In principle it's very easy - get a critical mass of radioactive material, sit back and watch the runaway nuclear reaction go. But luckily for us it's the first part - getting the radioactive material - that is the biggest stumbling block.

Earlier this week, Iran joined the growing list of countries suspected of developing nuclear weapons. On Monday the European Union said Iran's nuclear reactors could make the radioactive raw materials needed for a nuclear bomb and demanded weapons inspectors be allowed in.

"You cannot make a nuclear bomb without fissile material," says Andrew Furlong, of the Institute of Chemical Engineers. And for an average thermonuclear device, the necessary material is plutonium or enriched uranium.

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.

You could use plutonium instead. According to Keith Barnham, a physicist at Imperial College, this is the preferred material because it makes much lighter weapons that can be mounted on to missiles.

Plutonium is produced as a by-product in nuclear reactors and only around 10kg is needed for a bomb. An average power plant needs about a year to produce enough and expensive reprocessing facilities are required to extract the plutonium from the fuel.

With the basic material, life gets easier. The bomb will explode once the critical mass of uranium or plutonium is brought together. So, to begin with, and to make sure that it doesn't explode in the hands of its owners, the bomb needs to keep the metal separated into two or more parts. When the weapon is in place and ready to go off, these sub-critical masses need only be thrown together - and this can be done with conventional explosives.

The chain reaction, explosion and familiar mushroom cloud then take care of themselves.