Chain Reactions
- One of the many decay reactions uranium-235 can undergo is shown below:
Uranium-235 decay chain from nuclear fission
- Neutrons involved in induced fission are known as thermal neutrons
- Thermal neutrons have low energy and speed meaning they can induce fission
- This is important as neutrons with too much energy will rebound away from the uranium-235 nucleus and fission will not take place
- Only one extra neutron is required to induce a Uranium-235 nucleus to split by fission
- During the fission, it produces two or three neutrons which move away at high speed
- Each of these new neutrons can start another fission reaction, which again creates further excess neutrons
- This process is called a chain reaction
The neutrons released by each fission reaction can go on to create further fissions, like a chain that is linked several times – from each chain comes two more
- The products of fission are two daughter nuclei and at least one neutron
- The neutrons released during fission go on to cause more fission reactions leading to a chain reaction, where each fission goes on to cause at least one more fission
Only one thermal neutron is used to create another fission reaction in a controlled chain reaction
- Nuclear reactions are designed to be self-sustaining yet very controlled
- This can be achieved by using a precise amount of uranium fuel, known as the critical mass
- The critical mass is defined as:
The minimum mass of fuel required to maintain a steady chain reaction
- Using exactly the critical mass of fuel will mean that a single fission reaction follows the last
- Using less than the critical mass (subcritical mass) would lead the reaction to eventually stop
- Using more than the critical mass (supercritical mass) would lead to a runaway reaction and eventually an explosion
Subcritical, critical and supercritical mass