The chemistry behind the Iran port explosion

An explosion on April 26 at Iran’s largest port, near the southern city of Bandar Abbas killed at least 70 people and injured more than 1,000, according to Iranian state media.

The Islamic Republic News Agency, a state-run outlet, quotes an official who says containers of chemicals set off the explosion, though they do not specify the chemicals’ identity. The New York Times reports that a person with ties to Iran’s Islamic Revolutionary Guard Corps, who spoke on the condition of anonymity, says the explosion was caused by sodium perchlorate—a strong oxidizer that is used as part of rocket fuel.

But that seems unlikely, according to Andrea Sella, a chemist at University College London. Sella says that although sodium perchlorate is a powerful oxidizer, it needs fuel to generate gaseous products that could cause an explosion like the one seen at the Iranian port. “Sodium perchlorate, on its own, to my knowledge, is not explosive and certainly doesn't detonate like that,” he says.

Sella initially thought the material might have been ammonium nitrate because of the intensity of the blast and the orange-brown plume of smoke—a hallmark of nitrogen dioxide from the burning of that bulk commodity chemical—that appears in videos of the incident. But he now suspects that it may have been ammonium perchlorate, a material that’s used as a solid-state propellant for missiles.

The combustion of ammonium perchlorate—a salt that combines the oxidant perchlorate and the reductant ammonium—is quite complex. It produces what Sella calls “a menagerie of gaseous products.” One of those is nitrogen dioxide, which could account for the orange-brown plume. Videos of the early stages of the fire also show black smoke, which Sella says indicates organic material also caught on fire. As the fire intensifies, the flame is a bright orange, which suggests that sodium is present.

Sella says it could be that both sodium perchlorate and ammonium perchlorate were in the container, which either heated to the point of ignition or caught fire another way. The fire’s high temperature would have caused the explosion.

“Having a single container of a material like that in the middle of a container port is absolutely shocking,” Sella says. Most ports segregate hazardous materials. The New York Times has also reported that Iranian officials say the materials’ documentation contained “false statements” that belied the dangerous nature of the shipment.