Fission

Most power stations get the energy they need by burning fuel (e.g. coal) or from the natural motion of something (e.g. waves, tides). Nuclear power stations do it a bit differently...

What does nuclear fission mean? [1]
- "splitting" the nucleus of an atom.
  - Why would you want to do that? [1]
    - To release the energy stored within the atom. Fission gives out a lot of energy - a lot more than you get with a chemical bond between two atoms. This energy is in the form of kinetic energy, which can be converted to heat energy in the reactor via collisions with other atoms.
      - Which atom is usually used for fission (splitting) in nuclear reactors (and atomic bombs)? [1]
        
        Uranium-235. (U-235 is actually quite stable, so it needs to be made unstable before it'll split)
        
        How is it made unstable? [1]
        
        You can bombard stable nuclei with various types of particles to make unstable nuclei
        
        In this case, slow-moving (low-energy) neutrons (called "thermal neutrons") are fired at the nucleus of U-235
        
        Why must the neutrons be low-energy? [1]
        
        Higher energy neutrons just bounce off the nucleus because they are travelling too fast
        
        What does the U-235 change into once the thermal neutron is captured and joins the nucleus? [1]
        
        Uranium-236
        
        Uranium-236 is really unstable!
        
        So it fissions into two smaller atoms, plus a few fast moving neutrons
        
        The two new lighter atoms are called "daughter nuclei" and they are usually radioactive because they have the 'wrong' number of neutrons in them (this is radioactive waste which is difficult and expensive to dispose of). There are many different pairs of radioactive daughter nuclei that U-236 can split into - What is one possibility? [2]
        
        E.g. Krypton-90 + Barium-144 (+ 3 neutrons)
        
        E.g. Rubidium-90 + Caesium-144 (+ 2 neutrons)
        
        E.g. Strontium-90 + Xenon-144 (+ 2 neutrons)
        
        E.g. Bromine-87 + Lanthium-146 (+ 3 neutrons)
        
        E.g. Zinc-72 + Samarium-160 (+ 4 neutrons)
        
        Can you describe the whole process of Nuclear Fission again? [5]
        
        What would the nuclear equation for this fission reaction look like? [1]
        
        Can you give an alternative nuclear equation? [1]

What is a Chain Reaction? [2]
- Each time a Uranium-235 nucleus splits up, it spits out a small number of neutrons.
- - These might go on to hit other U-235 nuclei, causing them to split and release even more neutrons. ...Which might hit even more nuclei ...and so on and so on.
  - - What might happen if this chain-reaction were allowed to continue and exponentially increase without being controlled (an uncontrolled chain reaction)? [2]
      - A nuclear meltdown (like the power plant in Chernobyl)
      - A nuclear explosion (like the bomb in Nagasaki)

What goes on inside a Nuclear Power Plant's Core Reactor? [3]
- Components 1 & 2 - What is this? [1]
  - Control Rods, often made of boron
    - What do they do? [1]
      - limit the rate of fission by absorbing excess neutrons
- Components 3 & 4 - What is this made of? [1]
  - Thick concrete walls
    - Why? [1]
      - To prevent the release of radiation from the radioactive waste
- Component 5 - What is this? [1]
  - The moderator, usually graphite or water
    - What does it do? [1]
      - slows neutrons to make them "thermal neutrons" (low-energy neutrons) so that they can successfully collide with uranium nuclei and sustain the chain reaction
- Component 6 - What is this? [1]
  - Uranium rods
    - What do they do? [1]
      - these contain the nuclear fuel - the U-235 nuclei. Free neutrons in the reactor 'kick start' the fission process. The atoms produced then collide with other atoms, causing the temperature in the reactor to rise.
- Component 10 - What is this? [1]
  - A gas, typically carbon dioxide
    - What is it used for? [1]
      - It is pumped through the reactor to carry away the heat generated
- Components 7 & 8 - What is this? [1]
  - Heat exchanger
    - How does it work? [3]
      - The heat exchanger pumps a gas through it which gives its energy to the cold water (component 13) being pumped (component 9) to transport the energy away from the reactor
      - This water is heated and turned into steam (component 12)
      - The steam is used to turn a generator which can convert the heat energy into electrical energy

Classwork
- 1) Give one reason for using nuclear power rather than fossil fuels?
- 2) What is nuclear fission?
- 3) What are the products of nuclear fission of Uranium-235?
- 4) Explain how the chain reaction in a nuclear reactor works
- 5) What is used in a nuclear reactor to slow down neutrons which are moving too quickly?
- 6) What job do control rods and moderators do in a nuclear reactor?
- 7) What is a thermal neutron?
- 8) Write a nuclear equation for the induced fission of uranium-235 into Lanthanum-145 and bromine-88
- 9) Describe how heat energy from the reactor is used to generate electricity
- 10) Explain how a nuclear fission chain reaction occurs, starting with a single plutonium nucleus absorbing a slow-moving neutron
Homework
- 1) The diagram shows control rods (green) inside a nuclear reactor.
  - Explain how the control rods control the rate of fission
  - What material are control rods usually made from?
  - Why is the reactor surrounded with a very thick layer of concrete?
- 2) Nuclear power is an example of how nuclear fission can be used peacefully
  - Give one destructive use of nuclear power
  - How is the chain reaction different in this case from that in a reactor?
- 3) Uranium-235 atoms are split in a nuclear reactor to release energy. Some products of the fission can also be used for medical applications. Uranium-235 must be converted to uranium-236 using a thermal neutron
  - What is a thermal neutron?
  - Describe how uranium-235 is converted into uranium-236
  - U-236 splits into two smaller atoms, which are often unstable. What makes these atoms unstable?
- 4) Bombarding stable elements with potons can produce radioactive isotopes. Complete the following passage using the words provided in brackets ( nucleus / electron / proton / element / mass )
  - A proton is absorbed by the ............................
  - This increases its ................. number so a new ............................ is produced

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Fission

Most power stations get the energy they need by burning fuel (e.g. coal) or from the natural motion of something (e.g. waves, tides). Nuclear power stations do it a bit differently...

What does nuclear fission mean? [1]

What is a Chain Reaction? [2]

What goes on inside a Nuclear Power Plant's Core Reactor? [3]

Classwork

Homework