Unit 10: Energy Challenges // Section 6: Nuclear Power
Nuclear energy, which generates about 17 percent of world electricity supplies (roughly 6 percent of total energy consumption), is produced by enhancing the radioactive decay of naturally fissile materials—elements whose atoms can be split by thermal (slow) neutrons, releasing energy. About 0.7 percent of natural uranium consists of the isotope uranium-235, which is fissile and is the most widely-used fuel in standard nuclear reactors. The remainder is the more stable uranium-238.
To exploit this energy source, companies mine uranium ore and, by a process called uranium enrichment, increase the concentration of U-235 to about 4 percent. Enriched uranium is formed into fuel rods or pellets, which are placed inside a nuclear reactor and bombarded by neutrons. This process causes U-235 atoms to split into two or more smaller atoms, called daughter products, and releases large amounts of energy. This process also releases excess neutrons, which split other U-235 atoms, causing a nuclear fission chain reaction.
Operators control the rate of fission using control rods and moderators that absorb excess neutrons and by adjusting the reactor temperature, which affects the reaction rate. Energy generated in the reactor heats water, steam, or some other fluid, which is pumped from the reactor and used to produce steam that drives electric turbines (Fig. 11).
Figure 11. Pressurized-water reactor
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Source: Courtesy University of Wisconsin-Madison.
Nuclear power is a well-established method of electric power generation. Uranium is abundant, and a number of countries are making substantial investments in new nuclear power reactors. The United States has more than 100 licensed commercial nuclear power reactors, but no new reactor has been ordered since 1978, although interest has revived in recent years.
Major obstacles to the expansion of nuclear power worldwide include concerns about safety and high capital costs compared to other energy sources. Nuclear accidents at the Three Mile Island plant in Harrisburg, Pennsylvania, in 1979 and the Chernobyl reactor in Ukraine in 1986 convinced many people that nuclear power was unsafe. Chernobyl caused more than 30 deaths in the days immediately following the accident (from acute radiation exposure), and widespread exposure to radioactivity from the accident over a large part of the Northern hemisphere may ultimately lead to tens of thousands of deaths from cancer over a period of decades. Although both accidents were largely results of human errors, and modern facilities have much more substantial safety procedures, these events demonstrated that nuclear accidents were possible.
Spent nuclear fuels remain highly radioactive for thousands of years, and finding appropriate sites to store radioactive waste is a highly contentious issue in virtually every nuclear nation. The United States is struggling to build and license a national repository at Yucca Mountain, Nevada, after decades of study (Fig. 12), but concerns persist about whether the site's complex geology can isolate nuclear waste from the environment until its radioactivity decays to background levels. This failure has forced many nuclear power stations to store their spent fuel onsite for years longer than owners planned and has undercut public support for new nuclear reactors in the United States.
Figure 12. Main tunnel shaft, Yucca Mountain repository site
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Source: Courtesy Daniel Mayer, 2002. Wikimedia Commons, GNU General Public License.
In addition to these environmental impacts, nuclear power also raises security concerns because it produces two types of fissile material that can be used in nuclear weapons. First, as noted above, uranium fuel for commercial power reactors is enriched to a concentration of about 4 percent U-235. Although this low-enriched uranium is not usable for weapons, the same facilities can often enrich uranium to 90 percent U-235 or higher, and this highly enriched uranium is the easiest material from which to make a nuclear weapon.
Second, when nuclear fuel is irradiated in a reactor, a portion is converted to plutonium, which is also fissile and, given somewhat greater skill on the part of weapon-makers than is needed for a uranium bomb, can also serve in the weapon role. The process of plutonium production is enhanced in certain modern reactor designs called fast breeder systems, which use plutonium as an additional source of nuclear fuel and are now in use in several nuclear nations. Plutonium fuel cycles pose increased proliferation risks because plutonium can be stolen or diverted while it is being handled in bulk quantities during fuel processing.