Can the S-300 defeat a 30 megaton air burst over Tehran?
As far as I know the US has nothing larger than a 100kt warhead...and I'm pretty darn sure the 50MT Titan was retired years ago.
We MIGHT have a few 1 MT device stashed somewhere, but I doubt it.
I don't think even the Russians have anything bigger than 10MT anymore, though they once had warheads as big as 100MT.
They do not have bombs that big in any inventory.