91A Few Final Comments on Energy from Nuclear ProcessesAt present, only nuclear fissionis used for the production of power. Reactions like those discussed on the previous page liberate their energy primarily in the kinetic energy of the products. That energy is transferred to a moderator, generally H2O, and the water is then boiled to make steam. The steam runs a turbine, and the turbine generates electricity. So, the energy from nuclear power is extracted as heat. The Carnot efficiency can be calculated for such a process. Suppose energy is made (i.e., heat is absorbed) at 373 K, and the energy is discharged at around 310K. The Carnot efficiency is ε- 1 –310/373 = 0.17 A boiling water reactor pressurizes the water and allows operation at Thof 530 K. Efficiency goes to about 32%. This is pretty low, but there is no CO2(greenhouse gas) emission from such a reactor. However, the biggest problem to deal with is the radioactive waste disposal. Half-lives of some of the fission products can be hundreds or thousands of years. Containing such waste for this period of time is an unsolved problem. Breeder ReactorsThe U-235 fuel needed for a conventional reactor is in short supply, so methods that start with the much more abundant U-238 isotope have been developed. U23892+ n10→⎯→⎯βU23992PuNp2399423993⎯→⎯βThis is a “breeding” process because the neutrons from U-235 fission reacts with U-238 (unused in a conventional reactor) to make Pu-239, which can also be used in a light water reactor. A major problem is the toxicityof Pu in non-nuclear applications. Poisoning of municipal water supplies is one such concern. Breeder reactors offer another advantage, in that the moderator is generally a liquid metal, like lithium or sodium. Sodium boils near 800 K, so the operating temperature First, a comment on calculating energetics of nuclear reactions: When we do these calculations involving nuclei, we use the masses of atoms including the electrons. Is this proper? The answer is YES– with one exception. The sole exception to this rule occurs for positron emission. In positron emission, a proton in the nucleus is converted to a neutron, and a positron (the electron’s antiparticle – same mass, spin, opposite charge) is emitted. We’ll see how this works later.
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