President Obama and Israeli Prime Minister Benjamin Netanyahu both alluded to military strikes against Iranian nuclear facilities at last week’s AIPAC meeting, and many commentators worry that war is becoming a more realistic possibility. With all that enriched uranium lying around, is there any chance a strike on one of Iran’s nuclear facilities could accidentally trigger a mushroom cloud?
No. A nuclear detonation cannot occur without a substantial amount of highly enriched uranium. (The International Atomic Energy Association says it would take at least 55 pounds of this material to create a weapon, but some experts say a modest nuclear bomb could be made with as little as 6 pounds using the most advanced technology.) Intelligence analysts believe that Iran hasn’t enriched enough uranium to pose an immediate risk, but even if they’re wrong, and Iran does have the capacity to build a bomb, it would still be incredibly unlikely for a missile strike to kick off a nuclear chain reaction. The explosion would scatter the fissile material instead.
The massive release of energy in a nuclear explosion comes from a chain reaction. A uranium atom splits, releasing three neutrons, which in turn strike neighboring uranium atoms and release more neutrons, accelerating the process. But if the uranium sample isn’t stored in a tightly packed, spherical container, escaping neutrons will fly off harmlessly and the reaction will fizzle. Nuclear enrichment facilities don’t store their uranium under these conditions unless they’re actually building a bomb. Based on everything we know, Iran is not at that stage.
But what if Iran had 55 pounds of 90-percent enriched uranium already assembled into a bomb, and a U.S. or Israeli missile strike hits that material dead-on? Even then, the strike isn’t likely to cause more than a minor nuclear detonation. The chain reaction in a nuclear bomb is carefully choreographed, with the most common strategy being to surround the fissile material with conventional explosives and then detonate the weapon with a finely tuned electrical charge. That has the effect of compressing the uranium simultaneously from all sides and preventing any uranium from escaping its container too quickly. All of the conventional explosives surrounding the fissile material must go off within microseconds of each other in order to contain the uranium. In the worse-case scenario, a missile strike on a facility containing nuclear weapons would almost certainly mess with the synchronization of the charges and the compression would be compromised. The nuclear chain reaction would either not occur at all, or it would be cut short.
A missile strike would be messy, though. An explosion could release gaseous uranium near the facility, which would cause kidney problems and possible cancer if inhaled or ingested. The strike might also release toxic fluorine gas. However, since the uranium would not have reacted to any significant extent, and therefore would not have given rise to cesium-137 or other hyper-radioactive particles, the disaster wouldn’t be as catastrophic as the breach of an active nuclear reactor or a true nuclear attack. Radiation sickness would be very unlikely.
Bonus Explainer: Can you set off a conventional bomb by shooting at it with a gun? It depends on the explosive. Some bomb materials, like dynamite, are highly sensitive to impact. Others, like C4 or TNT, can be shot with a gun or possibly even set on fire without exploding. (Read more on this topic in our archives.)
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Explainer thanks Bruce Blair of the World Security Institute, former Assistant Secretary of State Philip Coyle, Corey Hinderstein of the Nuclear Threat Initiative, Hans Kristensen of the Federation of American Scientists, and Steven Pifer of Brookings.