By Alex Kirby BBC News Online environment correspondent

British researchers say it would be frighteningly easy for terrorists to make a nuclear bomb.

They say the chemistry involved is simpler than in making illicit designer drugs.

They believe making a device would be no harder than building the bomb that destroyed the Pan Am aircraft over Scotland in 1988.

And they say the UK should stop reprocessing spent nuclear fuel soon, to prevent it being stolen.

The claims are made in a paper by the Oxford Research Group (ORG) called The Production Of Primitive Nuclear Explosives From Mox Fuel (Mox is a mixture of uranium and plutonium oxides).

It describes the ease with which a determined but technically unsophisticated group could make, not a "dirty bomb", but a genuine nuclear explosive.

The paper says a terrorist organisation could "relatively easily extract the plutonium and fabricate a nuclear explosive, having first acquired Mox fuel".

Accessible instructions

Both the 1988 Lockerbie bomb and the nerve gas weapon used in the Tokyo subway in 1995, it says, "required considerable planning and scientific skills".

It adds: "It is a sobering fact that the fabrication of a primitive nuclear explosive using reactor-grade plutonium, obtained from Mox, would require no greater skill.

A nuclear fuel vessel at sea

"None of the concepts involved in understanding how to separate the plutonium is difficult.

"A second-year undergraduate would be able to devise a suitable procedure by reading standard reference works, consulting the open literature in scientific journals and by searching the world wide web."

ORG says enough plutonium to check and refine procedures can easily be extracted from mud from the Ravenglass estuary in northwest England, which it says is contaminated by discharges from the nearby Sellafield reprocessing plant.

It would be easy, the paper says, for the bombmakers to refine their methods without arousing suspicion "by using environmental chemistry as a front".

Small and deadly

A plutonium oxide bomb would be an effective weapon, but one made of metallic plutonium might produce a bigger explosion. The paper says it would be a job for two or three people.

The emergency services... would find it difficult even to deal effectively with the dead

Oxford Research Group

The completed bomb - the plutonium, a beryllium shell, and a plastic explosive container - would have a diameter of about 80 centimetres (31 inches).

ORG says: "The size of the nuclear explosion from such a crude device is impossible to predict.

"But even if it were only equivalent to the explosion of a few tens of tonnes of TNT, it would completely devastate the centre of a large city.

Sellafield's Mox plant

"Such a device would, however, have a strong chance of exploding with an explosive power of at least 100 tonnes of TNT. Even 1,000 tonnes or more equivalent is possible, but unlikely."

A 100-tonne equivalent explosion would be "catastrophic", with anyone caught in the open within 600 metres (650 yards) likely to be killed by the direct effects of radiation, blast or heat.

Out of bounds

"An explosion of this size, involving many hundreds of deaths and injuries, would paralyse the emergency services. They would find it difficult even to deal effectively with the dead."

The report adds: "Even if the device, when detonated, did not produce a significant nuclear explosion, the explosion of the chemical high explosives would disperse the plutonium widely."

So much of the stricken city would remain uninhabitable until decontaminated, which could take years.

ORG concludes that the risk of terrorists stealing the material for a nuclear device is "a terrifying possibility".

It is urging a halt to reprocessing at Sellafield as soon as possible. Two vessels returning rejected Mox fuel from Japan are due to set sail for Sellafield this week.