Chapter 45 - 45 Applications of Nuclear Physics CHAPTER...

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45 CHAPTER OUTLINE 45.1 Interactions Involving Neutrons 45.2 Nuclear Fission 45.3 Nuclear Reactors 45.4 Nuclear Fusion 45.5 Radiation Damage 45.6 Radiation Detectors 45.7 Uses of Radiation Applications of Nuclear Physics ANSWERS TO QUESTIONS Q45.1 A moderator is used to slow down neutrons released in the fission of one nucleus, so that they are likely to be absorbed by another nucleus to make it fission. Q45.2 The hydrogen nuclei in water molecules have mass similar to that of a neutron, so that they can efficiently rob a fast-moving neutron of kinetic energy as they scatter it. Once the neutron is slowed down, a hydrogen nucleus can absorb it in the reaction nH H 1 2 +→ 1 1 . Q45.3 The excitation energy comes from the binding energy of the extra nucleon. Q45.4 The advantage of a fission reaction is that it can generate much more electrical energy per gram of fuel compared to fossil fuels. Also, fission reactors do not emit greenhouse gasses as combustion byproducts like fossil fuels—the only necessary environmental discharge is heat. The cost involved in producing fissile material is comparable to the cost of pumping, transporting and refining fossil fuel. The disadvantage is that some of the products of a fission reaction are radioactive—and some of those have long half-lives. The other problem is that there will be a point at which enough fuel is spent that the fuel rods do not supply power economically and need to be replaced. The fuel rods are still radioactive after removal. Both the waste and the “spent” fuel rods present serious health and environmental hazards that can last for tens of thousands of years. Accidents and sabotage involving nuclear reactors can be very serious, as can accidents and sabotage involving fossil fuels. Q45.5 The products of fusion reactors are generally not themselves unstable, while fission reactions result in a chain of reactions which almost all have some unstable products. Q45.6 For the deuterium nuclei to fuse, they must be close enough to each other for the nuclear forces to overcome the Coulomb repulsion of the protons—this is why the ion density is a factor. The more time that the nuclei in a sample spend in close proximity, the more nuclei will fuse—hence the confinement time is a factor. 595
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596 Applications of Nuclear Physics Q45.7 In a fusion reaction, the main idea is to get the nuclear forces, which act over very short distances, to overcome the Coulomb repulsion of the protons. Tritium has one more neutron in the nucleus, and thus increases the nuclear force, decreasing the necessary kinetic energy to obtain D–T fusion as compared to D–D fusion. Q45.8 The biggest obstacle is power loss due to radiation. Remember that a high temperature must be maintained to keep the fuel in a reactive plasma state. If this kinetic energy is lost due to bremsstrahlung radiation, then the probability of nuclear fusion will decrease significantly.
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Chapter 45 - 45 Applications of Nuclear Physics CHAPTER...

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