Chapter 20: Nuclear Energy and the Environment

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Chapter 20 Nuclear Energy and the Environment
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Nuclear Energy

• The energy of the atomic nucleus
• Two processes can be used to release that energy
• Fission splitting of atomic nuclei
• Fusion fusing or combining of atomic nuclei
• Nuclear reactors-devices that produce controlled
  nuclear fission.
• Used for commercial energy production

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Fission Reactors

• First demonstrated in 1942
• Led to development of nuclear energy to produce
  electricity.
• Also power submarines, aircraft carriers, and
  icebreaker ships.
• Nuclear fission produces much more energy than
  fossil fuels
• 1 kilogram of uranium oxide produces heat
  equivalent to 16 metric tons of coal

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Fission Reactors

• Three types (isotopes) of uranium occur in nature
• Uranium-238
• Uranium-235 (only naturally occurring fissionable
  material)
• Uranium-234
• Enrichment necessary
• Processing to increase concentration of U235

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Fission Reactors

• Split U-235 by neutron bombardment
• Reaction produces neutrons, fission fragments and
  heat.
• Starts a chain reaction
• Steam produced runs a turbine that generates
  electricity.
• Similar to coal or oil burning power plants

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Coal or Oil Power Plant
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Nuclear Power Plant
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Fission Reactors

• Main components of a reactor
• Core fuel and moderator
• Control rods control the rate of reaction or
  stop it
• Coolant remove heat
• Reactor vessel
• The entire reactor is contained in a reinforced
  concrete building

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Fission Reactors

• Meltdowns
• Occur when heat not removed fast enough
• Nuclear fuel becomes so hot it forms a molten
  mass
• Breaches containment of reactor and contaminates
  surrounding area
• Design philosophy emerged to build smaller
  reactors
• W/ passive stability (gravity feed cooling
  systems)
• Or helium gas cooled which cant meltdown

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Sustainability and Nuclear Power

• Two aspects
• Nuclear powers role in creating alternative fuel
  supplies
• Could help the US transition from oil to other
  energy sources
• The sustainability of nuclear fuel itself
• Is a nonrenewable resource

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Sustainability and Nuclear Power

• Currently light water reactors are very
  inefficient
• Breeder reactors
• Designed to produce new nuclear fuel
• Transform waste or low-grade uranium into
  fissionable material
• Future of nuclear power if sustainability of fuel
  an objective

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Pebble-Bed Reactors

• A gas cooled reactor
• Centered around fuel elements called pebbles
• As a spent pebble leaves the core another one is
  feed in
• Means the reactor has just the right amount of
  fuel for optimal production

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Fusion Reactors

• Involves combining the nuclei of light elements
  to form heavier ones
• Heat energy is released
• Source of energy in sun and stars
• In a hypothetical fusion reactor
• Two isotopes of hydrogen injected into reactor
  chamber
• Products include helium and neutrons

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Fusion Reactors

• Several conditions necessary
• 1. Extremely high temperatures
• 100 million degrees C
• 2. High density of fuel elements
• 3. Plasma must be confined
• Potential energy available if developed nearly
  inexhaustible.
• Many obstacles remain to be solved

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Nuclear Energy and the Environment

• Nuclear fuel cycle includes
• Mining and processing of uranium to controlled
  fission
• Reprocessing of spent fuel
• Decommissioning of power plants
• Disposal of radioactive waste
• Throughout the cycle radiation can enter and
  affect the environment.

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Problems with Nuclear Power

• Uranium mines and mills produce radioacitve waste
  material that can polluted the environment.
• U-235 enrichment and fabrication of fuel
  assemblies also produces waste materials.
• Site selection and construction controversial.
• Power plants also the site of past accidents or
  partial meltdowns.

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Problems with Nuclear Power

• Handling and disposal of waste
• Decommissioning expensive
• Terrorists could collect plutonium for dirty bomb

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Effects of Radioisotopes

• Radioisotopes is a isotope of a chemical element
  tht spontaneously undergoes radioactive decay.
• Affect environment in two ways
• By emitting radiation that affects other
  materials
• By entering the normal pathways of mineral
  cycling and ecological food chains

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Effects of Radioisotopes

• Explosion of nuclear atomic weapon does two types
  of damage
• Directly from blast
• Dispersal of radioactive isotopes
• Fallout
• Can enter ecological food chain
• Biomagnifies in the food chain (e.g. reindeer
  moss, caribou, humans)

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Radiation Doses and Health

• Determining the point at which the exposure or
  dose becomes a hazard to health.
• 5,000 mSv is considered lethal in 50 of people
• 1,000-2,000 mSv sufficient to cause health
  problems
• 50 msv maximum allowed dose for workers in the
  industry (30 times ave. natural background)

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Radiation Doses and Health

• Studies have shown a delay of 10-25 years between
  the time of exposure and the onset of disease.
• Most scientist agree that radiation can cause
  cancer
• But dont agree on relationship
• Linear vs. some threshold level

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Nuclear Power Plant Accidents

• US Nuclear Regulatory Commission
• Sets performance goal for a single reactor at
  0.01
• If there were 1,500 plants a melt down couls be
  expected every seven years
• Unacceptable risk

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Three Mile Island

• Occurred March 28, 1979 near Harrisburg, PA
• Malfunction of a valve resulted in partial core
  meltdown
• Intense radiation released to interior of
  containment structure
• Small amount of radiation released into
  environment

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Three Mile Island

• Because long-term chronic effects of exposure to
  low levels of radiation are not well understood,
  the effects of TMI difficult to estimate.
• Major impact of the incident was fear.

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Chernobyl

• Occurred April 26, 1986
• Worst accident in history of nuclear power
  generation
• Failure in cooling waters
• Reactor overheated melting the uranium fuel
• Explosions removed top of building
• Fires produced a cloud of radioactive particles

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Chernobyl

• 30 km zone surrounding Chernobyl evacuated
• City of Prypyat, pop. 48,000 at time of accident,
  now a ghost city.
• Thyroid cancer increased in Belarus, Ukraine and
  the Russian Federation
• Trees and vegetation damaged
• Ultimately be responsible for 16,000 deaths

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Radioactive Waste Management

• By-products that must be expected when
  electricity is produced at nuclear reactors.
• Three general categories
• Low-level waste
• Transuranic waste
• High-level waste
• In addition, tailings from uranium mines and
  mills considered hazardous.

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Low-Level Radioactive Waste

• Contains sufficiently low concentrations that it
  does not present a significant environmental
  hazard
• If handled properly
• Includes variety of residual and solutions from
  processing
• Solid and liquid plant waste, sledges, and acids
• Slightly contaminated equipment

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Low-Level Radioactive Waste

• Buried in near surface burial areas
• Where geologic and hydrologic conditions thought
  to limit migration
• However 3 of the 6 closed due to leaks
• Controversy remains as to whether low-level
  radiation can be disposed of safety.

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Transuranic Waste

• Composed of human-made radioactive elements
  heavier than uranium.
• Plutonium, americum, and einsteineum
• Most is industrial trash that has been
  contaminated.
• Generated from production of nuclear weapons

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Transuranic Waste

• Now being transported and stored in Carlsbad, NM
  salt beds
• Salt is 225 million years old and geologically
  stable
• No flowing ground water and easy to mine
• Rock salt flows slowly into mined openings,
  naturally sealing the waste.

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High-Level Radioactive Waste

• Consists of commercial and military spent nuclear
  fuel
• Uranium and plutonium derived from military
  reprocessing
• Other nuclear weapons material
• Extremely toxic
• Sense of urgency surround its disposal
• Total volume of spent fuel accumulating

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High-Level Radioactive Waste

• A comprehensive geologic disposal development
  program should have the following objectives.
• Identification of sites that meet broad geologic
  criteria.
• Intense subsurface exploration of possible sites.
• Predictions of future changes to sites
• Evaluation of risk associated with various
  predictions.
• Political decision making based on risks
  acceptable to society.

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Yucca Mountain Nuclear Waste Depository

• Nuclear Waste Policy Act of 1982
• Initiated high-level waste disposal program
• DOE investigated several sites
• To be disposed of underground in deep geologic
  waste repository
• Yucca Mountain only site being evaluated
• Use of site could begin in 2010

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Yucca Mountain Nuclear Waste Depository

• Extensive scientific evaluations of Yucca
  Mountain carried out
• Remain controversial
• Concerned that natural processes might allows
  radiation to escape
• Major question as to how accurate long-term
  predictions are

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The Future of Nuclear Energy

• Advocates argue that nuclear power is good for
  the environment
• It does not produce potential global warming
  through release of carbon dioxide.
• It does not cause acid rain.
• If breeder reactors are developed the amount of
  fuel will be greatly increased.
• Arguments for standardization,
• Safer and smaller plants to provide more energy

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The Future of Nuclear Energy

• Argument against
• Based on political and economic consideration
• As well as uncertainty of safety issues
• Known reserves would be used up quickly
• May be a path to nuclear weapons