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CHAPTER 15

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CHAPTER 15

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  1. CHAPTER 15 Nuclear Power

  2. An introduction to nuclear power • On July 16, 2007 a large earthquake struck Japan • Damaged the Kashiwazaki-Kariwa nuclear power plant, the world’s largest nuclear facility • Its four reactors were automatically shut down • A fire started in the transformer building • 400 barrels of low-level nuclear waste were knocked over • Some radioactive water leaked into the Sea of Japan • The plant was shut down for over a year to check for damage • More earthquake proofing was installed

  3. Fire at the Kashiwazaki-Kariwa nuclear plant

  4. Japan wants to increase nuclear power • Japan wants to increase electricity from nuclear power from 30% to 43% by 2010 • Japan is experiencing problems with its nuclear program • The 2007 earthquake • In 1999, two died when workers started a nuclear chain reaction by mixing uranium fuel in buckets • A ruptured steam pipe killed five workers in 2004 at the Mihama Nuclear Power Plant • The public is protesting expansion of the nuclear program • The government and officials have been embarrassed

  5. Nuclear energy in perspective • An energy future based on fossil fuels is not an option • Climate change, pollution, rising prices • Non-fossil-fuel energy sources must be developed • Nuclear power has few emissions • There is enough uranium for years of nuclear power • 31 countries either have or are planning nuclear power • It was anticipated that nuclear power could provide lots of extremely cheap electricity • After World War II, the U.S. government led the world into the Nuclear Age

  6. The Nuclear Age • The government researched, developed, and promoted plants • Companies constructed the plants • Utilities ordered and paid for the plants • Through the Price-Anderson Act (1957) the government guaranteed insurance coverage for liabilities • The Nuclear Regulatory Commission (NRC) sets and enforces safety standards • By 1957, 53 U.S. plants were operating • Up to a thousand were planned by 2000 • Other countries built or bought plants

  7. Nuclear power in the United States

  8. Curtailed • After 1975, utilities stopped ordering nuclear plants • Construction was terminated even after spending billions of dollars • Long Island’s (New York) Shoreham Nuclear Plant • Was completed and licensed at a cost of $5.5 billion • Was turned over to the state in 1989 • After operating only 32 hours, it was dismantled • People could not be evacuated if an accident happened • California’s troubled Rancho Seco plant was shut down • 28 U.S. units have been permanently shut down

  9. The Shoreham Nuclear Power Plant

  10. Currently . . . • By 2008, the U.S. had 104 operating plants • The Watts Bar Unit 1 reactor (in Tennessee) was the last one completed, in 1996 • The Watts Bar Unit 2 reactor will be the first reactor completed in the 21st century • U.S. plants generate 19% of U.S. electrical power • The U.S. Energy Information Administration projects increased generating capacity • From 100 gigawatts (GW) to 115 GW in 2030 • The Energy Policy Act (2005) has incentives for nuclear

  11. Global picture • Countries reconsidered nuclear power after the 1986 catastrophic accident at Chernobyl, in the Soviet Union • Other sources of electricity have their own problems • Coal: generates the most greenhouse gases and pollution • Oil and natural gas: are limited • Oil: vital for transportation • Hydroelectric: already heavily developed • Wind and solar: provide only a small amount of electricity

  12. Nuclear power plants are still being built • There are 439 operating power plants worldwide • 42 more are under construction • Nuclear provides 14% of electrical generation • This is growing at 3.2%/yr • France produces 77% of its electricity • Will increase to 80% • China and India have made large investments • Each nation is building six more units

  13. Nuclear share of electrical power generation

  14. How nuclear power works • The objective of nuclear power: to control the nuclear reaction • Energy is released as heat • Heat is used to boil water to steam • The steam turns a conventional turbogenerator • Nuclear plants are baseload plants • They always operate (except during refueling) • They are large (up to 1,400 MW)

  15. From mass to energy • Nuclear energy is different from burning fuels or other chemical reactions • Nuclear energy involves changes at the atomic level • Fission: a large atom of one element is split into two atoms of different elements • Fusion: two small atoms join to form a larger atom of a different element • The products of both have less mass than the starting material • The small mass is multiplied by the speed of light squared, resulting in a tremendous release of energy

  16. Nuclear fusion and fission

  17. The fuel for nuclear power plants • All nuclear plants use fission (splitting) of uranium-235 • Uranium occurs naturally in the Earth’s crust • It exists in two forms (isotopes): uranium-238 (238U) and uranium-235 (235U) • Isotopes: contain different numbers of neutrons but the same number of protons and electrons • Mass number = protons + neutrons • Different mass numbers come from different numbers of neutrons (238U = 146, 235U = 143 neutrons) • 235U readily undergoes fission, but not 238U

  18. Fission • Fission occurs when a neutron hits the nucleus of 235U at just the right speed • Some atoms of 235U undergo radioactive decay and release neutrons • These neutrons can hit other 235U atoms, producing highly unstable 236U • 236U undergoes fission into lighter atoms (fission products) • More neutrons are given off, releasing lots of energy • This domino effect causes a chain reaction

  19. Nuclear fuel • U.S. uranium deposits are in Wyoming and New Mexico • The ore is mined from underground or open pits • The ore is milled: crushed, treated chemically, and turned into yellowcake • Yellowcake = 80% UO2 • It is purified and enriched • Enrichment: separates 235U from 238U to produce 3%–5% 235U (the rest is 238U) • Technical difficulties prevent less developed countries from advancing their nuclear programs

  20. Nuclear bombs • When 235U is highly enriched, spontaneous fission of an atom triggers a chain reaction • Nuclear weapons have small amounts of pure 235U • Or other fissionable material • Two or three neutrons from a spontaneous fission cause two or three other neutrons to undergo fission • The whole mass undergoes fission in a fraction of a second • Releases all energy in one huge explosion

  21. Fission reactions

  22. The nuclear reactor • A nuclear reactor has a continuous chain reaction • But does not amplify it into an explosion • Control is through enriching uranium to 3–5% 235U • Faster neutrons absorbed by 238U convert it to 239Pu • Plutonium also undergoes fission and releases energy • Moderators surround the enriched uranium • A moderator slows down neutrons to the right speed to trigger another fission • Light-water reactors (LWRs): moderator is near-pure water

  23. Fuel rods • Enriched uranium is arranged in a suitable geometric pattern surrounded by the moderator • Uranium pellets are inserted into long metal tubes (fuel elements, fuel rods) • Fuel rods are placed close together to form a reactor core • The core is inside a water-holding vessel (the moderator and coolant) • Neutron-absorbing fission products accumulate in the rods • They slow down the rate of fission and heat production • Radioactive spent-fuel rods are replaced with new ones

  24. A nuclear reactor

  25. Control rods • Control rods: neutron-absorbing material inserted between the fuel elements • Control the chain reaction in the reactor core • Withdrawing and inserting control rods starts and controls the chain reaction • The fuel rods and moderator become intensely hot • A nuclear reactor is an assembly of fuel elements, moderator-coolant, and control rods

  26. The nuclear power plant • Water is boiled to make steam to drive turbogenerators • Boiling-water reactors: water circulates through the reactor • Pressurized-water reactors: high-pressure water circulates through the reactor without boiling • The superheated water then circulates through a heat exchanger and boils other, unpressurized water • Isolating hazardous reactor materials • Both reactor types have a serious drawback • If a reactor cracks, there would be a sudden loss of water from around the reactor

  27. LOCA • Loss of cooling accident (LOCA): occurs when a cracked reactor loses water • The missing moderator would stop fission • The fuel core could still overheat • Meltdown: enough energy is released to melt the core • Molten material falling into remaining water could cause a steam explosion • Backup cooling systems keep the reactor in water • The entire assembly is encased in a concrete containment building

  28. Pressurized nuclear power plant

  29. Nuclear vs. coal power: fuel • A 1,000 MW coal-fired plant • Uses 2–3 million tons of coal • Stripmining causes environmental damage, acid leaching • Deep mining causes deaths and harm to health • A 1,000 MW nuclear plant • Uses 30 tons of uranium • Coming from 75,000 tons of ore • Energy from fission of 1 lb of uranium equals 50 tons of coal

  30. Nuclear vs. coal power: pollution • A 1,000 MW coal-fired plant • Emits 7 million tons of CO2 • Emits 300,000 tons SO2, particulates, other pollution • Releases 100 times more radioactivity than a nuclear plant • Produces 600,000 tons of ash • Accidents could kill people or lead to fires • A 1,000 MW nuclear plant • Does not emit CO2 when operating • Produces no acid-forming pollutants or particulates • Low levels of waste gas • 250 tons of radioactive wastes require storage and disposal • Accidents can range from minor to catastrophic

  31. Nuclear power vs. coal-fired power

  32. Hazards and costs of nuclear power: emissions • Radioactive emissions: fission of uranium or any other material creates new atoms • Radioisotopes: unstable direct products of fission • Radioisotopes become stable by ejecting subatomic particles (alpha and beta particles, neutrons) • Or high-energy radiation (gamma and X-rays) • Curie: a measure of radioactivity • 1 gram of pure radium-226 gives off 1 curie/second • Radioactive emissions: particles + radiation

  33. Radioactive wastes • Indirect products of fission: materials in and around the reactor can become radioactive by absorbing neutrons • Radioactive wastes: direct + indirect products of fission • High-level wastes: direct products of fission • Are highly radioactive • Low-level wastes: indirect products of fission • For example, reactor materials • Are much less radioactive • Include material from hospitals and industry

  34. Radioactive wastes and emissions

  35. Hazards and costs of nuclear power: biological effects • Radioactive emissions can penetrate biological tissue • Results in radiation exposure • Absorbed dose: measures the exposure in joules/kg (J/kg) • Grays (GY): units for low-level radiation • Sieverts (Sv): units for high-level radiation • Rem: an old term; equals 0.01 Sv • Ionizing radiation: displaces electrons from tissues • Leaves behind charged particles (ions) • Breaks chemical bonds or changes molecular structures • Is not felt or seen, but impairs molecular functions

  36. High and low doses • High doses: radiation can prevent cell division • Radiation sickness: exposure (> 1 Sv) prevents replacement or repair of blood, skin, other tissues • Can lead to death in days or months • Low doses: can damage DNA • Cells can form tumors or leukemia • Damaged eggs or sperm can cause birth defects • Effects of exposure may go unseen for 10–40 years • Other effects: weakened immune system, mental retardation, cataracts

  37. Exposure • Health effects are directly related to the level of exposure • Doses between 100 and 500 milliSieverts increase the risk of developing cancer • 1 mSV = 1/1,000 of a Sievert • The National Research Council (NRC) found no safe level of radiation • Federal agencies also use this concept • Assume a relationship between exposure and cancer • Federal standards set 1.7 mSv/yr as the maximum permitted exposure (excluding medical X-rays)

  38. Sources of radiation • Humans are exposed to radiation from many sources • Normal background sources: uranium and radon gas from Earth’s crust • Cosmic rays from outer space • Medical and dental exposure: X-rays, CT scans • Deliberate exposure is one-fifth of background sources • Normal operation of a nuclear power plant is less than 1% of natural background radiation • The main concern is not from normal operations, but from wastes and the potential for accidents

  39. Radioactive wastes • Radioactive decay: unstable isotopes become stable by ejecting particles and radiation • Is harmless as long as it is kept away from organisms • Half-life: the time for half of the amount of a radioactive isotope to decay • It is always the same for the isotope • Ranges from a fraction of a second to thousands of years • Reprocessing: most 235U and 239Pu is recovered and recycled • Until recently, it was forbidden in the U.S.

  40. Disposal of radioactive wastes • Development of nuclear power went ahead without solving the issue of waste disposal • Assumed geologic burial: burying solidified wastes • Short-term containment: allows radioactive decay of short-lived isotopes • In 10 years, fission wastes lose 97% of radioactivity • Long-term containment: the EPA recommends a 10,000 year minimum • Government standards require isolation for 20 half-lives (e.g., plutonium has a half-life of 24,000 years!)

  41. Tanks and casks • Spent fuel is stored in deep swimming-pool like tanks • Short-term containment • Dissipates head and prevents escape of radiation • Can hold 10–20 years of spent fuel • Pools were 50% filled in 2004 and will be 100% filled by 2015 • Air-cooled dry casks hold spent fuel for the short term • Engineered to withstand floods, tornadoes, etc. • The world has 290,000 tons (9,000 tons/yr) of waste • The U.S. has 57,000 tons

  42. Radioactivity and the military • Some of the worst failures in handling radioactive waste occurred at U.S. and former Soviet Union military facilities • The U.S. nuclear program has killed 4,000 and sickened 36,500 people • Leaking liquid wastes have contaminated water, wildlife, sediments, and groundwater • Activities have been shrouded in secrecy • Many places have deliberately released materials • The DOE has already spent $50 billion in cleanup • The final cost could be $250 billion

  43. From Russia, with Curies • Russian military weapons facilities were irresponsible • For 20 years, nuclear wastes were discharged from Chelyabinsk-65 into the Techa River and Lake Karachay • Standing for 1 hour on the shore of Lake Karachay causes death within a week • In 1967, the lake dried up and the radioactive dust contaminated hundreds of thousands • This is the most polluted lake on Earth • Authorities filled the lake with concrete, rocks, soil

  44. Megatons to Megawatts • The end of the Cold War caused the U.S. and former Soviet Union to dismantle nuclear weapons • Plutonium weapon reactors were closed • Money helped Russia destroy missile silos • Nuclear submarines and bombs were dismantled • Megatons to Megawatts program: a U.S. company diluted weapons-grade uranium to power-plant uranium • This uranium is sold to U.S. power plants • The Russian government received $5.7 billion • Half of U.S. power plant uranium comes from this program

  45. Megatons to Megawatts

  46. High-level nuclear waste disposal • Geologic burial is the safest option for disposing of highly radioactive spent fuel • No nation has buried the fuel • Many nations haven’t even found burial sites • Selected sites have questionable safety • We can’t guarantee a stable rock formation for tens of thousands of years • Possible volcanoes, earthquakes, groundwater leaching • Still-radioactive wastes could escape

  47. Locating a waste storage site • NIMBY (not in my backyard) syndrome has prevented location of a long-term containment site in the U.S. • States prohibit nuclear waste disposal • Developing a long-term repository is critical • Every nuclear plant has increasing spent fuel • The Nuclear Waste Policy Act (1982): in 1998, the federal government must start receiving nuclear waste from commercial power plants • Congress selected Yucca Mountain, Nevada as the nuclear waste site for the U.S.

  48. Yucca Mountain, Nevada

  49. Yucca Mountain • In 1989, Nevada prohibited nuclear waste storage • The U.S. government overrode the prohibition • $4 billion has been spent studying Yucca Mountain • The EPA sets the safety standards for nuclear waste • Protection for 1 million years • The safety standard is below the level of exposure • In 2002, the DOE submitted a license application • The Obama administration stopped the Yucca program (2008) • A Blue Ribbon Commission is exploring other options • Nevada is delighted

  50. Nuclear power accidents • In Pennsylvania, on March 28, 1979, Three Mile Island Nuclear Power Plant suffered a partial meltdown • A lack of power shut down the steam generator • An open valve drained water from the reactor vessel • Operators shut down the emergency cooling system • Gauges incorrectly reported the reactor had water • The uncovered core suffered a partial meltdown • Released 10 million curies of radioactive gas • No injuries or deaths have been reported • The badly damaged reactor will not be reopened