This is Soviet propaganda. The real number from Nagasaki and Hiroshima was about half of the casualties were instant. Furthermore fallout is much more understood: after a few short days of hiding inside, the radiation levels will have fallen to where normal life can largely resume without fear, reducing the number of slow casualties.

Do you have any sources to back these claims? Also, what specifically do you mean by "half the casualties were instant"--is it that "of those who died, half of them died instantly" or "of those killed and injured, half of them received their injuries instantly". Or is it some other thing?

I think you're falling into exactly the sort of trap I was talking about, that the enormity of the devastation is so unimaginably great that it's difficult to imagine what it would actually be like, and to (somewhat lazily) conclude "well, it'd probably be instantaneous". But, for example, this analysis doesn't support that idea at all: https://thebulletin.org/2020/08/counting-the-dead-at-hiroshi...

In Scandinavia we're still sending samples of hunted wild boars to check for cesium. Large parts of Belarus are quite contaminated and the local tyrant is the reason we know very little about how it affects the population in those regions.