首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
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
admin
2013-06-17
33
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.
Uranium has to be enriched if a nuclear reactor is built.
选项
A、Y
B、N
C、NG
答案
A
解析
该句的句意为:如果要建立一个核反应堆,必须要对铀进行浓缩。在“Insider a Nuclear Power Plant”一节中,作者介绍建立核反应堆必须要有浓缩铀。
转载请注明原文地址:https://kaotiyun.com/show/E4M7777K
0
大学英语四级
相关试题推荐
Imagineyouwenttoarestaurantwithadate;hadaburger,paidwithacreditcard,andleft.Thenexttimeyougothere,thew
Teenageboys,regardlessofrace,aremorelikelytodiefromgunshotwoundsthanfromallnaturalcausescombined.Bythet
A、Shecanreturntheloanontime.B、Shepossessedacostlyear.C、Sheborrowedalotofmoney.D、Sheisamultimillionaire.D题目
A、Theoryclass.B、ProfessorSanford.C、Classicalliterature.D、BasicsinEnglishclass.B选项均为概括性很强的名词或名词性短语,故本题可能考查对话话题。根据对话开头部分
Thedoctor’spracticehas______becauseofhiscarelessness.
A、OxfordStreet.B、BuckinghamPalace.C、TrafalgarSquare.D、King’sCross.B信息明示题。男士让女士到白金汉宫时提醒自己,由此可知,男士要去白金汉宫,所以B正确。
Theworldisnotonlyhungry,but【B1】______forwater.Thatwayseemsstrangetoyou,since【B2】______75%oftheearth’ssurfacei
ThehelicoptersofNATOpeacekeepingforcewerespottedhoveringoverasteepmountainsidebutnowreckagewasvisibleand______
A、Hewouldbepromotedifhedidn’tleave.B、Hemustresignreluctantly.C、Heshouldn’tleaveundersomepressure.D、Hehaspote
随机试题
常顶回流罐长8m,直径3m(头盖体积忽略不计),装水试漏每小时进水20m3,几小时能装满?
制成了世界第一台金属印刷机的是【】
A.氨酚氢可酮片B.哌醋甲酯C.麦角酸D.氢可酮属于麻醉药品的是
下列不属于识别社会因素的是()。
依据《消防法》的规定,火灾扑灭后,为隐瞒、掩饰起火原因,推卸责任,故意破坏现场或者伪造现场,尚不构成犯罪的,可以给予的行政处罚包括()。
2008年度实现净利润2600万元,2008年度发生的一些交易和事项及其会计处理如下:(1)1月1日,为了对外提供更可靠、更相关的会计信息,该公司将设备的折旧年限延长了1/3,同时将折旧方法由双倍余额递减法改为直线法,该公司对此项变更采用追溯调整法
非公有制经济是促进我国社会生产力发展的重要力量,其重要作用表现在()。
“绿色食品”是指:
我国《宪法》第126条规定:“人民法院依照法律规定独立行使审判权,不受行政机关、社会团体和个人的干涉。”请分析该条文的含义。
设随机变量Y服从参数为1的指数分布,a为常数且大于零,则P{Y≤a+1|Y>a}=________.
最新回复
(
0
)