Freesound Forums

Looking for something that sounds like a distant bomb - engulfing fire sound - a very short special effect.

https://freesound.org/people/zee_zack/sounds/521542/

Try "nuclear" rather than "atomic" ...
https://freesound.org/search/?q=nuclear

... and if it isn't the real thing you want, try layering some of what you find by searching for "distant explosion", "cinematic boom", and "continuous thunder".