> No nuclear weapon has ever been built that can do that to 460 square miles.

460mi ^2 = 1192km^2

sqrt(1192km^2 / pi) = 19.5km

Tsar Bomba (largest one), from Wikipedia: 'total destruction _radius_ 35km.'

So, yes, such a single weapon surpassing that several times has been built. And it can as well be argued that a MIRVed ICBM is in a way a single weapon.