Sneed's Feed and Seed - I am become Chuck, the sneeder of feed.

"Now we are all sneeds of bitches"-Bainbridge

I suppose you could call it a "Oh god, what have I done?" sort of moment. They understood that this bomb would be bigger than anything ever done before (as they were afraid of setting the atmosphere on fire) but on paper all it was was theory. Seeing that mushroom cloud made all the hypothesizing into reality and the realization that there was no going back.