In nuclear power plants, the fission process is used as an energy source to generate electricity.
- When a neutron collides with the nucleus of uranium-235, the uranium nucleus becomes very unstable.
- This causes the nucleus to split into two daughter nuclei and release two to three neutrons.
- These neutrons can then collide with other uranium nucleus, triggering other fission reactions, which in turn release more neutrons, causing more reactions to occur.
This chain reaction is crucial to generate electricity and is achieved in the reactor with the use of a moderator. The neutrons released by the fission reaction must be moving at the right speed to be captured by the other uranium nucleus. The moderator slows down fast-moving neutrons so that the neutrons can be absorbed by the nucleus and carry on the chain reaction. The chain reaction, however, if uncontrolled, may lead to too much energy release and cause the reactor to explode.
That is why there is a control rod in all nuclear power plants! Control rods are made from elements that are good at absorbing neutrons but do not undergo fission reaction, such as boron. If the control rods are lowered, the control rods will absorb neutrons, thus reducing the number of neutrons available for fission reaction.