Fission

Questions:

  1. Discuss the release of energy by fission and fusion reactions.
  2. Explain the role of the reflector in a fission bomb.
  3. What is critical mass?
  4. Why can't plutonium be used in a gun-type bomb?
  5. How does the implosion technique work?
  6. What is the initiator and why is it necessary?
  7. Energy release from one fission reaction is about 200 MeV. How many fissions are required to release an energy of 10 Kt?

Answers:

  1. Discuss the release of energy by fission and fusion reactions.

    Fission is the splitting of one heavy nucleus into two or more lighter nuclei. Fusion is the union of two light nuclei to form a heavy nucleus. Both processes usually involve the release of energy.

  2. Explain the role of the reflector in a fission bomb.

    Neutrons produced in the first few stages of fission are reflected back to cause more reactions. This helps decrease the time it takes to fission the bomb material, yielding increased efficiency.

  3. What is critical mass?

    A critical mass is that mass required to just sustain a chain reaction.

  4. Why can't plutonium be used in a gun-type bomb?

    The rate of spontaneous fission for plutonium exceeds the assembly time for a gun-type bomb to reach critical mass. Thus, a plutonium bomb built in this manner would not be able to reach critical mass before a stray neutron from spontaneous fission prematurely ignited the plutonium.

  5. How does the implosion technique work?

    The implosion technique works as follows:

    1. A high explosive is ignited.
    2. A compressional shock wave begins to move inwards.
    3. The core density increases to supercritical levels.
    4. The chain reaction continues until the energy generated inside the bomb becomes greater than the implosion pressure, and the bomb explodes.
  6. What is the initiator and why is it necessary?

    An initiator is a device that produces neutrons at just the right instant, when the assembly process has reached the stage at which the fissile material is compressed into a supercritical mass.

  7. Energy release from one fission reaction is about 200 MeV. How many fissions are required to release an energy of 10 Kt?

    200 MeV must be converted into joules as follows:
    (200 × 106 ev / MeV) x (1.6 × 10-19 J / ev) = 3.2 × 10-11 J / fission
    1 Kt = 4.18 × 1012 J
    10 Kt = 4.18 × 1013 J
    number of fissions = 4.18 × 10-13 J / 3.2 × 10-11 J / fission =1.306 × 1024 fissions