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  3. Nuclear power plants provide about 17 percent of the world’s electricity. Some countries depend more on nuclear power for electr

Nuclear power plants provide about 17 percent of the world’s electricity. Some countries depend more on nuclear power for electr

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问题     Nuclear power plants provide about 17 percent of the world’s electricity. Some countries depend more on nuclear power for electricity than others.  In France, for instance, about 75 percent of the electricity is generated from nuclear power, according to the International Atomic Energy Agency. In the United States, nuclear power supplies about 15 percent of the electricity overall, but some states get more power from nuclear plants than others. There are more than 400 nuclear power plants around the world, with more than 100 in the United States.
    Have you ever wondered how a nuclear power plant works or how safe nuclear power is? In this article, we will examine how a nuclear reactor and a power plant work.
                                  Uranium
    Uranium is a fairly common element on Earth, which was taken into the planet during the planet’s formation. Uranium is originally formed in stars. Old stars exploded, and the dust from these shattered stars aggregated together to form our planet. Uranium-238 (U-238) has an extremely long life (4.5 billion years), and therefore is still present in fairly large quantities. U-238 makes up 99 percent of the uranium on the planet. U-235 makes up about 0.7 percent of the remaining uranium found naturally, while U-234 is even rarer and is formed by the decay of U-238.
                                    Nuclear Fission (裂变)
    A nuclear fission happens when a uranium-235 nucleus with a neutron captures another neutron. The nucleus then splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom happens to split). The two new atoms then produce gamma radiation as they settle into their new states. There are three things about this induced fission process that make it especially interesting:
    The probability of a U-235 atom capturing a neutron as it passes by is fairly high. In a reactor working properly (known as the critical state) , one neutron ejected from each fission causes another fission to occur.
    The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds ( 1×1012seconds).
    An incredible amount of energy is released, in the form of heat and gamma radiation, when a single atom splits. The two atoms that result from the fission later release beta radiation and gamma radiation of their own as well. The energy released by a single fission comes from the fact that the fission products and the neutrons, together, weigh less than the original U-235 atom. The difference in weight is converted directly to energy at a rate governed by the equation E = mc2.
    In order for these properties of U-235 to work, a sample of uranium must be enriched so that it contains 2 percent to 3 percent or more of uranium-235. Three-percent enrichment is sufficient for use in a civilian nuclear reactor used for power generation. Weapons-grade uranium is composed of 90-percent or more U-235.
                                  Inside a Nuclear Power Plant
    To build a nuclear reactor, what you need is some mildly enriched uranium. Typically, the uranium is formed into pellets (芯块) with approximately the same diameter as a coin and a length of an inch or so. The pellets are arranged into long rods, and the rods are collected together into bundles. The bundles are then typically submerged in water inside a pressure vessel. The water is used to lower the heat.  In order for the reactor to work, the bundle, submerged in water, must be slightly supercritical. That means that, left to its own devices, the uranium would eventually overheat and melt.
    To prevent this, control rods (棒) made of a material that absorbs neutrons are inserted into the bundle. Raising and lowering the control rods allow operators to control the rate of the nuclear reaction. When an operator wants the uranium core to produce more heat, the rods are raised out of the uranium bundle. To create less heat, the rods are lowered into the uranium bundle. The rods can also be lowered completely into the uranium bundle to shut the reactor down in the case of an accident or to change the fuel.
    The uranium bundle acts as an extremely high-energy source of heat. It heats the water and turns it to steam. The steam drives a steam turbine, which spins a generator to produce power. In some reactors, the steam from the reactor goes through a secondary, intermediate heat exchanger to convert another loop of water to steam, which drives the turbine.
                                  Outside a Nuclear Power Plant
    Once you get past the reactor itself, there is very little difference between a nuclear power plant and a coal-fired or oil-fired power plant except for the source of the heat used to create steam.
    The reactor’s pressure vessel is typically housed inside a concrete liner that acts as a radiation shield. That liner is housed within a much larger steel containment vessel. This vessel contains the reactor core as well the hardware that allows workers at the plant to refuel and maintain the reactor. The steel containment vessel is intended to prevent leakage of any radioactive gases or fluids from the plant.
    Finally, the containment vessel is protected by an outer concrete building that is strong enough to survive such things as crashing jet airliners. These secondary containment structures are necessary to prevent the escape of radiation/radioactive steam in the event of an accident like the one at Three Mile Island. The absence of secondary containment structures in Russian nuclear power plants allowed radioactive material to escape in an accident at Chernobyl.
                                  What Can Go Wrong
    Well-constructed nuclear power plants have an important advantage when it comes to electrical power generation--they are extremely clean. Compared with a coal-fired power plant, nuclear power plants are a dream come true from an environmental standpoint.  A coal-fired power plant actually releases more radioactivity into the atmosphere than a properly functioning nuclear power plant.  Coal-fired plants also release tons of carbon, sulfur and other elements into the atmosphere.
    Unfortunately, there are significant problems with nuclear power plants:
    Mining and purifying uranium has not, historically, been a very clean process.
    Improperly functioning nuclear power plants can create big problems. The Chernobyl disaster is a good recent example.
    Spent fuel from nuclear power plants is toxic (有毒的) for centuries, and, as yet, there is no safe, permanent storage facility for it.
    Transporting nuclear fuel to and from plants poses some risk, although to date, the safety record in the United States has been good.
    These problems have largely derailed the creation of new nuclear power plants in the United States. Society seems to have decided that the risks outweigh the rewards.
The present solution to dispose ______ is yet to be found as it remains toxic for a fairly long time.
选项
答案spent fuel或spent nuclear fuel．
解析 在“What Can Go Wrong”一节中，作者提到核废料的毒性会持续数百年，现在没有一个安全而永久的设施可以储存核废料。
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