It is scaremongering at its worst based on such sensational scenarios that it borders on science fiction

Jamie Reed, BNFL

Gordon Thompson, of the Institute for Resource and Security Studies in Cambridge, Massachusetts, USA, said the accompanying explosion from any crash would fling radioactive particles into the atmosphere.

Afterwards the burning fuel would continue to pump radioactive material into the air - and putting out the fire would be difficult.

In his Doomesday-scenario, the crash would be likely to destroy the cooling systems, causing surviving tanks to heat up and spew out more radioactivity within hours.

But Jamie Reed, of BNFL, told BBC News Online: "At a heightened time of national security such as the present, the New Scientist article is a grossly irresponsible piece of writing.

'Increased vigilance'

"It is scaremongering at its worst based on such sensational scenarios that it borders on science fiction."

He said there was no reason to believe Sellafield, or any BNFL sites, were a specific target for terrorists.

BNFL has reviewed security at all its buildings

Mr Reed said: "However, the company takes its responsibilities for the safety of its buildings very seriously and the site, along with other government buildings, has increased its security and vigilance.

"All plants on the site are subject to safety cases approved by the regulators.

"In the light of the World Trade Center attacks, BNFL has been reviewing those safety and contingency plans."

After the Chernobyl disaster in Ukraine in 1986, an exclusion zone of 4,800 square kilometres had to be set up around the plant.

Radiation spread so far that sheep in some parts of Wales still have to be tested to check that they are safe to eat.

So far, 11,000 cases of thyroid cancer have been reported in the Ukraine and Belarus.

Mr Thompson, who has been investigating the storage of high level waste in the UK for the past five years, believes as much as half the highly radioactive caesium-137 in the tanks at B215 could escape into the air.

Writing in New Scientist he said: "That would be 44 times more caesium-137 than was released by the Chernobyl disaster.

Widespread chaos

"Four million terabecquerels of radioactivity would contaminate large parts of Britain and, depending on which way the wind was blowing, Ireland, continental Europe and beyond."

Immediately after the attack there would be "widespread chaos" as authorities tried to organise mass evacuations.

He said: "In years to come, the death toll might be terrible."

There were safety concerns raised about the Sellafield site in September, ahead of Environment Secretary Margaret Beckett's decision to allow the �460m Mox plant to be commissioned.

The plant will process mixed-oxide, a blend of plutonium and uranium that has been extracted from the spent fuel rods already "burnt" in reactors.

Nuclear bomb

Concern was raised about the possible risks associated with moving spent nuclear fuel, such as uranium and plutonium, from customers in Germany and Japan, and then in exporting the Mox fuel.

Dr Frank Barnaby, of the Oxford Research Group, said: "If they can organise to capture four commercial airlines and fly them into the World Trade Center and the Pentagon, then they can certainly capture a ship on the high seas I would have thought."

He said the material could then be used to make a nuclear bomb.

But a BNFL spokesman said nuclear fuels could not be easily separated, and that terrorists would need their own nuclear reprocessing plant.