Uranium, a naturally-occurring heavy metal, comes as uranium 238 or 235. Both are radioactive and will decay into other elements, given time, but only the latter can be forcibly split when neutrons are fired at it. This is the basis of a nuclear bomb.

When an atom breaks apart, it gives out energy and more neutrons, which can then split other atoms. Get enough atoms splitting and you have the chain reaction needed for a bomb blast.

But natural uranium overwhelmingly consists of the 238 isotope, which bounces back any neutrons striking it - useless then for a bomb. To make a bomb, natural uranium needs to be treated to concentrate the 235 isotope within it.

And this is where the problems re ally begin. For every 25,000 tonnes of uranium ore, only 50 tonnes of metal are produced. Less than 1% of that is uranium 235. No standard extraction method will separate the two isotopes because they are chemically identical.

Instead, the uranium is reacted with fluorine, heated until it becomes a gas and then decanted through several thousand fine porous barriers. This partially separates the uranium into two types. One is heavily uranium 235, and called "enriched" while the rest is the controversial "depleted" uranium used to make conventional weapons.

To make a nuclear reactor, the uranium needs to be enriched so that 20% of it is uranium 235. For nuclear bombs, that figure needs to be nearer 80 or 90%. Get around 50kg of this enriched uranium - the critical mass - and you have a bomb. Any less and the chain reaction would not cause an explosion.