1、Nuclear EnergyEnvironmental Science LectureOutline:Nature and history of nuclear energyWorkings of a nuclear reactorAlternative types of reactorsNuclear fuel cycleNuclear weapons Concerns about nuclear energyWaste disposalGood and HarmMedical uses: X ray, cancer treating, diagnosis診斷17% electrical e

2、nergyDemolition拆遷 by nuclear explosionsNuclear bombsRadioactive wasteThe 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. Since positively charged part

3、icles in the nucleus repel one another.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 radiationNuclear Fission核裂變 - Occurs when

4、 neutrons impact and split the nuclei of certain atoms.Nuclear Chain Reaction核鏈式反應 - Splitting nuclei release neutrons, which themselves strike more nuclei, in turn releasing even more neutrons.Only certain kinds of atoms are suitable for development of a nuclear chain reaction. The two most common

5、are U 235 and Pu 239.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 built worlds first nuclear power plant in 1951.Dwight D. Eisenhower1953:“Nuclear reac

6、tors 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 producing electricity, and several accidents have caused worldwide concern.Workings of A Nuclear

7、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 turns a turbine thus generating electricity.Controlling The ReactionFuel rods - When slowly moving neut

8、ron 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 absorb neutrons. Decrease rate of fission.Moderator - A substance that absorbs energy, slowing neutro

9、ns, enabling them to split the nuclei of other atoms more efficiently. Light-water reactor Heavy-water reactor Gas-cooled reactor (UK )All need modifications.Other ReactorsBreeder Reactors增殖堆 - Nuclear fission reactor that forms a new supply of radioactive isotopes during operation. Fast-moving neut

10、ron 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.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 enoug

11、h 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.DrawbacksSodium reacts violently when contact with air or water, a faster rate to damage reactor. Sodium fire in France in 1981. Reaction rates are

12、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 bombs. Dangerous! development has slowed significantly in most regions of the world. Only 5 LMFBR in the world. Fusion Reactors聚變堆 - When two lightweigh

13、t 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 1km3water fossil fuel in the world last 1 million years Nuclear Fusion cont.but technical difficulties of attaining necessary conditions make this an u

14、nlikely fuel candidate for the immediate future.T, center of the sun, 40,000,000KWall protect from heatConquer the repulsion 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-g

15、rade material.(difficult and expensive process)Material is fabricated into a powder and then into pellets. 粉碎制彈Pellets are sealed into metal rods and lowered into the reactor. (Fuel Rods)As fission occurs, U 235 concentration decreases.After about 3 years of operation, fuel rods dont have enough rad

16、ioactive material remaining to sustain a chain reaction, thus spent fuel rods are replaced by new ones.These fuel rods are a major source of radioactive waste. Reprocessing ? Provide new fuel Reduce the amount of nuclear waste Cost is greater than ore processingAccident?TransportStoreEnvironmental I

17、ssue of Nuclear Material and Weapons ProductionNuclear 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.U. S. Department of Energy - has bec

18、ome steward of a large number(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.Nuclear Power ConcernsThree Mile Island - PennsylvaniaMarch 14, 1979 valves closed, fau

19、lty gauge indicationFission stopped, partial Core Melt-DownNo DeathsVery Little Radiation VentedChernobyl - UkraineApril 25, 1986A series of operating mistakesReactor Explodes31 deaths116,000 people evacuatedMore than 500,000 exposed to dangerous levels of radiation27 villages and cities within 40 k

20、m contaminatedThyroid Cancer甲狀腺癌 in childrenA 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.Radiation ExposureType and degree

21、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, radiation effects may build up over year

22、s and only appear later in life.Human exposure usually expressed in rems雷姆. (roentgen equivalent man，倫琴當量)The higher the dose, the more observable the results. 1000, death 10, birth defect, cancerControversy on low-lever, chronic radiationThermal PollutionAddition of waste heat to the environment.Es

23、pecially 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.Decommissioning CostsLife expectancy of most electrical generating plants is 30-40 years.Unlike other plants, nuclear plants are decommissione

24、d, not demolished.By 2000, 62/110 US plants will be at least 20 years old.Two Major Decommission Uncertainties:Method and CostUtilities 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