A CRASH involving a British nuclear weapon convoy would risk the release of highly radioactive debris contaminating a radius of up to 12 miles, a secret military report has revealed.

The potential scale of a disaster - greater than previously admitted by the government - would mean residents, motorists and shoppers as far as three miles away would all have to be given immediate shelter from the shower of radioactive particles.

Under contingency plans drawn up by the Ministry of Defence (MoD), food bans would be implemented in the affected zone, a 900-strong military team would be sent into the area and local residents would be rehoused. The subsequent decontamination operation would be likely to take months.

Exact details of the military's emergency response to a nuclear convoy accident involving a radioactive leak, which is given the codename Primrose, have been revealed in a confidential safety report published by the Nuclear Accident Safety Response Organisation (Naro), which is part of the MoD.

The document, compiled after a safety review at RAF Brampton in Cambridgeshire last year, is the first giving a detailed assessment of the risk posed by a crash involving a nuclear weapon to be revealed. While a full-scale atomic explosion would be impossible because one of the essential components for fusion is removed from the weapon, the 15-page report states: "There could be a chemical explosion which scatters radioactive debris."

Leaked plutonium would pose the main threat to those in the vicinity of the crash. The report warns of "cell damage, cancer induction and genetic damage".

Road convoys transport about five Trident weapons a year from the Atomic Weapons Establishment at Burghfield, near Reading, to the naval base at Coulport in Scotland. Last year about 10 convoys also transported Chevaline warheads from storage in East Anglia to AWE Aldermaston for decommissioning.

The Naro document reveals the convoy has an escort of 50 armed soldiers and comprises about 15 vehicles travelling at 50mph. Up to four load-carriers transport weapons; each are fitted with anti-hijack systems and fire explosive locking bolts on the doors in the event of an attack.

"The convoys go along the M25, around St Albans, up the A1, past the Metro Centre in Newcastle, and they don't even clear the traffic," said Di McDonald of Nukewatch, which monitors such movements.

The most serious threat would be a high-speed crash with another vehicle. "The trouble is that Trident used normal high explosive, and if that goes off the danger is that it scatters 4kg of plutonium around the surrounding area," said William Peden, of Greenpeace.

In the event of a radioactive release, an advance military team airlifted into the area would assess the extent of the leak. A Follow-on Force (FOF) would "render the weapon safe" and recover debris. Local police would be involved. The document, however, says that police have no protective clothing for a radioactive incident.

MoD officials say in the report that the public response to a leak would be "no confirmation nor denial". The document, however, reveals a military lorry involved in an accident in 1987 was carrying a nuclear weapon, which has never previously been admitted.

The report states the intention is to reduce the movement of nuclear weapons to just five a year.

"The government needs to be more open about the movement of convoys and the potential risks," said Norman Baker, Liberal Democrat MP for Lewes. "This document shows why they need to be kept well away from population centres."