Nuclear fission and fusion: (g) induced nuclear fission; chain reaction

The idea

Induced fission occurs when a nucleus absorbs a neutron and becomes unstable, splitting into two smaller nuclei plus two or three further neutrons and releasing energy. A typical example is uranium-235 absorbing a slow neutron to form uranium-236, which splits into fragments such as barium-141 and krypton-92 with three neutrons. Because each fission releases more neutrons, these can induce further fissions, giving a self-sustaining chain reaction. The energy released comes from the difference in binding energy per nucleon between the heavy parent and the lighter products. In a reactor the chain reaction is controlled so that on average only one neutron from each fission goes on to cause another.

What earns the marks

• Describe induced fission as absorption of a neutron by a nucleus, making it unstable so it splits into two smaller nuclei.
• State that each fission releases two or three neutrons and a large amount of energy.
• Explain that released neutrons can induce further fissions, producing a chain reaction.
• Recognise that the energy released arises from the increase in binding energy per nucleon of the products compared with the parent.
• Distinguish a controlled chain reaction in a reactor from an uncontrolled one in a weapon.

Common mistakes and how to put them right

• Saying fission is the same as radioactive decay.

  Put it right by noting that induced fission requires neutron absorption, whereas decay is spontaneous.

• Claiming the products have greater total mass than the parent.

  Put it right by stating the products have slightly less mass, the difference appearing as energy.

• Assuming every released neutron causes another fission.

  Put it right by explaining that some escape or are absorbed without fission, so control is needed.

Exam tips

• Use the term 'slow neutron' or 'thermal neutron' when describing fission of uranium-235.
• Balance nucleon numbers and proton numbers in the fission equation to check the products.
• Link the chain reaction to the multiplication of neutrons rather than to a single fission event.