25.3 Fission and Fusion > 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 25 Nuclear Chemistry 25.1 Nuclear Radiation 25.2 Nuclear Transformations 25.3 Fission and Fusion 25.4 Radiation in Your Life

25.3 Fission and Fusion > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Where does the sun’s energy come from? CHEMISTRY & YOU The sun is about halfway through its life cycle. It has been producing energy for about 5 billion years and is expected to continue to produce energy for about 5 billion more.

25.3 Fission and Fusion > 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission What happens in a nuclear chain reaction?

25.3 Fission and Fusion > 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission When the nuclei of certain isotopes are bombarded with neutrons, the nuclei split into smaller fragments. This process is called fission.

25.3 Fission and Fusion > 5 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission The figure below shows how uranium-235 breaks into two smaller fragments of roughly the same size when struck by a slow-moving neutron. More neutrons are released by the fission. These neutrons strike the nuclei of other uranium-235 atoms, which causes a chain reaction. U Uranium-235 (fissionable) U Uranium-236 (very unstable) Ba Barium Kr Krypton n 1010 Neutron

25.3 Fission and Fusion > 6 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission In a chain reaction, some of the emitted neutrons react with other fissionable atoms, which emit neutrons that react with still more fissionable atoms.

25.3 Fission and Fusion > 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Nuclear fission can release enormous amounts of energy. The fission of 1 kg of uranium-235 yields an amount of energy equal to that produced when 20,000 tons of dynamite explode. An atomic bomb is a device that can trigger an uncontrolled nuclear chain reaction.

25.3 Fission and Fusion > 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Nuclear reactors use controlled fission to produce useful energy. The reaction takes place within uranium- 235 or plutonium-239 fuel rods. A coolant absorbs heat produced by the controlled fission reaction and transfers the heat to water, which changes to steam. The steam drives a turbine, which drives a generator that produces electricity.

25.3 Fission and Fusion > 9 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Nuclear reactors use controlled fission to produce useful energy.

25.3 Fission and Fusion > 10 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Neutron Moderation Nuclear moderation is a process that slows down neutrons so the reactor fuel can capture them to continue the chain reaction.

25.3 Fission and Fusion > 11 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Neutron Moderation Nuclear moderation is a process that slows down neutrons so the reactor fuel can capture them to continue the chain reaction. Moderation is necessary because most of the neutrons produced move so fast that they would pass right through a nucleus without being captured.

25.3 Fission and Fusion > 12 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Neutron Moderation Nuclear moderation is a process that slows down neutrons so the reactor fuel can capture them to continue the chain reaction. Moderation is necessary because most of the neutrons produced move so fast that they would pass right through a nucleus without being captured. Water and carbon in the form of graphite are good moderators.

25.3 Fission and Fusion > 13 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Neutron Absorption To prevent the chain reaction from going too fast, some of the slowed neutrons must be trapped before they hit fissionable atoms.

25.3 Fission and Fusion > 14 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission 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 slow-moving neutrons.

25.3 Fission and Fusion > 15 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Neutron Absorption Control rods, made of materials such as cadmium, are used to absorb neutrons.

25.3 Fission and Fusion > 16 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Neutron Absorption Control rods, made of materials such as cadmium, are used to absorb neutrons. When 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.

25.3 Fission and Fusion > 17 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Neutron Absorption Control rods, made of materials such as cadmium, are used to absorb neutrons. If the chain reaction were to go too fast, heat might be produced faster than the coolant could remove it. Ultimately, a meltdown of the reactor core might occur.

25.3 Fission and Fusion > 18 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission 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.

25.3 Fission and Fusion > 19 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission 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. During fission, the amount of fissionable isotope in each rod decreases. Eventually the rods no longer have enough fuel to ensure that the output of the power station remains constant.

25.3 Fission and Fusion > 20 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Nuclear Waste Spent fuel rods are classified as high-level nuclear waste. 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.

25.3 Fission and Fusion > 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Nuclear Waste Spent fuel rods are classified as high-level nuclear waste. At some nuclear power plants, the storage pool has no space left.

25.3 Fission and Fusion > 22 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fission Nuclear Waste Spent fuel rods are classified as high-level nuclear waste. At some nuclear power plants, the storage pool has no space left. Finding appropriate storage sites is difficult because high-level waste may need to be stored for a long time. –Plutonium-239, for example, will not decay to safe levels for 20,000 years.

25.3 Fission and Fusion > 23 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. How does the fission of a uranium-235 nucleus cause a chain reaction?

25.3 Fission and Fusion > 24 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. How does the fission of a uranium-235 nucleus cause a chain reaction? When slow-moving neutrons bombard uranium- 235, the atom splits and releases more neutrons. These neutrons then collide with more uranium atoms, and so on.

25.3 Fission and Fusion > 25 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion How do fission reactions and fusion reactions differ?

25.3 Fission and Fusion > 26 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion The energy emitted by the sun results from nuclear fusion. Fusion occurs when nuclei combine to produce a nucleus of greater mass.

25.3 Fission and Fusion > 27 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion The energy emitted 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.

25.3 Fission and Fusion > 28 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion The energy emitted 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. The reaction also produces two positrons.

25.3 Fission and Fusion > 29 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large nuclei split apart and form smaller nuclei.

25.3 Fission and Fusion > 30 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large nuclei split apart and form smaller nuclei. However, fusion reactions occur only at very high temperatures—in excess of 40,000,000°C.

25.3 Fission and Fusion > 31 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion The use of controlled nuclear fusion as an energy source on Earth is appealing. The potential fuels are inexpensive and readily available.

25.3 Fission and Fusion > 32 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion The use of controlled nuclear fusion as an energy source on Earth is appealing. The potential fuels are inexpensive and readily available. The problems with fusion lie in achieving the high temperatures needed to start the reaction and in containing the reaction once it has started.

25.3 Fission and Fusion > 33 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Nuclear Fusion The high temperatures required to start 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. This process is of no use, however, as a controlled generator of power.

25.3 Fission and Fusion > 34 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The high temperatures needed to support fusion exist within the sun’s core. Late in the sun’s life cycle, other fusion reactions will occur. What element would form when two helium nuclei fuse? CHEMISTRY & YOU

25.3 Fission and Fusion > 35 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The high temperatures needed to support fusion exist within the sun’s core. Late in the sun’s life cycle, other fusion reactions will occur. What element would form when two helium nuclei fuse? CHEMISTRY & YOU Two helium nuclei fuse to form an unstable isotope of beryllium. Be 8484 He + He →

25.3 Fission and Fusion > 36 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Choose the correct words for the spaces. In solar fusion, _______ nuclei fuse to form _______ nuclei.

25.3 Fission and Fusion > 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Choose the correct words for the spaces. In solar fusion, _______ nuclei fuse to form _______ nuclei. Hydrogen nuclei fuse to form helium nuclei.

25.3 Fission and Fusion > 38 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts In a chain reaction, some of the emitted neutrons react with other fissionable atoms, which emit neutrons that react with still more fissionable atoms. Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large nuclei split apart to form smaller nuclei.

25.3 Fission and Fusion > 39 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms fission: the splitting of a nucleus into smaller fragments, accompanied by the release of neutrons and a large amount of energy neutron moderation: a process used in nuclear reactors to slow down neutrons so the reactor fuel captures them to continue the chain reaction

25.3 Fission and Fusion > 40 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms neutron absorption: a process that decreases the number of slow-moving neutrons in a nuclear reactor; this is accomplished by using control rods made of a material such as cadmium, which absorbs neutrons fusion: the process of combining nuclei to produce a nucleus of greater mass

25.3 Fission and Fusion > 41 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrons and the Structure of Atoms During fission and fusion, atoms change their chemical identity as the number of protons in their nuclei change. In fission, large nuclei split into two or more smaller nuclei. In fusion, smaller nuclei combine to form larger nuclei at extremely high temperature and pressure. BIG IDEA

25.3 Fission and Fusion > 42 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. END OF 25.3