Explainer: how do nuclear submarines work?

Australia will be building, using – and crewing – nuclear submarines under a new deal with the United Kingdom and United States.

There are many interesting economic and geopolitical implications from this, but it’s worth taking a closer look at the subs themselves: what exactly is a nuclear submarine, how does it differ from our current subs, and does it present any extra safety risks?

What is a nuclear submarine?

First up, the basics: what do we mean when we say ‘nuclear submarine’?

A nuclear submarine has its electricity supplied by a nuclear reactor.

“The nuclear reactor is used to heat water, to generate steam,” explains Tony Irwin, technical director at SMR Nuclear Technology.

“[The steam] then drives the steam turbine generator, so that produces electric power.”

This is in contrast to diesel-electric submarines, which use a diesel engine to charge electric batteries.

The ‘nuclear’ term refers to the power alone – nuclear submarines don’t necessarily carry nuclear weapons (although they can). In a statement, prime minister Scott Morrison said that the Australian submarines would not carry nuclear weapons.

“Australia is not seeking to acquire nuclear weapons or establish a civil nuclear capability,” said Morrison.

“We will continue to meet all our nuclear non-proliferation obligations.”

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Read more: Does Australia have the expertise to operate nuclear-powered submarines?

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How does the reactor work?

“Practically all nuclear-powered submarines use what’s called a PWR, a pressurised water reactor,” says Irwin.

“This is exactly the same technology as the majority of power reactors worldwide.”

Nuclear reactors work by generating heat from nuclear fission. Atoms of uranium-235 are bombarded with neutrons, causing some of them to split, releasing energy in the form of heat and more neutrons in the process.

This presents a hurdle: uranium-235 is hard to come by, so submarines require “enriched” uranium.

“Most of the uranium that you dig out of the ground, in fact over 99%, is uranium-238, which is not fissile. You can’t run a reactor with it,” explains Andrew Stuchbery, a professor of nuclear physics at the Australian National University.

Uranium-238’s atomic structure prevents it from splitting in the same way uranium-235’s does.

“Only 0.7% of the uranium that we get out of the ground is uranium-235. So you have to go through this complicated enrichment process,” says Stuchbery.

Enrichment entails reacting the uranium with fluorine and then separating it out based on mass, allowing the uranium-235 to become concentrated.

Submarine reactors require highly enriched uranium to operate with the level of power they have.

“The fuel in a nuclear submarine is actually of an enrichment grade that could be used in a weapon,” says Stuchbery.

Submarine reactors are smaller than large-scale land-based nuclear power stations. Still, they’re significantly larger than Australia’s current only nuclear reactor – ANSTO’s OPAL reactor, which is used to make radiomedicine products and assist with research.

“The power of a nuclear-powered submarine is around the 150-200 MW thermal power,” says Irwin.

“For comparison, a very large power generator is about 3400 MW, generating about 1100 MW of electrical output.

“The OPAL research reactor at Lucas Heights, the Australian one, that’s about 20 MW. And it doesn’t generate any electricity.”

Why would you want to build a nuclear-powered submarine?

Nuclear reactors famously require a lot of technology and expertise to manage safely, so why would you want to put one on an underwater vessel?

A nuclear reactor allows the submarine to be less reliant on external supplies. Diesel-electric subs need to refuel regularly, while a nuclear submarine – particularly one based on UK and US designs – might never need to refuel.

“They use what’s called highly enriched uranium, which enables them to run for the whole life of the submarine – so a 25-30 year life – without being refuelled,” says Irwin.

Diesel-electric submarines also need to resurface often to turn on their diesel engines to charge their batteries.

With nuclear submarines, however, “the length of time underwater is only limited by the endurance of the crew and the amount of food that can be carried in it”.

The UK defence force claims their submarines can circumnavigate the world without resurfacing over a 90-day period.

This means there are functional advantages to keeping a nuclear reactor onboard, despite the additional safety risks.

Speaking of safety…

What are the safety issues?

Because submarine reactors are similar but smaller versions of land-based reactors, a lot of the technical and safety considerations are the same. But there’s one key difference between submarine and land-based reactors: movement.

“A land-based reactor doesn’t rock and roll. And of course a submarine does,” says Irwin.

This is particularly important when considering the addition and removal of control rods, which keep the reactor stable and generating power at a regular pace.

“Normally, in a land-based power reactor, the control rods drop in under gravity, so if you de-latch them they’ll just drop in,” says Irwin. “You can’t rely on that with a submarine.”

Instead, you need extra controllers, “to make sure that the reactor can always be shut down, within the submarine, at any angle”.

On the other hand, submarines do always have access to water, which is critical to cool the reactor down if it’s heating too much.