Lecture 10 nuclear

Download Lecture 10 nuclear

Post on 07-May-2015

1.926 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

<ul><li>1.Environmental ScienceA Study of InterrelationshipsEleventh Edition Enger &amp; Smith Chapter 10Nuclear EnergyCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.</li></ul><p>2. Nuclear Energy 3. Outline The Nature of Nuclear Energy The History of Nuclear Energy Development Nuclear Fission Reactors Investigating Nuclear Alternatives The Nuclear Fuel Cycle Nuclear Concerns The Future of Nuclear Power 4. The Nature of Nuclear Energy The nuclei of certain atoms are unstable andspontaneously decompose. These isotopes areradioactive. Neutrons, electrons, protons, and other largerparticles are released during nucleardisintegration, along with a great deal of energy. Radioactive half-life is the time it takes for halfthe radioactive material to spontaneouslydecompose. 5. The Nature of Nuclear Energy Nuclear disintegration releases energy from thenucleus as radiation, of which there are threemajor types: Alpha radiation consists of moving particles composedof two neutrons and two protons. It can be stopped by the outer layer of skin. Beta radiation consists of electrons from the nucleus. It can be stopped by a layer of clothing, glass, oraluminum. 6. The Nature of Nuclear Energy Gamma radiation is a form of electromagneticradiation. It can pass through your body, several centimeters of lead, or a meter of concrete. The absorbed dose is the amount of energyabsorbed by matter. It is measured in grays orrads. The damage caused by alpha particles is 20 timesgreater than that caused by beta particles orgamma rays. 7. The Nature of Nuclear Energy The dose equivalent is the absorbed dosetimes a quality factor. When alpha or beta particles or gammaradiation interact with atoms, ions are formed.Therefore, it is known as ionizing radiation. Ionizing radiation affects DNA and can causemutations. Mutations that occur in some tissues of the body maymanifest themselves as abnormal tissue growthsknown as cancers. 8. The Nature of Nuclear Energy Large doses of radiation are clearly lethal. Demonstrating known harmful biological effectsfrom smaller doses is much more difficult. The more radiation a person receives, the morelikely it is that there will be biologicalconsequences. Time, distance, and shielding are the basicprinciples of radiation protection. Water, lead, and concrete are common materialsused for shielding from gamma radiation. 9. The Nature of Nuclear EnergyProtective equipment 10. The Nature of Nuclear Energy Nuclear fission occurs when neutrons impact andsplit the nuclei of certain other atoms. In a nuclear chain reaction, splitting nucleirelease neutrons, which themselves strike morenuclei, in turn releasing even more neutrons. 11. The Nature of Nuclear EnergyNuclear fission chain reaction 12. The Nature of Nuclear Energy Only certain kinds of atoms are suitable fordevelopment of a nuclear chain reaction. The two most common are uranium-235 and plutonium-239. There also must be a certain quantity of nuclear fuel(critical mass) for the chain reaction to occur. 13. The History of Nuclear EnergyDevelopment The first controlled nuclear chain reactionoccurred at Stag Field in Chicago in 1942. That event led directly to the development of theatomic bombs dropped on the Japanese cities ofHiroshima and Nagasaki. Following WW II, people began exploring otherpotential uses of nuclear energy. The U.S. built the worlds first nuclear powerplant in 1951. 14. The History of Nuclear Energy Development In December 1953, President Dwight D. Eisenhower made the following prediction in his Atoms for Peace speech: Nuclear reactors will produce electricity so cheaply that it will not be necessary to meter it. Todays Reality: Accidents have caused worldwide concern about the safety of nuclear power plants. As of 2006 there were 24 nuclear power plants under construction in 11 countries. Many experts predict an increase in building as energy demands increase. 15. Nuclear Fission Reactors A nuclear reactor is a device that permits acontrolled fission chain reaction. When the nucleus of a U-235 atom is struck by aslowly moving neutron from another atom, thenucleus splits into smaller particles. More rapidly-moving neutrons are released, whichstrike more atoms. The chain reaction continues to release energy untilthe fuel is spent or the neutrons are prevented fromstriking other nuclei. 16. Nuclear Fission Reactors Control rods made of a non-fissionable material(boron, graphite) are lowered into the reactor toabsorb neutrons and control the rate of fission. When they are withdrawn, the rate of fissionincreases. A moderator is a substance that absorbsenergy, which slows neutrons, enabling them tosplit the nuclei of other atoms more effectively. 17. Nuclear Fission Reactors In the production of electricity, a nuclear reactorserves the same function as a fossil-fuel boiler: itproduces heat, which converts water to steam,which turns a turbine, generating electricity. Lightwater reactors make up 90% of current reactors; use ordinary water to cool reactor cores. The two types of lightwater reactors are: Boiling Water Reactors (BWR) Pressurized Water Reactors (PWR) Heavy water reactors and gas-cooled reactors are two other kinds of nuclear reactors. 18. Nuclear Fission ReactorsBoiling water reactor 19. Nuclear Fission ReactorsPressurized-water reactor 20. Nuclear Fission ReactorsAdvanced gas-cooled reactor 21. Investigating Nuclear Alternatives Breeder reactors are nuclear fission reactors thatproduce heat to be converted to steam to generateelectricity. They also form a new supply of radioactiveisotopes during operation. (i.e., U238 turns into Pu239) The liquid metal fast-breeder (LMFBR) appears tobe the most promising model. Because Pu-239 is very hazardous to humans, and can be made into nuclear weapons, development has slowed significantly in most regions of the world. 22. Investigating Nuclear AlternativesFormation of Pu-239 in a breeder reactor 23. Investigating Nuclear Alternatives Nuclear fusion is a process in which twolightweight atomic nuclei combine to form aheavier nucleus, resulting in the release of alarge amount of energy. The energy produced by the sun is the result offusion. Nuclear fusion has a huge potential for energy,but technical difficulties of attaining necessaryconditions make this an unlikely fuel candidatefor the immediate future. 24. Investigating Nuclear AlternativesNuclear fusion 25. The Nuclear Fuel Cycle The nuclear fuel cycle begins with the mining oflow-grade uranium ore. It is milled, and crushed and treated with a solventto concentrate the uranium. Milling produces yellow-cake, a materialcontaining 70-90% uranium oxide. 26. The Nuclear Fuel Cycle Naturally occurring uranium contains about99.3% nonfissionable U238, and .7% fissionableU235. It must be enriched to 3% U235 to be concentratedenough for most nuclear reactors. Material is fabricated into a powder and then intopellets. The pellets are sealed into metal rods (fuel rods) andlowered into the reactor. 27. The Nuclear Fuel Cycle As fission occurs, U-235 concentration decreases. After about three years of operation, fuel rodsdont have enough radioactive material remainingto sustain a chain reaction, thus spent fuel rodsare replaced by new ones. Spent rods are still very radioactive, containingabout 1% U-235 and 1% plutonium. 28. The Nuclear Fuel Cycle Initially, scientists proposed spent fuel rods couldbe reprocessed and used to manufacture new fuelrods. This process would reduce the amount of nuclearwaste. At present, India, Japan, Russia, France and theUnited Kingdom operate reprocessing plants as analternative to storing rods as waste. 29. The Nuclear Fuel CycleSteps in the nuclear fuel cycle 30. The Nuclear Fuel Cycle All of the processes involved in the nuclear fuelcycle have the potential to generate waste. Each step in the nuclear fuel cycle involves thetransport of radioactive materials. Each of these links in the fuel cycle presents thepossibility of an accident or mishandling thatcould release radioactive material. 31. Nuclear Concerns Nuclear power provides a significant amount ofelectricity for the people of the world. However, many factors have caused somecountries to reevaluate their commitment tonuclear power: Accidents have raised questions about safety. Use of nuclear material by terrorists. Concern about worker and public exposure to radiation. Weapons production. Contamination and disposal problems. 32. Nuclear Concerns Reactor Safety The Three Mile Island nuclear plant in Pennsylvaniaexperienced a partial core meltdown on March 28,1979. It began with pump and valve malfunction, but operator error compounded the problem. The containment structure prevented the release of radioactive materials from the core, but radioactive steam was vented into the atmosphere. The crippled reactor was defueled in 1990 at a cost of about $1 billion. Placed in monitored storage until its companion reactor reaches the end of its useful life. 33. Nuclear Concerns Reactor Safety Chernobyl is a small city in Ukraine, north of Kiev. It is the site of the worlds largest nuclear accident,which occurred April 26, 1986. Experiments were being conducted on reactor. Operators violated six important safety rules. They shut off all automatic warning systems, automatic shutdown systems, and the emergency core cooling system for the reactor. 34. Nuclear ConcernsChernobyl 35. Nuclear Concerns Reactor Safety In 4.5 seconds, the energy level of the reactorincreased 2000 times. The cooling water converted to steam and blew the1102-ton concrete roof from the reactor. The reactor core caught fire. It took 10 days to bring the burning reactor undercontrol. 36. Nuclear Concerns Reactor Safety There were 31 deaths; 500 people hospitalized (237with acute radiation sickness); 116,000 peopleevacuated. 24,000 evacuees received high doses of radiation. Children or fetuses exposed to fallout are showing increased frequency of thyroid cancer. Some scientists fear the worst is yet to come.Whether people living in the shadow ofChernobyl remain at risk for health problems is aquestion under close scrutiny. 37. Nuclear ConcernsThe accident at Chernobyl 38. Nuclear Concerns A consequence of the Chernobyl accident inparticular has been a deepened public concernover nuclear reactor safety. Increased public opposition after Chernobyl: United Kingdom: from 65% to 83% opposed. Germany:from 46% to 83% United States:from 67% to 78% The number of new nuclear plants beingconstructed has been reduced, and many havebeen shut down prematurely. 39. Nuclear Concerns After Sept. 11, 2001, fear arose regardingnuclear plants as potential targets for terroristattacks. The consensus is that there is not great causefor concern of an airliner attack on a reactor orcontainment building. Probably the greatest terrorism-related threat isfrom radiological dispersal devices (RDDs), ordirty bombs. The point of a dirty bomb is to cause disruption andpanic, rather than kill large numbers of people. 40. Nuclear Concerns Each step in the nuclear fuel cycle poses aradiation exposure problem. Prolonged exposure to low-level radiationincreases workers rates of certain cancers. Enrichment, fabrication, and transport of nuclearmaterials offer potential exposure hazards andrisks of accidental release of radioactivematerials into the environment. 41. Nuclear Concerns Producing nuclear materials for weapons andother military uses involves many of the samesteps used to produce nuclear fuel for powerreactors. The U.S. Department of Energy (DOE) isresponsible for nuclear research for both weaponsand peaceful uses. Historically, research and production dealt withhazardous chemicals and minor radioactivewastes by burying them, storing them in ponds, orreleasing them into rivers. 42. Nuclear Concerns Despite environmental regulations that prohibitedsuch behaviors, the DOE (formerly the AtomicEnergy Commission) maintained they wereexempt from such laws. As a result, DOE has become steward of: 3,700 contaminated sites. 330 underground storage tanks with high-levelradioactive waste. More than one million 55-gallon drums of waste. 5,700 sites with wastes moving in soil. Millions of cubic meters of radioactive wastes. 43. Nuclear Concerns Clean-up will take decades and cost billions ofdollars. Local residents and host states are not completely trustful. Clean-up is a new mission for DOE. Additional problem of disposing of dismantled nuclearweapons. 44. Nuclear Concerns There are four general categories of nuclearwastes: Transuranic wastes are highly radioactive waste thatcontain large numbers of atoms that are larger thanuranium with half-lives greater than 20 years. Uranium mining and milling wastes have low levelsof radioactivity but are above background levels. High-level radioactive wastes are spent fuel rodsand highly radioactive materials from the reprocessingof fuel rods. Low-level radioactive wastes have low levels ofradioactivity and are not classified into one of theother categories. 45. Nuclear Concerns Transuranic waste from former nuclear weaponssites is transported to the Waste Isolation PilotPlant (WIPP) near Carlsbad, NM, which beganaccepting waste in March, 1999. Waste management activities of mining andmilling waste include: Building fences. Putting up warning signs. Establishing land use restrictions. Covering the wastes with a thick layer of soil and rockto prevent erosion, windblown particles, andgroundwater contamination. 46. Nuclear ConcernsDOE high-level radioactive transuranic waste sites 47. Nuclear Concerns Disposal of high-level radioactive waste is amajor problem for the nuclear power industry. In the U.S., about 30,000 metric tons of highlyradioactive spent fuel rods are stored in specialstorage ponds at nuclear reactor sites. There is no permanent storage facility, and manyplants are running out of temporary storage. Most experts feel the best solution is to bury waste ina stable geologic formation. 48. Nuclear Concerns In 1982, Congress called for a high-levelradioactive disposal site to be selected by March1987, and to be completed by 1998. In 2002, the Secretary of Energy indicated thechoice of a site at Yucca Mountain in Nevada wasbased on scientifically sound and suitablescience. Current work is primarily exploratory and isseeking to characterize the likelihood ofearthquake damage and water movement. 49. Nuclear Concerns If completed, the facility would hold about 70,000metric tons of spent fuel rods and other highlyradioactive material. Not to be completed before 2015. By that time, waste produced by nuclear power plantswill exceed the storage capacity of the site. 50. Nuclear Concerns In February 2002, President George W. Bushnotified Congress that he considered the YuccaMountain site qualified to receive a constructionpermit, and in July 2002, he signed a jointresolution allowing construction to proceed. Members of Congress have vowed to continueto fight this designa...</p>