Nuclear fission occurs when an unstable nucleus splits into two or more pieces (two small nuclei fuse to form a heavier nucleus, releasing a lot of energy).

When the nucleus splits, two smaller nuclei are produced. These will be of different elements than the original nucleus. The new nuclei are also unlikely to be the same size as one another. All of the fission products have kinetic energy.

If an uncontrolled chain reaction occurs, a huge amount of energy may be released in an explosion. This is how nuclear weapons work.

Key Points:

• Spontaneous fission is rare. Usually, for fission to occur, the unstable nucleus must first absora neutron.
• When the unstable nucleus absorbs a neutron, it splits into two smaller nuclei, roughly equal in size, and it then emits two or three neutrons and gamma rays.
• Energy is released by the fission reaction.
• This neutron may collide with another radioactive nucleus.
• This nucleus absorbs the neutron and becomes unstable.
• This nucleus splits, releasing another neutron and producing more energy.
• This is a chain reaction, as energy is being released, and one 'split' causes another to occur.
• A neutron needs to be absorbed initially before it splits because it needs a little extra energy before it can split and release lots of energy – like a ramp on a cliff, the neutron gives it the kick to get over the ramp to fall.
• After the nucleus splits, it leaves two smaller nuclei roughly equal in size.
• And they both (along with the neutrons) have kinetic energy.
• If the chain reaction is not controlled, it will increase at an exponential rate – which is what happens in a nuclear weapon.
• This process is used in nuclear fission with uranium nuclei.