Making nuclear weapons

Nuclear weapons are not new, but because they are so powerful there is still a lot of interest in them. That's especially true if it's thought that a country might be trying to make them for the first time.

It's not always clear what people are up to. Part of the reason for this is that governments are secretive about their nuclear programmes.

Another reason is that making the fuel for a nuclear bomb is similar to making the fuel for a nuclear power station. For both you can start with the ingredient uranium.

This is how nuclear fuel is made:

• Rocks containing uranium are dug up from mines and then purified and made into a solid known as yellowcake. The world's biggest uranium producers are: Australia, Canada, China, Kazakhstan, Namibia, Niger, Russia and Uzbekistan.

• The yellowcake is heated up, this makes it turn into a gas.

• The gas is spun round in a machine called a centrifuge, which separates out the most useful bits (called isotope U-235). By keeping those bits in the centrifuge and spinning them again and again you end up with something called enriched uranium.

• Uranium containing five times as much U-235 as normal, called low-level enriched uranium, can be used as fuel at nuclear power stations. This type of uranium is not explosive.

• Uranium containing 130 times as much U-235 as normal, called highly enriched uranium, and can be used in nuclear weapons.