nuclear physics

• binding energy is the energy required to separate a nucleus into its nucleons
• the mass defect is the change in mass between the nucleus and its separate nucleons
• the mass of the nucleus will always be lower than the mass of the nucleons as the mass defect is the energy used to fuse the nucleons
• fission is when a large unstable nucleus is hit by a slow moving neutron and splits into 2 smaller nuclei and 3 neutrons
• fission can occur naturally or be induced
• fusion is when 2 smaller nuclei fuse to form 1 larger nucleus
• fusion can only occur at extremely high temperatures as it is needed to overcome the electrostatic force of repulsion
• energy is released in fission and fusion because the products have a higher binding energy per nucleon than the reactants
• iron has the highest binding energy per nucleon
• elements smaller than iron undergo fusion
• elements heavier than iron undergo fission
• a thermal neutron is one with low energy and so will be moving at the right speed to be absorbed rather than rebounded
• critical mass is the minimum mass of fuel required to maintain a steady chain rection
• the disposal process for high level waste

  • rods placed in cooling ponds for up to a year
  • remaining plutonium / uranium is removed
  • rods placed in glass case and put in thick steel case
  • buried in a deep cavern in a geologically stable location
• moderator

  • slows down neutrons to right pace
  • water / graphite
  • because they are unreactive
• control rods

  • absorb neutrons to stop fission
  • boron / cadmium
  • because they absorb neutrons without undergoing fusion
• coolant

  • absorbs heat from reactions to create the steam that turns the turbines
  • water / helium
  • because they have a high specific heat capacity
• decay leads to an increase in binding energy as the nucleus is more stable
• work is required to overcome the strong force when separating nucleons. this added energy corresponds to a mass so the mass of the nucleons is greater than the mass of the nucleus.
• u is 1/12th of the mass of a carbon 12 atom
• higher binding energy = more stable nucleus
• both fission and fusion create nuclei that are more stable and closer in mass to iron
• fuel in a reactor is mainly U-238 with 2-3% U-235
• only U-235 is fissile so uranium needs to be enriched to have more U-235
• proton mass = 1.00728u
• neutron mass = 1.00867u