MoD: The risk of nuclear warheads' "popcorning"

Nuclear warheads have been carefully designed not to go off by accident. Or have they? More than 1,700 of them have design flaws that could conceivably cause multiple warheads to explode one after another — an effect known as “popcorning” — according to a U.K. Ministry of Defense (MoD) safety manual. New Scientist’s Rob Edwards writes that a typical Trident nuclear missile contains from three to six warheads, and a U.S. submarine may carry up to twenty-four missiles. Weapons builders aim to prevent accidental explosions of warheads by designing them to be “single-point safe,” meaning that a sudden knock at a single point — say, if it were dropped from a crane while being unloaded from a submarine — should not detonate the plutonium core. A nuclear-weapons safety manual drawn up by the MoD’s internal nuclear-weapons regulator argues, however, that this standard single-point design might not be enough to prevent popcorning. The document was declassified last month. The manual says that warheads should be capable of resisting multiple simultaneous impacts. This “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. A single knock may not detonate the core, but could set off this 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.

The effects of a popcorning accident would be dire. According to the manual, in the worst-case scenario, people a kilometer away would receive a radiation dose of 100 sieverts, which sixteen times the lethal dose. The seriousness of the accident would depend on the pattern of warhead explosions, though. The U.S. government’s National Nuclear Security Administration (NNSA) agrees that redesigning warheads to resist multiple impacts and switching to less-sensitive explosives would “enhance” safety, but it stresses that the current warheads “were, are and continue to be assessed as safe.” A spokeswoman for the MoD told New Scientist that although it is “a theoretical possibility”, popcorning is “a scenario that is not credible.” Any risk is mitigated by the way in which missiles are handled, transported and stored, she says.

Some nuclear-weapons specialists, however, say an accident could still happen. Philip Coyle from the Center for Defense Information, an independent think tank in Washington, D.C., points out that people sometimes forget safety procedures. He cites the example of last August, when nuclear weapons were unknowingly flown from North Dakota to Louisiana. Stefan Michalowski, a senior scientist at the OECD in Paris, France, who researched warhead safety at Stanford University in California 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 says. “It’s a very, very scary thought.”