FIGURE 5.1
5.2), the heat from the reaction is carried away by water under high pressure, which heats a second water stream, producing steam. The steam runs through a turbine (similar to a jet engine), making it and the attached electrical generator spin, which produces electricity. The steam is then cooled and recirculated. The large cooling towers associated with nuclear plants are used to cool the steam after it has run through the turbines. A boiling water reactor works much the same way, except that the water surrounding the core boils and directly produces the steam, which is then piped to the turbine generator.
FIGURE 5.2
The key problems in operating a nuclear power reactor include finding material (uranium 235) that will sustain a chain reaction, maintaining the reaction at a level that yields heat but does not escalate out of control and explode, and coping with the radiation produced by the chain reaction.
Radioactivity
Radioactivity is the spontaneous emission of energy and/or high-energy particles from the nucleus of an atom. One type of radioactivity is produced naturally and is emitted by radioactive isotopes (or radioisotopes), such as radioactive carbon (carbon 14) and radioactive hydrogen (H-3, or tritium). The energy and high-energy particles that radioactive isotopes emit include alpha rays, beta rays, and gamma rays.
Isotopes are atoms of an element that have the usual number of protons but different numbers of neutrons in their nuclei. For example, twelve protons and twelve neutrons comprise the nucleus of the element carbon. One isotope of carbon, C-14, has twelve protons and fourteen neutrons in its nucleus.
Radioisotopes (such as C-14) are unstable isotopes and their nuclei decay, or break apart, at a steady rate. Decaying radioisotopes produce other isotopes as they emit energy and/or high-energy particles. If the newly formed nuclei are radioactive as well, they emit radiation and change into other nuclei. The final products in this chain are stable, nonradioactive nuclei.
Radioisotopes reach our bodies daily, emitted from sources in outer space, and from rocks and soil on earth. Radioisotopes are also used in medicine and provide useful diagnostic tools. Figure 5.3 shows the sources of radiation.
FIGURE 5.3
As shown in Figure 5.3, radon is the largest source of radiation to which humans are exposed. This gas is formed in rocks and soil from the radioactive decay of radium. Most prevalent in the northern half of the United States, radon can enter cracks in basement walls and remain trapped there. Prolonged exposure to high levels of radioactive radon is thought to lead to lung cancer.
Decades after the discovery of radiation at the turn of the century by Antoine Henri Becquerel, Marie Curie, and Pierre Curie, other scientists determined that they could unleash energy by "artificially" breaking apart atomic nuclei. Such a process is called nuclear fission. Scientists learned that they could produce the most energy by bombarding the nuclei of an isotope of uranium called uranium 235 (U-235). The fission of U-235 releases several neutrons, which can then penetrate other U-235 nuclei. In this way the fission of a single U-235 atom could begin a cascading chain of nuclear reactions, as shown in Figure 5.4. If this series of reactions is regulated to occur slowly, as it is in nuclear power plants, the energy emitted can be captured for a variety of uses, such as generating electricity. If this series of reactions is allowed to occur all at once, as in a nuclear (atomic) bomb, the energy emitted is explosive. (Plutonium 239 can also be used to generate a chain reaction similar to that of U-235.)
Mining Nuclear Fuel
In the United States U-235, found in the form of ore, is used as nuclear fuel. The majority of uranium in the United States is found in two states: Wyoming and New Mexico. Ore that contains uranium is first located by geological methods such as drilling. Uranium-bearing ores are mined by methods similar to those used for other metal ores. However, uranium mining has the unique danger
FIGURE 5.4
of exposure to radioactivity. Uranium atoms split by themselves at a slow rate, causing radioactive substances such as radon to accumulate slowly in the deposits.
After it is mined, uranium must be concentrated, because uranium ore generally contains only 0.1% uranium metal by weight. To concentrate the uranium, it goes through a process called milling. The nuclear fuel cycle, which is shown in Figure 5.5, begins with the milling of uranium ore. In milling the ore is first crushed, and then various chemicals are poured slowly through the crushed ore to dis-solve out the uranium. The uranium is then precipitated from this chemical solution. The resulting material, called yellowcake because of its color, is 85% pure uranium by weight. But this uranium is 99.3% nonfissionable U-238 and only 0.7% fissionable U-235. Other processes result in enriched uranium, which is uranium that has a higher percentage of fissionable uranium than does yellowcake.
To produce enriched uranium, yellowcake is converted into uranium hexafluoride gas (UF6). This gas is loaded into cylinders, which are sent to a gaseous diffusion plant, shown in Figure 5.5, where uranium is made into reactor fuel (enriched). The enriched uranium is converted into oxide powder (UO2), which is made into fingertip-sized fuel pellets. The small pellets are less than one-half inch in diameter, but each one can produce as much energy as 120 gallons of oil. The pellets are stacked in tubes about twelve feet long, called rods. Many rods are bundled together in assemblies, and hundreds of these assemblies make up the core of a nuclear reactor.
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