What if a nuclear bomb hit a city?

The name Robert Oppenheimer may not have been familiar to many people before the release of the Hollywood blockbuster bearing his name. But he was the physicist who worked to realise the industry of British, American and other scientists in creating the atom bomb. It’s now the anniversary of the date, seventy-eight years ago, when the United States dropped two nuclear bombs on the Japanese cities of Hiroshima and Nagasaki to end the Second World War. In the present, the Russians have continued to rattle the nuclear sabre following their invasion of Ukraine. So how do these weapons work, what’s their history, and what would be the fallout, both literally as well as metaphorically, if Vladimir Putin were to press the button?

Will - "I am become death, the destroyer of worlds." It's a quote etched in history. Originally from the Hindu scripture the 'Bhagavad Gita', Robert Oppenheimer, dubbed the father of the atomic bomb, gave this quote when he recalled witnessing the first successful nuclear detonation on the 16th of July, 1945. About three weeks later, on the 6th and 9th of August, the US dropped nuclear bombs on Hiroshima and Nagasaki respectively. The death toll of the combined attacks varies by source, but they could have killed as many as 220,000 people. Most of them civilians. The bombs dropped on Hiroshima and Nagasaki and the deaths they caused stand as decades old reminders as to the horrifying powers of nuclear weapons and the utmost importance of them never being used again. But for a moment, let us imagine the unimaginable, what would happen if a nuclear bomb was detonated in a populated city? I asked Alex Wellerstein, historian of science at the Stevens Institute of Technology and creator of Nukemap. And when studying the effects of a nuclear blast, everything has to be taken into account. We refer to nukes as 'the bomb', but nowadays there are far more than just one.

Alex - You sort of have on the menu very low end would be like a terrorist nuclear weapon. Maybe a kilotonne, maybe 10 kilotonnes. 10 kilotonnes is somewhat like the World War II nuclear weapons. So even though we're talking about low, that's still pretty powerful. If you're talking about, say the Russians or the Chinese, you could range probably in the hundreds of kilotonnes potentially to the low megatonnes. So millions of tonnes of TNT. And if you're talking about a terrorist, you're talking about a weapon that's going to be set off on the ground and that changes things. If you're talking about a state and your target is like a city, then the weapon is probably going off in the air because that maximises the kind of damage that would be done to a city. So there's a lot of possibilities here, both in terms of the weapon itself, but also how it's delivered and how it's even targeted. And those change pretty dramatically what kinds of consequences you're worrying about.

Will - So with all that taken into consideration, what happens in the moments after detonation?

Alex - When the weapon goes off, it's going to be essentially almost instantly transformed into a fireball. It's not instant. It takes about a millisecond, but from a human perspective it's pretty instant. And that fireball is going to be putting out an intense amount of heat and an intense amount of radiation inside that fireball that's now going to be turning into that mushroom cloud. That's what the mushroom cloud is. It's the sort of cooling rising fireball, and there's going to be a lot of really nasty radioactive things inside there. If it's detonated on the ground, it's going to have sucked up a bunch of dirt and debris and who knows what inside of it. The radioactive bits are going to attach to that and sort of fall out of that over the next few hours. And that's going to be the delayed radiation. The nuclear fallout, which will be contaminating and go wherever the wind blows. That radiation is going to be a relatively quick blast. The heat is going to be searing for several seconds depending on how powerful it is. Searing enough that if you're within vast distances of it, you could get a burn just by being in the line of sight to it. There's going to be an interaction between the radiation and heat of the fireball and the atmosphere that is going to create a blast wave. You're basically dumping a lot of energy into the air and that's going to create this sort of expanding wave of superheated, super compressed air. And this is like a wall of pushing pressure. And as that expands out, it's going to be sort of shedding its energy, it's going to be getting weaker the further it goes, but depending on how far you are, that's a lot of pressure being put out sort of in a big wave or wall. And that's going to be doing a lot of breaking windows. If you're close it's going to be knocking buildings over. So those are your sort of four major effects.

Will - A truly horrifying concept on all levels then. But the danger isn't over once the mushroom cloud clears. Alex mentioned the immediate blast of radiation from the bomb, but that radiation hangs around afterwards and can travel very far from the source.

Alex - So the radiation can go very far from ground zero, depending on the size of the bomb used. It can go hundreds and hundreds of miles and be still pretty dangerous. The radiation that's coming down in fallout is going to look something like ash or snow. It could be a rain depending on the atmospheric conditions and depending on where you are, some of that in the first few hours in particular is going to be so radioactive that it's deadly from a very short exposure. The really tricky part about this is that the decay of that radiation is relatively fast compared to what people think it is. It's not going to stay hot, hot, hot for hundreds of years or anything like that. Within 48 hours. It's less than 1% of what it was at one hour, for example. But it creates this sort of chronic risk in that some of that stuff is radioactive enough that you wouldn't want to live with that radiation. You might be able to travel through that zone and not have any harm come to you. But if you lived there, especially large numbers of people, especially vulnerable populations like pregnant women and children, then you're going to start seeing the cancer rate go up, up, up over time. So you have a large area that is going to have a very short term danger from the immediate hours after the bomb, but then also have this long-term contamination problem.