You could create a good atomic bomb explosion by dropping some specialized paint into a water tank and film it upside down. It would cost you about $60 if you've already got a good camera. TOHO used to do that for their SFX movies back in the 60's and they flawlessly recreated it about a decade ago for a documentary on the special effects of Godzilla films.
*Time stamped
That's also how they created the atomic bomb effects in The Day After too.
I would venture to say that Nolan and company did something a bit more expensive though.