Nuclear Energy And Elementary Particles

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Nuclear Energy And Elementary Particles

• Chapter 30

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Introduction

• What are the two methods of producing energy by
  means of nuclear reactions?
• Nuclear Fission
• Nuclear Fusion

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Nuclear Fission

• A large nucleus splits into two medium sized
  nuclei.
• Nuclear bombs
• Nuclear power plants

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Klaatu barada nikto! (1951)
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Nuclear Fusion

• Two smaller nuclei join to become a larger
  nucleus.
• Hydrogen bombs
• Source of the suns energy
• Future energy source for us?

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Hydrogen Bomb
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Nuclear Fission Reaction
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• In nuclear fission, a heavy nucleus, such as
  Uranium-235 can split into two smaller nuclei
  with the release of energy.
• The total mass of the products is less than the
  mass of the heavy nucleus.

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Fission Products

• Products resulting from thefission of
  uranium-235
• Barium, krypton
• Strontium, xenon
• Iodine, yttrium
• The atomic numbers mustadd up to 92.

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• Nuclear fission was first observed in 1939 by
  Hahn and Strassman
• They bombarded uranium with neutrons.
• Large kinetic energies are involved.
• Meitner and Frisch explained what happened.
• 200 MeV was released from each fission event.

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• How did nuclear fission change the course of
  human history?

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Hiroshima, JapanAugust 6, 1945
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Nagasaki Peace Park
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The Fission Sequence

• Uranium-235 captures a thermal (slow-moving)
  neutron.
• 30.1

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• Uranium-236 is formed and begins to oscillate
  violently.
• 296

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• The uranium-236 nucleus becomes distorted
• Repulsive forces between the protons encourage
  this.

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• The uranium-236 nucleus splits into two
  fragments.
• 73, 40-1

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• The energy released is about one hundred million
  times greater than that released by the
  combustion of gasoline.
• 30.2

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Future Energy Sources
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Chain Reaction

• Neutrons emitted during nuclear fission can
  trigger other fission reactions.
• A chain reaction can take place.
• If uncontrolled, a violent explosion can take
  place.
• 297, 76

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Chain Reaction Animation
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Nuclear Weapons

• The energy contained in one kilogram of
  uranium-235 is equal to the energy released by
  the detonation of 20,000 tons of TNT.
• Nuclear bombs are rated in kilotons and megatons.

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• Uranium 238 is used in radiation shielding and in
  armor-piercing ammunition.
• It is a very hard metal that is 1.5 times as
  dense as lead.

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The First Controlled Nuclear Reaction

• Nuclear reactors maintain what is called a
  self-sustaining chain reaction.
• First achieved by Enrico Fermi in 1942 at the
  University of Chicago
• The Italian navigator has landed in a brave new
  world. The natives are friendly.
• 30.4

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Nuclear Fuel Rods

• Nuclear reactor fuels must be enriched because
  natural uranium only contains 0.7 of the
  fissionable form of uranium, uranium-235.

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• The uranium-235 is made into fuel pellets that
  are 1 cm in diameter.

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• These are stacked in zirconium rods that are
  several meters long.

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• These fuel rods are bundled together to make a
  fuel bundle.
• A reactor will contain many bundles.

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Production of Plutonium

• Complete this equation Uranium-238 absorbs a
  neutron and decays by ?? emission.

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• Complete this equation Neptunium-239 decays by
  ?? emission.

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Breeder Reactors

• What is a breeder reactor?
• A breeder reactor is a nuclear reactor that
  creates fissile material (typically Pu-239 by
  irradiation of U-238) at a faster rate than it
  uses another fissile material (typically U-235)
  as fuel.

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• Breeder reactors can convert Uranium-238 into
  Plutonium-239.

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• It is possible that a nuclear powerprogram may
  be used as a meansto produce nuclear weapons.
• Iran?
• North Korea?

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The Reproduction Constant

• The reproduction constant, K, is defined as the
  average number of neutrons from a fission event
  that will cause another event.

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K values

• K 1 is the value for a self-sustained chain
  reaction (desirable).
• The reactor is said to be critical
• If K is less than 1, the reaction is said to be
  subcritical.
• The reaction soon dies out
• If K is greater than 1, the reaction is said to
  be supercritical.
• It is out of control

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Neutron Leakage

• The correct surface area/volume ratio must be
  maintained or too many neutrons will escape.
• 231

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Regulating Neutron Energies

• A moderator is used to slow down the neutrons.
• Graphite was originallyused.
• Chernobyl
• Modern reactors useheavy water D2O.

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Nuclear Moderators
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Neutron Capture

• Slowing down the neutrons makes them more likely
  to be captured by uranium-235.

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Controlling Nuclear Power

• The K value is controlled by rods made up of
  cadmium which absorb neutrons.
• These are held, magnetically, from above.

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US Nuclear Reactors

• Reactors in the US are pressurized water
  reactors.
• Three water circulation systems are involved.
• 298, 30.4

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• Cooling towers or cooling ponds are used to
  remove excess heat energy from the water before
  it is returned to the lake or river.

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Reactor Safety

• Reactor safety is often sensationalized by the
  media and misunderstood by the public.
• The China Syndrome movie (1979)
• Released 12 days before the 3-mile Island
  incident.

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Three Mile Island Nuclear Plant (1979)
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Chernobyl Nuclear Plant (1986)
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Nuclear Plant Safeguards

• There are 3 levels of safety.
• The fuel and the radioactive fission products are
  contained in the reactor vessel.
• The reactor vessel is inside a reinforced
  concrete structure.
• The facility is usually located away from the
  general public.

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The China Syndrome

• Nuclear meltdown
• Emergency core cooling systems are in place to
  prevent this from happening.

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Safe Disposal of Nuclear Waste

• Safe disposal of nuclear waste is a major
  environmental concern.
• Nevada vs. Michigan?

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• Nuclear materials from hospitals and industry
  must also be disposed of.
• Nuclear waste is sealed in glass in waterproof
  containers and placed in salt mines.

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Two Types of Nuclear Waste

• Low level
• Mining, fuel enrichment and hospital waste
• High level
• Spent fuel rods

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Transporting Nuclear Waste

• Transportation of nuclear materials is another
  environmental concern.
• Containers must meet stringent crash tests.

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• How can we dispose of high level nuclear waste?
• Send it to the sun?
• Drop it into a volcano?
• Drop it into the ocean?
• Bury it at a polar icecap?
• Drop it into a deep hole?

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• Current solutions
• Store the fuel rods on site, under water.
• Reprocess the fuel.

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• Future solutions
• Store it in a cave in Yucca Mountain.

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• Problems associated with the mining of uranium
• Mining puts lives in danger.
• Mining can release radioactive material into the
  air and water.

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Nuclear Fusion

• Two light nuclei combine to form a heavier
  nucleus.
• Nuclear fusion is a much sought after energy
  source.
• 299, 40-2

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Nuclear Fusion

• Fusion on the sun
• 90 of the stars fuse hydrogen into helium.

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Two Conditions for Fusion

• The temperature must be about 107 K for hydrogen
  nuclei to overcome the Coulomb forces.
• The density of the nuclei must ensure a high rate
  of collision.

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Thermonuclear Reactions

• Energy liberating fusion reactions are called
  thermonuclear fusion reactions.

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Nuclear Fusion Reactors

• Deuterium could be used for fuel.
• Neutrons are needed to produce helium.
• 1 gallon of water contains 0.06 g of deuterium.
• The waste product would be helium.
• Tritium could also be used for fuel, but it is
  radioactive and rare.

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Confining the Plasma

• Magnetic fields can be used to contain a plasma.
• The tokamak fusion reactor has been shut down.
• The National Spherical Torus experiment is
  currently underway at Princeton.
• 30.5a

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Uncontrolled Nuclear Reactions
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Nuclear Weapons

• The atomic bomb is an out of control fission
  reaction.

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• The amount of uranium needed to cause an
  explosion is called the critical mass.

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• In an atomic bomb, the critical mass is kept in
  two separate halves.
• These are brought together to detonate the bomb.

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• Only 20 enrichment of the uranium-235 is
  required but 85 is considered weapons grade.

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• How much material is needed to build an atomic
  bomb?
• The uranium-235 needed would be about the size of
  a pop can.

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Elementary Particles

• The word atom comes from the Greek word atomos
  meaning indivisible.
• The Greek model persisted for two centuries.
• 27

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The Electron

• J.J. Thomson discovered the electron in 1897 and
  demonstrated that the atom was not indivisible.

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The Neutron

• In 1932, the neutron was discovered by Chadwick.
  Now there were three particles associated with
  the atom.

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Sub-Atomic Particles

• In 1945 several smaller particles were
  discovered.
• More than 300 have been documented to date.
• 98

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Quarks

• Most particles are now known to be made up of
  smaller particles called quarks.
• Protons and neutrons aremade up of quarks.
• 300

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The Fundamental Forces In Nature

• The key to understanding particles is to
  understand the four basic forces which govern
  them.
• 301

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Electromagnetic Force

• Responsible for the binding of atoms and
  molecules
• 1/100 the strength of the strong force
• Involves photons

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Nuclear Weak Force

• Produces instability in the nucleus
• Involves bosons

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Nuclear Strong Force

• Responsible for the binding of protons and
  neutrons in the nucleus.
• The strongest of the four forces.
• Involves gluons.

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Gravitational Force

• Responsible for the attraction between matter
• The weakest of the four forces
• Involves gravitons???

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Positrons And Other Antiparticles

• For every particle, there is an antiparticle.
• They have the same mass but opposite charge.
• proton, antiproton
• electron, positron
• neutron, antineutron

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The Positron

• The positron was discovered by Carl Anderson in
  1932.
• He received the Nobel Prize in 1936.
• A cloud chamber was used.

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Pair Production and Pair Annihilation

• The positron is involved in pair production and
  in pair annihilation.

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PET Scans

• Positrons are involved in Positron Emission
  Tomography (PET) scans.
• Used to study Alzheimers disease.
• Also used to identify which regions of the brain
  are involved in processes such as language,
  music, and vision.

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A PET Scan Image
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• Questions
• 2, 4, 5
• Pg. 957