Thanks to their state-of-the-art, built-in reactors, modern nuclear submarines never have to surface to refuel. When the submarine goes into service, it has all the nuclear fuel (such as uranium) it will need for its projected lifetime, which can extend as long as 33 years. Just as in a nuclear powerplant on land, nuclear fission in the reactor generates heat, which produces steam, which turns a turbine, which provides electricity. The reactor makes the submarine completely self-sufficient, with enough juice to keep all the equipment running day in, day out.
Modern subs don’t need to come up for air, either. Chemical processes continually remove carbon dioxide from the air on board, while oxygen generators use electrolysis to extract oxygen from surrounding seawater. As long as the equipment is working properly, the air is always fresh and breathable. Similarly, on-board distillation plants continually turn seawater into clean drinking water.
In fact, the only limiting factor for staying submerged is the food supply. Submarines typically carry about 90 days’ worth of food, which is kept in storerooms, refrigerated rooms and freezers. For exceptionally long missions, when the storerooms fill up, submarine crews may have to stack canned goods
along floors and passageways as well.
Answered by Tom Harris.