[Lower Deck]

I’m guessing there is a similar provision for refueling a nuclear sub.

I wouldn't know that. But when you refuel a nuclear sub you just replace the fuel rods, don't you? You don't take the whole reactor assembly out. How much space does that require?

[null and void]

I’m not certain, but on a commercial power plant they remove essentially the whole top of the reactor to pull and replace the rods. Not a trivial task even with full access, I can’t imagine how much of a pain in the neck it would be to feed the rods through a small hatch using remote TV and waldoes.

[PeterPrinciple]

You don’t take the whole reactor assembly out.

http://holbert.faculty.asu.edu/eee460/jtc/index.html

Would appear the fuel lasts as long as the boat?

http://www.gizmag.com/go/7292/

[rlmorel]

I have heard the pile in a nuclear sub is the size of an office desk.