First off - the nuclear power plants on USN vessels all require gravity to work. Even without that problem, power generation is problematic without a heat sink to keep cooling the condensers. And without the heat sink, there’s no way to keep the condensers as low pressure, so again, you’ve lost power very quickly.
Then there’s the batteries. AIUI nuke power subs have much less battery endurance than the old D/E boats. I think we’re talking on the order of a few hours of emergency power - not 12-24 at near normal operations. That’s assuming that the batteries still work without gravity. Again, this is just AIUI, but I believe that sub batteries are still unsealed lead acid batteries. Once inertia gets involved at all, you’ll have concentrated sulfuric acid running through the battery compartment, with a corresponding loss of power to the rest of the boat.
Then there’s the heat issue - modern subs spend most of their time in near freezing water - 34-35 F. They’re black to be harder to spot from the air. There is some insulation, but again AIUI, a sub is more usually putting heat into the environment from the reactor plant one way or another than it’s worried about cooling - because the sea makes for such a lovely heat sink. The sun would be putting a huge amount of heat into the hull, with no where for it to go. And on the inside, there’s the reactor doing much the same - even if it’s been shut down the moment they find themselves in orbit! In space, the heat generated by human bodies is a not insignificant issue, too.
My vote is going to go to heat stroke killing the crew before CO and CO[sub]2[/sub] poisoning can do them in. With smoke from the battery fire being a distant third place.
But, Tripler, I think that you’d be better off using sub crews to build your spaceship crews from, than air crew. I’ve heard stories about what air crew can do when they get off their shift and they have to fit an extra body into the barracks room. And I’ve never met an airman who knows how to ration water use.