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Nuclear fission and nuclear fusion sheet
7-mark answer frame

What is nuclear fission? Explain the chain reaction with a diagram. How is it used in an atom bomb?

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Define. Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei, with the release of energy and neutrons (Hahn and Strassmann, 1939).
  2. 2 · Write the equation. ₉₂U²³⁵ + ₀n¹ → ₅₆Ba¹⁴¹ + ₃₆Kr⁹² + 3 ₀n¹ + about 200 MeV.
  3. 3 · Explain the chain reaction. The 2 or 3 neutrons released split more U-235 nuclei, so the fissions multiply: 1, 3, 9, 27 and so on.
  4. 4 · Draw. [Diagram: a neutron hitting U-235, splitting into two fragments and 3 neutrons, each hitting new U-235 nuclei, for 3 generations]
  5. 5 · Controlled and uncontrolled.
    • Controlled: one neutron per fission carries on; used in nuclear reactors.
    • Uncontrolled: all the neutrons cause fission; used in the atom bomb.
  6. 6 · Atom bomb. Pieces of U-235 or Pu-239 below critical mass are pushed together by an explosive; above critical mass, an uncontrolled chain reaction releases huge energy.
  7. 7 · Real-life link. Kudankulam and Kalpakkam use controlled fission to generate electricity.
  8. 8 · Close. The energy comes from the mass defect: E = mc².

Scoring tip: Balance the fission equation (236 on both sides, and 92 = 56 + 36): examiners check it.