8.3 Atomic and Nuclear - Nuclear

Nuclear Fission is the splitting of a large, unstable nucleus (like Uranium-235) into two smaller nuclei.

The process is:

1. A slow-moving neutron is absorbed by the large nucleus.
2. The nucleus becomes highly unstable and splits.
3. This releases a huge amount of energy, plus two or three more neutrons.

These new neutrons can then go on to hit other nuclei, which also split, releasing more neutrons and more energy. This is called a chain reaction.

Source: Wikimedia Commons

In a nuclear power station, this chain reaction is controlled using "control rods" (which absorb neutrons) to generate heat at a steady rate.

Nuclear Fusion is the joining (fusing) of two light nuclei (like hydrogen isotopes) to form a heavier nucleus.

This process releases even more energy than fission.

Fusion is the process that powers our Sun and all other stars.

Source: Wikimedia Commons

For fusion to happen, you need extremely high temperatures and pressures. This is needed to overcome the electrostatic repulsion between the positive nuclei.

Scientists are trying to build fusion reactors on Earth, but it is very difficult to create and contain the high temperatures needed.