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Nuclear Reactions: Fission and Fusion

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energy required to break up 1 mol of nuclei into its individual nucleons ... 5) At 3 x 109 K, nuclei release neutrons, protons, and a particles, and recapture them; ... – PowerPoint PPT presentation

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Title: Nuclear Reactions: Fission and Fusion


1
Chapter 24
Nuclear Reactions Fission and Fusion
2
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)

4
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

5
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

6
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

7
Variation in Binding Energy Per Nucleon
8
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

9
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)
10
A Chain Reaction of 235U Fission
11
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

12
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

13
The Aftermath of the Bombing of Hiroshima
14
The Aftermath of the Bombing of Nagasaki
15
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

16
Controlled Fission Reactions
A light-water reactor
17
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?

18
Generating Energy Through Nuclear Fusion
The tokamak design for magnetic containment of a
fusion plasma.
19
Element Synthesis in the Life Cycle of a Star
20
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

21
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

22
  • A view of Supernova 1987A.
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