Containment buildings are built to withstand tremendous forces

US nuclear power plants are designed to withstand earthquakes, hurricanes and flying debris from hurricanes, the Nuclear Control Institute, the policy organisation of the American nuclear industry, says.

This is the so-called defence-in-depth safety strategy. It also means nuclear plants must be able to protect themselves from direct assault by armed groups who might want to force their way into a power station and set off a bomb.

In Europe, governments have been very reluctant to divulge the details of extra security measures implemented at nuclear facilities after the 11 September attacks.

France, which has the largest nuclear programme in the EU, made the high-profile statement of placing ground-to-air missiles near the plant at La Hague. France said it was prepared to use warplanes to shoot down hijacked aircraft.

In the UK, last weekend, two Tornado fighter jets were reported to have been scrambled to patrol the sky above the Sellafield plant in northwest England for several hours in response to a telephone threat.

Contingency plans

La Hague and Sellafield could be prime targets for terror groups because they store waste fuel, which has more concentrated, higher levels of radioactivity.

British Nuclear Fuels Limited (BNFL), which operates Sellafield, declined to comment on its security arrangements on Thursday but said it had put in place whatever measures were required by the nuclear watchdog, the Office for Civil Nuclear Security (OCNS).

"We do what we're told by our regulator. For security reasons, we cannot give details of measures taken," said a BNFL spokesman.

Last month, when it was asked to comment directly by New Scientist Magazine on the possibility of a plane strike on the Sellafield complex, BNFL said: "Major nuclear facilities, including for example reactors and highly active waste stores, are constructed to extremely robust engineering standards and incorporate large quantities of reinforced concrete as an integral part of the construction."

It added: "These facilities are resistant to many terrorist threats including aircraft impact. Safety cases and contingency plans take these events into account."

But John Large, an independent nuclear consultant in the UK, believes the nuclear industry is making claims about the integrity of its buildings it cannot possibly support.

"Some of the British plants are now 40 to 50 years old. Back then a direct terrorist attack with an aircraft would have been inconceivable. And even in the 50s and 60s when [the plants] were designed and built, aircraft were much smaller, the fuel loads were much smaller, so they do not have built-in features that are resistant to aircraft crashes."

'Dirty bombs'

However, Mr Large said a would-be suicide pilot would still need information about the most vulnerable parts of a power station to hit, and he suggested that it would be prudent if public access was temporarily denied to detailed plans on nuclear installations until a thorough security review had been carried out.

The IAEA is most concerned about the dispersal or low-level radioactive materials

Whilst most commentators have concentrated on the possibility of an airliner being flown into a nuclear facility, the UN's International Atomic Energy Agency has identified the dispersal of low-level radioactive material as the weak point in nuclear security.

The IAEA said there should be better control over radionucleotides used in medical, industrial, and food processing equipment.

The concern is that this material could be used to make so-called "dirty bombs", in which radioactive isotopes are mixed with conventional explosives. These bombs would not cause nuclear explosions, but could distribute radioactive debris over a wide area.

"The effects of a dirty bomb would not be devastating in terms of human life. But contamination in even small quantities could have major psychological and economic effects," said IAEA official Abel Gonzalez.