D 2 O is a more efficient moderator than H 2 O because D 2 O absorbs fewer neutrons. Section 25.3 Fission and Fusion of Atomic Nuclei Neutrons produced by fissionable atoms react with other fissionable atoms, producing more neutrons that react with other fissionable atoms. 16. Water cools spent fuel rods and provides a radiation shield. Section 25.3 Assessment 17. Fission reactions involve splitting nuclei. In fusion reactions, small nuclei combine and release much more energy. 18. slows down neutrons 19. unused nuclear fuel and fission products 20. Potential fuels are inexpensive and readily available. If your class subscribes to the Interactive Textbook, use it to review key concepts in Section with ChemASAP Answers to... Figure positrons Nuclear Chemistry 813

5 Dating a Fossil Discuss Discuss with students the limits of using carbon-14 to date fossils. First, some organic material must be present, as the material tested must be from an organism that once lived. If only an imprint remains and all organic material is gone, therefore, carbon-14 cannot be used in dating the fossil. Second, after a certain time, so little carbon-14 remains that it cannot be detected. Experts disagree as to what this age is, but it is commonly accepted that carbon-14 dating cannot be used reliably on objects that are more than 75,000 years old. Ask, To the nearest whole number, how many half-lives of carbon-14 is 75,000 years? (75,000 years/5730 years per half-life = 13 halflives. ) What percent of the carbon-14 remains after this number of half lives? (0.0012%) Dating a Fossil Direct information about earlier life on Earth comes from fossils and artifacts. The age of an artifact of biological origin, such as bone, wood, cloth, and plant fibers, from 200 to about 50,000 years old can be determined by carbon-14 dating. All living organisms contain carbon-12 and carbon-14 in a fixed ratio. After an organism dies, the ratio of carbon-12 to carbon-14 changes. Carbon-14 is radioactive with a half-life of 5730 years and decays to nitrogen-14, while carbon-12 remains constant. Calculating An unearthed wooden tool was found to have only 50% of the carbon-14 content of a sample of living wood. How old is the wooden tool? Archaeology Archaeologists study the life and culture of the past, especially ancient peoples, by excavating ancient cities, relics, and artifacts. The Wooly Mammoth The fossil remains of a woolly mammoth are found in Pleistocene deposits (the Pleistocene epoch was from 2,500,000 to 10,000 years ago). The abundance of well-preserved carcasses in the permanently frozen ground of Siberia have provided much information about these extinct animals. C. 23,000 BC C. 17,300 BC C. 11,600 BC C. 5,900 BC At the time of death, the carbon-12 to carbon-14 ratio of the woolly mammoth was very similar to that of an animal living today. The carbon-14 in the dead mammoth was one-half of its value in 23,000 BC. The carbon-14 was 25% of its initial value. After another 5,730 years, the carbon-14 is now 12.5% of its 23,000 BC value. 814 Chapter 25 Facts and Figures Relative Dating Fossils can also be dated by relating them to known geological events. For example, during a large volcanic eruption, vast amounts of volcanic ash are spread over large areas of the surrounding area. This ash will settle and eventually become a distinctive clay layer. Any fossil in or near this layer can be dated according to the date of the event that formed the layer. 814 Chapter 25

Mummies This mummy from Peru was dated between the late 13 th and early 14 th century by carbon-14 dating of the decorative cloth around the remains.

6 Mummies This mummy from Peru was dated between the late 13 th and early 14 th century by carbon-14 dating of the decorative cloth around the remains. Use Visuals L1 Note in the large photo that the fish fossil is in a distinct rock layer. Ask, What methods other than carbon-14 dating might be used to date the fossil? (Answers will vary but might include looking for other fossils in the layer and dating them.) Discuss how organisms found in the same rock layer probably formed at the same time. Some of these fossils might be dated by using carbon-14, and others might be from organisms that existed only during a known, short period of time. Relate Ask students to relate the half-lives in Table 25.3 on page 805 to the usefulness of isotopes in dating materials. For example, uranium-238 is used to date rocks that are quite old. If U-238 were used to date a sample that is only 50,000 years old, not enough of the U-238 would have decayed for the change to be noticed. Ask, Which of the radioisotopes listed in the table might be used to date a rock that is 20 million years old? (Answers will vary, but students might suggest potassium-40.) C. 200 BC The carbon-14 is 6.25% of its initial value. Today, AD 2005 In what year will the carbon-14 level have decreased to 3.125% of its 23,000 BC value? Woolly Mammoth This well-preserved frozen skin of a baby mammoth was discovered in frozen ground in Siberia in It had been completely refrigerated for about 25,000 years. Technology and Society 815 Differentiated Instruction Gifted and Talented L3 Have students write down the exact procedure used to work problems involving wholenumber half-lives. Then have them use scientific calculators to expand this procedure to include partial half-lives. Provide students with problems to work that involve partial half-lives. Examples might include problems such as the following: A fossil was found to contain 60% of its original carbon-14. How old is the fossil? (k = ln 2/t 1 /2 = 0.693/5,730 yr = yr 1 ; t = 1/k ln A o /A t = (1/ yr 1 )ln(1/0.60) = 4,200 yr) Answers to... Calculating 5,730 years. Nuclear Chemistry 815

Name Date Class NUCLEAR CHEMISTRY

Name Date Class NUCLEAR CHEMISTRY 25 NUCLEAR CHEMISTRY SECTION 25.1 NUCLEAR RADIATION (pages 799 802) This section describes the nature of radioactivity and the process of radioactive decay. It characterizes alpha, beta, and gamma radiation