They did not understand radiation sickness and fallout well.
In fact, after the war, the US set up clinics with the ulterior motive of studying the survivors. They talk about it in this documentary, that I’d highly recommend:
https://www.pbs.org/articles/hibakusha-stories-of-survivors-of-hiroshima-and-nagasaki
We knew what radium poisoning was for quite a while the question was would this work the same way.
It was possible, but both bombs - Fat Man and Little Boy were in the kiloton range of size, and not like the modern nuclear arsenal that are in megatons of size, so the nuclear fallout at the time was much more limited.
Also, given the prevailing winds flow from west to east, much of the radioactive material fell into the Pacific just east of Japan. Now the rampant nuclear testing (once the scientific community worked out that a nuclear detnation would NOT set the atmosphere on fire - like some suspected it might..) which happened in the Pacific and in the Southwestern US during the 50's and 60's, was an entirely different animal.
That caused a lot of problems in the US and on the Pacific Islands and atolls downwind.
It's back down to kilotons, actually, albeit hundreds. The tens of megatons thing was to make up for the inaccuracy of early delivery methods; now firing more small bombs is preferred. And the Tsar Bomba was undeliverable - purely for show.
They're also a lot cleaner now, too - Fat Man and Little Boy were quite dirty, so significant fallout did happen. Little Boy was used in an airburst, and so it's mess was distributed pretty globally through the stratosphere, but Fat Man's came right back down in black rain.
Undeliverable? They dropped it from a plane.
I always love that bit .... scientists wondering if the first test would set the entire planet's atmosphere on fire ...
Scientist getting ready to pull the trigger on the first bomb: .... if we don't fuck around .... how are we ever gonna find out?
It wasn’t a real concern. Calculations showed very minimal chance. But I guess it’s not zero until you try it
For real. The first one that incorporated Lithium ended up being like 2x the power that was calculated, and something new was discovered about Lithium lol
Of course the military wasn't worried much. After all, they had just finished the test at Trinity, near Alamogordo, New Mexico, a month before Hiroshima. (July in New Mexico, August in Japan, 1945.)
If they weren't worried in July in the U.S., of course they wouldn't worry about August in Japan. And what neighboring countries were they going to possibly theoretically accidentally hurt? China, Korea, Russia, I guess possibly some Pacific islands ... all of which were considered far less important than the U.S. itself.
The two dropped bombs were detonated at altitude. This causes much less radiation near the blast than detonating on the ground because the radioactive contamination is dispersed in the atmosphere rather than in the soil and groundwater.
This is why the destroyed cities didn't have to be abandoned like Pripiat.
Basically not a real issue. As both detonations were air bursts under 100kt anything bad was swept into the upper atmosphere and dispersed mostly over the Pacific. It did rain back down over years but in miniscule amounts world wide. In short any of those hot particles interacting with life is not great but the concentration and exposure period is what is actually bad. In short there was no fallout risk.
Well, the 300 000 civilians who died because of the bombs weren't involved, so I'm guessing they didn't care.
Of the roughly 270k fatalities most were from the blunt force trauma (shockwave) and heat wave created by igniting the bombs not radiation. The lethal doses received at Hiroshima and Nagasaki were only possible during initial exposure when the immediate area was saturated by high energy particles. Radiation levels rapidly dropped and had returned to normal in a month on the ground.
When they first conceptualized the bomb some scientists weren't even sure the explosion would stop at all, or if it might create an unstoppable chain reaction that would just continue infinitely and consume the whole earth.
*consume the whole atmosphere
They weren't concerned about rock getting involved.
The concern was that the extreme temperature and pressure caused by the fission event would trigger fusion events of the nitrogen in the atmosphere, which would lead to a chain reaction of fusion of the atmosphere.
https://www.advancedsciencenews.com/could-a-nuclear-explosion-set-earths-atmosphere-on-fire/
...and then they did it anyway?
Yeh, they did.
They were extremely smart people.
And they considered the possibility of that happening.
They calculated the probability of it happening, considered their known-unknowns and unknown-unknowns in their calculations, and concluded the possibility (including their error margin) was so incredibly low that it wouldn't happen.
And they were right.
A scary prospect, to be sure.
But ultimately, that's what experts do.
Anyone can build a bridge that will stay up, but it takes an engineer to build a bridge that only barely stays up.
And besides, if they were wrong, it would very quickly not matter any more.
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