A typical Trident nuclear missile contains three to six warheads, and some submarines carry up to 24 missiles, meaning the potential for disaster could be huge.  Defence companies try to prevent accidental explosions of warheads by designing them to be "singlepoint safe" which means that a sudden knock at a single point should not  detonate the plutonium core.

The typical scenario would see the weapon being dropped from a crane while being loaded on or off a submarine.

 Source : Telegraph UK

However, a nuclear-weapons safety manual drawn up by the MoD's internal nuclear-weapons regulator, and declassified last month, argues that this standard single-point design might not be enough to prevent "popcorning".

The manual, seen by the New Scientist, says that warheads should be capable of resisting multiple simultaneous impacts which "would contribute to the prevention of popcorning and should be a design objective".

It also recommends replacing the highly sensitive explosive that surrounds the warheads' plutonium cores because a single knock may not detonate the core, but could set off the explosive.

Less-sensitive explosives are available but they are heavier and bulkier than those currently in use so the warheads would have to be redesigned.

According to the manual, in the worst-case scenario people a kilometre away would receive a radiation dose of 100 sieverts – 16 times the lethal dose – although the seriousness of the accident would depend on the pattern of warhead explosions.

The US government's National Nuclear Security Administration said that redesigning warheads to resist multiple impacts and switching to less-sensitive explosives would "enhance" safety but stressed that current warheads "were, are and continue to be assessed as safe".

A spokesman for the MoD said popcorning was only "a theoretical possibility" and in fact it was "a scenario that is not credible".

Any risk is mitigated by the way in which missiles are handled, transported and stored, she added.

Nuclear-weapons experts say an accident could still happen.

Philip Coyle from the Center for Defense Information, an independent think tank in Washington, said there was always a risk that safety procedures are ignored and pointed to the accidental loading of nuclear weapons onto a flight last year from North Dakota to Louisiana.

Stefan Michalowski, a senior scientist at the OECD in Paris, who researched warhead safety in the 1990s, is concerned about the risks of an extreme event such as a firefight with direct gunshots.

"The explosion of a boatload of missiles in a port would be an unimaginable catastrophe," he said. "It's a very, very scary thought."