Nuclear Reactions: Fission and Fusion

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Chapter 24
Nuclear Reactions Fission and Fusion
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Nuclear Reactions and Nuclear Chemistry

• Nuclear fission
• A heavy nucleus splits into 2 lighter nuclei
• Emission of radioactive particles
• Nuclear fusion
• Two lighter nuclei combine into 1 heavier nucleus
• Both processes release large amount of energy

3
Nuclear Reactions Mass Defect

• The Interconversion of Mass and Energy
• The total quantity of mass-energy in the universe
  is constant
• E mc2 (Einsteins Relativity Theory)
• When any reaction releases or gains energy, there
  must be an accompanying loss or gain of mass
• Chemical reaction
• H2O(g) ? 2H2(g) O(g) DHrxn 934 kJ/mol
• Using E mc2 ? Dm DE/c2 1.04 x 10-8 g/mol
• Negligible amount when change in mass compared to
  1 mol of H2O (18.0016 g)

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Nuclear Reactions Mass Defect

• Nuclear reaction
• Consider 12C 6 neutrons and 6 protons
• Mass of 6 H atoms mass of 6 neutrons
• six 1H atoms 6.046950 amu
• six neutrons 6.051990 amu
• Total mass 12.090940 amu
• Mass of 12C 12.000000 amu
• Dm 0.098940 amu/12C 0.098940 grams /mol 12C
• Not a negligible amount of mass
• Using E mc2 ? Dm DE/c2 8.8921 x 109 kJ/mol
  
• Nuclear Binding Energy energy required to break
  up 1 mol of nuclei into its individual nucleons

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Nuclear Reactions Nuclear Binding Energy

• Nuclear Binding Energy
• energy required to break up 1 mol of nuclei into
  its individual nucleons
• nucleus nuclear binding energy ? nucleons
• Often expressed in electron volts
• energy that 1 electron acquires when it moves
  through a field of 1 Volt
• 1 eV 1.602 x 10-19 J
• 1 amu 931.5x106 eV 931.5 MeV
• Binding energy of one 12C nucleus 0.098940 amu
  92.16 MeV
• Binding energy per nucleon in 12C 92.16 MeV/12
  nucleons 7.680 MeV/nucleon

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Nuclear Reactions Nuclear Binding Energy

• Compare Nuclear Binding Energy
• 56Fe nucleus 26 1H atoms 30 neutrons
• 26 1H atom 26.203450 amu
• 30 neutron 30.259950 amu
• 56Fe atom 55.934939 amu
• Mass defect, Dm 0.528461 amu
• Binding energy 0.528461 amu x 931.5 MeV/amu
  492.26 MeV
• Binding energy per nucleon 492.26 MeV/56
  nucleons 8.790 MeV/nucleon
• Compare binding energy for 12C 7.680
  MeV/nucleon
• 56Fe is more stable than 12C, because it requires
  more energy per nucleon to break up the nucleus

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Variation in Binding Energy Per Nucleon
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Nuclear Reactions Nuclear Binding Energy

• Compare Nuclear Binding Energy
• 56Fe 8.790 MeV/nucleon
• 12C 7.680 MeV/nucleon
• 238U 7.57012 MeV/nucleon
• Fission or fusion is a means to increase binding
  energy, and thus form a more stable nuclide
• The greater the binding energy, the more stable
  the nuclide
• Nuclides with less than 10 nucleons have low
  binding energy
• Nuclides increase binding energy to elements with
  60 nucleons (A 60)
• Larger elements have decreasing binding energy
  and become more unstable
• Nuclear fission
• A heavy nucleus splits into 2 lighter nuclei with
  A closer to 60
• Nuclear fusion
• Two lighter nuclei combine into 1 heavier nucleus
  with A closer to 60
• In both cases release of energy

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Induced Fission of 235U
Bombard 23592U with 10n ? unstable 23692U (t½
10-14s) ? 14156Ba 9236Kr 310n
energy Energy released 2.1 x 107 MJ/mol ( 106
more energy than burning an equal amount of coal)
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A Chain Reaction of 235U Fission
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Controlling a Chain Reaction of 235U

• 235U fission creates more and more neutrons
• Neutrons collide with other 235U nuclei
• Generate more neutrons ? self-sustaining process
• Whether a chain reaction occurs depends on the
  mass and the volume of the available fissionable
  sample
• if mass and volume of a sample is too low
• generated neutrons will fly out of the sample
  before colliding with another nucleus
• If the mass (and the volume) is high enough
• the statistical change of colliding with another
  nucleus is higher than flying out of the sample
• result is chain reaction
• Critical Mass

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Uncontrolled Fission Reaction of 235U
Diagram of an atomic bomb.

• 2 subcritical masses are separated
• Explosion brings them together
• Threshold of critical mass is surpassed
• Nuclear reaction starts
• Hiroshima, Aug 6, 1945
• Little Boy, 1 kg of fissionable 235U
• 600 milligrams of mass were converted into energy
• estimated 13 to 16 kilotons of TNT
• approximately 140,000 people were killed
• Blast
• Fire
• radiation
• Nagasaki, Aug 9, 1945
• Fat Man, plutonium bomb
• bomb had a yield of about 21 kilotons of TNT, or
  8.781013 joules 88 TJ (terajoules)
• an estimated 39,000 people were killed outright
• about 25,000 were injured

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The Aftermath of the Bombing of Hiroshima
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The Aftermath of the Bombing of Nagasaki
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Controlled Fission Reactions

• Generating nuclear energy
• Controlled fission of uranium can generate
  electricity
• Nuclear power plant generates heat that produces
  steam
• Steam turns a turbine that generates electricity
• Fuel rods of uranium(IV) oxide (UO2) in the
  reactor core
• enriched in 235U from naturally 0.7 to 3-4
• Moveable control rods control the amount of
  fission
• control rods are cadmium or boron
• absorption of neutrons when lowered between the
  fuel rods
• fewer neutrons bombard the uranium
• fission slows down ? less heat
• Reflectors (beryllium alloy) reflect the neutrons
  into the fuel rods and speed up the reaction
• Moderator slows neutrons to increase fission
  instead of leaving the fission reaction
• Light-water reactors, 1H2O, as moderator and
  coolant - 1H absorbs neutrons
• Heavy-water reactors use 2D2O as moderator
  absorbs less neutrons, more available for reaction

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Controlled Fission Reactions
A light-water reactor
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Generating Energy Through Nuclear Fusion

• Sun generates energy through nuclear fusion
• all elements larger than H are formed through
  fusion and decay processes in stars
• Nuclear fusion
• reaction between deuterium and tritium
• 21H 31H ? 42He 10n DE 1.7 x 109 kJ/mol
• 21H can be generated from electrolysis of water
• 31H is generated through cosmic radiation
• 147N 10n ? 31H 126C
• The amount of 31H is very small
• Alternative reaction
• 63Li 10n ? 31H 42He
• Energy required to start the reaction is 108 K
• Hotter than the core of the Sun
• Possible with thermonuclear reaction (hydrogen
  bomb)
• 63Li 21H ? 2 42He
• But do you want to start with a hydrogen bomb?

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Generating Energy Through Nuclear Fusion
The tokamak design for magnetic containment of a
fusion plasma.
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Element Synthesis in the Life Cycle of a Star
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Generating Elements in the Stars

• 1) Hydrogen burning produces helium
• stellar contraction creates temperatures of
  107 K, resulting in hydrogen fusion
• 4 11H ? 42He 2 01b 2 g energy
• Helium burning produces C, O, Ne and Mg
• dense 42He core, 2 x 108 K, starts to fuse 42He
  to heavier elements
• absorption of a particles
• 2 42He ? 84Be ? 126C ? 168O ? 2010Ne ? 2412Mg
• Formation of elements through Fe and Ni
• carbon and oxygen burning at 7 x 108 K
• 126C 126C ? 23Na 1H
• 126C 16O ? 28Si g
• Absorption of a particles by 126C
• 126C ? 168O ? 2010Ne ? 2412Mg ? 2814Si ? 3216S
  ? 3618Ar ? 4020Ca

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Generating Elements in the Stars

• 5) At 3 x 109 K, nuclei release neutrons,
  protons, and a particles, and recapture them
• creation of nuclei with highest nuclear binding
  energy, 56Fe and 58Ni
• 6) Formation of heavier elements through neutron
  capture
• 68Zn 10n? 69Zn g ? 69Ga 0-1b
• 69Ga 10n? 70Ga ? 70Ge 0-1b
• 5626Fe 23 10n ? 7926Fe ? 7935Br 9 0-1n

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• A view of Supernova 1987A.