First, take some uranium...

How easy is it to build a weapon of mass destruction? According to a new book by a man who made his career in nuclear technology, it's not that difficult. But don't be alarmed, says Terry Kirby, this is no do-it-yourself guide for terrorists

"I am become death, the destroyer of worlds." With those words from the Bhagavadgita, J Robert Oppenheimer, the director of the Manhattan Project, greeted the sight of the first nuclear test in New Mexico, the moment that heralded the dawn of the nuclear age. Little more than half a century later, it seems that almost anyone with a relatively low-level knowledge of physics, and a slightly larger amount of ready and untraceable cash, can at least attempt to "become death" and plan their very own destruction of worlds.

Indeed, given the proliferation of internet sites with names such as "Nuclear Bomb Making Made Easy" that purport to contain step-by-step descriptions, it seems incredible that Saddam Hussein, with access to both the science and the money, tried for many years and, so far as we know, failed to "become death". Similarly, Osama bin Laden; files suggesting that al-Qa'ida was planning to build a nuclear weapon were also discovered in Afghanistan.

Perhaps Saddam should just have sent somebody to look at the Ministry of Defence files in the Public Record Office at Kew, which are said to contain enough precise and detailed instructions, including measurements and diagrams, to make the kind of atomic bomb tested by Britain in the 1950s. Or maybe he already has.

Forty years ago, already alarmed at the prospect of nuclear proliferation, the United States military set two young scientists the theoretical task of building an atomic bomb from scratch, with no prior knowledge but assuming access to the necessary funding and materials. According to recently declassified documents, it took them around 30 months, although they were never told whether the device was actually built or just existed on paper.

This summer, millions of Britons have been gripped by the US-made real-time drama series 24, in which federal agent Jack Bauer (Keifer Sutherland) races against time to stop Arab terrorists exploding a nuclear bomb in the middle of Los Angeles. It can be done, the programme-makers imply. But is it really that easy? And just how scared should we be? A new book, to be published this autumn, entitled How to Build a Nuclear Bomb - And Other Weapons of Mass Destruction (Granta), taps directly into the post-Iraq war appetite for spin-free analysis, and acts as a warning of the dire consequences of ignorance.

But although the book provides a straightforward and unhysterical guide to some of the more hair-raising nuclear, chemical and biological threats that have dominated the post-11 September world, it does not exactly do what it says on the cover. While there are descriptions of the basic principles of nuclear fission and what constitutes an atomic bomb, there is no do-it-yourself guide to creating apocalypse.

"That was deliberate, of course," explains its British author, Frank Barnaby, a man who actually does know how to make a nuclear bomb. "I didn't want to be accused of giving too much away that might help terrorists." Still, his book guides the reader through toxic substances such as ricin, anthrax and sarin; the well-documented Iraqi programmes to develop chemical and nuclear weapons; the rise of al-Qa'ida and other terrorist groups, and how they might go about staging an attack.

It also describes, for the benefit of the post-Cold War generation, exactly what the awesome sight experienced by Oppenheimer involves in terms of human destruction, and what it would mean for a city such as London.

Barnaby, now 75, writes from the heart as one of a small - and dwindling - number of people who have witnessed a nuclear explosion at first hand. He was director of the Stockholm International Peace Research Institute in the 1970s, and is currently working for the Oxford Research Group on military technology, nuclear energy, and weapons of mass destruction.

But it was in 1953, in Maralinga, in the south Australian desert, as a young nuclear physicist working for the Atomic Weapons Research Establishment at Aldermaston, Berkshire, that he understood the awesome power of nuclear fission. It convinced him that he had to work to stop its spread.

"The observer at first stands with his back to the explosion to avoid being blinded by the initial flash of light and ultraviolet radiation. After the flash, he can turn towards the nuclear explosion to watch the fireball grow. The initial flash of light is followed by a weird, very short period of silence," he writes.

"Any exposed skin then feels a wave of heat. Just as one gets over the surprise of the heat wave, one is shaken by the blast wave, accompanied by a loud noise. The body is shaken again by a wind travelling away from the explosion, raising a cloud of dust. A short time later, one is shaken yet again by another wind blowing in the opposite direction.

"Experiencing the heat, blast, noise, and the winds, seeing the brilliantly coloured fireball growing to a tremendous size, and watching the mushroom cloud rise to a high altitude, combine to give a sense of the immense power of a single nuclear explosion. It is an experience that one does not forget. The most awesome thing is that this huge explosion, powerful enough to destroy a city, is produced by a piece of plutonium about the size of a tennis ball."

It is this final image of something that can fit in the palm of your hand being able to lay waste to whole cities with the frightening force that he experienced for himself, that now most disturbs Barnaby. It is what differentiates the terrorist threat from that posed by "rogue states" such as Iraq or North Korea. Whole countries, he argues, need to produce sufficient numbers of nuclear devices to satisfy the demands of their own military and to ensure, at the very least, a balance of power with their neighbours - otherwise, he says, why bother at all.

It is terrorists such as Bin Laden who can achieve their aims with little more than a small, primitive device, nuclear or "dirty" (that is, a piece of radioactive material blown apart by conventional explosive), strategically placed to create mass destruction of a population and a panic-inducing spread of radioactivity.

While it is generally accepted that the principle of a nuclear bomb is simple, and that the technical assembly is relatively easy, for people such as Bin Laden, the most difficult bit is obtaining sufficient enriched uranium or plutonium to build your bomb.

For the needs of a country such as Iraq or North Korea, you need the massive infrastructure of nuclear reactors or gas centrifuges to create the enriched uranium, coupled with power stations, testing sites, and so forth. Barnaby says: "I met the Iraqi nuclear scientists a few years ago, and they were certainly capable of building a bomb - after all, they had been trained at places such as Cern in Switzerland and Imperial College London. What they lacked was the material, so that is why they had to embark on the programme to obtain sufficient amounts." With lower demands, it is assumed that al-Qa'ida would have to seek uranium on the black market. Quite what is out there is, of course, difficult to calculate. There are an estimated 1,300-2,100 tonnes of highly enriched uranium in the world, together with 200-300 tonnes of weapons-grade plutonium. The best estimate is that 100 tonnes - enough for about 20,000 nuclear weapons, are now surplus.

Despite efforts by the United States to secure Russian supplies of uranium and plutonium since the collapse of the Soviet Union, there are still persistent reports of material being offered for sale, together with the expertise of former Warsaw Pact nuclear scientists. "The evidence suggests that the potential supply is greater than the demand of terrorist groups or countries," says Barnaby.

He quotes Luis Alvarez, a leading American nuclear-weapons physicist who says that a "high-yield" explosion could be achieved by simply dropping one half of the material on to the other half, the "gun-type weapon" used in Little Boy, the bomb dropped on Hiroshima. Alvarez adds: "Most people seem unaware that if highly enriched uranium is at hand, it's a trivial job to set off a nuclear explosion - even a high-school kid could make a bomb in short order."

Barnaby describes the process, with chilling precision, thus: "Such a primitive gun-type weapon could use a thick-walled cylindrical 'barrel' with an inner diameter of about 8cm and a length of about 50cm. A cylindrical mass of highly enriched uranium... weighing about 15kg, would be placed at the top of the barrel. The larger mass of uranium, weighing about 40kg would be placed at the bottom of the barrel. This mass would have hollowed out of it a cylinder of the same size as the smaller uranium mass.

"A high-explosive charge would be placed at the top of the barrel, behind the smaller mass of uranium. This charge could be fired from a distance by a remote-control device operated by an electronic signal. When the two masses of uranium are brought together, the total mass becomes greater than critical, and a nuclear explosion takes place."

The total size of such a device, he says, is likely to be no more than about 1m, about 25cm in diameter, and weigh about 300kg. "It could be transported by, and detonated in, an ordinary van." Which is, in fact, roughly what you see in the TV series 24, for which Barnaby was consulted by the producers.

But that's about as close as Barnaby gets to describing the method. Those keen to know more, he says, should bear in mind that a lot of the material on nuclear bomb-making on the internet is either inaccurate or insufficiently precise. Determined bomb-makers should turn to the specialist scientific journals, he suggests.

Frank Barnaby has not been watching 24, in which, of course, the hero ensures that the fictional nuclear bomb is located and transported, in the nick of time, to the Nevada desert, where it explodes harmlessly, but still terrifyingly.

Somehow, one feels that he doesn't need to.