chapter30 - Chapter 30 Nuclear Energy and Elementary...

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Chapter 30 Nuclear Energy and Elementary Particles
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Processes of Nuclear Energy Fission A nucleus of large mass number splits into two smaller nuclei Fusion Two light nuclei fuse to form a heavier nucleus Large amounts of energy are released in either case
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Nuclear Fission A heavy nucleus splits into two smaller nuclei The total mass of the products is less than the original mass of the heavy nucleus
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Fission Equation Fission of 235 U by a slow (low energy) neutron 236 U* is an intermediate, short-lived state Lasts about 10 -12 s X and Y are called fission fragments Many combinations of X and Y satisfy the requirements of conservation of energy and charge 1 235 236 0 92 92 * n U U X Y neutrons
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More About Fission of 235 U About 90 different daughter nuclei can be formed Several neutrons are also produced in each fission event Example: The fission fragments and the neutrons have a great deal of KE following the event 1 235 141 92 1 0 92 56 36 0 3 n U Ba Kr n
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Sequence of Events in Fission The 235 U nucleus captures a thermal (slow- moving) neutron This capture results in the formation of 236 U*, and the excess energy of this nucleus causes it to undergo violent oscillations The 236 U* nucleus becomes highly elongated, and the force of repulsion between the protons tends to increase the distortion The nucleus splits into two fragments, emitting several neutrons in the process
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Sequence of Events in Fission Diagram
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Energy in a Fission Process Binding energy for heavy nuclei is about 7.2 MeV per nucleon Binding energy for intermediate nuclei is about 8.2 MeV per nucleon Therefore, the fission fragments have less mass than the nucleons in the original nuclei This decrease in mass per nucleon appears as released energy in the fission event
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Energy, cont An estimate of the energy released Assume a total of 240 nucleons Releases about 1 MeV per nucleon 8.2 MeV 7.2 MeV Total energy released is about 240 Mev This is very large compared to the amount of energy released in chemical processes
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Chain Reaction Neutrons are emitted when 235 U undergoes fission These neutrons are then available to trigger fission in other nuclei This process is called a chain reaction If uncontrolled, a violent explosion can occur The principle behind the nuclear bomb, where 1 kg of U can release energy equal to about 20 000 tons of TNT
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Chain Reaction Diagram
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Nuclear Reactor A nuclear reactor is a system designed to maintain a self-sustained chain reaction The reproduction constant , K, is defined as the average number of neutrons from each fission event that will cause another fission event The maximum value of K from uranium fission is 2.5 In practice, K is less than this A self-sustained reaction has K = 1
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K Values When K = 1, the reactor is said to be critical The chain reaction is self-sustaining When K < 1, the reactor is said to be
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This note was uploaded on 04/12/2011 for the course PHYS 1410 taught by Professor Staff during the Spring '08 term at North Texas.

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chapter30 - Chapter 30 Nuclear Energy and Elementary...

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