Nuclear submarines are powered by nuclear reactors which produce heat that creates high-pressured steam to spin turbines and power the boat's propeller.
They can run for about 20 years before needing to refuel, meaning food supplies are the only limit on time at sea.
The boats are also very quiet, making it harder for enemies to detect them and can travel at top speed - about 40kmh - for longer than diesel-powered subs.
The first nuclear submarines were put to sea by the United States in the 1950s. They are now also in use by Russia, France, the United Kingdom, China, and India.
A senior US defence official told reporters in Washington DC: 'This will give Australia the capability for their submarines to basically deploy for a longer period, they're quieter, they're much more capable.
'They will allow us to sustain and to improve deterrence across the Indo-Pacific.'
Zack Cooper, a senior fellow with the American Enterprise Institute, said nuclear submarines would hugely boost Australia's military capability.
'They are going to be much, much more capable in the large, expansive ocean that is Australia has to deal with,' he told the ABC.
Will Australia have nuclear weapons?Scott Morrison made it clear that the nuclear-power submarines will not have nuclear missiles on board.
Australia has never produced nuclear weapons and signed the Treaty on the Non-Proliferation of Nuclear Weapons in 1973 which prevents non-nuclear states which don't already have them from developing nuclear weapons.
Mr Morrison also said the Australia has no
