The sections of ship are designed to be isolable from each other. Close a door, shut some duct work to isolate air, and you're fat dumb and happy back in the engine room!
Uh, except commenters and Wikipedia both say that's not true - that the Los Angeles class has only 1-2 bulkhead doors and they most likely had cables and plumbing passed through, making them impossible to seal.
I guess you're just full of shit, then, and lying about serving on one of these subs. Nobody knows you're a dog on the internet, huh?
The compartment door between the forward compartment and the engine room is generally always closed. The engine room is a restricted area (more so than the forward compartment). Anything running through this hatch does have quick disconnects, even in an extended overhaul (which I have done). In addition, the actual reactor control room (Called Maneuvering) is equip with additional systems that maintain positive air flow into the room, which also assist with cooling that space, all designed to keep that room inhabitable during the worst of emergencies to include a steam line rupture. You will never find this room unmanned on a ship that is still commissioned with a reactor, whether it is shut down or not. SSBN (Ohio Class) and SSN (LA Class) veteran
To do nothing is to be nothing.