Trident nuclear warheads damaged in a vehicle pile-up or a plane crash could partially detonate and deliver a lethal radiation dose, according to a newly declassified report from the UK Ministry of Defence obtained by New Scientist.
The MoD has also revealed that an attack by terrorists on a nuclear weapons convoy could produce an even more disastrous outcome. "The consequences of such an incident are likely to be considerable loss of life," says a senior MoD official.
Trident warheads are regularly transported to weapons facilities in the US and the UK, where they are inspected to make sure that ageing materials don't render them unreliable or unstable. The MoD has always insisted that an accidental nuclear explosion could not happen in transit, because a warhead's plutonium core must be compressed symmetrically by conventional explosives. Bombs are designed to be "single point safe" so a knock at a single point should not trigger all the explosives around the core.
But according to the report extreme accidents could result in a nuclear explosion. A serious vehicle collision or an aircraft crash combined with multiple failures of the MoD's secret protective measures could mean that the weapon might not remain single-point safe. The report puts the overall yearly risk of an "inadvertent yield" in the UK at 2.4 in a billion, mainly due to the possibility of an aircraft crashing onto a convoy. Inadvertent yield suggests a partial nuclear explosion, also called fizzle yield, smaller than the full yield of up to 100 kilotons.
The report judges this risk to be "tolerable". Nevertheless, the MoD has drawn up contingency plans for responding to such an event, which has "potentially high off-site consequences". They say that radiation doses could range from 1 to 10 sieverts. According to the UK Health Protection Agency, people exposed to 4 sieverts have a 50 per cent chance of dying from acute radiation poisoning, while 6 sieverts or more will kill everyone exposed. The report concludes that emergency arrangements are adequate, though it does not spell them out.
US experts agree that the risk of an accidental explosion is real. "You can't rule it out," says Philip Coyle, from the Center for Defense Information think tank in Washington DC. "If we are going to have nuclear weapons, we have to live with the risks."
A less predictable danger of moving nuclear weapons is terrorist attack. When David Mackenzie, a Scottish anti-nuclear activist concerned about bomb convoys driving over weight-restricted bridges, filed a freedom-of-information request, he was told that the MoD could not release information on convoy routes or axle weights because that might help terrorists plan an attack.
"Such an attack has the potential to lead to damage or destruction of a nuclear weapon," wrote the MoD's director of information, David Wray, in May. "The consequences of such an incident are likely to be considerable loss of life and severe disruption both to the British people's way of life and to the UK's ability to function effectively as a sovereign state."
Despite this, the MoD stuck to its line this week that neither a terrorist attack nor an accident could trigger a full nuclear explosion, because each warhead is transported with "vital parts of its final configuration removed".
"A nuclear-bomb-type explosion is therefore impossible," said an MoD spokesman. Though he accepted that an inadvertent yield was theoretically possible, he said it was incredibly improbable, and would not be greater than a few kilograms of TNT equivalent.
This is disputed by Frank Barnaby, a nuclear physicist who worked on the UK nuclear weapons programme and is now a consultant with the independent Oxford Research Group. "The MoD report confirms what many scientists have long suspected - that nuclear bombs can go off by accident," he says.
"They have also effectively admitted that a terrorist attack could cause a nuclear explosion. 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."
The MoD's declassified safety assessment, with sections blacked out for national security reasons, can be downloaded here (8MB pdf).
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