Induced Fission
- Induced nuclear fission occurs:
When a stable nucleus splits into small nuclei from the bombardment of a slow-moving neutron
- For example, when a uranium-235 nucleus absorbs a neutron, it becomes a uranium-236 nucleus
- This uranium-236 nucleus is highly unstable and will decay almost immediately, which is why it is not usually shown in nuclear decay equations
- This isotope can then decay into smaller nuclei
- 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
Chain Reactions & Critical Mass
- 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
