Nuclear Fission and Fusion

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  • Created on: 22-03-13 16:20
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  • Nuclear Fission and Fusion
    • Nuclear Binding Energy
      • The measured mass of a nucleus is always less than the total mass of its constituent nucleons.
        • The difference in mass is the mass deficit / mass defect.
      • Mass deficit occurs because a small amount of the mass of the neucleons is converted into the energy needed to hold the nuclus together.
        • This is the binding energy.
      • E= Mc^2 (if the mass is in KG)
        • If the mass is in atomic mass units then convert this into binding energy in mega electron volts (MeV)
          • 1u= 931.5 MeV
    • Binding Energy Per Nucleon
      • To find out how much energy is needed to remove one nucleon from a nucleus the binding energy and the number of nucleons must be known.
      • From the binding energy per nucleon we can determine how strongly different nucleo are held together
      • A graph of binging energy per nuclei against mass number allows us to compare hot tightly different nuclides are bound together.
      • Iron 56 is the isotope with the highest binding energy.
    • Nuclear Fusion
      • Joining two light nuclei together will make a new heavier nucleu.
        • The mass of the new nucleus will be less than the mass of the constituent parts, as some mass is converted into energy
      • Not all the energy created is used as binding energy, some is released.
      • The binding energy per nucleon afterwards is higher than at the start
      • -Two protons fuse - one prton changes into a neutron (beta plus decay) to leave a deuterium nucleus - another proton  joins on making a nucleus of He 3 - two He 3 nuclei fuse -Two protons break off leaving He 4
        • Scientists have not yet maintained a controlled nuclear fusion reaction as it takes million kelvin temperatures and high density to overcome electrostatic repulsion of protons.
    • Nuclear Fission
      • In this process a large nucleus breaks up into two smaller nuclei, with the release of some neutrons and energy.
      • If a reaction went a head without moderators it would run uncontrolled and would produce energy continuously at an ever increasing rate until all fuel was used up
    • Nuclear reactors
      • uranium 235 is the most common nuclear fission reaction used in power stations.
        • A nucleus of uranisum 235 hit by a slow moving neutron will absorb it to from u 236. which is unstable
          • The nucleus will break up, the products will be two medium sized nuclei with roughly half the nucleons each.
            • There is a difference in binding energy of uranium 235 and the daughter products which is given off as kinetic energy.
      • Only one neutron is required but it will produce two/three chain reactions.
      • To control the reactions, control rods absorb excess neutrons


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