The warheads, designed to produce a blast equivalent to up to 100,000 tonnes of TNT, are deployed aboard Royal Navy submarines but are regularly transported to weapons facilities in Britain and the United States for checks.

The Ministry of Defence has always insisted that these transit operations are safe because a warhead's plutonium core must be compressed symmetrically by conventional explosives in order to deliver a nuclear blast.

Bombs are designed to be "single-point safe," meaning that a knock on a single point should not trigger all the explosives around the core.

But a report to be published in this Saturday's New Scientist, citing what it says is a newly-declassified defence ministry document, says that extreme accidents could result in a partial nuclear explosion, an event called an "inadvertent yield".

The report estimates the annual risk of an "inadvertent yield" in Britain at 2.4 per billion, New Scientist says. Most of the risk would come from the possibility of a plane smashing into a convoy.

Even though the Ministry of Defence in this document quantifies the risk as "tolerable", it also acknowledges that if such an incident happened there would be "potentially high off-site consequences," inflicting radiation doses of up to 10 sieverts to people in the vicinity, New Scientist says.

According to British health standards, people exposed to four sieverts of radiation have a 50-percent risk of dying, while six sieverts is lethal.

The document concludes that contingency plans for responding to an "inadvertent yield" are adequate, although it does not spell them out, according to the New Scientist report.

Reacting to the report, a Ministry of Defence spokesman told AFP: "A nuclear bomb-type explosion is not possible because the warheads are transported unarmed.

"Britain's safety record on nuclear transportation is second-to-none."

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