The Docklands device was delivered by truck, killed two people and wrecked buildings in a radius of 100 yards. A nuclear explosion would require only enough plutonium to fill a standard tin can, but could have removed all of Docklands and probably much else from the map: thousands would have died.
It was the growing fear that sooner or later this might become a reality, if not in Britain then in the Middle East or elsewhere, that helped prompt this weekend's summit on nuclear safety and security in Moscow.
The collapse of the Soviet Union may have ended the Cold War, but it drastically increased the danger of nuclear materials, civil or military, falling into the wrong hands. The leaders meeting in Moscow take the reasonable view that just one serious lapse could have horrific consequences; and yet the past four years have seen no fewer than 1,000 documented claims that such materials were being secretly smuggled and sold.
How real is the risk? Is it possible to get hold of those few kilos of plutonium? And what else is needed to make a bomb? Might it be possible to steal or even buy a finished weapon?
According to Patricia Lewis, director of Vertic, an independent body which monitors disarmament, smuggling is probably not as rife as we fear. Of the 1,000 claims, all but six have proved false or to have involved radioactive materials with no conceivable nuclear weapons use.
But six is still plenty to worry about, says Dr Lewis. She cites the discovery on the back seat of a car in Prague in 1994 of 6lbs of weapons- grade uranium. This may only be about one-tenth of what is needed to make a weapon, but it shows that the former Soviet system is leaking.
And the six verified cases of smuggling are only what we know about. Writing in the journal Scientific American, Phil Williams, an expert on international crime at the University of Pittsburgh, has warned that "in almost all illicit markets, only the tip of the iceberg is visible and there is no reason why the nuclear-materials black market should be an exception. Police seize at most 40 per cent of the drugs coming into the USA. Law-enforcement officers are less experienced at stopping shipments of uranium than ... in seizing marijuana or hashish". Moreover, since Russia has a stockpile of more than 100 tons of plutonium and 1,200 tons of weapons- grade uranium, there is an alarming quantity of material to keep tabs on.
It is a possibility, then, that enough plutonium or uranium for a bomb might get into terrorists' hands. Does that mean they could produce a Hiroshima-type explosion?
Scientifically there is no obstacle, for there are no secrets in atomic science any more. Anyone with a reasonable physics degree and access to a good technical library could design a workable atomic bomb within about six months. (It has been done. In the 1970s, for example, two British students, Mike Sweatman and Bob Lowe, published a pamphlet setting out the scientific requirements for a bomb based only on their own intuition, their calculations and information in the public domain.)
But a design is a piece of paper; making it into a nuclear fission bomb that would actually explode is another matter. It is the engineering involved that is difficult.
The two bombs dropped on Japan were of fundamentally different design. In the Hiroshima bomb, the long, narrow casing contained a modified "gun" which fired one lump of weapons-type uranium at another. When they collided, the two lumps made up a "critical mass" capable of sustaining a runaway nuclear chain reaction and releasing vast energy in the form of heat and blast - a full-scale nuclear explosion.
To make such a device you would need a large quantity of the rare isotope uranium-235 and facilities to machine it to very high specifications. You would need other, highly sophisticated components to make it explode - for example, an "initiator" to start the chain reaction. Omit these components, or get your calculations wrong by the tiniest margin, and the device will simply fizzle. The uranium will break up, ceasing to be a critical mass, and the result will be equivalent to a nuclear spill or bad laboratory accident.
The Nagasaki design, using plutonium, is even more demanding. It incorporated different types and shapes of high explosive, multiple, simultaneous detonations, and a complex initiator. All had to be produced with only infinitesimal margins for error: the components to fit to measurements of a few millionths of a metre, and the detonators - at least 30 - to fire perfectly within a few millionths of a second.
Even now, 50 years after Hiroshima, only a fairly large and sophisticated engineering organisation with a great deal of money could replicate such weapons. The backyard A-bomb, put together by terrorists using stolen plutonium, student physics and everyday workshop facilities, is a myth.
This does not mean, however, that we have nothing to worry about. Even if a terrorist group was unable to provoke an explosive nuclear reaction, it might, using conventional explosives, spread plutonium over a wide area of a city. It could enter the water supply and air-conditioning systems, contaminate buildings and streets, and drift invisibly through the air. Although, contrary to popular belief, plutonium is far from being the most toxic substance in the world, it is still very poisonous if ingested. The mass panic that would be caused by such an attack can be imagined.
Finally, what you can't make, you steal. It is conceivable that terrorists could steal a complete, ready-made bomb.
Since 1993 there have been persistent rumours that one political group within South Ossetia in the former Soviet republic of Georgia has actually got hold of a battlefield nuclear weapon.
That is denied by the Russians, who claim that what had been stolen was a simulator for training troops to fight on a nuclear battlefield - it would explode and make a mushroom cloud but there was no nuclear component.
The truth is impossible to know. And that is what is so worrying about the situation in the former USSR.Reuse content