15-5 What Are the Advantages and Disadvantages of Nuclear Energy?

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15-5 What Are the Advantages and Disadvantages of
Nuclear Energy?

• Concept 15-5 Nuclear power has a low
  environmental impact and a very low accident
  risk, but high costs, a low net energy yield,
  long-lived radioactive wastes, vulnerability to
  sabotage, and the potential for spreading nuclear
  weapons technology have limited its use.

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How Does a Nuclear Fission Reactor Work? (1)

• Controlled nuclear fission reaction in a reactor
• Light-water reactors
• Fueled by uranium ore and packed as pellets in
  fuel rods and fuel assemblies
• Control rods absorb neutrons

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How Does a Nuclear Fission Reactor Work? (2)

• Water is the usual coolant
• Containment shell around the core for protection
• Water-filled pools or dry casks for storage of
  radioactive spent fuel rod assemblies

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Light water reactors

• 85 of worlds nuclear generated electricity
  (100 in US)
• High inefficient in terms of energy conversion
  (up to 83 lost as waste heat)

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Nuclear fuel

• Made from uranium ore
• Enriched to 3 of radioactive isotope U-235
• Made into pellets, size of pencil, energy
  equivalent to 1 ton of coal
• Pellets are packed into large pipes-fuel rods
• Rods are grouped together into fuel assemblies,
  these assemblies are placed into reactor core

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Controlling the reaction

• Control rods placed between rods
• Control rods moved in and out of the assemblies,
  absorbing neutrons which trigger the chain
  reaction
• Water circulates through the assemblies, removing
  the heat, keeping the rods from melting

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What happens if there is no water?
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How nuclear reactors generate electricity

• Superheated water turns into steam
• Steam passed through turbine
• Physical motion of the turbine is converted into
  electrical energy

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Steam from Nuclear power plants

• Superheated water after used in the turbines goes
  into a condenser
• Condenser requires cold water source which is why
  most plants are located next to water
• Pipes with the hot water are circulated through a
  container filled with cold water , heat is
  exchanged
• Hot water is either discharged into river, ocean
  or vented into the atmosphere as steam

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Light-Water-Moderated and -Cooled Nuclear Power
Plant with Water Reactor
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Small amounts of radioactive gases
Control rods
Containment shell
Heat exchanger
Waste heat
Generator
Turbine
Steam
Uranium fuel input (reactor core)
Hot coolant
Useful electrical energy 2530
Hot water output
Pump
Pump
Shielding
Pump
Waste heat
Coolant
Pump
Cool water input
Moderator
Pressure vessel
Coolant passage
Water
Condenser
Water source (river, lake, ocean)
Periodic removal and storage of radioactive
wastes and spent fuel assemblies
Periodic removal and storage of radioactive
liquid wastes
Fig. 15-17, p. 387
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Reactor in Japan
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After 3 or 4 Years in a Reactor, Spent Fuel Rods
Are Removed and Stored in Water
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Spent Fuel rods

• After about 3-4 years of use, the Fuel rods
  become spent-level of fission drops beneath a
  certain level
• Rods are taken out of reactor stored nearby in
  water filled pools or dry casks
• Stored until they cool down enough to be shipped
  for permanent storage or to be recycled
• These storage facilities are next to the reactor
  plants, vulnerable to terrorist attack or
  accidents

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Spent fuel reprocessing

• The spent fuel rods are sent to a facility which
  separates plutonium from spent fuel for further
  use as a new generation of fuel or as material
  used to make atomic weapons.
• First the fuel is chopped up, by remote control,
  behind heavy lead shielding.
• These chopped-up pieces are then dissolved in
  boiling nitric acid, releasing radioactive gases
  in the process.
• The plutonium is separated from the acid solution
  by chemical means, leaving large quantities of
  high-level radioactive liquid waste and sludge
  behind.
• After it has cooled down for several years, this
  liquid waste will have to be solidified for
  ultimate disposal, while the separated plutonium
  is fabricated into nuclear fuel or nuclear
  weapons.

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Japan and the Soviet Nuclear program connection

• As a result of a successful program between the
  Soviet Union and USA, many nuclear weapons have
  been destroyed.
• The fuel from a number of Soviet weapons was sold
  to Japan to be used for fuel in their reactors
• So plutonium is used in at least one of the
  damaged reactors in Japan

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What Is the Nuclear Fuel Cycle?

• Mine the uranium
• Process the uranium to make the fuel
• Use it in the reactor
• Safely store the radioactive waste
• Decommission the reactor

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Decommissioning of reactor
Fuel assemblies
Reactor
Enrichment of UF6
Fuel fabrication
(conversion of enriched UF6 to UO to UO2 and
fabrication of fuel assemblies)
Temporary storage of spent fuel assemblies
underwater or in dry casks
Conversion of U3O8 to UF6
Uranium-235 as UF6 Plutonium-239 as PuO2
Spent fuel reprocessing
Low-level radiation with long half-life
Geologic disposal of moderate- and high-level
radioactive wastes
Open fuel cycle today Recycling of nuclear fuel
Fig. 15-19, p. 389
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What Happened to Nuclear Power?

• Slowest-growing energy source and expected to
  decline more
• Why?
• Economics
• Poor management
• Low net yield of energy of the nuclear fuel cycle
• Safety concerns
• Need for greater government subsidies
• Concerns of transporting uranium

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Case Study Worst Commercial Nuclear Power Plant
Accident in the U.S.

• Three Mile Island
• March 29, 1979
• Near Harrisburg, PA, U.S.
• Nuclear reactor lost its coolant
• Led to a partial uncovering and melting of the
  radioactive core
• Unknown amounts of radioactivity escaped
• People fled the area
• Increased public concerns for safety
• Led to improved safety regulations in the U.S.

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Case Study Worst Nuclear Power Plant Accident in
the World

• Chernobyl
• April 26, 1986
• In Chernobyl, Ukraine
• Series of explosions caused the roof of a reactor
  building to blow off
• Partial meltdown and fire for 10 days
• Huge radioactive cloud spread over many countries
  and eventually the world
• 350,000 people left their homes
• Effects on human health, water supply, and
  agriculture

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Oak Ridge Reactor
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Remains of a Nuclear Reactor at the Chernobyl
Nuclear Power Plant
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Level of contamination
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Human casualties of Chernobyl

• 56 people lost their lives as a direct result of
  radiation poisoning or fire
• Thyroid cancer
• From drinking
• Milk killed
• 10-12 thousand

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Nuclear Power Has Advantages and Disadvantages

• Advantages
• Disadvantages

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TRADE-OFFS
Conventional Nuclear Fuel Cycle
Advantages
Disadvantages
Large fuel supply
Cannot compete economically without huge
government subsidies
Low environmental impact (without accidents)
Low net energy yield
High environmental impact (with major accidents)
Emits 1/6 as much CO2 as coal
Environmental costs not included in market price
Moderate land disruption and water pollution
(without accidents)
Risk of catastrophic accidents
Moderate land use
No widely acceptable solution for long-term
storage of radioactive wastes
Low risk of accidents because of multiple safety
systems (except for Chernobyl-type reactors)
Subject to terrorist attacks
Spreads knowledge and technology for building
nuclear weapons
Fig. 15-21, p. 391
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TRADE-OFFS
Coal vs. Nuclear
Coal
Nuclear
Ample supply
Ample supply of uranium
High net energy yield
Low net energy yield
Very high air pollution
Low air pollution
Low CO2 emissions
High CO2 emissions
Much lower land disruption from surface mining
High land disruption from surface mining
Moderate land use
High land use
Low cost (with huge subsidies)
High cost (even with huge subsidies)
Fig. 15-22, p. 392
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Nuclear Power Plants Are Vulnerable to Terrorists
Acts

• Explosions or meltdowns possible at the power
  plants
• Storage pools and casks are more vulnerable to
  attack
• 60 countries have or have the ability to build
  nuclear weapons

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Dealing with Radioactive Wastes Produced by
Nuclear Power Is a Difficult Problem

• High-level radioactive wastes
• Must be stored safely for 10,000240,000 years
• Where to store it
• Deep burial safest and cheapest option
• Transportation concerns
• Would any method of burial last long enough?
• There is still no facility NIMBY scenario
• Can the harmful isotopes be changed into harmless
  isotopes? (working on it, )

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Case Study Experts Disagree about What to Do
with Radioactive Wastes in the U.S.

• 1985 plans in the U.S. to build a repository for
  high-level radioactive wastes in the Yucca
  Mountain desert region (Nevada)
• Problems
• Cost 58100 billion
• Large number of shipments to the site protection
  from attack?
• Rock fractures
• Earthquake zone
• Decrease national security

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What Do We Do with Worn-Out Nuclear Power Plants?

• Decommission or retire the power plant
• At least ½ of US plants are scheduled to close by
  next year
• Some options
• Dismantle the plant and safely store the
  radioactive materials
• Enclose the plant behind a physical barrier with
  full-time security until a storage facility has
  been built
• Enclose the plant in a tomb
• Monitor this for thousands of years

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Can Nuclear Power Lessen Dependence on Imported
Oil, Reduce Global Warming?

• Nuclear power plants no CO2 emission
• Complete Nuclear fuel cycle emits CO2 (not like
  Fossil fuels)
• Opposing views on nuclear power and global
  warming
• Nuclear power advocates
• 2003 study by MIT researchers low grade ore
• 2007 Oxford Research Group reactor a week for
  70 years still only produce 20

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Will Nuclear Fusion Save Us?

• Nuclear fusion is the power of the future and
  always will be
• Still in the laboratory phase after 50 years of
  research and 34 billion dollars energy
• 2006 U.S., China, Russia, Japan, South Korea,
  and European Union
• Will build a large-scale experimental nuclear
  fusion reactor by 2040

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Experts Disagree about the Future of Nuclear Power

• Proponents of nuclear power
• Fund more research and development
• Pilot-plant testing of potentially cheaper and
  safer reactors
• Test breeder fission and nuclear fusion
• Opponents of nuclear power
• Fund rapid development of energy efficient and
  renewable energy resources

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Science Focus Are New and Safer Nuclear Reactors
the Answer? (1)

• Advanced light-water reactors (ALWR)
• Built-in passive safety features
• High-temperature-gas-cooled reactors (HTGC)
• Avoids problems with present water based systems,
  but still not 100 reliable

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Alternatives

• Pebble bed modular reactor (PBMR)
• Pros no need to shut down for refueling
• Cons graphite protective coatings, vulnerable
• China, South Africa
• Breeder nuclear fission reactors
• Creates plutonium from uranium
• France built one then shut it down 2 years later

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Pebble Bed Reactors
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Breeder reactors
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Science Focus Are New and Safer Nuclear Reactors
the Answer? (2)

• New Generation nuclear reactors must satisfy
  these five criteria
• Safe-runaway chain reaction is impossible
• Fuel can not be used for nuclear weapons
• Easily disposed of fuel
• Nuclear fuel cycle must generate a higher net
  energy yield than other alternative fuels,
  without huge government subsidies
• Emit fewer greenhouse gases than other fuels

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Exit questions for 15.5

• Describe the nuclear fuel cycle
• How do we deal with the highly reactive
  radioactive wastes produced by nuclear power
  plants? How should we?
• What is the difference between nuclear fusion and
  fission? What is fusions potential as a energy
  source
• Summarize arguments for and against nuclear power