Fission and Fusion

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Chapter 6 : Fission and Fusion

Fission arrow_upward

  • Fission: Splitting a heavy nucleus into 2 nuclei with smaller mass numbers.
  • Chain Reaction: Self-sustaining fission process caused by the production of neutrons that proceed to split other nuclei.
  • Critical Mass: Mass of fissionable material required to produce a chain reaction.

  • How Fission Works arrow_upward

  • Water or other moderator slow down neutrons, thermalizing them.
  • Thermal neutron collides with U-235.
  • Unstable nucleus splits in two.
  • Energy and neutrons are released.
  • Reaction repeats.

  • Types of Fission Reaction arrow_upward

  • There are two types of fission reactions:
    • Uncontrolled fission reaction,
    • Controlled fission reaction,

  • Uncontrolled Fission Reaction:
    • A fission reaction whereby the reaction is allowed to proceed without any moderation is called an Uncontrolled Fission Reaction.
    • Used for: Nuclear bombs,
  • Example: In a fission of .
    • By slow neutrons, three more neutrons are created.



    • The emitted neutrons can create further fission reactions and will give rise to nine more neutrons and so on.
    • This is the chain reaction.
    • Each fission reaction gives out an enormous amount of energy.
  • Controlled Fission Reaction:
    • To maintain a sustained controlled nuclear reaction, for every 2 or 3 neutrons released, only one nucleus must be allowed to strike another nucleus.
    • A neutron absorbing element must be present to control the amount of free neutrons in the reaction space.
    • Used for: Nuclear reactors that produce power and generate electricity.
  • Example:


  • On removing two of the three neutrons then, instead of having three more    to fission, we will have only one more of such a reaction.
  • In this way, the rate of reaction can be controlled.
  • The neutrons can be removed by moderators (Cadmium) which are made up of materials that absorb neutrons.

  • Types of Fission Reactors arrow_upward

  • Fast Breeder Reactors (FBR),
  • Aqueous Homogeneous Reactors (AHR),
  • Heavy Water Moderated Reactors (HWR) (the CANadian Deuterium Uranium (CANDU) reactor),
  • Pressurized Water Reactors (PWR),
  • Boiling Water Reactors (BWR),
  • Organic-Cooled Power Reactors (OCPR),
  • Sodium Graphite Reactors (SGR),
  • Gas-Cooled Reactors (GCR).

  • Conclusion arrow_upward

  • Nuclear fission is a viable energy source.
  • It cuts down on CO2 emissions, improving air quality.
  • New designs have made nuclear power safer and more economically feasible.

  • Nuclear Fusion arrow_upward

  • The process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus.
  • This is usually accompanied by the release or absorption of large quantities of energy.
  • Nuclei are positively charged so they repel each other.
  • Energy has to be provided to overcome this repulsive force.
  •  Hydrogen Fusion Reaction:



  • Example:
    • Most efficient reaction known is fusion of Hydrogen isotopes, Deuterium and Tritium to form Helium.


    • This reaction releases 17.6 MeV of energy.
    • Unlike fission, there is no limit on the amount of fusion that can occur.
    • Fusion is the source of energy for stars and the sun.
    • High temperatures (> 100 million Kelvin) needed for fusion on earth.

    Advantages of Nuclear Fusion Reactions arrow_upward

  • Abundant fuel supply.
  • No risk of a Nuclear Accident since any malfunction results in a rapid shutdown.
  • The carbon dioxide emitted by this reaction contributes to the global warming (so-called "Greenhouse Effect").
  • No high-level Nuclear Waste.
  • Does not emit harmful toxins into the atmosphere.

  • Conditions for a Fusion Reaction arrow_upward

  • Plasma Temperature:
    • About 100 million Kelvin,
    • Needed to overcome natural positive repulsive forces of plasma ions.
  • Energy Confinement Time:
    • 4 to 6 seconds,
    • The Energy confinement time is a measure of how long the energy in the plasma is retained before being lost.
  • Central Density in Plasma:
    • 1 to 2 x 1020 particles m-3 ,
    • Large density needed because number of fusion reactions per unit volume is roughly proportional to the square of the density.

    Problems with Fusion arrow_upward

  • For fusion to occur, reactor temperatures would have to be on the order of 200 million Kelvin.
  • No material on earth can withstand 200 million Kelvin without melting.
  • Two basic strategies are:
    • Magnetic Confinement
    • Inertial Confinement
  • Magnetic Confinement:
    • Confine the plasma with magnetic fields so that the plasma will not touch the containment walls.
  • Inertial Confinement:
    • Supply large amounts of energy very quickly, so that the fuel is burned before it has time to expand and touch the walls.

    Potential Solutions arrow_upward

  • High Temperature:
    • Large current heating à7 million amperes,
    • High speed neutral beam heating-140,000 volts,
    • Radio-Frequency Heating-20MW: Current driven by microwaves heating.  
  • Isolation-Magnetic Field:
    • Plasma consists of two types of charged particles, so magnetic fields can be used to isolate it from the vessel.

    Difference between Nuclear Fusion and Fission arrow_upward

    Nuclear Fusion

    Nuclear Fission

    Two light nuclei combine to form heavier nucleus.

    A heavy nucleus breaks up to form light nuclei.

    Can never be spontaneous.

    Can be spontaneous

    No chain reaction is present.

    Chain reaction is present.

    Cannot be sustained in the laboratory conditions.

    Can be sustained and controlled in practical situations.

    Fusion happens when two nuclei come very close together.

    Fission requires large critical mass and a slow neutron to initiate the process.

    Fusion takes place only under massive energy.

    Fission only needs a low energy for splitting the atom.

    The radioactive waste that comes with fusion is very low.

    The radioactive waste that comes with fission is more as compared to fusion reactions.

    Is it Safe and Environment Friendly? arrow_upward

  • Fusion Reactors are safe and environment friendly.
    • Fusion reactors cannot meltdown because there is very little fuel in the reactor at a time, so to stop the reaction, the fuel valve can be closed or a “poison” valve can be opened to kill the reaction.
    • Neutrons cannot be confined in the magnetic field because they are uncharged so they will cause the first containment structure to be radioactive.
    • The structure will only need to be stored for 100 years and constitutes much less material than nuclear fission.

    Thank You from Kimavi arrow_upward

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