"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....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"

 http://www.newscientist.com/article/mg19125594.300-road-crash-could-set-off-nuclear-blast.html

 http://www.nukewatch-scotland.org.uk/index.php