How a nuclear submarine works: Why Australia’s AUKUS boats are faster, stealthier and stay down for months

It’s early in the 2030s and an Australian nuclear-powered submarine slips away from HMAS Stirling on Garden Island, submerges and disappears into the Indian Ocean.

Within days it’s conducting surveillance or posing a threat to an adversary in an ever-expanding area in the Indo-Pacific region. Within a week it could be anywhere.

It’s undetected for weeks or months, confusing or threatening an adversary who will have to divert resources — ships, its own submarines, aircraft, satellites — to look for it. An entire fleet could be at-risk, or fear it’s at-risk, from a single Australian submarine.

This scenario is outlined in a research paper produced by the Australian Parliamentary Library on the advantage of nuclear-powered (but conventionally-armed) submarines in deterring adversaries in an increasingly contested region.

All submarines — conventional ones, like the Collins-class boats currently operated by the Royal Australian Navy, or the nuclear-powered boats it’s acquiring — rely on stealth.

“Stealth, the ability to remain undetected in the undersea environment, is probably the greatest tactical attribute of any submarine,” says Peter Scott, a former Collins-class commanding officer. He was the RAN’s director-general submarines when he retired with the rank of Commodore from full-time service in 2017. He’s currently vice-president of the Submarine Institute of Australia.

“Stealth creates doubt in an adversary’s mind,” Scott says.

“And the longer the submarine’s range, and the greater its endurance (the length of time it can stay at sea), the wider the doubt about where that submarine might be and what it might be doing.”

While all subs are stealthy, nuclear boats — the US Virginia-class vessels the RAN is due to obtain in the early 2030s, followed by the new Australian-built AUKUS subs projected to be in service in the early 2040s — are stealthier. In the language of submariners, they are more discreet.

Conventional subs are powered by diesel generators and electric motors. No matter how silent when submerged, they have a higher indiscretion rate and are more vulnerable to detection and attack, since they must come close to the surface on a routine basis to draw in air to recharge batteries and renew their internal atmosphere. It’s known as “snorting”.

Nuclear boats don’t snort operationally.

At the heart of a nuclear-powered sub is a pressurised water nuclear reactor designed to run for the entire 30-plus-year life of the submarine – without refuelling.

The reactors create steam that spins turbines connected to a propeller shaft. The AUKUS subs will be powered by a Rolls-Royce reactor which, instead of driving a traditional screw propeller, uses pump-jet technology involving rotating blades enclosed in a cylindrical shroud. The result makes the submarine quieter and harder to detect. Reactors also create power for lighting, refrigeration, air-conditioning, air purification and all the other systems on the vessel.

They are not just stealthier. Nuclear submarines are faster, bigger, more flexible, better armed, and able to stay submerged for longer — as long as the crews’ stamina and food supply holds out.

They offer, as the parliamentary library paper noted, “a game-changing upgrade in deterrence”.

Nuclear boats can travel at around 30 knots when submerged, compared to perhaps 20 knots in a Collins, and they can maintain that speed indefinitely.

“That translates to far greater options about where and when and for what purpose a submarine can be deployed,” Scott says.

Because they’re bigger, they carry more weapons and other equipment, such as underwater drones, and more people, including special forces teams.

The AUKUS subs will displace more than 10,000 tonnes, larger than the Virginias (just over 7000 tonnes), and the Collins-class (about 3300 tonnes).

Crucial to any sub’s operations is the combat management system which gives the crew a picture of their environment. It’s effectively the sub’s brain.

“It brings together all the sensory inputs the submarine has,” Scott says. “You have sensors like sonars, sensors that pick up electronic transmissions, and periscopes. The combat management system brings all those inputs together so the crew can build a correlated picture of where you are in the world and what’s going on around you.”

Australia’s nuclear-propelled submarines will use an advanced version of an American combat management system, the AN/BYG-1, already in use in the Collins-class.

The nuclear subs will require bigger crews — up from 58 in the Collins to more than 130 in the Virginias — and they will be better armed.

The Collins boats carry torpedoes for attacking other subs and surface ships, and anti-ship Harpoon missiles. The Virginias can carry more torpedoes and Harpoon missiles, as well as Tomahawk missiles that can hit land targets 1250km to 2500km away. The Government has indicated the AUKUS subs will be similarly armed.

The main limit on the endurance of a nuclear-powered submarine is the food supply it can carry.

“The ultimate constraint is feeding the people onboard,” Scott says. “The people can stay out there for a very long time. You can generate power continuously, and you can regenerate your atmosphere continuously. But what you can’t do is generate food.”

A Collins submarine can remain at sea for a maximum of 70 days without resupply, although 30 to 40 days is more usual. Virginias, on the other hand, can carry enough food for at least 90 days and even longer before needing to be resupplied.

Space is the scarcest resource on any submarine. While the nuclear subs are bigger, space is still tight.

“If you were to step onboard a Collins or Virginia, in many respects they would be quite similar,” Scott says. “The Collins might be a little bit tighter for space, but they’d both be pretty jam-packed. Space would be at a premium.”

He says crews will face the same challenges: “They’re working in a confined space. They’re at sea for extended periods of time. They’re working in close proximity and in close teamwork with others.”

When they’re not at sea, the submarines will require specialised maintenance. That’s one of the key reasons why the Federal and Western Australian governments are developing the Henderson Defence Precinct, south of Perth.

To be built in stages, it will eventually become one of the nation’s main naval maintenance and shipbuilding centres — and the major facility for maintaining the nuclear-powered submarines, providing servicing, repair and upgrade work. It will include large dry docks where submarines can be taken out of the water for extensive maintenance.

Henderson will also support naval shipbuilding and surface-fleet maintenance.

Vice-Admiral Jonathan Mead, director-general of the Australian Submarine Agency, says the new submarines will strengthen Australia’s ability to deter threats and protect its national interests.

“Nuclear-powered submarines provide Australia with the stealth, endurance, mobility and striking power to complicate an adversary’s calculations, hold a potential adversary at-risk and help prevent conflict before it begins,” he says.

Former submarine commander Scott says the advantage of nuclear-powered submarines particularly applies to Australia because of its geographic circumstances.

“When you look at the extent of our coastline, when you look at the vastness of the Indian and the Pacific oceans, and the distances a submarine needs to travel to find itself in strategically significant waters, nuclear-powered submarines with substantially more sustainable speed have a tremendous advantage,” he says.

“They give government a much greater option set of where and when and how it might deploy its submarines.

“That creates a much greater, and more difficult, problem for adversaries, because they just don’t know where a conventionally powered submarine is. And they absolutely don’t know where a nuclear-powered submarine is or where it could be.”