If that need ever arises, it won't matter. A conflict that big isn't going to end well for anybody.
Are you speaking about nukes? I don't think a large conflict guarantees the use of nukes. Historical example: One of the greatest fears in WWII was that the Nazis would use poison gas. Britain handed out tens of thousands of gas masks to her citizens because of that fear. The Nazis never used them. They knew that act would be reciprocated. Gas attacks didn't happen in WWII.
I think there are 2 basic scenarios the US employs nuclear weapons again: 1. Nukes are used on us, then we return the favor 1,000 fold. 2. The US finds itself in imminent danger of ceasing to exist. Outside these, I can't think of a situation where we go down the nuclear road. I'm not saying that their couldn't be other scenarios; just saying that without deeper thought and analysis of the matter, those are it. Conversely, I can see the need to have a carrier or two in reserve to replace losses and prevent the imminent danger of our nation falling from coming about.
Hypothetical situation: if we find ourselves in a conflict with Iran, I can see the possibility of losing a carrier in the Straights of Hormuz. It's narrow and Iran has missiles that can sink our ships. Now, does that place us in a situation for defeat? I don't think so. Our air, land, and sea power would tear them apart. But, they as I understand it, have some capabilities to make it costly on us. If we lose a carrier, I know we can bring in more. However, losing one multi-billion dollar carrier to a insanely less expensive missile attack is a possibility. That would smart a bit.
Perhaps someone more knowledgeable about Iran's missle capabilities and our fleets' anti-missile capabilities could bring some facts to the discussion and end my hypotheticals and speculation. I would appreciate that information.