Nuclear-Energy原子能課件(PPT 49頁)

1、Nuclear EnergyEnvironmental Science Lecture第1頁，共49頁。Outline:Nature and history of nuclear energyWorkings of a nuclear reactorAlternative types of reactorsNuclear fuel cycleNuclear weapons Concerns about nuclear energyWaste disposal第2頁，共49頁。Good and HarmMedical uses: X ray, cancer treating, diagnosis

2、診斷17% electrical energyDemolition拆遷 by nuclear explosionsNuclear bombsRadioactive waste第3頁，共49頁。The Nature of Nuclear EnergyRadioactive - Nuclei of certain atoms are unstable and spontaneously自發(fā) decompose.Some isotopes同位素 are radioactive.Energy is needed to hold the protons and neutrons together. Si

3、nce positively charged particles in the nucleus repel one another.第4頁，共49頁。Half-Life - Time it takes for 1/2 of the radioactive material to spontaneously decompose. (rate of decomposition is constant)Nuclear disintegration分解 releases radiation and neutrons. Alpha radiation Beta radiation Gamma radia

4、tion第5頁，共49頁。第6頁，共49頁。Nuclear Fission核裂變 - Occurs when neutrons impact and split the nuclei of certain atoms.Nuclear Chain Reaction核鏈式反應(yīng) - Splitting nuclei release neutrons, which themselves strike more nuclei, in turn releasing even more neutrons.第7頁，共49頁。Only certain kinds of atoms are suitable fo

5、r development of a nuclear chain reaction. The two most common are U 235 and Pu 239.第8頁，共49頁。第9頁，共49頁。Nuclear HistoryFirst 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 buil

6、t worlds first nuclear power plant in 1951.第10頁，共49頁。Dwight D. Eisenhower1953:“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 produc

7、ing electricity, and several accidents have caused worldwide concern.第11頁，共49頁。Workings of A Nuclear reactorA 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 tu

8、rns a turbine thus generating electricity.第12頁，共49頁。Controlling The ReactionFuel 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 r

9、eactor to absorb neutrons. Decrease rate of fission.第13頁，共49頁。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頁，共49頁。第15頁，共49頁。第

10、16頁，共49頁。第17頁，共49頁。Other ReactorsBreeder Reactors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 rapidl

11、y.第18頁，共49頁。Liquid 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 materi

12、al, lasts 20,000 years.第19頁，共49頁。DrawbacksSodium 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 amo

13、unt can be made into nuclear bombs. Dangerous! development has slowed significantly in most regions of the world. Only 5 LMFBR in the world. 第20頁，共49頁。Fusion Reactors聚變堆 - When two lightweight atomic nuclei combine to form a heavier nucleus, a large amount of energy is released. - Nuclear Fusion - i

14、e Sun.(H2, H3)Huge potential for energy 1km3water fossil fuel in the world last 1 million years第21頁，共49頁。第22頁，共49頁。 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, 40,000,000KWall pro

15、tect from heatConquer the repulsion 第23頁，共49頁。第24頁，共49頁。Nuclear Fuel CycleMining 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

16、 into a powder and then into pellets. 粉碎制彈Pellets are sealed into metal rods and lowered into the reactor. (Fuel Rods)第25頁，共49頁。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, th

17、us spent fuel rods are replaced by new ones.These fuel rods are a major source of radioactive waste. 第26頁，共49頁。Reprocessing ? Provide new fuel Reduce the amount of nuclear waste Cost is greater than ore processingAccident?TransportStore第27頁，共49頁。Environmental Issue of Nuclear Material and Weapons Pr

18、oductionNuclear Power Industry is an outgrowth of 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頁，共49頁。U. S. Department of Energy - has become steward of a large number

19、(3700) of contaminated sites. Greater than 330 underground storage tanks currently exist with high-level radioactive waste.Clean up may take many years and tens of billions of dollars.第29頁，共49頁。第30頁，共49頁。Nuclear Power ConcernsThree Mile Island - PennsylvaniaMarch 14, 1979 valves closed, faulty gauge

20、 indicationFission stopped, partial Core Melt-DownNo DeathsVery Little Radiation Vented第31頁，共49頁。第32頁，共49頁。Chernobyl - UkraineApril 25, 1986A series of operating mistakesReactor Explodes31 deaths116,000 people evacuatedMore than 500,000 exposed to dangerous levels of radiation27 villages and cities

21、within 40 km contaminatedThyroid Cancer甲狀腺癌 in children第33頁，共49頁。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頁，共49頁。第35

22、頁，共49頁。Radiation ExposureType and degree of damage vary with radiation form, dosage and duration of exposure, and type of cells irradiated.Mutation (變異) abnormal offspring-mutated eggs or sperm（卵子和精子）; abnormal tissue growth-cancer (breast cancer and leukemia，乳腺癌和白血病)Because mutations are permanent,

23、 radiation effects may build up over years and only appear later in life.第36頁，共49頁。Human exposure usually expressed in rems雷姆. (roentgen equivalent man，倫琴當(dāng)量)The higher the dose, the more observable the results. 1000, death 10, birth defect, cancerControversy on low-lever, chronic radiation第37頁，共49頁。

24、第38頁，共49頁。第39頁，共49頁。第40頁，共49頁。Thermal PollutionAddition 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頁，共49頁。Decommissioning CostsLife expectancy of m

25、ost electrical generating plants is 30-40 years.Unlike 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頁，共49頁。Utilities Have (3) Options:Decontaminate and dismantle (拆除) plant as soon as possibleMothball plant for short period, allowing some radiation to dissipate (消散), then dismantleEntomb plant within concrete barrierBest?Least favorable, long half-life, endanger groundwaterMay cost more to decommis