Trident convoys carry risk of nuclear blast

· MoD says accident could cause partial detonation
· Explosion unlikely, but result would be lethal
An accident involving Trident nuclear warheads being moved on Britain's roads could lead to a partial nuclear blast, an internal Ministry of Defence report admits.

The document, obtained by New Scientist magazine, says such an explosion could potentially deliver a lethal radiation dose to the surrounding area. The MoD describes the risk as significant enough for drawing up contingency plans to deal with such an event.

The warheads are regularly transported by road convoy between military facilities in the UK. The weapons are moved between submarine bases in Scotland and repair and maintenance facilities at the Berkshire sites of Aldermaston and Burghfield. One convoy round trip happens roughly every two to three months.

The January 2005 report, by the MoD's defence logistics organisation, which has its headquarters in Bath, assessed the risks posed by the transportation of the weapons at 2.4 in a billion.

The 36-page declassified summary is entitled Operational Safety Case for Transport of Nuclear Weapons. Large chunks of the text had been censored before release under the Freedom of Information Act. Publicly the MoD has always insisted that transportation is safe because a plutonium core must be compressed symmetrically for a nuclear explosion to occur; a knock from one side, through a collision, would not set off a warhead, it says. The MoD also told New Scientist that each warhead was moved with "vital parts of its final configuration removed".

But according to the report a large accident, for example a plane landing on a convoy, could trigger a partial detonation which would have "potentially high off-site consequences". It says radiation doses from such a blast could range from one to 10 sieverts. Exposure to four sieverts would kill 50% of exposed people through acute radiation poisoning, while six sieverts would kill everyone exposed.

The document concludes that "adequate safeguards exist to restrict the risk from such hazards to a level that is as low as reasonably practicable".

Jane Tallents, an anti-nuclear campaigner with Nuke Watch, said: "They might think that's a risk worth taking [for having a nuclear deterrent] but have they asked all the people on the route?"

Richard Stokes, leader of Slough borough council, said the weapons convoys were "easy targets for terrorists".

Frank Barnaby, a nuclear physicist who has worked on the UK's nuclear weapons programme, told New Scientist: "A Trident warhead exploded in a densely populated area could kill hundreds of thousands of people. However small the risk, that is too horrifying to contemplate."