Nuclear power has revolutionized the submarine industry, allowing vessels to remain submerged for extended periods of time without the need for refueling. Food supply is now the only limitation for a nuclear submarine at sea. With the capability to stay underwater for three or four months in a row and cross oceans with ease, nuclear submarines have become the most reliable and resilient vessels in the world. Navy nuclear submarines can remain submerged for long periods of time without any air-related issues.
They produce their own oxygen and keep the air clean, so the only limits for how long they can stay underwater are food and supplies. Submarines generally have a 90-day food supply, meaning they can spend three months underwater without resurfacing. At the height of the Cold War, approximately five to ten nuclear submarines were being commissioned at each of the four Soviet submarine yards (Sevmash in Severodvinsk, Admiralteyskiye Verfi in St. Petersburg, Krasnoye Sormovo in Nizhny Novgorod and Amur Shipyard in Komsomolsk-on-Amur).
Nuclear power proved to be ideal for the propulsion of strategic ballistic missile (SSB) submarines, greatly improving their ability to remain submerged and undetected. Nuclear reactors are quieter, do not consume air and produce greater energy production, allowing nuclear submarines to remain submerged for months instead of days while traveling at higher speeds underwater. Nuclear submarines still have a clear advantage in terms of resistance over AIP ships, as their engines are practically silent. The weakness of nuclear submarine stealth technology is the need to cool the reactor even when the submarine is not moving; about 70% of the reactor's output heat dissipates into seawater.
In 1960, the second nuclear submarine in the United Kingdom was commissioned from Vickers Armstrong and, equipped with the Rolls-Royce PWR1 nuclear plant, the HMS Valiant was the first British nuclear submarine. From the late 1950s to the end of 1997, the Soviet Union, and later Russia, built a total of 245 nuclear submarines, more than all other nations combined. A nuclear submarine can stay and operate underwater for a couple of decades, as long as it has enough supplies and rations for its crew on board to survive for so long. However, keep in mind that AIP submarines are mostly small or medium-sized vessels with crews of around 30 and 60 respectively, while nuclear submarines are usually larger with crews of 100 or more. So, if appearing on the surface of the water is so dangerous for a submarine and its crew, why does it have to resurface? What prevents a submarine from being submerged indefinitely? Can't you stay underwater for the entire mission? The answer is no; while nuclear power allows submarines to operate for about twenty years without refueling, food supply remains the only time limit for a nuclear submarine at sea.