The limiting factor tends to be F44/JP5 usage rate. CVN lets you have bigger bunkers for a given size of ship, but a big CVN flying an intensive airplan uses F44 at a massive rate.
That tends to mean UNREP from an MSC tanker every 4-5 days and they don't p1ss about - it's not unusual to have 6 hoses chucking JP5 across at 500 cubes an hour each.
The PWR2 reactor we currently use has a relatively low power output and one of its limiting factors is core life. If you put something like that in a CVN you'd need a number of reactors to get enough power and potentially core life, given the significantly different power loading and operational profiles on a CVN compared to a submarine.
I'm no nuclear engineer, but you also get into safety, shielding and heat transfer issues on a ship that you don't necessarily get on a boat. If you look at the USN A4W powering their current CVN it's a very different beast from the S6G and S8G that power their boats.