Summary and Key Points: A nuclear submarine does not need the surface. Its reactor does not consume oxygen, so it never has to come up for air as a diesel boat does, and the electrical power it produces is enough to run two systems that keep a sealed steel hull habitable indefinitely. One splits purified fresh water into hydrogen and oxygen with an electric current, keeps the oxygen for the crew, and vents the hydrogen overboard. The other boils seawater using heat from the reactor and condenses the steam back into drinking water. An Ohio-class ballistic missile submarine carries 15 officers and around 140 enlisted sailors, and it can supply every one of them with air and water for as long as the reactor runs. What it cannot do is feed them forever.
The Wonder of US Navy Nuclear Submarines
The advent of nuclear-powered submarines revolutionized naval technology.
While legitimate concerns remain about nuclear reactors in the event of a submarine being damaged or destroyed, the advantages of nuclear technology are immense.
Whereas conventionally powered submarines have to surface frequently for air or to recharge their batteries, this is not necessary for a nuclear submarine.
Nuclear reactors provide exponentially more energy than conventional power plants, enabling additional systems to turn the ship into a self-contained environment that generates its own oxygen, fresh water, and power, keeping the ship going.
The limits of a modern submarine are how much food it can carry and the sanity of its crew.
How Nuclear Submarines Get Their Oxygen While Underwater
Prior to the development of naval nuclear reactors, diesel-electric power plants required atmospheric oxygen to operate and often needed their batteries recharged (the same is still true of modern conventionally powered submarines).
Nuclear reactors, on the other hand, don’t need oxygen and instead use nuclear fission to generate heat and power the ship’s steam turbines.
As a result, nuclear submarines don’t need to surface to resupply air to keep their generators running.
Instead, the reactor can keep running indefinitely as long as the instruments that maintain it remain intact.
That said, a submarine still needs oxygen for the crew, so how do modern submarines handle that problem?
Nuclear submarines use a process called electrolysis, which splits water molecules into hydrogen gas and oxygen gas.
These electrolytic oxygen generators pass an electric current through purified water, which breaks the chemical bond holding hydrogen and oxygen together.
The system collects oxygen and introduces it into the ship’s atmosphere, while handling hydrogen and other harmful gases separately and venting them safely overboard.
(July 9, 2018) – Multi-national Special Operations Forces (SOF) participate in a submarine insertion exercise with the fast-attack submarine USS Hawaii (SSN 776) and combat rubber raiding craft off the coast of Oahu, Hawaii during Rim of the Pacific (RIMPAC) exercise, July 9. Twenty-five nations, 46 ships and five submarines, about 200 aircraft, and 25,000 personnel are participating in RIMPAC from June 27 to Aug. 2 in and around the Hawaiian Islands and Southern California. The world’s largest international maritime exercise, RIMPAC provides a unique training opportunity while fostering and sustaining cooperative relationships among participants critical to ensuring the safety of sea lanes and security of the world’s oceans. RIMPAC 2018 is the 26th exercise in the series that began in 1971.` (U.S. Navy photo by Mass Communication Specialist 1st Class Daniel Hinton)
Western Australia, Australia (Feb. 25, 2025) The Virginia-class fast-attack submarine USS Minnesota (SSN 783) prepares to moor at HMAS Stirling, Western Australia, Australia, Feb. 25, 2025. Minnesota arrived in Western Australia kicking off the first of two planned U.S. fast-attack submarine visits to HMAS Stirling in 2025. Minnesota is currently on deployment supporting the U.S. 7th Fleet, the U.S. Navy’s largest forward-deployed numbered fleet, operating with allies and partners in preserving a free and open Indo-Pacific region. (U.S. Navy photo by Lt. James Caliva)
U.S. Navy SSN Submarine. Image Credit: Creative Commons.
However, the electrolytic oxygen generator requires a lot of energy to run continuously, which is why it is available only on nuclear submarines.
These generators require fresh water rather than saltwater to avoid potentially dangerous byproducts.
Where Do They Get Fresh Water
This raises the question of where submarines get their fresh water.
Modern submarines operate onboard desalination plants to turn salt water into drinkable fresh water.
These plants use thermal distillation.
Salt water is added to the plant and heated until it becomes steam, separating out the salt and other non-drinkable materials.
The steam then condenses back into water, which is collected, stored, and eventually distributed across the ship.
Whatever is left of the remaining salt and brine is discharged back into the ocean. The ship’s onboard nuclear reactor powers and enables these systems. In fact, the reactor is the primary heat source for distilling seawater.
As a result, the crew has more than enough water for drinking, cooking, bathing, and anything else that needs water.
An artist’s concept of the nuclear-powered submarine SEAWOLF (SSN-21).
However, anyone familiar with nuclear submarines knows they must make several port calls during their deployment.
Why is this necessary if the ship’s onboard systems can generate infinite water and oxygen?
The answer is food.
While nuclear submarines can keep their onboard freezers running for eternity, they still need food to feed their crews.
Ohio-class ballistic missile submarines (SSBNs), for example, house a crew of 15 officers and about 140 enlisted personnel. That’s a lot of mouths to feed.
On long, multi-month deployments, fresh food is consumed remarkably quickly, and ships have limited capacity to store it. This is why no patrol can last forever.
Keeping the Crew Sane
As someone who has set foot in a few nuclear submarines, I can confirm that spending months on end inside one of them would be physically and mentally challenging.
Nuclear submarines, even the massive Ohio-class, are extremely claustrophobic.
Puget Sound Naval Shipyard, Wash. (Aug. 14, 2003) — USS Ohio (SSGN 726) is in dry dock undergoing a conversion from a Ballistic Missile Submarine (SSBN) to a Guided Missile Submarine (SSGN) designation. Ohio has been out of service since Oct. 29, 2002 for conversion to SSGN at Puget Sound Naval Shipyard. Four Ohio-class strategic missile submarines, USS Ohio (SSBN 726), USS Michigan (SSBN 727) USS Florida (SSBN 728), and USS Georgia (SSBN 729) have been selected for transformation into a new platform, designated SSGN. The SSGNs will have the capability to support and launch up to 154 Tomahawk missiles, a significant increase in capacity compared to other platforms. The 22 missile tubes also will provide the capability to carry other payloads, such as unmanned underwater vehicles (UUVs), unmanned aerial vehicles (UAVs) and Special Forces equipment. This new platform will also have the capability to carry and support more than 66 Navy SEALs (Sea, Air and Land) and insert them clandestinely into potential conflict areas. U.S. Navy file photo. (RELEASED)
Each ship must accommodate numerous delicate instruments, weapon systems, life support systems, and the crew simultaneously.
The result is a functional but not-so-elegant living space.
Any submariner will gladly testify to the challenges of living aboard a nuclear submarine. Although newer submarines include facilities like gyms and other recreational amenities to improve quality of life, sailors are still stuck in a submerged tin can, devoid of sunlight and fresh air.
Port calls matter not only to resupply the ship with fresh food, but also to give the crew a literal breath of fresh air.
With all that in mind, the endurance of a nuclear submarine is nearly limitless.
The nuclear reactor enables systems that generate oxygen and fresh water, and because it doesn’t need its batteries replaced, the ship can, on paper, remain submerged indefinitely.
The only limit to their endurance is the ship’s food supply and the sanity of the crew.
As a result, nuclear submarines can remain at sea substantially longer than their conventionally powered counterparts.
This allows the U.S. to keep its strategically valuable SSBNs at sea longer, thereby increasing its undersea deterrence.
About the Author: Isaac Seitz
Isaac Seitz, a Defense Columnist, graduated from Patrick Henry College’s Strategic Intelligence and National Security program. He has also studied Russian at Middlebury Language Schools and has worked as an intelligence Analyst in the private sector.
