1、Nuclear EnergyEnvironmental Science Lecture第1页，共49页。Outline:Nature and history of nuclear energy Workings of a nuclear reactor Alternative types of reactors Nuclear fuel cycle Nuclear weapons Concerns about nuclear energy Waste disposal第2页，共49页。Good and HarmMedical uses:X ray,cancer treating,diagnos
2、is诊断诊断17%electrical energyDemolition拆迁拆迁 by nuclear explosionsNuclear bombsRadioactive waste第3页，共49页。The Nature of Nuclear Energy Radioactive-Nuclei of certain atoms are unstable and spontaneously自发自发 decompose.Some isotopes同位素同位素 are radioactive.Energy is needed to hold the protons and neutrons tog
3、ether.Since positively charged particles in the nucleus repel one another.第4页，共49页。Half-Life-Time it takes for 1/2 of the radioactive material to spontaneously decompose.(rate of decomposition is constant)Nuclear disintegration分解分解 releases radiation and neutrons.Alpha radiation Beta radiation Gamma
4、 radiation第5页，共49页。第6页，共49页。Nuclear Fission核裂变核裂变-Occurs when neutrons impact and split the nuclei of certain atoms.Nuclear Chain Reaction核链式反应核链式反应-Splitting nuclei release neutrons,which themselves strike more nuclei,in turn releasing even more neutrons.第7页，共49页。Only certain kinds of atoms are sui
5、table for development of a nuclear chain reaction.The two most common are U 235 and Pu 239.第8页，共49页。第9页，共49页。Nuclear History First controlled fission-Germany 1938.1945-US dropped atomic bombs on Hiroshima and Nagasaki.Following WW II,people began exploring other potential uses of nuclear energy.US b
6、uilt worlds first nuclear power plant in 1951.第10页，共49页。Dwight D.Eisenhower 1953:“Nuclear reactors will produce electricity so cheaply that it will not be necessary to meter it Atoms will provide a safe,clean and dependable source of electricity.”Reality:Nuclear is the most expensive method of produ
7、cing electricity,and several accidents have caused worldwide concern.第11页，共49页。Workings of A Nuclear reactor A nuclear reactor is a device that permits a controlled fission chain reaction.It serves the same function as any fossil-fuel boiler.It produces heat,which converts water to steam,which turns
8、 a turbine thus generating electricity.第12页，共49页。Controlling The Reaction Fuel rods-When slowly moving neutron strike the fissionable U-235,two or three rapidly moving neutrons and energy are released.Control rods-Made of a non-fissionable material(boron,graphite)and are lowered into the reactor to
9、absorb neutrons.Decrease rate of fission.第13页，共49页。Moderator-A substance that absorbs energy,slowing neutrons,enabling them to split the nuclei of other atoms more efficiently.Light-water reactor Heavy-water reactor Gas-cooled reactor(UK)All need modifications.第14页，共49页。第15页，共49页。第16页，共49页。第17页，共49页
10、。Other Reactors Breeder Reactors增殖堆增殖堆-Nuclear fission reactor that forms a new supply of radioactive isotopes during operation.Fast-moving neutron hits a U 238 nucleus and is absorbed,fissionable Pu 239 is produced.(p.231,fig 11.7)Moderator allows the neutrons to move more rapidly.第18页，共49页。Liquid
11、metal fast-breeder reactor.(LMFBR)Reactor core:rods of U 238 and liquid sodium,620.Question whats the function of U and Na?10 years,produced enough Pu to operate a second reactor.Naturally occurring Uranium contains about 99.3%U 238,and 0.7%U 235.More abundant raw material,lasts 20,000 years.第19页，共4
12、9页。Drawbacks Sodium reacts violently when contact with air or water,a faster rate to damage reactor.Sodium fire in France in 1981.Reaction rates are extremely rapid and very difficult to regulate.Because Pu is very hazardous to humans,can be stolen and even small amount can be made into nuclear bomb
13、s.Dangerous!development has slowed significantly in most regions of the world.Only 5 LMFBR in the world.第20页，共49页。Fusion Reactors聚变堆聚变堆-When two lightweight atomic nuclei combine to form a heavier nucleus,a large amount of energy is released.-Nuclear Fusion-ie Sun.(H2,H3)Huge potential for energy 1k
14、m3water fossil fuel in the world last 1 million years第21页，共49页。第22页，共49页。Nuclear Fusion cont.but technical difficulties of attaining necessary conditions make this an unlikely fuel candidate for the immediate future.T,center of the sun,40,000,000K Wall protect from heatConquer the repulsion 第23页，共49
15、页。第24页，共49页。Nuclear Fuel Cycle Mining of low-grade Uranium ore(0.2%).Naturally occurring Uranium contains about 99.3%U 238,and.7%U 235.Much be enriched to 3%U 235 to produce weapons-grade material.(difficult and expensive process)Material is fabricated into a powder and then into pellets.粉碎制弹粉碎制弹 Pe
16、llets are sealed into metal rods and lowered into the reactor.(Fuel Rods)第25页，共49页。As fission occurs,U 235 concentration decreases.After about 3 years of operation,fuel rods dont have enough radioactive material remaining to sustain a chain reaction,thus spent fuel rods are replaced by new ones.Thes
17、e fuel rods are a major source of radioactive waste.第26页，共49页。Reprocessing?Provide new fuel Reduce the amount of nuclear waste Cost is greater than ore processing Accident?Transport Store第27页，共49页。Environmental Issue of Nuclear Material and Weapons Production Nuclear Power Industry is an outgrowth o
18、f the weapons industry.Research and production have typically dealt with hazardous and radioactive materials in the same manner as other waste eg.dumping in ground,water,etc.第28页，共49页。U.S.Department of Energy-has become steward of a large number(3700)of contaminated sites.Greater than 330 undergroun
19、d storage tanks currently exist with high-level radioactive waste.Clean up may take many years and tens of billions of dollars.第29页，共49页。第30页，共49页。Nuclear Power Concerns Three Mile Island-Pennsylvania March 14,1979 valves closed,faulty gauge indication Fission stopped,partial Core Melt-DownNo Deaths
20、 Very Little Radiation Vented第31页，共49页。第32页，共49页。Chernobyl-UkraineApril 25,1986A series of operating mistakesReactor Explodes 31 deaths116,000 people evacuatedMore than 500,000 exposed to dangerous levels of radiation27 villages and cities within 40 km contaminatedThyroid Cancer甲状腺癌甲状腺癌 in children第
21、33页，共49页。A consequence of both the Three Mile Island and Chernobyl accidents has been a deepened public concern over nuclear reactor safety.In 1996 there were 34 nuclear reactors under construction,before Chernobyl there were 160.第34页，共49页。第35页，共49页。Radiation Exposure Type and degree of damage vary
22、with radiation form,dosage and duration of exposure,and type of cells irradiated.Mutation(变异变异)abnormal offspring-mutated eggs or sperm（卵子和精子）卵子和精子）;abnormal tissue growth-cancer(breast cancer and leukemia，乳腺癌和白血病乳腺癌和白血病)Because mutations are permanent,radiation effects may build up over years and o
23、nly appear later in life.第36页，共49页。Human exposure usually expressed in rems雷姆雷姆.(roentgen equivalent man，伦伦琴当量琴当量)The higher the dose,the more observable the results.1000,death 10,birth defect,cancer Controversy on low-lever,chronic radiation第37页，共49页。第38页，共49页。第39页，共49页。第40页，共49页。Thermal Pollution
24、Addition of waste heat to the environment.Especially dangerous in aquatic systems.In a nuclear power plant,1/3 of heat used to generate electricity,other 2/3 is waste heat.vs.fossil fuel plant,1/2.第41页，共49页。Decommissioning Costs Life expectancy of most electrical generating plants is 30-40 years.Unl
25、ike other plants,nuclear plants are decommissioned,not demolished.By 2000,62/110 US plants will be at least 20 years old.Two Major Decommission Uncertainties:Method and Cost第42页，共49页。Utilities Have(3)Options:Decontaminate and dismantle(拆除拆除)plant as soon as possibleMothball plant for short period,al
26、lowing some radiation to dissipate(消散消散),then dismantle Entomb plant within concrete barrier Best?Least favorable,long half-life,endanger groundwater May cost more to decommission than to build.第43页，共49页。第44页，共49页。Waste Disposal At this time,NO country has a permanent storage solution for high-level
27、 radioactive waste.Fuel rod Obsolete废弃 nuclear weapon Method?Politics of disposal are as crucial as disposal method.Not in my territory.Low-level radioactive waste:Cooling water,material from reactor,medical waste,protecting cloth,etc.第45页，共49页。第46页，共49页。Disposal?high-level radioactive disposal site
28、 in 1989.(Yucca Mountain,Nevada)第47页，共49页。Summary Nuclear Fission-splitting of an atom Nuclear Fusion and Breeder Reactors Nuclear Waste Contamination and Disposal Failure of initial promises,accidents,and dangers of low-level radiation exposure have sent public acceptance into decline worldwide.第48页，共49页。Would You Rather Live With Energy Conservation Efforts Or Nuclear Power Plants and their Associated Baggage?第49页，共49页。
