We know of no specific threat to the Sellafield site

British Nuclear Fuels

An area hundreds of miles square could be made uninhabitable by radioactive fallout, warns the US-based academic behind the report.

But the findings, which have been handed to the Commons defence committee, were branded "alarmist" by Sellafield's operator British Nuclear Fuels (BNFL).

According to the report's author, Gordon Thompson of the Institute for Resource and Security Studies in Cambridge, Massachusetts, the buildings could not withstand an impact from a passenger jet.

The main threat, it is claimed, comes from a part of the Sellafield plant storing high level radioactive waste, which dates back to the 1950s.

The highly radioactive material in the buildings has to be constantly cooled and stirred to prevent a chain reaction, Dr Thompson claims.

'No secret'

A spokesman for British Nuclear Fuels said Dr Thompson was a "well known opponent of the nuclear industry" who had "never had his claims about the releases from these buildings independently verified."

He said it was "no secret" that highly radioactive material was stored at Sellafield but BNFL was in the process of reducing its stocks.

Security at the plant was paramount but there was no evidence that terrorists were planning an attack.

"We know of no specific threat to the Sellafield site," he told BBC News Online.

Irish opposition

Nevertheless, Dr Thompson's report is likely to fuel the arguments of the Irish government, which is bitterly opposed to plans to build a new reprocessing plant at Sellafield.

The Dublin administration had wanted the UK to block the �470m mixed-oxide (Mox) fuel development just across the sea, claiming it would break international laws on sea pollution.

It also claimed the BNFL plant posed safety and security risks, which have been heightened since 11 September.

But a United Nations maritime tribunal rejected the challenge to the plant, which will turn useless plutonium and uranium into a powerful energy source.

Safety assurances

Last week, John Clarke, BNFL's head of environment, health, safety and quality at Sellafield, attempted to reassure the Irish at a special conference on security.

Dealing with the storage of highly active liquid (HAL) waste, Mr Clarke said: "We have now considered what the full impact of a deliberate commercial aircraft crash and the fire that would ensue would be on the HAL facility."

He said BNFL had re-examined its safety arrangements and was satisfied that storage facilities would remain intact.

"Our emergency arrangements would work to mitigate the offsite impact of any radiological release that might result from such an attack," said Mr Clarke.

Sabotage threat

Dr Thompson also claims Sellafield is vulnerable to sabotage of the cooling equipment which keeps the material stable.

One scenario could see a large-scale release of radioactive waste into the Irish Sea, making fishing waters in Western Scotland unusable.

But his concerns were dismissed as unfounded by BNFL.

The Ministry of Defence is understood to have plans in place to deal with terrorist action at Sellafield.

It has declined to comment on security matters.