M8-S11: Nuclear Fission and Reactors

  • predict quantitatively the energy released in nuclear decays or transmutations, including nuclear fission and nuclear fusion, by applying: (ACSPH031, ACSPH035, ACSPH036)

    • the law of conservation of energy
    • mass defect
    • binding energy
    • Einstein’s mass–energy equivalence relationship

Nuclear Fission

  • Nuclear fission is the process where a heavy or large nucleus or atom splits to form two lighter nuclei. The binding energy of the products is lower than that of the parent nucleus because they have higher stability. As a result, energy is emitted during the process.

Nuclear fission of Uranium-235

Diagram shows nuclear fission of uranium-235 to form two smaller nuclide fragments: krypton-92 and barium-141. This process involves the formation of an intermediate (uranium-236) and emission of 3 neutrons.

 

  • Nuclear fission can occur in the form of radioactive decay, but it is very rare. Radioactive decay is only considered a nuclear fission reaction when two fragments of daughter nuclei are formed.
  • Nuclear fission reactions require large amounts of energy to overcome the strong nuclear force holding all the nucleons together in the nucleus.
  • Nuclear fission is often induced by colliding a neutron with a heavy isotope such as uranium. The speed of the neutron must be high enough such that its kinetic energy equals or exceeds the required energy for fission to occur.

 

Mass defect and binding energy

  • The actual mass of a nucleus is always less than the sum of its constituent masses. This implies there is missing mass and this missing mass is called mass defect.
    • For a nucleus, the mass defect is the difference between the total mass of its nucleons and its actual nuclear mass.
    • For an atom, the mass defect is the difference between the total mass of nucleons as well as electrons as its actual atomic mass.
Mass defect of a hydrogen atom
  • The binding of nucleons to form a nucleus involves loss of energy. The energy that is lost is converted into mass using Einstein’s mass-energy equivalence principle.
  • Similarly, if we were to do the opposite, i.e. split a nucleus into its individual protons and neutrons, sufficient energy would need to be applied to overcome the binding energy. At the same time, the mass defect would need to be restored to produce each sub-atomic particle.
  • Mass-energy equivalence and the concept of binding energy are also evident in nuclear fission reactions.
    • The total mass of products will be less than that of reactants.
    • The binding energy of products will be greater than that of reactants.
    • This implies that energy must have been emitted in the splitting process (increase in binding energy) - this is the energy release from fission.
  • Mass defect is typically measured in atomic mass unit (amu). Using mass-energy equivalence, the binding energy of an atom can be found, which is typically measured in MeV
  • Binding energy per nucleon equals to the total binding energy of a nucleus divided by the number of nucleons in the nucleus.
  • Equations to remember for calculating binding energy:

     

    For example, the binding energy of carbon-12 (nuclide mass = 12.0000 u) nucleus can be calculated:

     

    The binding energy of carbon-12 (atomic mass = 12.0000 u) atom can be calculated:

      

    • model and explain the process of nuclear fission, including the concepts of controlled and uncontrolled chain reactions, and account for the release of energy in the process (ACSPH033, ACSPH034)

    Controlled and Uncontrolled Chain Reactions

    • A chain reaction in nuclear physics refers to the process during which nuclear fission reactions are made self-sustaining. In other words, one fission can induce another without any further action.

    • Critical reaction - each fission reaction produces exactly one further fission reaction and maintains constant reaction rate. This kind of perfect reaction is called a critical reaction and the amount of fissionable material used to facilitate this one-for-one sustainability is called the critical mass (smallest amount of fissile material that will facilitate sustained chain reaction).
    An example of uncontrolled chain reaction
    • Uncontrolled Chain Reaction - when more than one neutron induces further nuclear fissions. The amount of fissionable material required to facilitate this kind of reaction must exceed the critical mass; uncontrolled chain reactions are created in atomic bombs.

    • A controlled chain reaction refers to a chain reaction whose rate of fission is controlled and thus the generated energy is also controlled. While a controlled chain reaction is conducted in such a way that it remains self-sustaining, it is also deemed safe through the use of control rods. The number of control rods required to achieve this depends on the quantity of fissionable material present.

     

    Enrico Fermi’s demonstration of controlled chain reaction (1942)

    • Fermi realised that since the fission of a Uranium atom released 3 neutrons and only one was needed to cause fission in uranium, a chain reaction of nuclear fission could be produced which could supply large amounts of energy.

    • He used bricks made of graphite to act as moderator materials which separated the 50 tonnes of natural uranium he had gathered.
    • The graphite moderators were used to slow down neutrons so that they are more likely to be captured by uranium nuclei. This increases the frequency of nuclear fission and efficiency of energy production.
    • During the experiment, heavy water (water containing hydrogen-2 isotope instead of normal hydrogen-1) was also tested as moderators. Heavy water was found to be more effective but more expensive, so graphite was used instead.
     

    Diagram illustrates a simple set-up of moderated and controlled chain reaction of Uranium-235. Moderators are positioned after each 'row' of fissile material (Uranium-235) to slow down emitted neutrons. Controlled rods are dispersed throughout the reaction chamber to reduce reaction rate.

    • He used control rods made of cadmium. When the control rods were removed, the reactor heated up and produced increasing amounts of radiation. The number of control rods determines the rate of nuclear fission and more importantly and stability of the experiment.
    • The reaction became self-sustaining when the control rods were removed sufficiently – with the primitive reactor producing 0.5 W in a self-sustaining reaction.

     

    Nuclear Reactor

    • A fission reactor uses a nuclear reaction to generate electricity. As with all generators, this involves producing rotation to turn a generator.
    • In a nuclear reactor, heat from the reaction is used to produce steam which turns a turbine which will generate electricity.
    • There are essentially five major components which make up a fission reactor.
    • These maintain a controlled chain reaction which ensures that reaction continues and the reactor does not explode (due to uncontrolled chain reaction). 

    Fuel Rods

    • Fuel rods are hollow rods made of metal, typically alloy steel, and contain a sub-critical mass of fuel or fissionable material (usually enriched uranium oxide or plutonium).
    • They are inserted into the reactor in a grid pattern and their combined fuel provides enough fissionable material for critical mass.

    Moderator

    • Moderators, typically heavy water or graphite, slow down neutrons produced in the fission reactions from the reactor.
    • The slowing down of neutrons increases the chance of more fission reactions. 

    Control Rods

    • Commonly made of cadmium, control rods are inserted into the reactor core to slow/stop the rate of fission by absorbing neutrons.

    Coolant

    • The heat produced by the fission reaction is transferred to the coolant, which will transfer its heat to another liquid circuit as the coolant is radioactive.
    • The water that is heated at the heat exchange will turn into steam which drives a turbine and produces electricity.

    Shielding

    • Multiple layers of shielding surround the reactor.

    A graphite shield reflected neutrons back into the core, followed by a thermal shield to prevent unwanted heat loss, a pressure vessel to isolate and contain everything inside the core and a biological shield of 3 metres of concrete mixed with lead pellets to absorb gamma rays and neutrons.

     

     

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