Submariners are all volunteers rigorously tested before acceptance.
Even more than in conventional submarines, the physical and psychological stamina of the crew on nuclear-powered submarines becomes a crucial factor. They must also deal, while on patrol, with being largely cut off from the outside world, including their families, for long periods of time. Crew members live inside a pressure hull filled with the machinery required to keep them alive and allow the ship to function. They must make do with the cramped spaces between the machines, enjoying little stowage space or privacy. Then it's back to the duty watch.