Australia is to build at least eight nuclear-powered submarines in a new partnership with the UK and United States known as AUKUS.
The reactors on board will not be built in Australia but bought from the US or UK.
What is a nuclear-powered submarine?
A nuclear submarine is a boat powered by a nuclear reactor on board.
It does not mean it carries nuclear weapons. The Australian submarines will not have nuclear warheads mounted on missiles to be fired from the deep.
A nuclear-powered submarine can be armed with nuclear weapons - as US and UK submarines are - but the government is adamant that this will not happen with the Australian subs.
On this, we can trust the word of politicians because nuclear-tipped missiles are not the kind of thing which can be introduced secretly.
The Australian subs will be nuclear-powered but not nuclear-armed.
What's the advantage?
In a nuclear-powered submarine, nuclear reactors create energy which generates steam which powers the turbines (the engines).
Because of the huge amount of energy generated by a nuclear reactor, a nuclear-powered submarine can stay underwater for months. The constraint is not the engine but how long other supplies - particularly food for the crew - can last.
In a conventional submarine, a diesel engine charges batteries which then drive the submarine when it's underwater.
But batteries can only be recharged on the surface and that means the vessel has to come up every two or three days, according to Dr Malcolm Davis, a senior strategist at the Australian Strategic Policy Institute.
On the surface, the vessel is vulnerable to enemy detection and potentially attack.
It also slows the vessel down, according to a former Royal Australian Navy submarine commander. The effectiveness of a submarine depends on "endurance at speed" - the ability to go far as quickly as possible and undetected.
"When it runs its engines to charge, it has to slow down and it makes noise so it's vulnerable. Its overall speed can be a third of that of a nuclear submarine," the former submarine commander who wanted to remain anonymous told this paper.
Are they safe?
There have been two significant accidents involving nuclear-powered submarines.
On April 10, 1963, the USS Thresher sank during diving tests in the Atlantic, about 350 kilometres from the coast of the United States. All 129 crew members died.
An investigation indicated that a leak of water in the engine room caused the vessel's electrical system to short circuit and that shut down the reactors as well as its ability to get back to the surface.
The Thresher remains at the bottom of the sea in six pieces, about 3,000 metres down.
No leak of radioactivity has been detected, according to the US government.
"The reactor fuel elements are made of materials that are extremely corrosion resistant, even in seawater," a spokesman for the Navy told a trade magazine. "The reactor could remain in seawater for centuries without releasing fission products."
And on August 12, 2000, the Russian nuclear-powered submarine, Kursk, sank with the loss of all 118 people on board.
Its two reactors were eventually recovered and "defueled".
Advocates for nuclear-powered submarines (and indeed for nuclear power on land) say that it is relatively safe - the number of people who have died in nuclear accidents are few compared to those killed, for example, through the use of fossil fuels.
They also say that conventional submarines also suffer terrible accidents. An Indonesian submarine sank in June, with the loss of all 53 crew.
Dr Davis says that Australia's nuclear-powered submarines will not need refueling. The fuel will be inserted at the start of their lives and it will last for the 30 years of their service, so there will not be a transfer of nuclear fuel to docks in Australia and then onto the vessels.
"The nuclear reactor is self-contained so it never needs refueling," Dr Davis said.
But it's not just about the hardware
Some analysts of international relations mark the agreement as a very significant change in the way Australia sees itself and in the way Australia behaves.
There is already an alliance between the United States and Australia - ANZUS - but it is much broader and doesn't imply such tight cooperation and mutual commitment.
The new alliance, inelegantly called AUKUS, is much tighter.
"This is hugely important," Dr Davis said.
"It's a recognition that Australia has a global role. We are laying our cards on the table. We are saying, 'We are going to be there with the UK and the US in any crisis and they are going to be there for us'."
The unmentioned elephant in the room is China. The former submarine commander said that the submarines were crucial for Australia's position in the South China Sea, much of which is contested by China, with a myriad disputes over which countries have sovereignty over important trade routes.
"Ninety-nine per cent of Australia's trade comes by sea and 60 per cent of that comes through the South China Sea," the submariner said.
"The submarine is the only platform which gives the government options when you don't control the sea and the air-space.
"If you don't control the sea and the air-space, you can always put in a submarine."
That is what the Australian government has decided to do.
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