The reactor solves the fuel problem—not every problem
A nuclear submarine carries a compact reactor that releases heat by splitting uranium atoms. That heat makes steam, which drives turbines: some turn the propeller, while others generate electricity for the vessel’s equipment. Unlike a diesel engine, the reactor does not need oxygen from the air to keep producing power. The boat can therefore travel underwater without surfacing to take in air for propulsion.
The long refueling interval comes from the energy packed into nuclear fuel. A reactor can use that fuel over years of operation, so the submarine’s practical schedule is not set by frequent trips to a fuel depot. “Up to twenty-five years” describes a possible interval without reactor refueling, not a promise that a boat can stay at sea that long.
Keeping a crew alive is a separate system
Power also lets the submarine manage the air and water inside its sealed hull. Electrolysis uses electricity to split purified water, producing oxygen for the crew to breathe. Equipment also has to remove carbon dioxide exhaled by people; otherwise, oxygen production alone would not make the air safe. These systems make a submerged patrol possible, but they do not make the boat self-sufficient in every respect.
Food is finite, machinery wears, and equipment can fail. The crew also needs maintenance and resupply, and a vessel may have operational reasons to return to port. A reactor that still has usable fuel cannot replace a meal, a spare part, or an inspection.
Endurance is a chain, not a single number
The striking distinction is between how long the reactor can go without refueling and how long the submarine can remain deployed. The reactor can remove routine fuel stops from the equation; provisions, repairs, crew needs, and mission demands still set the limits. Nuclear power changes what forces a submarine to surface, but it does not turn the boat into a sealed world that can last indefinitely.
