The aim of all nuclear bomb designers is to create a supercritical mass which will sustain a chain reaction and violently release vast amounts of heat.
One of the simplest is a so-called 'gun' design.
Here, a smaller subcritical mass is fired at a larger one, causing the combined mass to go supercritical triggering a nuclear explosion.
The process occurs in less than a second.
To make fuel for a uranium bomb, highly-enriched uranium hexafluoride is first converted into uranium oxide, and then uranium metal ingots.
This can be done using relatively simple chemical and engineering processes.
The most powerful basic fission weapon - an atom bomb - will detonate with an explosion the force of 50 kilotons.
This force can be increased by a technique called boosting, which harnesses the properties of nuclear fusion.
Fusion consists of the joining together of the nuclei of atoms of hydrogen isotopes to produce nuclei of helium. This process occurs when hydrogen nuclei are subjected to intense heat and pressure, both of which are produced by a nuclear bomb.
Nuclear fusion has the effect of injecting more energetic neutrons into the fission reaction, resulting in a bigger explosion.
Such fission-fusion-fission devices are known as hydrogen bombs, or thermonuclear weapons.