That’s the point. The impact of Hiroshima was not the number of casualties, or even the destruction, but the way it happened - and to some extent the radioactivity which killed after the bomb had been dropped.
Which the US Government years and much money studying, especially in Hiroshima, where the main office of theAtomic Bomb Casualty Commission was established. There was a lot of research done on the survivors, and a lot of other research done on explosion effects at the Nevada Test Site trying to simulate some of the effects noted in Hiroshima and Nagasaki. (Things like building simulated Japanese houses and running a naked reactor up a REALLY tall pole to simulate explosion height, and then calculating exposures inside the structures. This really was done.)
A Torch to the Enemy: The Fire Raid on Tokyo. Balantine Books. ISBN 0-553-29926-3. D767.25.T6 C35. Coffey, Thomas M. (1987).
As I mentioned, there is an excellent Nuclear Testing Museum in Vegas which I encourage anyone there to devote a few hours to. (You’ll save on gambling losses.) This has a lot about such studies, though we weren’t there long enough to see everything.
Here is the Amazon page. This is non-fiction, not a story, so it doesn’t count.
Accepted.
Vonnegut’s Slaughterhouse Five.
I wonder what took everyone so long. 
Still, I was thinking of sf stories about future attacks. I can actually think of a story about biowarfare against crops from 1920. There have been numerous stories about poison gas. There was an article in the New Yorker, I think, about someone who was a child in Germany at the time on the impact of firebombing on the population. I just can’t think of any fiction about it happening in the future.