Nuclear Fission

Nuclear fission is the splitting up of large and unstable nuclei into smaller nuclei. This process releases a lot of energy. There are two main ways that nuclear fission can take place:

  • Spontaneously, which takes place naturally, without the help of a neutron
  • By absorbing a neutron, which makes the nucleus more unstable

In a nuclear reactor, we use neutrons to start the process, because spontaneous fission is extremely rare. Let’s break down how the process works:

1. A slowed neutron is fired at a large, unstable nucleus (usually uranium-235).

2. The large nucleus absorbs the neutron, which makes it become uranium-236 (which is unstable).

3. As the nucleus becomes more unstable, it splits up into smaller nuclei, which we call daughter nuclei.

4. In the process, the nucleus fires out two or three more neutrons and releases energy in the form of gamma radiation.

5. The neutrons released collide with more uranium nuclei, which will lead to more fission reactions. This is known as a chain reaction.

If this process is not controlled, the whole system will not function properly, which could lead to the release of large amounts of energy. This is what happens in a nuclear bomb.

In nuclear reactors, the rate of fission has to be controlled, which can be achieved by using control rods. They are inserted and removed from the reactor to increase or decrease the rate of reaction and absorb neutrons.

The energy released in the process is used to heat water, which turns into steam. The steam rises and drives turbines, which are connected to an electricity generator.

Advantages and Disadvantages of Nuclear Fission


  • Uranium and plutonium fuel is quite cheap, so there are low running costs
  • Nuclear fission produces a large and steady supply of energy
  • There are no gas emissions produced by nuclear fission


  • Nuclear power plants are quite expensive to build and dismantle
  • They produce radioactive waste, which is expensive to dispose of – It has to be buried underground in metals containers, which are then stored in concrete-lined vaults
  • Nuclear fission emits ionising radiation that is harmful to humans
  • Although rare, there is a risk of catastrophe if the process goes wrong – This makes many people hesitant about using nuclear energy

Comparing Nuclear Fission and Nuclear Fusion

Try not to confuse nuclear fission with nuclear fusion.

Nuclear fission is when unstable nuclei split into smaller nuclei. This process releases a lot of energy but not as much as nuclear fusion. We use nuclear fission on Earth to generate electricity because it can take place at much lower temperatures and pressures than nuclear fusion.

Whereas, nuclear fusion is the joining of lighter nuclei to form heavier nuclei, which releases a large amount of energy in the process. Nuclear fusion takes place in stars at very high temperatures and pressures.