The terrifying physics behind Beirut’s deadly explosion

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On Tuesday evening at around 6pm local time, a fire started in a warehouse in the Lebanese capital, Beirut. A large white mushroom cloud of smoke billowed out from the warehouse near the port, shortly before a fireball erupted and a supersonic explosion rocketed through the city. Footage of the fire and subsequent explosion show a shockwave scorching a path across the city, destroying buildings, lifting cars and people off the ground and shattering windows up to ten kilometres away.
At the time of writing, 135 people have been confirmed dead and more than 4,000 have been injured. But how did the explosion happen – and why was it so big?


The chemical compound responsible for the catastrophic explosion is thought to be ammonium nitrate, a compound which is most commonly used in agricultural fertiliser and mining explosives. According to Michel Aoun, Lebanon’s president, 2,750 tonnes of the stuff had been stored in a port warehouse for six years without any intervening safety measures. The impact from the blast was so severe that it was felt 200 kilometres away in Cyprus.
While it’s still unclear how the ammonium nitrate ignited, the warehouse fire interacting with the chemical compound was most likely responsible for the blast. Ammonium nitrate is relatively stable, but it melts at 170 degrees Celsius. When the large quantity of ammonium nitrate ignited in Beirut, it created a large fireball due to the subsequent chemical reaction and detonated to create a devastating explosion. Angela Sella, a professor of chemistry at University College London says that ammonium nitrate’s explosive qualities come from its core components: ammonium is a little bit like hydrogen, while nitrate is a little bit like oxygen.
“You have one part which is fuel, one part which is oxidiser,” says Sella. An oxidiser draws oxygen to a fire, making it more extreme. “When you get them together, and you combine them with a source of ignition, you really have a recipe for a problem.”

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The detonation of the ammonium nitrate causes a rapid pressurisation of the atmosphere. This resulted in the supersonic shockwave we see rippling outward from the warehouse. The temperature is extremely high in front of the shockwave, but cool behind it, displacing and compressing the air around the sonic boom. “It’s a little bit like what happens when you pop the cork off a bottle of fizzy wine, and you look inside, and you see that little cloud of condensation. It’s exactly the same process which results in very, very rapid cooling,” says Sella.
The blast is one of the largest non-nuclear explosions in recent history says Andy Tyas, a civil engineering professor at the University of Sheffield. Tyas and a team of researchers in the Blast and Impact Dynamics Research Group at the University of Sheffield analysed a number of videos and images from the explosion to estimate the size of the explosive charge that produced the explosion. The team estimate that the explosion was equivalent to something around the order of 1,000 to 1,500 tonnes of TNT. That’s around ten per cent of the intensity of the Hiroshima bomb.
But why was the explosion so severe? That’s down to a combination of factors. One was the sheer amount of ammonium nitrate and another was the length of time it had been sitting in the warehouse, where it was improperly stored.
The quantity of ammonium nitrate in the Beirut blast isn’t unusual by itself – similar amounts are frequently shipped around the world to make fertiliser. But the compound isn’t often stored at such large quantities for the very reason witnessed in Beirut. “If anybody has any basic understanding of process safety in the industry, the number one rule is that you never store large quantities of hazardous materials in one place, especially in a confined environment,” says Haroun Mahgerefteh, a professor of chemical engineering at University College London. “The second no-no is that it’s never near a populated area.”


Steve Kershaw, director of Haztech Consultancy, a UK firm which advises on the safe storage of ammonium nitrate, says that while the compound is relatively stable on its own, if it’s housed in large quantities for a long period of time, or becomes contaminated, it will have compounded and become a large mass. Over time, moisture will cause the ammonium nitrate to stick together which would makes the potential risk for an explosion to be both larger and more intense.
If that mass of ammonium nitrate is then ignited, which is what happened in Beirut, then it could be disastrous. “It was because it was such a large pile, poorly stored and compacted that there was a detonation,” says Kershaw, who adds that usually just a couple of hundred tonnes of ammonium nitrate is stored in small segregated bins, with no combustible material around to interact with the compound. If there’s a small quantity of the material, then there would only be a small detonation if something went wrong. “The shock and detonation just set this huge quantity virtually all off instantaneously,” he says.
According to media reports, the ship housing the 2,750 tonnes of ammonium nitrate arrived in Beirut in 2013, but was prevented from leaving the port in 2014 because either the ship was deemed not seaworthy or because the owner refused to pay fees to Beirut port. The ammonium nitrate was then confiscated and stored in the warehouse. A report from The Guardian says that inspectors had warned as recently as six months ago that if the ammonium nitrate was not moved, it would blow up the whole of Beirut. “It was just a matter of time, in my opinion,” says Mahgerefteh. “It should never have happened.”
Alex Lee is a writer for WIRED. He tweets from @1AlexL
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